Hi all,

I'm working on a VHF receiver design with a VCO and PLL. I'm using SA602 MIXER/VCO from NXP and LMX2306 PLL from National.
I'm having some spurs on the VCO. They appear as simmetric at +/-9.6kHz from the fundamental frequency (172MHz). The PLL phase frequency comparator is set to 25kHz (same as channel spacing). I tried to reduce pll feeback loop filter from 2.5kHz to 1.5kHz and nothing happened. I also turned off the PLL and let the VCO run freely and the kept appearing.
The first IF on the circuit is at 10.7MHz and they appear there as +/-3.1kHz, what I consider strange (Or I'm not seing it correctly).
My questions at this point are:

1) Any idea on what might cause this spurs?
2) What can usually be done on schematics/layout to remove this problem?
3) The purpose is to decode GMSK data with 9k6bitrate and M=0.5 (this means I have 2k4 and 4k8 sinusoids on my data signal). Can this interfere with it and decrease overall circuit THD, which is now around 5.5 to 6.5% (too much)?

Any suggestions?

Thanks for your help!!

Regards,
Rodrigo

Hi pancho,

  Thank a lot for your answer.
  In fact yesterday I though about the GMSK modem and its crystal. I use a 9.8304MHz crystal divided internally by 1024 putting it at 9.6kHz. I found a part of the VCC line on the layout being shared with the RF part of the circuit. After I shutdown the modem CLOCK, the spurs disappeared. I double checked on the 10.7MHZ IF also and nothing was there.

  In the picture attached you can see in the YELLOW trace the VCO with spurs. In the GREEN trace, it's the VCO with the modem CLOCK in shutdown, so no spurs.

  You tip was right! Thank a lot for you help.

Regards,
Rodrigo Matias