A useful S-Meter Design for Direct Conversion Radios
Rolf Heine, DL6ZB (ex DL6MBI)


This S-meter circuit indicates the fieldstrength which is derived from an AF signal of a direct conversion receiver. The logarithmic amplifier SA614 offers 90 dB of dynamic range which is enough for simple receivers. If properly constructed, the accuracy of this s-meter amplifier should be better than of most s-meters in commercial amateur radio equipment.

S-Meter amplifier mounted into transceiver


Best results are obtained with low impedance sources. With DC receiver Type 002 a suitable point for decoupling is at the end of the tschebycheff bandpass filter 0.5-1.5 kHz, at the emitter of transistor T4 (a decoupling capacitor of several microfarads is necessary). 

Decoupling of the AF Signal to S-Meter amplifier
Picture: Decoupling of the AF signal to the S-Meter amplifier via capacitor C1 (see DC Receiver Type 002)



The SA614 logarithmic amplifier is available from several sources. One source is RS Components. Google it!

The SA614 is a SMD part. It is mounted at the bottom side of the circuit board. The picture shows a TL072 dual-opamp, although in the meanwhile I substituted it to a LM358 because of the single source circuit. I use a 100µA meter as a indicator. The user is obliged to resample the output resistor network for suitable output current. With no input signal a output current of a few microamperes remains. The meter needle is therefore not exactly at "zero". If you want maximum readout accuracy you need to compensate this remaining current by proper selcting R4; R6, R7, R8. The suggested resistor network in the circuit diagram is suitable for a 100µA instrument.

The instrument I used is from German distributor Bürklin (www.buerklin.com).
Part# 16K210

I printed a s-meter scale to a simple printer after I measured the signalstrengh. If you don't have a signal generator for calibration I recommend to use a homemade signal generator made of a NE555 signal generator and a variable resistor. The frequency should be tuned to about approx. 750 Hz.



AF TEST GENERATOR

Picture: Test circuit for S-Meter calibration of Direct Converstion Receivers.


The plastic cover of the instrument fortunately can (CAREFULLY!!!) be removed. After removing the plastic cover it is possible to remove the glued genuine scale. Glue the new scale into the instrument - and ready is a nice personal s- and vswr-meter. I also use the instrument to measure the supply voltage. Even more measurements are possible. It's a matter of requirements.

When the transceiver is switched on it usually takes a few seconds until the needle goes down to zero. I currently use a 78L08 voltage regulator instead of a 6V type because I couldn't find a 6V regulator in my junk-box... :-)

Circuit Diagram

Please contact me if you need a PCB layout of this S-Meter circuit.

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