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AMATEUR RADIO BY GARRY CRATT, VK2YBX PEP monitor circuit for transmitters & transceivers Althoug~_most analog meters are capable of making accurate measurements of the constant power modes used in amateur stations, they're no good for measuring peak envelope power (PEP) ~The circuit presented here is designed to give accurate PEP readings. Because peak power equ ates to "talk" power when using SSB as a transmission mode, being able to measure this output power accurately is important to SSB operators on HF, VHF and UHF alike. The inability of a standard analog power meter to correctly measure peak power is largely due to the inertia of the meter movement used. By necessity, the movement must be damped to some degree to ensure that it cannot easily be physically damaged. This means that the meter is unable to track rapid transients. Not only that, the peak indicated power is only visible for a fraction of a second, making visual recognition difficult. Indeed, this is also the case to a certain degree with transmission monitors using a CRT, although they are quite useful in ensuring that the transmitted signal is not unduly distorted , a common problem when linear amplifiers are driven into compression. Such units also require a significant financial investment. What is needed then is some system where the rapi d modulation transients can be monitored and the metering system modified to lengthen the response time, so that they can be easily seen. Of course oommercial PEP meters do exist, as do retrofit circuits designed for specific units such as the "Bird" wattmeter series, recognised as an industry standard. Once again , the cost cannot be justified for amateur use. The circuit described here is similar to one that has been previously publish ed overseas but includes several improvements. In essence, it takes the fluctuating DC signal which would normally be fed to the existing meter movement, feeds it to a "sample and hold" circuit and then drives the meter. Circuit details Now refer to Fig.1. The circuit uses a DC coupled op amp (ICla) and a voltage fo llower (IClb) to drive the original meter movement. The output of the first op amp charges a lµF capacitor through a l00kQ resistor, setting the circuit rise time to lO0ms. A 4.7MQ bleed resistor ensures that the decay time is set to several seconds, enough time to read the peak value on the meter. In more detail, the DC input signal to the circuit is fed across VRl, a 5kQ potentiometer in series with a lkQ resistor. This network effectively replaces the original meter movement. + ,ooI.- ,oOk 100k INPUT + VR1 5k 1M 1k VR2 50k - 0.1 03 1N914 04 1N914 ~ 4.7M 1M ":' 84 SILICON CHIP - - - ---<11----<.1 OUTPUT TO METER .,. ,. Fig.1: this circuit is intended to replace a normal analog meter for reading peak envelope power. It is essentially a sample and hold circuit. Note that it has no RF detector circuit but is intended to be driven by the fluctuating DC across a normal moving coil meter. Fig.2: the PC board overlay diagram. The board is intended to be mounted inside a normal RF power meter, possibly with switching to give "normal" and "PEP" readings. At right is the full-size PC pattern. The input signal is then coupled via a lOOkQ resistor to pin 3, the non-inverting (+) input ofICla. IC1a acts as voltage follower and charges the lµF "hold" capacitor via diode Dl. Dl is connected inside the feedback loop for IC1a so that the circuit can respond to quite small signals. Op amp IC1b buffers the lµF capacitor (ie, places negligible load on it) and drives the external meter via diode DZ . Diode D3 protects the meter movement against overdrive. Since one side of the meter is connected to the 0V line and the other side is driven by IC1b via DZ, it follows that the output of IC la and IC1 b must be biased so that they are normally about +0.6V (ie, one diode drop) above 0V. This is accomplished by th e biasing network involving trimpot VR2 , the two 1MQ resistors and diode D4. Any op amp capable of operating from a single supply rail _a nd allowing the output to go to ground can be used. The LM324 or LM358 are suitable, although the 324, being a quad amplifier, is somewhat wasted in this application. The entire circuit can be run from a 9 volt battery, which in our circuit feeds a 5 volt zener regulator. As the total current drain is less than 2mA, this arrangement could last six months or so in continuous use without a battery change. Construction of the unit is simple, with the idea being to fit the PC board inside a standard wattmeter case, if possible. Fig.2 shows the parts placement on the PC board. You can install the parts in any order you wish but take care with component orientatfon. Calibration Calibration of the circuit is straightforward . Set both trimpots to their midpoints and connect the transmitter to the unmodified meter in the standard manner. Set the transmitter so that the meter reads half-scale. This can be done by adjusting the actual RF output power or, in some cases, decreasing the DC supply voltage. Having set the transmitter to read half-scale, and without making any further adjustments, disconnect the power meter, open it up, disconnect the two wires leading to the meter movement (taking note of the positive lead), and connect the new circuit board output to the meter movement. This done, connect the circuit board input to the wires previously removed from the meter movement. Now turn the transmitter on and adjust trimpot VR1 until the meter reads the same as it did prior to the addition of the new circuit board. Reassemble the meter and recheck. In some cases, input matching of the circuit board can be assisted by replacing the lkQ series resistor with a lkQ potentiometer. This should allow finer adjustment. To adjust VR2, turn off the transmitter and check the meter reading in this "no signal" condition. Adjust VR2 so that the meter reads zero. Now recheck the accuracy of the meter and readjust VR1 if necessary. 'If desired, the unit can be switched in or out by using a DPDT switch to bypass the circuit board. Alternatively, as the gain of the circuit is unity, the decay response time can be shortened by reducing the value of the 4. 7MQ resistor, by switching a 100kQ resistor across it, when in the "normal" mode. References National Semiconductor Linear Data Book 1; Radio Communications magazine (USA) January 1989; Ham Radio magazine, September 1989; ARRL Handbook. SC ~ PARTS LIST ! 1 PC board, 60 x 65mm, code SC06107911 1 LM358 dual FET-input op amp (IC1) 4 1N9 14, 1N4148 signal diodes (0 1-04) 1 5. 1V 400mW zener diode (2 D1) , 1 50kQ trimpot (VR2) 1 5kQ trim pot (VR 1) 1 ~ 1, 1 , Capacitors 1 1 1OOµF 16VW electrolytic ; 1 1µF metallised polyester or polycarbonate 2 0. 1µ F metallised polyester or ceramic 1 .00 1µF metallised polyester or ceramic 1 Resistors (0.25W, 5%) 1 4.7MQ 2 1MQ 3 100kQ 1 1kn 1 470Q I JULY 1991 85