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Using the pulling figure of a VCO - RF Cafe Forums

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Martin H

Post subject: Using the pulling figure of a VCO Posted: Wed Mar 19, 2008 12:51 pm


Joined: Fri Dec 01, 2006 12:03 pm

Posts: 4

Hi all,

I'm designing 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.

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 line-up of their VCO, a 6dB pad and a sliding short. They vary the sliding short and measure the peak-to-peak frequency deviation.

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 calculations?

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.

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 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 is

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

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.


Martin H

Post subject: Posted: Tue Mar 25, 2008 10:12 am


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.



Post subject: vco/ lo chainPosted: Fri May 09, 2008 2:33 am


Joined: Fri Nov 02, 2007 3:45 am

Posts: 8

Location: germany


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




Posted  11/12/2012

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