frequency difference detector

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svensl

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I would like to build a frequency difference detector, using digital components only. It should be able to compare the signal frequency to a reference frequency and detect which frequency is faster. It should then output the difference of the two frequencies.
The signals will go in to a charge pump with a LP filter to obtain the average value.
Furthermore, it should be able to do the comparison within 1 period and also prevent slipping, meaning there should be almost no limitations on how much the reference and signal frequency can be apart for the circuit to work.

Can anyone suggest a circuit, name or paper that might address above issues?

Thanks
Svensl
 

If you tell us what you are making we may be able to suggest better ways to solve the overall problem

>It should then output the difference of the two frequencies.
Do you mean a number in hertz or a ratio or just 0 means F1 is higher 1 means F2 is higher?

>it should be able to do the comparison within 1 period

Since you are going to take an average why do you need to do it in 1 period?
 

The output of the circuit should be a signal like in a phase-frequency-detector. So when the input frequency is higher Qa goes high and when slower Qb goes high. With the charge pump and a LP filter the overall output should be a voltage proportional to the freq. difference. The discrimination range should be as much as possible. For example, in a quadricorrelaator the range is only 20% meaning the referenced and signal frequency have to be within 20% of each other.
The output of the FD should change with the rising edge of the input signal and the delay should be kept to a minimum as the circuit will be in a feedback loop where the output of the FD has to be available with the next reference clock rising edge.

Thanks
 

the circuit u need is a PFD .it will consist of two d ff's.now tie both the d inputs high.connect the signals to be compared to each of the clock inputs of the flip flops.again and the outputs of the flip flops and connect it back to the reset.

regards
amarnath
 

But a PFD also gives an output when the frequency of the two inputs is the same but the phase is different. In my case, when the frequency of the two signals is the same I want both outputs, Qa and Qb, to be zero. So far, the only circuits I have come accross are the rotational frequency detector and the quadricorrelator. I was just wondering what else there is as I am not very experienced. Thanks.
 

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