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. 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!

lw1ecp wrote:
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. 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!

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

I saw one with a modified Wien-bridge that had an AGC (...)
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.

On Wed, 25 Nov 2009 17:10:07 -0800, lw1ecp wrote:

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. 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!

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.

--
www.wescottdesign.com

lw1ecp wrote:
I saw one with a modified Wien-bridge that had an AGC (...)
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.


Many hundreds of MHz? Are you trying for D.C. to daylight? I could poke
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

In article , Bill Baka
wrote:

lw1ecp wrote:
I saw one with a modified Wien-bridge that had an AGC (...)
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.


Many hundreds of MHz? Are you trying for D.C. to daylight? I could poke
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


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.

"lw1ecp" wrote in message
...
I saw one with a modified Wien-bridge that had an AGC (...)
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.

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

I'm astonished at the tsunami of answers this arose!.
- 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.

Bill Baka wrote:
lw1ecp wrote:
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. 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!

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

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

lw1ecp wrote in news:a6888c2a-b312-443b-ae70-
:

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 softwa 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