PLL Sweep Lock Circuit?

I have a low frequency VCXO that I am trying to lock to a weak received signal. It works fine, except when the receiver output falls below -20 dBm the control loop loses lock and the analog integrator pegs at a supply rail. Then for some non-linear reason, it will not reacquire until the power is raised to 0 dBm.

The phase detector is a DC coupled mixer, so there is no frequency acquisition circuit yet. For various reasons, I would like to keep this simple analog phase detector instead of a digital phase/frequency detector.

I remember, from a long time ago, a sweep lock circuit where you can sense an out of lock condition and force the integrator to sweep like a sawtooth to reacquire. If I could sweep the band, it would probably phase lock at a lower power level. Does anyone have a schematic of such a circuit?

Thanks

a ramp-generator before vco may help u acquireing weak signal and also returning back the pll to lock condition.

As I remember you should also add a lock indicator ckt, to switch off ramp when lock is reached.
g579