Wide frequency range, arbitrary waveform DDS

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Stef

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Aug 16, 2022, 11:37:41 AMAug 16
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To generate frequencies from approximately 0.5 mHz to 12 MHz with a DDS
a minimum clock of >24, say 25 MHz, is required. To be able to go down
to 0.5 mHz, a phase accumulator of at least 36 bits is required. This
will give sub mHz resolution over the entire range. Nice for the low
frequencies, but not of much use for MHz frequencies (in this
application).

Is there any objection to using a smaller phase accumulator and a clock
pre-scaler to generate the lower frequencies?

I see Analog Devices has DDS chips up to 48 bits, so 36 bits would not
be a problem (except for cost maybe).

But al of the DDS chips I find from Analog seem only to implement a
fixed sine table/function. Do DDS chips exist that allow downloading an
arbitrary lookup table with 2^10 - 2^16 entries of 10 - 16 bit each?

If no such standard chips exist, I expect I need to implement the DDS
in an FPGA. Using a smaller accumulator would probably save some space
in the FPGA. Or am I just optoimizing prematurely?


--
Stef

Baker's First Law of Federal Geometry:
A block grant is a solid mass of money surrounded on all sides by
governors.

gnuarm.del...@gmail.com

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Aug 16, 2022, 9:42:50 PMAug 16
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On Tuesday, August 16, 2022 at 11:37:41 AM UTC-4, Stef wrote:
> To generate frequencies from approximately 0.5 mHz to 12 MHz with a DDS
> a minimum clock of >24, say 25 MHz, is required. To be able to go down
> to 0.5 mHz, a phase accumulator of at least 36 bits is required. This
> will give sub mHz resolution over the entire range. Nice for the low
> frequencies, but not of much use for MHz frequencies (in this
> application).
>
> Is there any objection to using a smaller phase accumulator and a clock
> pre-scaler to generate the lower frequencies?

There is a STRONG objection to shortening the phase word. It will cause problems with close in spurs (which can't be filtered) and reduces your resolution. See below.

Instead of a pre-scaler, use a post divider. A pre-scaler on the clock to the DDS requires the analog filter to be adjusted to suit.


> I see Analog Devices has DDS chips up to 48 bits, so 36 bits would not
> be a problem (except for cost maybe).
>
> But al of the DDS chips I find from Analog seem only to implement a
> fixed sine table/function. Do DDS chips exist that allow downloading an
> arbitrary lookup table with 2^10 - 2^16 entries of 10 - 16 bit each?

If you are looking to generate a non-sine waveform, it is not needed, or useful.


> If no such standard chips exist, I expect I need to implement the DDS
> in an FPGA. Using a smaller accumulator would probably save some space
> in the FPGA. Or am I just optoimizing prematurely?

Do you work at Highland Technology?

I've posted about an appropriate solution more than once in s.e.d. Here is a fair summary...

A DDS should be designed to generate a top frequency over a 2:1 range. This is easy stuff, with good accuracy and very low jitter if properly designed, use of a LONG phase word and a good filter after the DAC. It is important to NOT shorten the phase word as that introduces spurs close to the frequency being generated, which can not be filtered out. This is because the truncation of the phase word creates discontinuities which show up as close in spurs. Keep the phase word long and round the sine output to suit a 16 bit DAC. This generates harmonics which are much easier to filter.

You don't need to use a single large table for the sine. sin(a+b) = sin(a)cos(b) + cos(a)sin(b), where a is the msbs of the phase word and b is the lsbs. Use appropriately sized tables for sin(a), cos(b) and a single table for the product cos(a)sin(b). This is practical if (a) and (b) are each truncated a bit. Because sin(b) is always small, this term is always small and does not need to be calculated with full resolution on a and b. This product table also does not require the full word width. Three table lookups, one multiply and one add.

A programmable divider then divides the top frequency by 2**N by counting up to a settable value.

The top frequency can clock a final FF buffer register of your favorite technology to provide the actual pulse output with an appropriate jitter.

The settings of these two units can both be changed on a single clock cycle by writing to a buffer register and updating the actual operational registers simultaneously on a cue. The DDS will continue from the present phase, so will produce one top frequency clock pulse that is an intermediate period. The programmable divider will continue from the current count, either triggering right away, or continuing to count from the present value. Either way it will produce one output pulse that is within the range of the two settings, the prior setting and the new setting.

Does this make sense? Feel free to drop my name with your boss.

--

Rick C.

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Stef

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Aug 17, 2022, 7:36:42 AMAug 17
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On 2022-08-17 gnuarm.del...@gmail.com wrote in comp.arch.fpga:
> On Tuesday, August 16, 2022 at 11:37:41 AM UTC-4, Stef wrote:
>> To generate frequencies from approximately 0.5 mHz to 12 MHz with a DDS
>> a minimum clock of >24, say 25 MHz, is required. To be able to go down
>> to 0.5 mHz, a phase accumulator of at least 36 bits is required. This
>> will give sub mHz resolution over the entire range. Nice for the low
>> frequencies, but not of much use for MHz frequencies (in this
>> application).
>>
>> Is there any objection to using a smaller phase accumulator and a clock
>> pre-scaler to generate the lower frequencies?
>
> There is a STRONG objection to shortening the phase word. It will cause problems with close in spurs (which can't be filtered) and reduces your resolution. See below.
>
> Instead of a pre-scaler, use a post divider. A pre-scaler on the clock to the DDS requires the analog filter to be adjusted to suit.
>

Ah yes, changing the clock would require adjustment of the output
filter. Not something you would want to do.

