# Fast Frequency Measurement - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Tura
Post subject: Fast Frequency Measurement Posted: Sat May 20, 2006 12:47 am

Lieutenant

Joined: Sat May 20, 2006 12:29 am
Posts: 1
I need to track resonant frequency changes in a resonator. The resonant frequency is about 230 kHz. I would like to measure resonant frequency changes with noise level on the order of 1-5 mHz with a large dynamic range, and I would like to sample this frequency at about 100 Hz or better.

I was wondering if a PLL could help me. Essentially, I would like to take my signal (230kHz), convert it to a voltage proportional to the resonant frequency, so that I can measure this output voltage with a DAQ card. I'm not very competent when it comes to electronics, so it would be great if someone could suggest circuits and components.
Tura

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Charl
Post subject: Posted: Sat Jun 10, 2006 6:07 am

Colonel

Joined: Fri May 19, 2006 5:01 am
Posts: 25
Location: Netherlands
Hi Tura,

Just a thought, might be silly.. you could heterodyne the drifting 230KHz signal with a PLL'ed 230KHz (the calibration frequency so to speak). If the frequencies differ, their difference frequencies will be produced. This frequency should be so low that it's easily measurable.

Kind regards,
Charl

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IR
Post subject: Posted: Mon Jun 12, 2006 8:43 am

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello,

What I woud suggest is simply to use an FM detector. The detector provides a voltage relative to the frequency offset from the carrier (The center frequency of 230kHz in your case). You can adjust the center frequency by means of an inductor and capacitor (Variable). I worked with such an IC 8 years ago in an FM receiver circuit. I am sure that since then there are many new IC's availabel. Simply run a search in Google I am sure you will get up on something.

Good luck!

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Best regards,

- IR

Posted  11/12/2012

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