3:1 range VCO and varactor RF voltage swing
lw1ecp
possible. Think of this as the varactor version of the old general
purpose RF bench generators or grid-dip-meters. I know the penalties:
high phase noise, high drift, high harmonic content. I don't care,
this won't be made into a high dynamic range receiver.
What I do need is a reliable means to keep the peak-to-peak RF voltage
across the varactors (varicaps) below 1 or 0.5Vp-p. Otherwise, even
having back-to-back diodes they rectify the RF, create a DC component
into the 100k resistor that feeds the tuning voltage, and this
dramatically raises the bottom frequency. If I reduce the R, the f
goes lower, but the RF is nastily clipped by the diodes.
The oscillator is a FET Hartley (no Colpitts capacitive tap in order
to maximize C swing). All attempts I made on Spice and in real life to
AGC the amplitude always created a low frequency relaxation
oscillation.
Can anybody tell me about a proven way to accomplish this?. I have
already googled a little with no success.
Many thanks!
Bill Baka
See if you can find any of the old (now) op-amp cook books or general
applications manuals. I saw one with a modified Wien-bridge that had an
AGC, but that was in 1975 for a breadboard I was doing.
Good luck.
Bill Baka
lw1ecp
Thanks Bill for your immediate answer!. Yes, RC oscillators have
impressive f spans: you change R or C by 10:1 and f changes 10:1, not
its square root. But... my needs go into the many hundreds MHz, sorry,
I have to stick to LC oscillators.
Tim Wescott
How elaborate an AGC loop are you trying? The _really pedantic_ way to
do it would be to use a separate amplitude detector, an op-amp controller
(which would let your controller be if not arbitrarily then at least
really really slow), and whatever bias point on the oscillator you're
changing to effect the gain.
This should let you get a pretty fancy transfer function on your
controller, to relieve the motor-boating.
Bill Baka
around a bit and see if I could trip over something while I look for
parts for my own projects. Are you looking to build or buy? It seems you
want a circuit to build. Am I on the right track? Some of the older ARRL
handbooks may have a circuit or two. Lately I have seen them migrating
towards VHF and UHF and higher frequency stuff.
Good luck.
Bill Baka
artie
<bb...@comcast.net> wrote:
A typical way of getting this kind of range is to use a pair of
oscillators, at least one voltage controlled, a mixer, and a low pass
filter -- with the amplitude stabilizing loop described previously.
Use a pair of YIG tuned oscillators and a wideband mixer, and you can
get as wide a range as you want -- a lot of sweep generators work this
way.
You could use a pair of Mini Circuits VCOs in the 2 to 4 GHz range and
build a vco with a few hundred MHz range.
WA4VZQ
news:2f64fde9-0aa1-4305...@e23g2000yqd.googlegroups.com...
I remember what was known as a "seven league oscillator" which derived
its name from the folklore of "seven league boots". The design was
discussed in one of my old 1930's vintage vacuum tube books. If I
remember correctly, one such oscillator covered from a few Hertz to
several MHz. Unfortunately I am in SC and my library is back in
Tennessee.
My memory may be wrong, but I think the book was "Functional Circuits and
Oscillators" by Herbert Reich.
73, Dr. Barry L. Ornitz WA4VZQ
lw1ecp
- Tim: yes: detector + error amplifier (minimum gain, even a single
NPN) + FET bias shifting.
- Bill: I love old ARRL publications, 80's QSTs are my favorites, but
3:1 (continuous) coverage is not common on ham bands. I am looking to
build, and design (oh, well, guesstimate).
- Artie and Barry: I knew the heterodyne way of getting broad coverage
thanks to the old Wavetek 2000 Sweep/Signal Generator. This is also
used in tracking generators for spectrum analyzers. But keeping
unwanted mixing products low is an art. Maybe later.