How would you post-divide the (arbitrary waveform) output? Or do you
mean something else with the post-divide?

>
>> I see Analog Devices has DDS chips up to 48 bits, so 36 bits would not
>> be a problem (except for cost maybe).
>>
>> But al of the DDS chips I find from Analog seem only to implement a
>> fixed sine table/function. Do DDS chips exist that allow downloading an
>> arbitrary lookup table with 2^10 - 2^16 entries of 10 - 16 bit each?
>
> If you are looking to generate a non-sine waveform, it is not needed, or useful.

Could you explain this? How would I generate an arbitrary waveform
without using a table?


>> If no such standard chips exist, I expect I need to implement the DDS
>> in an FPGA. Using a smaller accumulator would probably save some space
>> in the FPGA. Or am I just optoimizing prematurely?
>
> Do you work at Highland Technology?

No, I do not.

> I've posted about an appropriate solution more than once in s.e.d. Here is a fair summary...
>

I don't read there, so missed that. But are you now talking about an
appropriate solution for a specific Highland Technology problem, or a
general solution?

> A DDS should be designed to generate a top frequency over a 2:1 range. This is easy stuff, with good accuracy and very low jitter if properly designed, use of a LONG phase word and a good filter after the DAC. It is important to NOT shorten the phase word as that introduces spurs close to the frequency being generated, which can not be filtered out. This is because the truncation of the phase word creates discontinuities which show up as close in spurs. Keep the phase word long and round the sine output to suit a 16 bit DAC. This generates harmonics which are much easier to filter.
>
> You don't need to use a single large table for the sine. sin(a+b) = sin(a)cos(b) + cos(a)sin(b), where a is the msbs of the phase word and b is the lsbs. Use appropriately sized tables for sin(a), cos(b) and a single table for the product cos(a)sin(b). This is practical if (a) and (b) are each truncated a bit. Because sin(b) is always small, this term is always small and does not need to be calculated with full resolution on a and b. This product table also does not require the full word width. Three table lookups, one multiply and one add.
>
> A programmable divider then divides the top frequency by 2**N by counting up to a settable value.
>
> The top frequency can clock a final FF buffer register of your favorite technology to provide the actual pulse output with an appropriate jitter.
>
> The settings of these two units can both be changed on a single clock cycle by writing to a buffer register and updating the actual operational registers simultaneously on a cue. The DDS will continue from the present phase, so will produce one top frequency clock pulse that is an intermediate period. The programmable divider will continue from the current count, either triggering right away, or continuing to count from the present value. Either way it will produce one output pulse that is within the range of the two settings, the prior setting and the new setting.
>
> Does this make sense?

Not really. I don't need just a sin(a+b) signal, I need to implement
arbitrary (not realy arbitrary, but various rather complex) waveforms. I
don't see how your solution will do this, unless I totally misunderstood
it.

> Feel free to drop my name with your boss.

I can do that, but I doubt it it will ring a bell, even with your full
name. I believe we live on different continents, so I don't think we
have ever met.


--
Stef

A sinking ship gathers no moss.
-- Donald Kaul

anti...@math.uni.wroc.pl

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Aug 17, 2022, 3:14:44 PMAug 17
to
In comp.arch.embedded Stef <m...@this.is.invalid> wrote:
> To generate frequencies from approximately 0.5 mHz to 12 MHz with a DDS
> a minimum clock of >24, say 25 MHz, is required. To be able to go down
> to 0.5 mHz, a phase accumulator of at least 36 bits is required. This
> will give sub mHz resolution over the entire range. Nice for the low
> frequencies, but not of much use for MHz frequencies (in this
> application).
>
> Is there any objection to using a smaller phase accumulator and a clock
> pre-scaler to generate the lower frequencies?

Well, your frequency will be less accurate. To see this let me
derive formula for DDS. Let t be DAC clock, T be period of desired
signal and assume that we have N samples at uniformly distributed
points. At n-th tick of DAC clock real time is nt. In the
scale of desired signal this corresponds to nt/T. To get
position within period we drop integer part of this, that is
take frac(nt/T). Then we need to round to closest sample point.
Actually instead of rounding we can multiply by N, add 0.5 as
bias and take integer part. So, sample index is:

[N frac(nt/T) + 0.5]

where [ ] denotes integer part. Assuming that N is power of 2,
say 2^m and all arthmetic is in fixed point binary frac above
is equvalent to dropping high bits, leaving only m bits before
binary point. Integer part means dropping bits after bianary
points. So formula simplified to

[nNt/T + 0.5] = [n*a + b]

where a = Nt/T and b = 0.5 is time shift. Note that taking

phi_n = n*a + b

we have phi_{n+1} = (n+1)*a + b = a + phi_n so single addition
is enough to adjust phase. What is effect of using smaller
number of bits to represent phase phi_n? Well, b needs only
1 bit, so if Nt/T fits into k bits with k bigger than m + 1,
then calculation using k bits gives exactly the same result
as calculation using infinite precision. In other words,
using k bits we get exact result but possibly for wrong
frequency.

In general acceptable frequency error depends on application.
But since good analog components are more expensive than
digital ones, simple heuristic says that resuluting of
phase accumulator should not degrade accuracy of
oscilator. Assuming few ppm quartz oscilator as source
of DAC clock, this means that we need about 20 significant
bits in parameter a. OTOH, at moderate freqences we do not
want to make big jumps, so parameter a should have m or more
zero bits at start. With m = 10 we arrive at 30 bits. Add
some margin for users that want slightly better results and
we arrive at 36 bits. In fact, if you want 0.5 mHz without
divisor on DAC clock you will have about 35 zero bits
at start of paramter a, so 55 bits phase accumultor would
be more appropriate. However, in in few hundreds Hertz
range and below pre-divisor on DAC clock seem quite
appropriate, so 36 bits + pre-divisor should be OK.

> I see Analog Devices has DDS chips up to 48 bits, so 36 bits would not
> be a problem (except for cost maybe).
>
> But al of the DDS chips I find from Analog seem only to implement a
> fixed sine table/function. Do DDS chips exist that allow downloading an
> arbitrary lookup table with 2^10 - 2^16 entries of 10 - 16 bit each?
>
> If no such standard chips exist, I expect I need to implement the DDS
> in an FPGA. Using a smaller accumulator would probably save some space
> in the FPGA. Or am I just optoimizing prematurely?

If you go for 25 MHz DAC clock your DDS should be doable using
sufficiently fast processor. My rough guesstimate is that
to produce single sample (addjust phase accumulator, extract
bits and copy value) you need about 10 machine instructions,
so 250 MIPS processor should be fast enough to generate
samples. You probably need a DMA channel to transmit them
to DAC. I am not aware of processor with fast enough DAC,
but I think that there are processors capable of driving
external DAC at that speed.

OTOH with 12 MHz signal and 25 MHz DAC clock you essentially
are limited to sinusoidal signals, to have more variety
you need more samples per period, so either lower signal
frequency or higher DAC clock. So you may end up with
much higher DAC freqency and censequenty be forced to
use FPGA.

As I wrote earler, skimming bits on phase accumulator seems
unwise, it is at most one instruction in critical loop
in CPU realization and has _much_ smaller impact on
FPGA (think about size of your tables, single counter
is tiny compared to that).

--
Waldek Hebisch

gnuarm.del...@gmail.com

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Aug 17, 2022, 10:20:09 PMAug 17
to
It's not without a table, but not one huge table, 2^n length where n is the number of phase bits. I described the process below.


> >> If no such standard chips exist, I expect I need to implement the DDS
> >> in an FPGA. Using a smaller accumulator would probably save some space
> >> in the FPGA. Or am I just optoimizing prematurely?
> >
> > Do you work at Highland Technology?
> No, I do not.

Ok, someone there was inquiring about the exact same thing and often has one of his people enquire about such problems here when they potentially involve FPGAs. Sorry I mistook you for that.


> > I've posted about an appropriate solution more than once in s.e.d. Here is a fair summary...
> >
> I don't read there, so missed that. But are you now talking about an
> appropriate solution for a specific Highland Technology problem, or a
> general solution?

The DDS is general. A single sine table is fine if you have the memory for it. But you can do very well with numerical methods and much smaller tables as well.


> > A DDS should be designed to generate a top frequency over a 2:1 range. This is easy stuff, with good accuracy and very low jitter if properly designed, use of a LONG phase word and a good filter after the DAC. It is important to NOT shorten the phase word as that introduces spurs close to the frequency being generated, which can not be filtered out. This is because the truncation of the phase word creates discontinuities which show up as close in spurs. Keep the phase word long and round the sine output to suit a 16 bit DAC. This generates harmonics which are much easier to filter.
> >
> > You don't need to use a single large table for the sine. sin(a+b) = sin(a)cos(b) + cos(a)sin(b), where a is the msbs of the phase word and b is the lsbs. Use appropriately sized tables for sin(a), cos(b) and a single table for the product cos(a)sin(b). This is practical if (a) and (b) are each truncated a bit. Because sin(b) is always small, this term is always small and does not need to be calculated with full resolution on a and b. This product table also does not require the full word width. Three table lookups, one multiply and one add.
> >
> > A programmable divider then divides the top frequency by 2**N by counting up to a settable value.
> >
> > The top frequency can clock a final FF buffer register of your favorite technology to provide the actual pulse output with an appropriate jitter.
> >
> > The settings of these two units can both be changed on a single clock cycle by writing to a buffer register and updating the actual operational registers simultaneously on a cue. The DDS will continue from the present phase, so will produce one top frequency clock pulse that is an intermediate period. The programmable divider will continue from the current count, either triggering right away, or continuing to count from the present value. Either way it will produce one output pulse that is within the range of the two settings, the prior setting and the new setting.
> >
> > Does this make sense?
> Not really. I don't need just a sin(a+b) signal, I need to implement
> arbitrary (not realy arbitrary, but various rather complex) waveforms. I
> don't see how your solution will do this, unless I totally misunderstood
> it.

Ok, if you need an AWG, then you need a large memory. Your original post didn't seem to mention that. It only talked about sine waves.


> > Feel free to drop my name with your boss.
> I can do that, but I doubt it it will ring a bell, even with your full
> name. I believe we live on different continents, so I don't think we
> have ever met.

Sorry, I was mistaken thinking you worked at Highland Tech.

--

Rick C.

+ Get 1,000 miles of free Supercharging
+ Tesla referral code - https://ts.la/richard11209

gnuarm.del...@gmail.com

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Aug 17, 2022, 10:25:10 PMAug 17
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In sine generation, it is often used in applications where spurs need to be minimized. A filter is good for a sine wave, but is limited on removing close in spurs. Truncation of the phase word (even just as an input to the sine generator) creates spurs close to the fundamental. It essentially creates phase discontinuities that manifest as close in spurs. These are virtually unfilterable. If jitter or close in spurs are not important, then shortening the phase word has other obvious limitations that can be evaluated readily.

