Steve,
I think the main reason for back-to-back diodes is to prevent rectification
of the RF. Rectification can cause several bad things, including pushing DC
current back out the tuning voltage line, instability, and increased phase
noise. Having said that, I tried back-to-back diodes a couple times and I
don't ever recall ending up with it in the final design, so it must not have
added all that much value. On the negative side, it halves the capacitance.
One technique to improve the phase noise in wide-band VCO's was shown by
Ulrich Rhode. He uses several diodes in parallel in order to decrease the RF
current in each diode, reportedly lowering losses and improving noise. I
never tried it myself.

Joe
W3JDR


"Steve Nosko" wrote in message
...
My comments are colored by experience desigining quite low noise,
relitavely
narrow bandwidth VCOs, so all the other comments are very valid.

I will note that there are two aspects of noise to consider, that I can
think of off hand. One is that:
1) Too low of an absolute DC voltage on the varactors will make for a
noisy
VCO and
2) A wide tuning range means that the varactor is relatively tightly
coupled into the oscillator which means that control line noise/spurs
you'll
have to watch control line noise & spurs.
3) This also means the Varactors have more effect on the oscillator Q and
therefore can also mean increased noise.
OK nobody expects the Spanish inquisition.

If you can determine the actual noise performance and keep it in mind that
the analyzer has a noise limit, you'll be ok.

Been away from it too long and can't seem to recall why we used to use
two,
back-to-back varactors...brain is full...seems like it was to reduce the
voltage across the varactor.

'guards, Steve K;9;D:C:I



"W3JDR" wrote in message
...
2:1 tuning range with varactor tuning is very doable in the frequency
range
he's considering. I've done 3:1 at lower frequencies. No steep hill at
all.

Joe
W3JDR


"Steve Nosko" wrote in message
...
You're climbing a steep hill here James. It is the % change in
frequency
that is the problem. Professional spec analysers use a 2 GHz LO with
a
2:1
range (to scan 0-2GHz), but I believe that is a magnetically tuned YIG
resonator. If you could get the frequency higher, so the sweep range
is
narrower, then mix to the desired freq with a balanced mixer...that
would
be
an easier VCO design.

Steve
K;9;d;c;i

The punctuation is my feeble attempt at spam-bot blocking.


"James Fenech" wrote in message
...
Hi Roy,

thanks for the suggestion. I am considering buying this book. Can I
ask
how
much theory, and how deep does it go? I am an engineer (digital
electornics
and software background) and actually like some theory to help me
understand
what I am doing.

I already have some "real" test equipment, 50MHz CRO, signal
generator,
multimeter, etc. So "simple" test equipment may not be too much of
an
improvement.

I've looked over the internet, and some books that I have, but found
no
real
example circuit on wideband VCOs. The only "sort of close enough"
circuit
I
found is at:




http://www.newwaveinstruments.com/resources/rf_microwave_resources/sections/oscillator_vco_theory_design_circuit.htm#Voltage%2 0Controled%20Oscillator%20(VCO)%20Circuits
The second one down - Colpitts. Are there any such examples in this
book?

Thanks,
James.


"Roy Lewallen" wrote in message
...
I highly recommend _Experimental Methods in RF Design_, by
Hayward,
Cambell, and Larkin. It's published by the ARRL and available from
them
and numerous other sources. Besides theory and a lot of real,
practical,
tested circuits and projects, it includes simple test equipment
you
can
build yourself.

Roy Lewallen, W7EL

James Fenech wrote:
. . .
I have the ARRL handbook (1997 or so) but this doesn't have much
in
the
way
of theory. Is there any other reference anyone can recommend?
. . .