Using the Pulling Figure of a VCO - RF Cafe Forums
Post subject: Using the pulling figure of a VCO Posted:
Wed Mar 19, 2008 12:51 pm
Fri Dec 01, 2006 12:03 pm
an LO-chain for a transciever that operates around 20GHz. I'm using
a VCO at 2GHz, so the chain has x10 multiplication and also contains
various amplifiers and filters.
Now I have a frequency-pulling
spec of the whole transceiver when the Tx power is switched on and
off as well as when the LO signal is modulated.
in a VCO module that has a pulling figure at 12dB return loss.
Now the simplest form of my question is: how would you calculate
the pulling figure at any other return loss? Manufacturers measure
the pulling figure by having a line-up of their VCO, a 6dB pad and
a sliding short. They vary the sliding short and measure the peak-to-peak
Now in a simulation, I have my whole
LO-chain (no VCO model; my input is a harmonic balance port). I
have a sliding short at the end of my LO-chain and I vary its phase.
At the LO-chain input, I note the change in return loss that the
VCO will see. What frequency deviation from the VCO will result
from this change in return loss?
Now I have an equation
and I have done calculations. I guess I'm asking the question: how
would YOU do it? The equation I'm using is from some 1982 MTT paper.
I find it hard to believe that the only equation I can find is in
some obscure paper. Surely many people will have to do such pulling
The equation is:
Df = [fo(S-1/S)]/2Qext
where Df is the p-p frequency variation of the VCO;
fo is the VCO's centre frequency;
S is the VSWR into which the
VCO is looking into;
Qext is the Q of the VCO.
the VCO's data sheet, giving Df at a 12dB return loss (converted
to VSWR), I calculate Qext. Putting this value back into the equation,
I can then calculate the frequency pulling for any VSWR.
For my LO-chain simulation, I put in my 2 values of return loss
I obtained from varying the sliding short, calculate Df for both
cases, then subtract the two Df values to give me how much the frequency
deviates. I then divided by 2 since Df is p-p and I only want peak.
When I get test circuits, I expect the actual frequency pulling
to be much less since I won't have a sliding short at the end of
my LO-chain, but a modulator that'll have some kind of matching.
Any comments will be welcome. A link to the paper I'm using
http://ieeexplore.ieee.org/Xplore/login ... pdf?temp=x
but you have to be a paid-up member to read it.
Happy Easter everyone.
Post subject: Posted: Thu Mar 20, 2008 3:12 am
Joined: Mon Jun 27, 2005 2:02 pm
You also have to take into account
that the pulling values varies as a function of the Vtune provided
to the VCO.
My intuition would be to order some samples
from this VCO module and test it under different load (VSWR) confitions.
Post subject: Posted: Tue Mar 25,
2008 10:12 am
Joined: Fri Dec 01,
2006 12:03 pm
I do have samples of the VCO I'll
be using. I guess I'll only know what pulling I get when I get to
test my final circuit. In the meantime, I have to make do with calculations.
I'm going on the assumption that if predictions/simulations show
that the pulling is acceptable when I have a sliding short on the
end of my LO chain, then it'll be more than enough given that the
output load won't change that much due to modulation and the PA
switching on and off.
vco/ lo chainPosted: Fri May 09, 2008 2:33 am
Joined: Fri Nov 02, 2007 3:45 am
i've read your posts -
so let me try a short
if you buffer your VCO shure s21 of the buffer
stage makes sense - but
shouldn't you take s12 into account
in all your calculations ? if it is high enough and you measure
pulling at the end of the lo chain
(including multiplier effects
x10?) do you have to bother about whats going on at the end of the
chain effecting the vco ??
just a quick comment