--

Rick C.

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gnuarm.del...@gmail.com

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Aug 17, 2022, 10:26:34 PMAug 17
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Opps, to correct myself... 2^(n-2) length table if you take advantage of the folding properties of the sine function.
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Stef

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Aug 18, 2022, 4:21:50 AMAug 18
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On 2022-08-17 anti...@math.uni.wroc.pl wrote in comp.arch.embedded:
> In comp.arch.embedded Stef <m...@this.is.invalid> wrote:
>> To generate frequencies from approximately 0.5 mHz to 12 MHz with a DDS
>> a minimum clock of >24, say 25 MHz, is required. To be able to go down
>> to 0.5 mHz, a phase accumulator of at least 36 bits is required. This
>> will give sub mHz resolution over the entire range. Nice for the low
>> frequencies, but not of much use for MHz frequencies (in this
>> application).
>>
>> Is there any objection to using a smaller phase accumulator and a clock
>> pre-scaler to generate the lower frequencies?
>
> Well, your frequency will be less accurate. To see this let me
> derive formula for DDS.

<snip detailed DDS math>

>>
>> If no such standard chips exist, I expect I need to implement the DDS
>> in an FPGA. Using a smaller accumulator would probably save some space
>> in the FPGA. Or am I just optoimizing prematurely?
>
> If you go for 25 MHz DAC clock your DDS should be doable using
> sufficiently fast processor. My rough guesstimate is that
> to produce single sample (addjust phase accumulator, extract
> bits and copy value) you need about 10 machine instructions,
> so 250 MIPS processor should be fast enough to generate
> samples. You probably need a DMA channel to transmit them
> to DAC. I am not aware of processor with fast enough DAC,
> but I think that there are processors capable of driving
> external DAC at that speed.

That is assuming the processor has not much else to do and that only a
single DDS channel is required. Both will not be true in the possible
application, I'm affraid. The additional DDS channels can be a bit
slower, so it may still be doable.


> OTOH with 12 MHz signal and 25 MHz DAC clock you essentially
> are limited to sinusoidal signals, to have more variety
> you need more samples per period, so either lower signal
> frequency or higher DAC clock. So you may end up with
> much higher DAC freqency and censequenty be forced to
> use FPGA.

Yes, this is understood. Read the 10 MHz as bandwidth, not as the max
frequency at which a complex waveform should be generated. So the
waveform will degrade to a sine when sped up to 10 MHz.

> As I wrote earler, skimming bits on phase accumulator seems
> unwise, it is at most one instruction in critical loop
> in CPU realization and has _much_ smaller impact on
> FPGA (think about size of your tables, single counter
> is tiny compared to that).

FPGA have memory blocks to hold such tables. A simple ripple counter
will indeed take a tiny amount of logic, a synchronous counter will take
more, certainly at 55 bits. But I think you need an adder if you want
variable accumulator steps and not only +1. Keeping the adder small
enough to fit in something like a 48-bit DSP slice will probably save
space.

The above is probably quite Xilinx specific, as that is the last FPGA I
have experience with. And even that was a while ago.

But again, I may be worrying too much about space already. When it comes
to it, I should first implement the 'best' solution. And then probably
find that this uses less than 10% of my FPGA. :-)


--
Stef

Don't shout for help at night. You might wake your neighbors.
-- Stanislaw J. Lem, "Unkempt Thoughts"

Stef

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Aug 18, 2022, 4:37:03 AMAug 18
to
On 2022-08-18 gnuarm.del...@gmail.com wrote in comp.arch.fpga:
> On Wednesday, August 17, 2022 at 7:36:42 AM UTC-4, Stef wrote:
>> On 2022-08-17 gnuarm.del...@gmail.com wrote in comp.arch.fpga:
>> > On Tuesday, August 16, 2022 at 11:37:41 AM UTC-4, Stef wrote:

<snip>

>> >> But al of the DDS chips I find from Analog seem only to implement a
>> >> fixed sine table/function. Do DDS chips exist that allow downloading an
>> >> arbitrary lookup table with 2^10 - 2^16 entries of 10 - 16 bit each?

'Hidden' AWG hint.

>> > If you are looking to generate a non-sine waveform, it is not needed, or useful.
>> Could you explain this? How would I generate an arbitrary waveform
>> without using a table?
>
> It's not without a table, but not one huge table, 2^n length where n is the number of phase bits. I described the process below.
>
>
>> >> If no such standard chips exist, I expect I need to implement the DDS
>> >> in an FPGA. Using a smaller accumulator would probably save some space
>> >> in the FPGA. Or am I just optoimizing prematurely?
>> >
>> > Do you work at Highland Technology?
>> No, I do not.
>
> Ok, someone there was inquiring about the exact same thing and often has one of his people enquire about such problems here when they potentially involve FPGAs. Sorry I mistook you for that.

I suspected something like this was the case, no problem. :-)

>
>> > I've posted about an appropriate solution more than once in s.e.d. Here is a fair summary...
>> >
>> I don't read there, so missed that. But are you now talking about an
>> appropriate solution for a specific Highland Technology problem, or a
>> general solution?
>
> The DDS is general. A single sine table is fine if you have the memory for it. But you can do very well with numerical methods and much smaller tables as well.

Yes DDS is general. But your method seemed to be aimed specifically at a
certain problem. But if I understand you correctly, the method you
describe is a general solution to generate sine waves without large
tables?

<snip>

> Ok, if you need an AWG, then you need a large memory. Your original post didn't seem to mention that. It only talked about sine waves.

