Precision Modulation Monitor - NOT!


   Author:   Max Carter
   Email:    mcarter@btigate.com
   Date:     1998/01/18
   Forums:   alt.radio.pirate
   _________________________________________________________________

If you plan to use the Modulation Monitor Circuit shown at
http://members.aol.com/amn92/Mod_mon.HTM as a foolproof way to keep
your station from over modulating, you might want to reconsider.
There are other choices out there - better and simpler.

When I first ran across the Precision Modulation Monitor Meter Circuit
it just didn't "look right".  To be specific, it looked as though the
Circuit, while working just fine with a symmetrical modulating (audio)
signal, probably would not accurately indicate when an FM transmitter
is being modulated ASYMMETRICALLY.  In other words, during conditions
when the transmitter is being heavily deviated in one direction, say
to +100 kHz, while at the same time being under-deviated in the other,
say to -50 kHz.  The basis for my doubt was the fact that the Circuit
uses two peak-holding detectors - one for positive peaks, one for
negative peaks - followed by a summing amp, instead of what seemed to
me the correct configuration - a full-wave precision rectifier,
followed by a single peak detector.

To check my suspicion, I built the Circuit on a solderless breadboard
and ran it through a rigorous test.  The test setup was as follows: A
1-kHz, adjustable duty-cycle squarewave signal from a function
generator was used to directly drive the paralleled inputs of a North
Country Radio MPX96 stereo transmitter operating in mono mode (no
pilot) and without pre-emphasis.  No compression, limiting or other
processing was used in the audio path.  An oscilloscope was used to
monitor and set the function generator.   The RF output from the
transmitter was fed to both a Heathkit AJ-15 FM tuner and an IFR model
COM120B service monitor, which served as a measurement standard.  The
baseband output from the tuner was connected to the Modulation Monitor
Circuit.

The setup was first tested with the generator set to 50% duty-cycle.
The generator amplitude was adjusted to produce a range of deviation
readings from 0 to +/-75 kHz.  With each setting-change the DC output
from the Modulation Monitor Circuit was noted.   Under these
circumstances, the Circuit always produced an output voltage
proportional to peak-to-peak transmitter deviation.

The function generator was then set to produce a reference reading on
the service monitor of +/- 50 kHz and the output voltage from the
Modulation Monitor again noted. The output signal from the function
generator was then varied through various duty-cycle settings in the
range of 10% to 90% while maintaining constant peak-to-peak amplitude.
The IFR COM120B reference standard always correctly indicated the
asymmetrical modulation of the transmitter in exact relation to the
duty-cycle of the modulating signal - a function generator setting of
75% / 25%, for example, produced a reading of +75 kHz / -25 kHz on the
IFR.

The output from the Modulation Monitor Circuit produced no such
indication.  As changes were made the output voltage from the Monitor
Circuit would spike upward for a fraction of a second - too brief to
get a meter reading - but would always quickly settle to the ORIGINAL
reading.  In other words, the Modulation Monitor Circuit, while
correctly indicating peak-to-peak deviation, was blind to sustained
asymmetrical deviation!

Thinking there was probably a better way, I rebuilt the circuit,
incorporating a standard full-wave precision rectifier (Fig 5-9, from
IC Op-Amp Cookbook, Walter G. Jung, First Edition, Howard W. Sams,
1974) followed by a standard peak-hold detector (Fig. 5-12, same
source) and a non-inverting buffer, then re-tested.  [Incidentally,
the standard circuits can be implemented using only two op-amps and
two diodes for the precision rectifier, and two op-amps and 1 diode
for the peak-hold detector/buffer.]  As expected, the standard circuit
responded precisely to any combination of modulating signal amplitude
and asymmetry with an accurate indication of peak deviation.

So, don't believe everything you read on the Internet.  I suppose it
could be argued that real-world conditions are different, that
asymmetrical modulation only takes place intermittently, not
continuously, as in my tests.  But, you see, the word "precision"
implies "foolproof" and the Circuit, as presented, isn't.  Not to say
it isn't useful, just that it isn't precise, and it certainly isn't as
simple as it could be.  A single AC-coupled half-wave peak-holding
detector would probably work as well and be a lot easier to build.
The foolproof, standard circuit is also easier to build.

I also disagree with the author's advice to use an electromechanical
meter movement, with its unknown ballistic characteristics, as the
indicating device in this application.  A better choice would be an
LED display driven by one or more LM3914 LED driver chips.  This would
allow the ballistics of the meter to be completely specified in the
electrical domain.  An unbeatable combination, I think, would be two
peak-holding detectors (one for positive and one for negative), each
driving its own LM3914 display.

Sincerely,

Max Carter