A useful S-Meter Design for Direct Conversion Radios
Rolf Heine, DL6ZB (ex DL6MBI)
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
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).
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
The instrument I used is from German distributor Bürklin (www.buerklin.com).
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.
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... :-)
Zip File PCB layout for EAGLE layout editor click here.