Single and dual conversion receivers?
Shelly/Dave Raines
Would someone, with the appropriate backround, please
explain the difference between single and dual conversion.
Specifically, why are different types of crystals needed for
these two types of receivers?
thanks in advance!
Insert stupid, unoriginal quote here
Fred McClellan
Shelly/Dave Raines (vort...@ix.netcom.com) wrote:
:
: Would someone, with the appropriate backround, please
: explain the difference between single and dual conversion.
: Specifically, why are different types of crystals needed for
: these two types of receivers?
Single and double conversion means about what you might think -
One design receiver converts the radio frequency signal down to an audio
(or an output signal) in one step. Dual conversion receivers do it in two
stages, although they achieve the same eventual output (audio, as in an
entertainment radio) and a low level digital signal, in the case of the
R/C systems, destined for the channel decoder section of the receiver.
The difference in crystal configurations has to do with the circuit
loading. There are a jillion (well, maybe a dozen) ways to cut and mount
a crystal, and the circuit designer sets the crystal parameters when the
circuit is designed.
You can sometimes swap in crystals with different characteristics
(although still having the same basic frequency) and get them to work, but
usually the incorrect loading destroys the odd-ball crystal in short
order.
What really matters is that you've got the correct modulation scheme for
your needs, i.e. AM, FM, or PCM/PPM. In most cases, unless you're an
amateur and can legally modify your own equipment, the issue of single or
dual conversion is largely a matter of advertising hype.
One manufacturer, JR, is presently touting their 'NEW' single-conversion
receiver design. That they've managed to get the performance in one stage
normally required by a dual conversion receiver is an achievement in
design, but it also means the receiver is operating a LOT closer to the
"edge" of its technology.
As a flyer, no one CARES how many stages the Rx has, just so it has how
ever many it NEEDS to keep the model from "mining without a permit".
--
Cheers,
Fred McClellan
f...@hiwaay.net
AMA 180201 IMAA 6512 FCC KA6OLG
Rob Thompson
In article <5n2mel$a...@sjx-ixn4.ix.netcom.com>, Shelly/Dave Raines <vort...@ix.netcom.com> writes:
|>
|> Would someone, with the appropriate backround, please
|> explain the difference between single and dual conversion.
|> Specifically, why are different types of crystals needed for
|> these two types of receivers?
|>
|>
|>
|>
|> Insert stupid, unoriginal quote here
A detailed explanation would fill a text book, so please excuse me if I have
skipped a lot, but here goes...
Single conversion receivers work as follows:
The signal from the aerial is mixed with a signal generated from the plug in
crystal (called the "local oscillator") to generate a signal at what is called
the "intermediate frequency" or IF. Crystals are chosen so that the difference
between the local oscillator frequency and transmitter frequency is exactly
equal to the intermediate frequency of the receiver. Most of the filtering of the
wanted signal from adjacent channel interferance is performed in the IF section
of the receiver.
Note that only the difference in frequencies matters. Suppose that you have a
receiver with a 1MHz IF, and want to receive a signal at 72MHz. Then the receiver
could use a 73MHz crystal in the local oscillator , and everything would work
just fine...
Well that is until a signal comes along at 74MHz. Note that we again have a 1MHz
difference between the signal coming in on the aerial and the local oscillator.
That means that the signal will be accepted into the intermediate frequency
stages of the receiver, and from then on there is no way to reject the signal.
This is an effect called "image interferance".
The only stages of a receiver that can reject image interferance are those before
the mixer with the local oscillator. In most receivers this is only one or two
tuned circuits or filters. You can improve image rejection by making these
circuits more sharply tuned, but then they become more critical (read harder to
design, more expensive or more sensitive to change over time), and the receiver
might need retuning if you change the crystal.
Alternatively you can make image rejection better by increasing the IF frequency.
This puts the image frequency further away from the wanted signal frequency so
its easier to separate the 2 with unsophisticated filters.
This works fine for image rejection, but with the higher IF its harder to design
for the same adjacent channel rejection. (The channel separation is a smaller
percentage of the IF frequency if you make the IF higher).
This means that single conversion receivers have to compromise between image and
adjacent channel interferance rejection, and getting it just right is difficult.
(But not impossible for most of the RC bands used today.).