Re-reading my original post, I see that was a bit hidden in the text and
may not have been clear to a reader not familiar with the application.
Sorry about that.

>
>> > Feel free to drop my name with your boss.
>> I can do that, but I doubt it it will ring a bell, even with your full
>> name. I believe we live on different continents, so I don't think we
>> have ever met.
>
> Sorry, I was mistaken thinking you worked at Highland Tech.

I thought you might have. ;-)


--
Stef

Hey, I had to let awk be better at *something*... :-)
-- Larry Wall in <1991Nov7.2...@netlabs.com>1

gnuarm.del...@gmail.com

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Aug 18, 2022, 3:25:36 PMAug 18
to
You keep calling it a DDS, which apparently is correct in that it can be used generically. But conventionally this refers to generating a sine wave. The term, AWG (Arbitrary Waveform Generator) is more commonly used for what you are doing.


> > OTOH with 12 MHz signal and 25 MHz DAC clock you essentially
> > are limited to sinusoidal signals, to have more variety
> > you need more samples per period, so either lower signal
> > frequency or higher DAC clock. So you may end up with
> > much higher DAC freqency and censequenty be forced to
> > use FPGA.
> Yes, this is understood. Read the 10 MHz as bandwidth, not as the max
> frequency at which a complex waveform should be generated. So the
> waveform will degrade to a sine when sped up to 10 MHz.
> > As I wrote earler, skimming bits on phase accumulator seems
> > unwise, it is at most one instruction in critical loop
> > in CPU realization and has _much_ smaller impact on
> > FPGA (think about size of your tables, single counter
> > is tiny compared to that).
> FPGA have memory blocks to hold such tables. A simple ripple counter
> will indeed take a tiny amount of logic, a synchronous counter will take
> more, certainly at 55 bits. But I think you need an adder if you want
> variable accumulator steps and not only +1. Keeping the adder small
> enough to fit in something like a 48-bit DSP slice will probably save
> space.

Ripple or synchronous would be the same, 1 FF and 1 LUT per bit. That's assuming an FPGA that supports addition in the LUT/FF cell, which most do. Even if your increment is +1, that uses the same logic because it's still 1 LUT. Of course, if you have other inputs, such as an enable or add/sub


> The above is probably quite Xilinx specific, as that is the last FPGA I
> have experience with. And even that was a while ago.
>
> But again, I may be worrying too much about space already. When it comes
> to it, I should first implement the 'best' solution. And then probably
> find that this uses less than 10% of my FPGA. :-)

Yes, unless you have *a lot* going on in the FPGA, you won't need to worry about a 55 bit counter, running at 50 MHz. It won't be significant in your greater design.

--

Rick C.

+- Get 1,000 miles of free Supercharging
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gnuarm.del...@gmail.com

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Aug 18, 2022, 3:32:01 PMAug 18
to
Yes, only sine waves. If you need an AGW, you need a large table, unless the waveform can be described mathematically in ways that can be synthesized in logic. Like combinations of sawtooth, triangle, sine, pulses, etc. That can be done with counters and added together.


> > Ok, if you need an AWG, then you need a large memory. Your original post didn't seem to mention that. It only talked about sine waves.
> Re-reading my original post, I see that was a bit hidden in the text and
> may not have been clear to a reader not familiar with the application.
> Sorry about that.

You can combine the table with logic to produce a "sparse" waveform. Such as a burst of arbitrary data, followed by no signal or some base signal that can be easily synthesized, followed by another burst of arbitrary data. There's no limit to the combinations you might think of to suit a given problem.

--

Rick C.

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Stef

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Aug 19, 2022, 5:13:11 AMAug 19
to
On 2022-08-18 gnuarm.del...@gmail.com wrote in comp.arch.fpga:

> You keep calling it a DDS, which apparently is correct in that it can be used generically. But conventionally this refers to generating a sine wave. The term, AWG (Arbitrary Waveform Generator) is more commonly used for what you are doing.

To me, the core of the DDS is the clock/phase accumulator/control word.
The phase output of this can be fed to any phase to amplitude conversion
you like. Complete DDS chips often include sine (table), sawtooth
(direct phase to DAC) and pulse (MSB of phase).

Although not an authoritive source, wikipedia agrees with this
interpretation.
https://en.wikipedia.org/wiki/Direct_digital_synthesis

AWG, to me, just means you can generate any waveform. But not how you
set the frequency of this waveform. It can be fixed or just use divide
by 2 stages on the clock, or wathever.

But this is all just personal interpretations, I think we both
understand what the other means right now. Maybe we should call it an
DDS AWG generator to cover everything. ;-)


--
Stef

The party adjourned to a hot tub, yes. Fully clothed, I might add.
-- IBM employee, testifying in California State Supreme Court

gnuarm.del...@gmail.com

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Aug 19, 2022, 5:38:49 AMAug 19
to
On Friday, August 19, 2022 at 5:13:11 AM UTC-4, Stef wrote:
> On 2022-08-18 gnuarm.del...@gmail.com wrote in comp.arch.fpga:
> > You keep calling it a DDS, which apparently is correct in that it can be used generically. But conventionally this refers to generating a sine wave. The term, AWG (Arbitrary Waveform Generator) is more commonly used for what you are doing.
> To me, the core of the DDS is the clock/phase accumulator/control word.
> The phase output of this can be fed to any phase to amplitude conversion
> you like. Complete DDS chips often include sine (table), sawtooth
> (direct phase to DAC) and pulse (MSB of phase).
>
> Although not an authoritive source, wikipedia agrees with this
> interpretation.
> https://en.wikipedia.org/wiki/Direct_digital_synthesis
>
> AWG, to me, just means you can generate any waveform. But not how you
> set the frequency of this waveform. It can be fixed or just use divide
> by 2 stages on the clock, or wathever.
>
> But this is all just personal interpretations, I think we both
> understand what the other means right now. Maybe we should call it an
> DDS AWG generator to cover everything. ;-)

Sorry, I didn't mean to make a big deal of the nomenclature. So what are you trying to do that isn't obvious, given a phase accumulator and a lookup table?

