×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

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

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

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

(OP)
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

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

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.
 

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

(OP)
Thanks benta that is a great suggestion.

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

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.

 

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

(OP)
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

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

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.

 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources