10.7MHz FM DETECTOR
by Harry Lythall - SM0VPO

Many times I have been experimenting with QRP and VHF FM and needed a very simple FM detector. Detecting FM requires an amplifying limitor stage as well as coils and diodes. Many IF chips have a built-in detector but these cost a couple of 'bucks/quid/spän', yet still require a bit of setting up. FMIF-DET is based upon a SN7401 quad NAND gate with open collector outputs and perform the same function. The 1K0 pot controls the centre frequency and the 'see text' resistor is selected for the PLL lock range, depending upon the amount of deviation you are to detect. Use 10K for 250KHz to 560 ohms for 1MHz. Increase the 22pf capacitor to 560pf if you want to use this PLL with a 455KHz IF sub-system. The IF input must be at least 350mV for good reliable locking of the PLL.

Alignment is easy, adjust the 1K0 preset pot with a 10.7 MHz input, whilst watching DC output of the filter (AF out). At some setting the PLL will be in-lock and the voltage will follow the movements of the control. Set the control for 1.8 - 2.2 volts DC.

This circuit can also be added to commercial HF receivers and transcievers not equiped with FM mode. Additional RF filtering may be necessary in the AF output if there is little or non in the following AF amplifier although this may only be necessary at 455KHz.

Have fun with QRP de Harry Lythall - Lunda, Sweden.

Dont forget to visit my messageboard if you have any questions about this or any other project. I always look forward to receiving feedback, positive or negative. You do not have to register to post messages.

Very best regards from Harry Lythall
SM0VPO (QRA = JO89WO), Märsta, Sweden.
EA/SM0VPO (QRA = IM86BS), Nerja, Spain.

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