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January 24, 2012


John Dunn

Hi, Tom.

With coaxial cable's velocity factor of approximately 2/3, the speed of signal propogation would be 200 million meters per second. That velocity amounts to 5 nSec of delay per meter so 10 nSec would take 2 meters of cable or approximately 6.56 feet, not an impossible number.

Some FM demodulation circuits that come to mind are phase-locked loops, the Foster-Seely discrminator and the ratio detector.


John Dunn

Oops! I left one out. FM singals can also be demodulated using a slope detector.

William Ketel

This is certainly a way to detect frequency modulation, much the same as most modern IC detectors do, with an external delay element. BUT it does not match the frequency discriminator description given in the explanations of Armstrong's original frequency discriminator.
So YES, it is an FM detector, but NO, it is not really a "Frequency discriminator" in the common understanding of the term.

Jerry Meyerhoff

Fun with FM demodulators . Names like Foster-Seely , Quadurature, Ratio detector & RCA's CA3089 (over-used) IC come rushing back to me from working on Motorola Two-way radios & entertainment automotive receivers of the 70's & 80's. Today its "quadrature detection" done in software at base-band.
Perhaps some application notes and textbooks survive to show "how it was done in the day". e.g. for 230 kHz bandwidth FM broadcast in stereo, tuning up (phasing in factory language) the CA3089's phase-shift coil (tuned circuit) was a snap. For Motorola 2-way radios, all's you did was tune the coil to "zero the discriminator" (meter 5) on the serviceman's test set.
## But the real trick was getting a 100 dB gain 10.7 MHz IF strip to NOT self-oscillate. Till that was achieved by applying all the black-arts of EMC control, the detected audio output at minimum sensitivity was often quite distorted. Thanks John for another opportunity to walk down memory lane. jdm 2/1/2012

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