But, the first answer from Tim made me think: what's the difference in
amplitude stabilizing methods on a Wien bridge and my Hartley?. In a
RC oscillator you control amplitude thru some voltage dependent
resistance (a lamp, a FET channel, etc.). But in my oscillator I vary
it thru gate bias, which varies the average transconductance of a
device working in a (nonlinear) class C. So, I Spice'd a FET damping
the coil (source) tap with its Rds being controlled by oscillations
amplitude, an voilà!, I got ~1Vp-p at the gate (and the varicaps), the
gate's 1N4148 almost does not conduct. I will try a more decent of
controlling the feedback (without ruining Q) and let you know.
brian whatcott
The old school method - for a wien bridge for example - was to place a
small filament bulb in the feedback path. This had a non linear response
to amplitude - and a low natural bandwidth. It's that slow response
that you have been lacking, apparently. Putting together an op amp
with variable amplitude output that is smoothed on a five+ second time
constant is what you need.
Brian W
Doug White
9af933...@l13g2000yqb.googlegroups.com:
<snip>
> - Artie and Barry: I knew the heterodyne way of getting broad coverage
> thanks to the old Wavetek 2000 Sweep/Signal Generator. This is also
> used in tracking generators for spectrum analyzers. But keeping
> unwanted mixing products low is an art. Maybe later.
No big deal. Use two high freqeuncy oscillators (one fixed, one
tunable), say around 2 GHz. When you mix them, the difference freqeuncy
gives you the low freqeuncy you want, and the sum freqeuncy will be
around 4 GHz & easily filtered out. The higher order terms that might be
an issue will be things like three times one minus twice the other. A
little care with your frequency plan & a good mixer & you should be fine.
There is a free DOS executable mixer spur calculator buried in with the
MARTHA/LLAMA APL CAD software: http://www.marthallama.org/
The program is MIXSPUR.EXE, and is included in the LLAMA distribution. It
is documented in the LLAMA manual. No APL is required.
If folks are interested, I could put together standalone documentation on
MIXSPUR & park it and the executable on my personal website.
Doug White
raypsi
Maybe you should looky at an old NTSC tv tuner. The brothers at RCA,
made sum tuners for the cable tv bands that went from 5Mhz all the way
up to 900Mhz
And they did it with quad matched varicaps, dual gate FET's, and a
PLL.
Not only did they control the VCO, but the TRF front end also.
Me thinks your OSC is way to efficient and your Q is way to high. Roll
the Q down with the right LC ratio or swamp it with sum resistance.
If you series varicaps you would increase the the amount of RF voltage
they could handle before starting to rectifry the RF.
73 OM
de n8zu
lw1ecp
- Thank you for the hint, the CATV tuners cover 54 to 900 MHz (5 to
~40 is for the upstream), an impressive 16:1 range, but the L.O. is
much narrower in relative values. Suppose 1st IF at 1200MHz, the L.O.
would sweep 1254 to 2100, 1.67:1, not enough for me.
- "The TRF front end also". Are you sure?. I dismantled a Jerrold and
the incoming CATV hit directly the balanced modulator thru a fixed
high-pass. But even if it really tuned 54-900, it would be easier than
my needs because the CATV amplitude is much lower than in an
oscillator.
- "Roll the Q down"?. It wouldn't help, I just need to vary 9:1 the LC
product.
- "Series varicaps"?. Sorry... they are back-to-back in series
already!.
Doug:
Thank you for encouraging me about the heterodyne way, and the links.
Yes, I think I will go this way for my gen-purp lab generator, but for
the dipper I still need the coil to oscillate right at the measured
frequency.
Brian:
I'm afraid this is more complicated than choosing a long time
constant, I have already tried in the simulation. The problem seems to
stem from the fact that the effective FET gm is dependent not only on
applied bias but also on RF amplitude. The error amplifier senses the
amplitude is high, applies more -Vgs, gm decreases, amplitude
decreases, but this lower amplitude makes effective gm to fall even
more, and as a result oscillation ceases and then starts again, and
again, and again. I think I will have to prevent the FET from working
in class C and forget about Vgs as a control means.
