What may cause VCO pulling?
I find the peak phase error for the first bite is very high . Maybe the power from power amplifier feedback to the VCO , cause the frequency pulling .But I don't know what cause this issue . Any guy, can provide some suggestion ?Best regards !
Need more information, but my first wild guess is that you are AC coupling the data onto the VCO tuning port. The data, in to form of a voltage, typically has a non-DC content. So when you superimpose the AC data with the DC tuning voltage of your phase locked loop, you get a momentary tuning of the VCO off of the steady state phase locked state. What some do is to sent in a meaningless header of "all F's" (0101010101...) data to let the VCO settle out before useful data is sent.
Try This Link For General Information About VCO And Pullability Issue
http://www.ecliptek.com/press/tech/faq_ev.html
Hi
If you're working in Direct conversion, the power amplifier can trouble your VCO depending on its position on your board and it's frequency. try shieldings to see if it's a radiated or conducted issue. If conducted, look carefully your lay-out.
hope it helps.
Rayan
Appreciation for the comments ,the frequency of the VCO is the same as transmitted frequency . So the vco is very sensitivity for the power feedback from the power amplifier .
The VCO pulling is due to the poor isolation from VCO tank circuit to its external circuitry(conducted or radiated). If your problem is really due to pulling, you can solve it by improving its isolation. One simple thing you can try is to add a buffer amplifier between the VCO and PA path. If you think it is costly, some people will add a pi/tee attenuator between VCO and PA path on condition you have some power can be trade-off.
Shielding, adding de-coupling cap (resonant freq at the TX/VCO freq.) on all power line and data line are also important.
First , many thanks for your kindly comments .
The tranceiver integrate the buffer in it . And we add 2 db attenuator to the power amplifier input .
Now I have no idea how to fix that issue ?
Have you checked
- VCO DC power supply lines
- VCO tuning voltage input line
during transmit, using an oscilloscope?