The frequency is normally set by the increment on the phase accumulator. But maybe that doesn't work so well for an AWG? What sort of waveforms are you interested in generating?

--

Rick C.

--- Get 1,000 miles of free Supercharging
--- Tesla referral code - https://ts.la/richard11209

gnuarm.del...@gmail.com

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Aug 19, 2022, 5:40:24 AMAug 19
to
On Friday, August 19, 2022 at 5:13:11 AM UTC-4, Stef wrote:
> On 2022-08-18 gnuarm.del...@gmail.com wrote in comp.arch.fpga:
> > You keep calling it a DDS, which apparently is correct in that it can be used generically. But conventionally this refers to generating a sine wave. The term, AWG (Arbitrary Waveform Generator) is more commonly used for what you are doing.
> To me, the core of the DDS is the clock/phase accumulator/control word.
> The phase output of this can be fed to any phase to amplitude conversion
> you like. Complete DDS chips often include sine (table), sawtooth
> (direct phase to DAC) and pulse (MSB of phase).
>
> Although not an authoritive source, wikipedia agrees with this
> interpretation.
> https://en.wikipedia.org/wiki/Direct_digital_synthesis
>
> AWG, to me, just means you can generate any waveform. But not how you
> set the frequency of this waveform. It can be fixed or just use divide
> by 2 stages on the clock, or wathever.
>
> But this is all just personal interpretations, I think we both
> understand what the other means right now. Maybe we should call it an
> DDS AWG generator to cover everything. ;-)

I suppose to control the sample rate of the AGW, you could use a DDS with a clock output. The DDS would give a wide range of frequency with high resolution. What range of sample rate are you interested in?

--

Rick C.

--+ Get 1,000 miles of free Supercharging
--+ Tesla referral code - https://ts.la/richard11209

Stef

unread,
Aug 19, 2022, 6:45:53 AMAug 19
to
On 2022-08-19 gnuarm.del...@gmail.com wrote in comp.arch.fpga:
> On Friday, August 19, 2022 at 5:13:11 AM UTC-4, Stef wrote:
>> On 2022-08-18 gnuarm.del...@gmail.com wrote in comp.arch.fpga:
>> > You keep calling it a DDS, which apparently is correct in that it can be used generically. But conventionally this refers to generating a sine wave. The term, AWG (Arbitrary Waveform Generator) is more commonly used for what you are doing.
>> To me, the core of the DDS is the clock/phase accumulator/control word.
>> The phase output of this can be fed to any phase to amplitude conversion
>> you like. Complete DDS chips often include sine (table), sawtooth
>> (direct phase to DAC) and pulse (MSB of phase).
>>
>> Although not an authoritive source, wikipedia agrees with this
>> interpretation.
>> https://en.wikipedia.org/wiki/Direct_digital_synthesis
>>
>> AWG, to me, just means you can generate any waveform. But not how you
>> set the frequency of this waveform. It can be fixed or just use divide
>> by 2 stages on the clock, or wathever.
>>
>> But this is all just personal interpretations, I think we both
>> understand what the other means right now. Maybe we should call it an
>> DDS AWG generator to cover everything. ;-)
>
> Sorry, I didn't mean to make a big deal of the nomenclature. So what are you trying to do that isn't obvious, given a phase accumulator and a lookup table?
>
I think it is obvious. Phase accumulator points to LUT, but LUT isn't a
sine table but sommething else.

> The frequency is normally set by the increment on the phase accumulator. But maybe that doesn't work so well for an AWG? What sort of waveforms are you interested in generating?
>
This is the DDS part (as I interpret it) and this is what I intend to
do. I see no reason why this would not work for AWG. Except when you get
to higher frequencies, you start losing your waveform, until it degrades
to a sine at FS/2 (providing the output filter is correct).

For now, I need to accomodate 'anything you can draw'. So a full phase
to amplitude LUT is required. Length and width TBD.


--
Stef

Don't read any sky-writing for the next two weeks.

Richard Damon

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Aug 19, 2022, 10:43:56 AMAug 19
to
Normally for a Arbitrary Waveform, the phase increment is limited to no
more that one LUT entry per clock cycle, and if there aren't an integral
number of increments per phase increment of one LUT entry, you want
there to be a moderate number to avoid "distortion" of the waveform.