Again, many thanks to all of you. As soon as I can come back to this
subject I will let you know the results.
brian whatcott
/snip/
> - "Series varicaps"?. Sorry... they are back-to-back in series
> already!.
>
I understood the suggestion was to place two varicaps in series
NOT back to back in order to extend the voltage range.
But that implies two series strings, in order to maintain the
capacitance range over a longer voltage swing??
Brian W
raypsi
Well the dual gate FET's RCA was using were sub 1Ghz range so not only
were they controlling the bias, also they controled the miller effect
with an isolated supply for the second gate.
I should has ment 50Mhz to 900Mhz it was the splitters we used were
rated 5Mhz to 900Mhz.
So 8 varicaps in back to back series would be to much? Giving you
double the voltage and the same C.
You may be right 54Vdc would be way too much to have floating around
to tune them.
I remember the ones RCA used were so small if you dropped one on the
floor you would never ever find it.
They diode switched in 2 sets of coils in both the OSC and TRF, to get
LO VHF, HI VHF, and UHF.
So like they had 3 coils in series, all 3 for the LO, 2 for the HI,
and 1 for the UHF
73 OM
de n8zu
Tim Shoppa
> Raypsi:
> - Thank you for the hint, the CATV tuners cover 54 to 900 MHz (5 to
> ~40 is for the upstream), an impressive 16:1 range, but the L.O. is
> much narrower in relative values. Suppose 1st IF at 1200MHz, the L.O.
> would sweep 1254 to 2100, 1.67:1, not enough for me.
I think maybe you're looking at the wrong generation of CATV tuners. I
think Raypsi's suggestion was for the little tin-can tuners common in
say 80's and 90's VCR's, these certainly did NOT have a 1200MHz IF,
closer to 45MHz.
Tim.
Tim Shoppa
> Hi!. I need to cover the HF and VHF ranges with as few VCOs as
> possible. Think of this as the varactor version of the old general
> purpose RF bench generators or grid-dip-meters. I know the penalties:
> high phase noise, high drift, high harmonic content. I don't care,
> this won't be made into a high dynamic range receiver.
> What I do need is a reliable means to keep the peak-to-peak RF voltage
> across the varactors (varicaps) below 1 or 0.5Vp-p.
Look at the Elecraft K2's VCO. A single VCO covers all the HF bands
(some with up and others with downconversion), with many narrow ranges
all cleverly switched using small relays to move the varactors/fixed
capacitors in and out. I can't promise that this is exactly what you
want but there's much to be learned by studying it if you want to
build wide-range VCO's with commodity parts.
The Elecraft K2 VCO also uses a clever scheme to do AGC such that the
VCO output remains constant through its wide range, this probably also
keeps the RF voltage across the varicap in check.
There's a lot of cleverness in the K2. Schematics are online, www.elecraft.com.
Tim N3QE
Ian Jackson
<40b6ddcc-b3be-4c7f...@1g2000vbm.googlegroups.com>, Tim
Shoppa <sho...@trailing-edge.com> writes
set-top boxes from one manufacturer. I would have thought that, even
after 10 years, I would remember to the nearest Hz what the oscillator
and IF frequencies were - but I'm afraid I can't!
However, the general plan was essentially that entire input spectrum was
bandpass filtered (via a cascaded highpass and lowpass filter), and
presented to a 4-diode ring double-balanced mixer. I'm pretty certain
that one model (50 to 550MHz) had a first IF at around 650MHz, with the
LO running from 700 to 1200MHz. This was then down-converted to a second
IF - the usual 45.75MHz (for NTSC).
It was fairly obvious that you could make a wide-range variable
oscillator (say 0 to 500MHz) by 'reversing things', using a variable
oscillator (a readily achievable 650MHz to 1150MHz), and mixing it with
a fixed oscillator on 650MHz. The output of the double balanced mixer is
taken from the IF port, which (of course) goes down to DC, and is
lowpass filtered to 500MHz. As has been stated, this is essentially the
principle used by many sweep oscillators.
