EEVblog Electronics Community Forum
Electronics => RF, Microwave, Ham Radio => Topic started by: philpem on January 25, 2024, 03:08:56 pm
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Question in short: what are the current "preferred chips" for superhet FM/FSK demodulators and FM/FSK modulators?
I've a need for an FM (more accurately FSK) modulator and demodulator with the following specs:
Demodulator - frequency 6MHz to about 12MHz in 100kHz steps.
Modulator - frequency 80MHz to 125MHz in 100kHz steps.
Signal spec - 15kbps data rate, Manchester biphase coding, +/- 75kHz FM deviation, roughly +/-200kHz bandwidth. (same on both the uplink and downlink)
RF interface - 75 ohm impedance, F-type connector
RX performance doesn't need to be magnificent, the 'transmitter' will be on the other end of a piece of co-ax. it just needs to not get swamped by CATV-level (5 to 15 dBmV, or about -40 to -30dBm) VHF and UHF signals - and I think a good front-end filter will do most of that.
Back in the day I'd have probably built a PLL-synthesized exciter out of a VCO and one of the Philips TV-tuner-style chips (TSA5511 probably). Set up the PLL loop to be far slower than the data rate (1/4th of it at the fastest), and use an opamp to sum the TX Data with the PLL before feeding it into the VCO's VIn.
For the receive side I'd have probably used an SA636 or SA615 with a similar PLL VCO arrangement for the LO, but without the TX-In. Probably order the IF filters and quad coil from Cirkit or rob them from a junked FM radio.
Problem is all these chips are obsolete, and quad coils are pretty hard to find too. Are there any modern alternatives - or more modern ways to build an RX/TX?
I'd considered using one of the lower-end Analog Devices DDS chips followed by some filtering and an upconverter (diode ring mixer?). I'd probably generate the LO with something like an Si5351... but the whole setup seems like overkill for generating FM, and very high component count.
For RX it seems like SDR is the way things are going, but that also feels like absolute overkill. The second-best option I came up with was a 74HC9046 PLL, but I'd still need a limiting amplifier.
Hams and RF gurus, what do you suggest?
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it looks like your far more experienced than i,
but you might find the QRP labs QDX FSK transmitter interesting since the design is reasonably well published as DIY kit.
its not FM, or VHF even. i got one and its designed for HF FSK. But you might find it an interesting study.
looks like it uses a Si5351A/B/C "ANY-FREQUENCY CMOS CLOCK" (8 kHz to 160 MHz) chip for signal generation.
https://qrp-labs.com/qdx.html