Martin H Post subject: Using the pulling figure of a VCO Posted:
Wed Mar 19, 2008 12:51 pm Lieutenant Joined: Fri
Dec 01, 2006 12:03 pm Posts: 4 Hi all, I'm designing
an LOchain 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 frequencypulling
spec of the whole transceiver when the Tx power is switched on and off
as well as when the LO signal is modulated. I'm buying 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 lineup of their VCO, a 6dB pad and a sliding short. They
vary the sliding short and measure the peaktopeak frequency deviation.
Now in a simulation, I have my whole LOchain (no VCO model;
my input is a harmonic balance port). I have a sliding short at the
end of my LOchain and I vary its phase. At the LOchain 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 calculations? The equation is: Df = [fo(S1/S)]/2Qext
where Df is the pp 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. From 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 LOchain 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 pp 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 LOchain, but a modulator that'll have some kind
of matching. Any comments will be welcome. A link to the paper
I'm using is http://ieeexplore.ieee.org/Xplore/login ... pdf?temp=x
but you have to be a paidup member to read it. Happy
Easter everyone. Martin. UK Top IR
Post subject: Posted: Thu Mar 20, 2008 3:12 am Site Admin
Joined: Mon Jun 27, 2005 2:02 pm Posts: 373 Location:
Germany 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. Top Martin H
Post subject: Posted: Tue Mar 25, 2008 10:12 am Lieutenant
Joined: Fri Dec 01, 2006 12:03 pm Posts: 4 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. Top mac Post subject: vco/ lo chainPosted:
Fri May 09, 2008 2:33 am Captain Joined: Fri Nov
02, 2007 3:45 am Posts: 8 Location: germany Hi, i've read
your posts  so let me try a short comment: 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
_________________ regards mac
Posted 11/12/2012