And you need to decide how the "Arbitrary" waveform is shaped between
points, normally either step held or lineally interpreted.

gnuarm.del...@gmail.com

unread,
Aug 19, 2022, 11:37:49 PMAug 19
to
On Friday, August 19, 2022 at 6:45:53 AM UTC-4, Stef wrote:
> On 2022-08-19 gnuarm.del...@gmail.com wrote in comp.arch.fpga:
> > On Friday, August 19, 2022 at 5:13:11 AM UTC-4, Stef wrote:
> >> On 2022-08-18 gnuarm.del...@gmail.com wrote in comp.arch.fpga:
> >> > You keep calling it a DDS, which apparently is correct in that it can be used generically. But conventionally this refers to generating a sine wave. The term, AWG (Arbitrary Waveform Generator) is more commonly used for what you are doing.
> >> To me, the core of the DDS is the clock/phase accumulator/control word.
> >> The phase output of this can be fed to any phase to amplitude conversion
> >> you like. Complete DDS chips often include sine (table), sawtooth
> >> (direct phase to DAC) and pulse (MSB of phase).
> >>
> >> Although not an authoritive source, wikipedia agrees with this
> >> interpretation.
> >> https://en.wikipedia.org/wiki/Direct_digital_synthesis
> >>
> >> AWG, to me, just means you can generate any waveform. But not how you
> >> set the frequency of this waveform. It can be fixed or just use divide
> >> by 2 stages on the clock, or wathever.
> >>
> >> But this is all just personal interpretations, I think we both
> >> understand what the other means right now. Maybe we should call it an
> >> DDS AWG generator to cover everything. ;-)
> >
> > Sorry, I didn't mean to make a big deal of the nomenclature. So what are you trying to do that isn't obvious, given a phase accumulator and a lookup table?
> >
> I think it is obvious. Phase accumulator points to LUT, but LUT isn't a
> sine table but sommething else.

Yes, it is obvious. That's why I'm asking what you are looking for from this discussion.


> > The frequency is normally set by the increment on the phase accumulator. But maybe that doesn't work so well for an AWG? What sort of waveforms are you interested in generating?
> >
> This is the DDS part (as I interpret it) and this is what I intend to
> do. I see no reason why this would not work for AWG. Except when you get
> to higher frequencies, you start losing your waveform, until it degrades
> to a sine at FS/2 (providing the output filter is correct).

I'm not following what you are saying about your filter. It only needs to filter artifacts above the Nyquist rate of fs/2. It would not need to distort your waveform.


> For now, I need to accomodate 'anything you can draw'. So a full phase
> to amplitude LUT is required. Length and width TBD.

Yes, if you want "anything you can draw", you need the LUT. But many waveforms can be constructed from simple shapes which can be generated digitally. The LUT won't be able to use the folding techniques that work for sine waves.

I don't know what your requirements are, but you can buy low cost AWG board level products and small box level products.

--

Rick C.

-+- Get 1,000 miles of free Supercharging
-+- Tesla referral code - https://ts.la/richard11209

Stef

unread,
Aug 22, 2022, 4:24:41 AMAug 22
to
On 2022-08-20 gnuarm.del...@gmail.com wrote in comp.arch.fpga:
> On Friday, August 19, 2022 at 6:45:53 AM UTC-4, Stef wrote:

...

>> I think it is obvious. Phase accumulator points to LUT, but LUT isn't a
>> sine table but sommething else.
>
> Yes, it is obvious. That's why I'm asking what you are looking for from this discussion.

In my original post, I had 2 questions

1) Are there objections to using a prescaler and a shorter phase
accumulator for generating frequencies over a wide range?

This has been discussed. Conclusions is that you should keep the
accumulator as long as possible for best performance.


2) Are the complete DDS chips available that have a downloadable LUT,
instead of the standard sine table?

If such chips are available, I may not have to develop a custom
(FPGA/CPU/DSP) solution. This question has not been answered and I have
found non myself sofar.

...


> I don't know what your requirements are, but you can buy low cost AWG board level products and small box level products.


At this time there are only general requirements (AWG, 12MHz BW, ...).
If the project continues, this will be detailed further.

Do you have an example of such a board level product? If it can do what
will be required, it is certainly an option.


--
Stef

If everything seems to be going well, you have obviously overlooked something.

gnuarm.del...@gmail.com

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Aug 22, 2022, 4:04:24 PMAug 22
to
On Monday, August 22, 2022 at 4:24:41 AM UTC-4, Stef wrote:
> On 2022-08-20 gnuarm.del...@gmail.com wrote in comp.arch.fpga:
> > On Friday, August 19, 2022 at 6:45:53 AM UTC-4, Stef wrote:
> ...
> >> I think it is obvious. Phase accumulator points to LUT, but LUT isn't a
> >> sine table but sommething else.
> >
> > Yes, it is obvious. That's why I'm asking what you are looking for from this discussion.
> In my original post, I had 2 questions
>
> 1) Are there objections to using a prescaler and a shorter phase
> accumulator for generating frequencies over a wide range?
>
> This has been discussed. Conclusions is that you should keep the
> accumulator as long as possible for best performance.

If any of that is from my comments, I retract them. I was thinking of a typical DDS generating a sine wave.

You only need the clock rate to suit the waveform you are generating, such as Nyquist considerations. Your memory is finite, so you have a fundamental trade off between clock rate and duration of the AWG pattern. I don't know your real requirements, so I can't advise you about how to optimize this. It will depend on your particular problem.


> 2) Are the complete DDS chips available that have a downloadable LUT,
> instead of the standard sine table?
>
> If such chips are available, I may not have to develop a custom
> (FPGA/CPU/DSP) solution. This question has not been answered and I have
> found non myself sofar.

Yeah, I've never used DDS chips, so I couldn't say. As I've mentioned, there are many AWG products at other levels of integration, modules and boards. eBay abounds with them.


> > I don't know what your requirements are, but you can buy low cost AWG board level products and small box level products.
> At this time there are only general requirements (AWG, 12MHz BW, ...).
> If the project continues, this will be detailed further.
>
> Do you have an example of such a board level product? If it can do what
> will be required, it is certainly an option.

Try punching AWG into eBay or one of the other sites.

