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Design >> RF Design >> VCO pulling,
https://designers-guide.org/forum/YaBB.pl?num=1189998616

Message started by aaron_do on Sep 16^th, 2007, 8:10pm

Title: VCO pulling,
Post by aaron_do on Sep 16^th, 2007, 8:10pm

Hi,

just wondering, how big of a problem is VCO pulling? I have a receiver which uses a direct conversion transmitter, and furthermore, the PA is not ON in receive mode. Therefore, i'm thinking that VCO pulling will not be a big issue.

cheers,
Aaron

hmm......seems direct conversion transmitters have a worse VCO pulling problem...

Title: Re: VCO pulling,
Post by Aigneryu on Sep 16^th, 2007, 9:47pm

Hi,

When your transmitter is on, the output of PA will be a carrier-modulated signal, which will be different from your VCO. So if you have some feedback path, the PA will still pull your VCO. So that is why the VCO frequency should avoid to be the same as your LO in a "integrated" "direct-conversion" (or low-IF). If you lock your VCO at 2x of the required LO, then you can prevent the pulling.

Sincerely,

Title: Re: VCO pulling,
Post by aaron_do on Sep 17^th, 2007, 6:27pm

thanks for the reply,

one thing though, if we have a direct conversion transmitter and the modulated signal is symmetric about 0 Hz, the upconverted signal will also be symmetric about the LO. So therefore, can we expect much pulling effect. i.e. which way would it pull?

thanks,
Aaron

Title: Re: VCO pulling,
Post by nxing on Sep 19^th, 2007, 4:20pm

It really depends on how strong your PA output and how linear it is. 2X LO is not necessary enough to prevent the pulling. THat's why some companies use 4X VCO or 3 X VCO.

Title: Re: VCO pulling,
Post by aaron_do on Sep 19^th, 2007, 6:34pm

thanks for the advice :)

I was quoting from memory, but i guess i was quite far off.

Aaron

Title: Re: VCO pulling,
Post by yang on Sep 20^th, 2007, 6:55am

Aigneryu wrote on Sep 16^th, 2007, 9:47pm:

Hi,

So if you have some feedback path, the PA will still pull your VCO. So that is why the VCO frequency should avoid to be the same as your LO in a "integrated" "direct-conversion" (or low-IF).

Sincerely,

Hi Aigneryu,

Could you please give us some examples of " So if you have some feedback path"? In my experience, the common VCC and Ground maybe a feedback path.

Best regards,
Yang