Motorola Micor S-meter Driver

Posted on Feb 5, 2014

The artwork style of the opamp and of the meter face both suggest that is is an ACC design and is from an old issue of ACC Notes. I have credited it as such. I scanned the second image and wrote the text below from scratch. If anybody can locate the original ACC Notes text I will add it, and a reference to the original issue. Provide an analog s-meter output proportional to signal input.

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This DC voltage can be fed to a analog input of a repeater controller. The controller can be programmed to give an indication of signal strength, perhaps by varying the frequency of the courtesy beep. Just remember that intermod, desense, and other extraneous RF noise will false the reading. Looking at the two schematics below you will see that the circuit point after the first IF amplifier and before the second IF filter is identified as "Point H" on the UHF schematic and as "Point J" on the VHF schematic. This is the point of interest. The circuit below provides a dc voltage from near zero to just over 5 volts dc depending on signal input to allow a local meter or the ACC controllers s-meter input to be driven. A gain (calibration) pot is provided to allow the user to set the full-scale s-meter calibration as desired. The 68pf capacitor provides DC isolation from the IF amplifier output, the two diodes and the 100pf capacitor produce the varying DC level, and the LM324 opamp acts as an variable gain buffer amplifier and prevents whatever load there is (an analog meter or the repeater controller analog input) from dragging down the DC level. Your target is a 0 to +5vDC voltage. The 68pf capacitor may need to have it`s value adjusted for a 10m or 6m receiver. A low band Micor uses 5. 26 MHz for an IF frequency (or 5. 36 if needed to dodge a birdie), whereas high band, UHF, 800MHz and 900MHz receivers use 11. 7 (or 11. 8)...

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