--

Rick C.

-++ Get 1,000 miles of free Supercharging
-++ Tesla referral code - https://ts.la/richard11209

Stef

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Aug 23, 2022, 10:59:51 AMAug 23
to
On 2022-08-22 gnuarm.del...@gmail.com wrote in comp.arch.fpga:
> On Monday, August 22, 2022 at 4:24:41 AM UTC-4, Stef wrote:
>> On 2022-08-20 gnuarm.del...@gmail.com wrote in comp.arch.fpga:
>> > On Friday, August 19, 2022 at 6:45:53 AM UTC-4, Stef wrote:
>> ...
>> >> I think it is obvious. Phase accumulator points to LUT, but LUT isn't a
>> >> sine table but sommething else.
>> >
>> > Yes, it is obvious. That's why I'm asking what you are looking for from this discussion.
>> In my original post, I had 2 questions
>>
>> 1) Are there objections to using a prescaler and a shorter phase
>> accumulator for generating frequencies over a wide range?
>>
>> This has been discussed. Conclusions is that you should keep the
>> accumulator as long as possible for best performance.
>
> If any of that is from my comments, I retract them. I was thinking of a typical DDS generating a sine wave.

Most of those comments (and of Waldek) will also apply to AWG in some
way, so no need to retract. Also the output filter argument is one
against switching sample frequency. Ofcourse I can keep the pre-scaler
option in mind, but I don't think it should be the initial approach.
Only an optimization when really necessary (speed, size, etc).


> You only need the clock rate to suit the waveform you are generating, such as Nyquist considerations. Your memory is finite, so you have a fundamental trade off between clock rate and duration of the AWG pattern. I don't know your real requirements, so I can't advise you about how to optimize this. It will depend on your particular problem.
>
>
>> 2) Are the complete DDS chips available that have a downloadable LUT,
>> instead of the standard sine table?
>>
>> If such chips are available, I may not have to develop a custom
>> (FPGA/CPU/DSP) solution. This question has not been answered and I have
>> found non myself sofar.
>
> Yeah, I've never used DDS chips, so I couldn't say. As I've mentioned, there are many AWG products at other levels of integration, modules and boards. eBay abounds with them.
>
>
>> > I don't know what your requirements are, but you can buy low cost AWG board level products and small box level products.
>> At this time there are only general requirements (AWG, 12MHz BW, ...).
>> If the project continues, this will be detailed further.
>>
>> Do you have an example of such a board level product? If it can do what
>> will be required, it is certainly an option.
>
> Try punching AWG into eBay or one of the other sites.

Searching on AWG alone will find you a lot of wire. ;-)
With "arbitrary waveform generator", I get a lot of complete bench top
generators. And a number of (usually the same) cheap bare board units
like this one:
https://www.ebay.com/itm/221506533974?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=MRtVGD4USYW&sssrc=2047675&ssuid=&widget_ver=artemis&media=COPY

This one is 8-bit 256 sample 100 kSPS. Not enough on every parameter and
it seems most of these modules have similar specs.

But this is the kind of board level product you meant? I hoped for
someting that can be placed on a board. Looks like I need to implement
something myself if this project continues.


--
Stef

Just type 'mv * /dev/null'.

gnuarm.del...@gmail.com

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Aug 23, 2022, 12:24:06 PMAug 23
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There are products out there that do exactly what you want. I found Bitscope, for example, which seems to have a board level product, but their web site is so goofy I can't find actual specs on it. The control panel example they show only provides standard waveforms, not arbitrary. So I'm not sure it is an AWG.

http://bitscope.com/product/BS05/

You might need to contact them.

How many do you need? Is it worth the effort to roll your own? How are you going to provide a waveform input/definition?

--

Rick C.

+-- Get 1,000 miles of free Supercharging
+-- Tesla referral code - https://ts.la/richard11209

Stef

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Aug 23, 2022, 12:47:35 PMAug 23
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On 2022-08-23 gnuarm.del...@gmail.com wrote in comp.arch.fpga:

...

> There are products out there that do exactly what you want. I found Bitscope, for example, which seems to have a board level product, but their web site is so goofy I can't find actual specs on it. The control panel example they show only provides standard waveforms, not arbitrary. So I'm not sure it is an AWG.
>
> http://bitscope.com/product/BS05/
>
> You might need to contact them.

Indeed a bit goofy, found no specs either. They say arbitrary but only
show standard and distorted standard waveforms. Also no indication how
to get truly arbitrary data in.

> How many do you need? Is it worth the effort to roll your own? How are you going to provide a waveform input/definition?
>

Number is still unknown (to me), but it is for a commercial product. So
just sticking in some fancy boards will not be an option. There will be
more generators and signal operations and a lot of other stuff, so it
will be custom board(s) and controllers etc. anyway. And then you have
things like form factor and design and more. The AWG bit may in the end
be the simplest problem to solve.

Getting the data in is another thing completely and that will probably
involve PC programs and cloud connections. Not my first choice, but
understandable from our customer and end user points of view.

--
Stef

Windows - From the people who brought you EDLIN!

gnuarm.del...@gmail.com

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Aug 23, 2022, 7:52:08 PMAug 23
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lf you want to provide a very large memory, you can attach an SDRAM to an FPGA and have literally GBs of RAM at very high sample rates with low cost. If the rest of the design does not require a large FPGA, you can do this is a small FPGA for less money than the memory chip.

--

Rick C.

+-+ Get 1,000 miles of free Supercharging
+-+ Tesla referral code - https://ts.la/richard11209
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