frequency jump
Thanks!
Hi,
I think that this is normal since the loop is open. In fact, with time the capacitance of the filter will see their voltage decreasing because of the leakage to VCO Vtune will decrease and consequenctly the Frequency change.
Each system with negative feedback (even a simple opamp) will loose its normal operating point when the loop is open. You should try to understand the meaning and the function of feedback loops.
In my understanding open-loop modulation does't mean breaking the PLL loop rather than additive insertion of the modulation signal into the loop. The loop filter has to be designed sufficient slow not to cancel the modulation.
Both of you are right.
1. with time the capacitance of the filter will see their voltage decreasing because of the leakage. The leakage satified my spec. It only decrease less than 100KHz in 50ms.
2. Each system with negative feedback (even a simple opamp) will loose its normal operating point when the loop is open. It's right. But open loop modulation can be used if only the frequency drift not too much in a fixed period.
My problem is the frequecy jump just on the time when the PLL is open. The frequeny jump up about 500KHz. It's too much. And I can't find which reason caused that.
perhaps u are using a strong current in the charge and decharge of the filter's capacitances. Try to minimise the current and tell what you get.
Thank you !
Hi! FvM!
Open loop modulation is often used in FSK/GFSK modulation(such as bluetooth). When the signal is modulating the PLL is in open state.
Hi! AdvaRes!
I had tried using small current. It didn't have any help!
There may be different flavours of open loop modulation. Clearly the modulation signal is added to loop. The phase detector can be either in effect (with a slow loop filter) or disconnected. But disconnection of the PD occurs at the loop filter input and should not inject a frequency step, if done correctly. You should show your circuit implementation to make the issue understandable.