One way to avoid this compromise is to design a receiver with 2 IFs, the first is
significantly higher than the second. This is called a dual conversion receiver.
The first IF gives good image rejection, and the second gives good adjacent
channel rejection. There needs to be a second local oscillator and mixer
stage to convert from the first IF to the second IF. (The main disadvantage of
dual conversion being complexity and hence cost. If badly done, there are other
problems too!)
It is the first IF frequency that is used to calculate the crystal frequency for
the receiver. The difference in IF frequencies used for single and dual
conversion mean that a different receiver crystal is needed.
--
Rob Thompson.
rob_th...@mentorg.com | "Nothing unreal exists."
Mentor Graphics Corp. | Tel: (503) 685 1823
8005 S.W. Boeckman Road | Fax: (503) 685 1654
Wilsonville, OR 97070-7777 | The number -e^( i * pi ) in test.
Bob Burrill
Fred McClellan wrote:
> Single and double conversion means about what you might think -
> One design receiver converts the radio frequency signal down to an audio
> (or an output signal) in one step. Dual conversion receivers do it in two
> stages, although they achieve the same eventual output
>
Actually, Rob's discussion is closer to being correct. The output of
either of these two designs is an intermediate frequency (IF) at
455KHz. That is then decoded to the audio signal. But dual conversion
first converts to around 10MHz - then converts the amplified 10MHz to
455KHz.
> the issue of single or dual conversion is largely a matter of advertising hype.
> One manufacturer, JR, is presently touting their 'NEW' single-conversion
> receiver design.
>
You should read up on the 23 Channel seperation issue with respect to
single conversion receivers. 23 channels are 460KHz apart, awfully
close to the 455KHz IF frequency. And these channels don't have to be
anywhere near your frequency either since a single conversion frequency
has a high gain amplifier at 455KHz.
I'd be very interested in knowing how JR gets around that problem. In
the meantime, I'll stick with dual conversion.
Thanks,
rb
Art1217
A dual conversion system uses two IF (intermediate frequency ) circuits.
For example, an commercial FM radio will use 10.7 mHz as the first IF and
then 455 kHz for the second. A single conversion will use a single IF
generally at 455 kHz, like the Commercial AM radios.The radios I am
talking about are for the US broadcast band. Basically the 455 kHz is fed
to the detector, either FM, AM, SSB, etc. There are other methods that can
be used, for example, regenative circuits were used in the earlier days,
and prior to that no conversion was done and only a diode or cat whisker
was used as a detector. Some of the radio kits at radio shack that teach
electronics use just the diode detector without conversion.
The amount of IF stages is independent of the type of crystal used and can
be implemented in 3rd overtone crystals or other cuts. Crystals can be
used in parrallel or series tuned circuits. The choice of crystal is
dependent on the circuit design. Crystals at the same frequency may or
may not work in a circuit depending on the design. End result may be a
stable circuit (no oscillation) and nothing will work.
Hope this helps,
Art Steinmetz
Fred McClellan
Bob Burrill (bob_b...@rd.qms.com) wrote:
: Fred McClellan wrote:
: > Single and double conversion means about what you might think -
: > One design receiver converts the radio frequency signal down to an audio
: > (or an output signal) in one step. Dual conversion receivers do it in two
: > stages, although they achieve the same eventual output
: >
:
: Actually, Rob's discussion is closer to being correct. The output of
: either of these two designs is an intermediate frequency (IF) at
: 455KHz. That is then decoded to the audio signal. But dual conversion
: first converts to around 10MHz - then converts the amplified 10MHz to
: 455KHz.
<INSERT>
And obviously much longer. What I wrote is accurate. The difference is
the number of stages of frequency conversion.
:
: > the issue of single or dual conversion is largely a matter of advertising hype.
: > One manufacturer, JR, is presently touting their 'NEW' single-conversion
: > receiver design.
: >
:
: You should read up on the 23 Channel seperation issue with respect to
: single conversion receivers. 23 channels are 460KHz apart, awfully
: close to the 455KHz IF frequency. And these channels don't have to be
: anywhere near your frequency either since a single conversion frequency
: has a high gain amplifier at 455KHz.
