This peak reading circuit will quickly go to the peak power level and hold. The decay of the reading is determined by an adjustable time constant. A diode is connected in series with a current proportional to the power and is used to charge a storage capacitor. A very high impedance voltage follower circuit (opamp) is used to read the capacitor voltage. A variable resistor across the capacitor determines the decay time of the peak reading. This is similar to an AGC circuit with a fast attack and slow (adjustable) decay time.
Most wattmeters, including the one in the Collins 312B-4/5, consist of a directional power detector circuit which applies a current proportional to power through a resistor in series with a meter (or a couple of resistors, switchable, for selecting more than one scale). In our modification, we break the connection between the meter and range resistors. The peak circuit, with an input resistance equal to the meter resistance, is installed between the range resistors and the meter. The original calibration of the range resistors is therefore unchanged. The peak circuit may be switched in and out as desired for peak and average reading.
The voltage (on the order of 0.2 volts) across the input resistance of the peak circuit is amplified to take full advantage of the dynamic range of the opamps. The nonlinearity of the diode drop is eliminated by putting the diode inside the feedback loop of the amplifier. A resistance in series with the meter and the output of the peak circuit restores the full scale current to it's original value.
This design can be used with any metering circuit, with modification of parts values depending upon the meter resistance and full scale current through the meter.
SSB and AM operators will find the PDC-1 essential for accurate voice power readings. Utilize your favorite wattmeter to read voice peaks!
QRP enthusiasts find this circuit indispensable for PEP QRP readings! Converts low power wattmeters to actually show QRP power peaks!
The PDC-1 is available directly from HI-RES COMMUNICATIONS, INC. for $28.95 postpaid (USA & Canada), in easy to assemble kit form.
Please add $3.95 postage for OVERSEAS orders. Michigan residents add 6% sales tax.