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