:
: I'd be very interested in knowing how JR gets around that problem. In
: the meantime, I'll stick with dual conversion.
I don't NEED to "read up on" single conversion receivers. Just because JR
can DO it doesn't mean it's a good idea. How they did it isn't a
technical achievement, is a manufacturing achievement - holding the
circuit tolerances close enough to do the job. Can't help wonder what
will happen when those circuit components start to age.
I do agree on one thing - single conversion receivers aren't for me and my
models.
Max Feil
Fred McClellan (f...@HiWAAY.net) writes:
> Single and double conversion means about what you might think -
> One design receiver converts the radio frequency signal down to an audio
> (or an output signal) in one step. Dual conversion receivers do it in two
Actually the "single/double" refers to the number of heterodyning steps
before the final filter, i.e. the number of frequency down-shifts
in front of the 455 kHz filter. The demodulation ("audio") comes after.
> The difference in crystal configurations has to do with the circuit
> loading. There are a jillion (well, maybe a dozen) ways to cut and mount
> a crystal, and the circuit designer sets the crystal parameters when the
> circuit is designed.
Not really. The crystal frequency required is determined by the local
oscillator frequency at each stage, plus or minus the first intermediate
frequency. For single conversion the IF is 455 kHz but for dual conversion
it is usually 10.7 MHz which explains the big difference in crystals.
For example if you are on 72.250 MHz, then the crystal frequency will be
72.250 MHz +- 455 kHz for single conversion but 72.250 MHz +- 10.7 MHz
for dual conversion. The plus or minus depends on whether the rx is
designed for high or low side injection. The second stage of a
dual conversion rx always is going from 10.7 MHz to 455 kHz so the
crystal is fixed and never needs to be swapped. I have an article
which explains all this theory if you are interested.
>
> You can sometimes swap in crystals with different characteristics
> (although still having the same basic frequency) and get them to work, but
> usually the incorrect loading destroys the odd-ball crystal in short
> order.
I have not heard of this.
> What really matters is that you've got the correct modulation scheme for
> your needs, i.e. AM, FM, or PCM/PPM. In most cases, unless you're an
The modulation scheme is totally beside the point. The question was
about single vs. dual conversion.
> amateur and can legally modify your own equipment, the issue of single or
> dual conversion is largely a matter of advertising hype.
I disagree. Dual conversion rx's are superior because of their almost
total rejection of image frequency and 2nd order intermodulation
products.
>
> One manufacturer, JR, is presently touting their 'NEW' single-conversion
> receiver design. That they've managed to get the performance in one stage
> normally required by a dual conversion receiver is an achievement in
> design, but it also means the receiver is operating a LOT closer to the
> "edge" of its technology.
I agree here. The JR ABC&W also does not have very good image rejection.
>
> As a flyer, no one CARES how many stages the Rx has, just so it has how
> ever many it NEEDS to keep the model from "mining without a permit".
As a flyer, I say the rx needs TWO.
Cheers,
Max
--
Max Feil
ah...@freenet.carleton.ca
Ottawa, Canada.
Dennis Collin
ah...@FreeNet.Carleton.CA (Max Feil) wrote:
>Actually the "single/double" refers to the number of heterodyning steps
>before the final filter, i.e. the number of frequency down-shifts
>in front of the 455 kHz filter. The demodulation ("audio") comes after.
>Not really. The crystal frequency required is determined by the local
>oscillator frequency at each stage, plus or minus the first intermediate
>frequency. For single conversion the IF is 455 kHz but for dual conversion
>it is usually 10.7 MHz which explains the big difference in crystals.
>For example if you are on 72.250 MHz, then the crystal frequency will be
>72.250 MHz +- 455 kHz for single conversion but 72.250 MHz +- 10.7 MHz
>for dual conversion. The plus or minus depends on whether the rx is
>designed for high or low side injection. The second stage of a
>dual conversion rx always is going from 10.7 MHz to 455 kHz so the
>crystal is fixed and never needs to be swapped. I have an article
>which explains all this theory if you are interested.
... and lots more good stuff clipped ...
Thanks for the very clear, concise and correct explanation, Max! As a
ham I'm often tempted to answer this question, but your answer should
be included in the R/C FAQ ... thanks ...
73 de KD1JT (Dennis)