For most 'amateur' purposes, there is no need to have any ALC applied to
the output signal. This is achieved by ensuring that the level of the
variable oscillator is relatively high (say 66dBmV or 2V), and that of
the fixed oscillator is relatively low (say 0.3V or 50dBmV). A normal
feature of a ring mixer is that the 'signal' loss through the mixer
(typically 6dB) is more-or-less independent of level of the local
oscillator - provided it is high enough. This means that, even if the
level of the variable oscillator varies by a few dB across its tuning
range (which it is bound to do), the mixer output level stays fairly
constant 44dBmV or around 0.15V) over the entire frequency range.
But things are not perfect. Unfortunately, in the mixer, you inevitably
get mixing of the harmonics of two oscillators. The effect is to produce
unexpected signals within the wanted band 0 to 500MHz (and greatly
dependant on the actual frequency of the variable oscillator). I know of
one commercial sweep oscillator which had, at the LF end of the band,
several unwanted mixer products only 25dB down. But it was still quite
usable for most lab purposes, provided you knew of its limitations. The
cure for this inherent problem is to have the frequencies of both the
fixed and variable oscillators as high as possible (not as stated
above), but this obviously makes amateur construction more difficult.
Once you have made a satisfactory wide range oscillator, it is very easy
to convert it to a sweep generator by driving the varicap of the
variable oscillator from a sawtooth.
--
Ian
Tim Shoppa
<ianREMOVETHISjack...@g3ohx.demon.co.uk> wrote:
> In message
> <40b6ddcc-b3be-4c7f-a8b9-c989c4ea1...@1g2000vbm.googlegroups.com>, TimShoppa<sho...@trailing-edge.com> writes
>
> In the 1980s and 90s, I was intimately involved in many types of CATV
> set-top boxes from one manufacturer. I would have thought that, even
> after 10 years, I would remember to the nearest Hz what the oscillator
> and IF frequencies were - but I'm afraid I can't!
>
> However, the general plan was essentially that entire input spectrum was
> bandpass filtered (via a cascaded highpass and lowpass filter), and
> presented to a 4-diode ring double-balanced mixer. I'm pretty certain
> that one model (50 to 550MHz) had a first IF at around 650MHz, with the
> LO running from 700 to 1200MHz. This was then down-converted to a second
> IF - the usual 45.75MHz (for NTSC).
All I know about the little tin-can tuners made by the billions in the
80's and 90's, is that antenna comes in one end, circa 45MHz IF came
out the other end :-). Maybe the ones I saw were not the DC-to-
daylight ones used for CATV. The generation I'm most familiar with
took a tuning voltage that went up to 30V or 40VDC for the varactors,
maybe that high voltage gave them a wider range with simpler
circuitry. Of course the line-operated ones had that high voltage
around for easy use in the chassis.
> For most 'amateur' purposes, there is no need to have any ALC applied to
> the output signal.
Look at the Elecraft K2's LO system. It uses a single VCO to cover all
bands, and one relevant factor that lets them use the same oscillator
for all bands is the ALC. It's a very very clever design, one that
minimizes not only parts count but the low parts count also means low
power consumption (less than 200mA for the whole rig in receive.)
Contrast that with, say, modern Japanese HF transceivers which draw
ten times as much power in receive and have these ginormously
complicated multi-loop synthesizers that in the end have worse phase
noise than the K2's simple scheme.
My gut feeling is that the Japanese rig philosophy of making sure
their receivers do DC to daylight drives up their parts count
enormously with no benefit (perhaps a negative effect) on ham band
performance. Maybe that's what the Japanese hams want. Heck, it's
probably what most US hams think they want, if for no other reason
than because the YaeKenCom ads have been pushing it as a feature (not
a bug) for decades, at least as long as they've been doing
upconversion ham receivers.
Of course the K2 does so well in comparison because it applies the ham-
band performance simplicity philosophy not just to the LO chain but
throughout.
Tim N3QE