FM modulation use PLL (Help)

I have simulated the analog base signal injected to VCO port(Vtune) to generate the FM modulation signal. But find a problem if the Kvco varies with the amplitude of input base signal due to the varactor?s nonlinear, the modulation quality is poor. Kvco varies 1%, the demodulated signal SNR degrade to about 40dB. Can anybody have designed this project or know this theory?Could you give me the advice or any information how to solve the Kvco nonlinear problem?Thanks much in advance.[/quote]

peak up myslef

I also need to know how to design PLLs.

What is the VCO Kv? It sounds like you have the VCO designed so that the tuning range is comparable to the modulation deviation you need. That's not typical. A Typical VCO might have a tuning range of 10 MHz/Volt, and with a modulation deviation of, say 200 kHz, you'd only be modulating over a very small portion of the tuning range, which will be linear enough.

As it is, you've got fairly significant tuning nonlinearity, so I'm not suprised that your SNR degrades a lot.

Dave
www.keystoneradio.com