HELP circuit compares 2 RF FREQS (fix & sweeping) = trigger when same

[gmcjetpilot]

NEED IC/circuit compares 2 RF FREQS, one fix and one sweeping. When they are same I want a trigger to send a 5 volt pulse. See picture below.

Is there a IC chip for RF comparison detecting between two signals.

The idea is compare a FIXED RF signal (any where from 455Khz to 12 Mhz at least, more range the better) with a sweeping Frequency (RF signal of fixed voltage). The idea is when they are the same, it triggers a pulse to the Z intensity input of the scope.

Do you have any part and circuit ideas?
Here are the design parameters.

Sweep - Rate about 60 Hz
Sweep of RF freq is constant voltage
Sweep - Width (Frequency) about 300 Khz to 1 Mhz or 3 MHz
Fixed Freq - close to center of Sweep width, symmetrical sweep
Signals are unmodulated & have control of attenuation of both

 

[benta]

Search for "phase/frequency detector", which is normally part of a PLL circuit. Most have a "lock" output that indicates when phase and frequency are equal.

Benta.

 

[gmcjetpilot]

Thanks benta that is a great suggestion.

 

[VEBill]

With some approaches you might run into latency issues, where the circuit tells you that the two frequencies *were* the same, but too late (after the fact). Some test equipment can be set-up to pre-trigger, but not usually on the Z input.

If such latency becomes a problem then another approach would be to mix the signals and look at the resultant beat note (simple audio). It would allow you effectively adjust an amount of pre-trigger by setting the beat note frequency trigger. It's a bit recursive because you still have choose the audio frequency detector circuit.

A simple mixer may produce double pulse (coming and going); use a one-shot to mask it, or an IQ mixer to prevent it.

You may also run into false triggering if your signals are not clean.

 

[gmcjetpilot]

Thanks VE1BLL. How late are we talking?

If the sweep rate is say 60 Hz
Sweep is 1 mhz wide
I'd like to keep the error to 100 Hz
so that is 0.0001 of the sweep
the time per sweep is 0.0166
So trigger time is 1.66 uS
You think it can trigger in that time?
I wounder if it will trigger at all?

The idea of using audio or beat freq makes sense.
I'll look at that. Thanks

 

[VEBill]

With phase/frequency detectors of the sort used in PLL circuits, you can trade-off between detection time and false triggering rate. But the "1.66us" is comparable to the period of the RF (order of 1 us), so there's a risk that latency would be an issue. Thus my suggestion to use an architecture such that you can turn a knob to effectively adjust the negative-latency pre-trigger.

Or at least check the 'scope manual to see if there's any built-in method to accomplish the same thing.

The main point is to think about latency.