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I am just wondering if there is any simple circuit to measure the frequency of a signal near 100MHz. A FM demodulator IC may do it but I need it to be simple and without ICs (aka, only containing discrete R, L, C, diodes, transistors and such). There is very loose requirement about the response time -- as long as the output can settle in less a second when frequency shifts by 20%.

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  • \$\begingroup\$ If you are wanting to measure the frequency, in what form are you hoping to have the measurement reading supplied to you - I don't see that any of R, L or C have a built-in display. \$\endgroup\$
    – Andy aka
    Commented Sep 13, 2015 at 8:37
  • \$\begingroup\$ Something easier to measure, such as slower voltage and such. \$\endgroup\$
    – shva
    Commented Sep 13, 2015 at 15:46

2 Answers 2

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A PLL is the best way, especially since your signal is in the middle of the FM broadcast band. But if you want the circuit to be passive, you can use a slope detector. Make a narrowband filter at 100 MHz. Then use a diode to make an envelope follower (also called an AM detector circuit, like a crystal radio.) The signal strength will be greater when the signal is closer to 100 MHz.

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Theremins did something like this for the volume control. The oscillator is shifted in frequency by the hand, and the output frequency is passed through a filter, and then diode rectified to create the control voltage.

You don't mention how much signal level or range of variation you have to work with.

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