Anyone operating on the A.M. mode needs to have an accurate, reliable, easy to use method of measuring their modulation. Furthermore, a good audio monitor should be available for listening to the sound of the transmitted signal. Ideally, the monitor should also immediately inform the operator when the signal is overmodulated in the negative direction – a violation of FCC rules. The modulation monitor described in this article accomplishes these objectives. It provides separate metering for positive and negative peaks, a negative overmodulation peak flasher, and has a built-in calibration oscillator allowing the monitor to be accurately calibrated without using an RF source.

The negative peak flasher consists of an op amp used is a comparator driving a peak storage network and LED driver. When the instantenaous voltage fed to the non-inverting input of the peak flasher input op amp drops below the voltage fed to the non-inverting input. The op amp will produce a positive output voltage. This is fed through a 1n34 rectifier to a R/C network which provides sufficient energy storage to keep the LED on for a long enough period of time to be plainly visible.

The DC level shift provides a 0 VDC referenced signal for the precision rectifiers. These rectifiers very accurately rectify the positive and negative portions of the modulated signal. The rectifier output is fed into a filter network which has a sufficient time constant to keep allow the meters to respond to the signal, and be visible to the operator. The output of the filter networks is buffered. The buffered signal is fed into the "meter kick" networks. These networks provide a voltage "kick" which gets the meters moving quickly. The time constant of these circuits is adjusted such that the meters are "kicked", but do not overshoot. The negative peak output is inverted before being fed to the negative peak meter.

Construction Notes

The monitor operates in an RF environment, and handles RF energy. The RF handling circuitry (detector circuit) should be constructed to minimize coupling to other parts of the circuit. Each power lead of each IC and OP Amp should be bypassed directly to the ground plane, as close to the IC as possible. The TL074 Op Amps are run at plus and minus 12 volts. The 7404 IC used in the calibration oscillator requires plus five volts. The power supply voltages MUST be regulated. Use 7812, 7912 and 7805 regulator ICs, one for each supply voltage. They are very stable and easy to use.

The meters I am using in my version of the monitor are large VU meters with a 1500 Ohm internal resistance and require 200uA full scale. Other meters may be used. You need to make a meter equivalent circuit which will put your meters at full scale when approximately .3 (3/10) volt is applied. As an example, a 1 mA meter with a 300 Ohm resistance in series (meter resistance plus any required external resistance) is suitable. You may want to change the Meter Calibration Adjustment pots from 1k to 100 ohm pots if a 1mA meter is used. For maximum usability, VU meters are suggested. 0 VU = 100% modulation.

Adjustment and Calibration

For proper calibration, an oscilloscope is required. With the monitor turned off, set the Pos. Peak and Neg. Peak meter kick adjustments to approximately 4k across the capacitors. Set the Carrier Internal Lev. adjustment to about half (7k). Now turn the monitor on keeping it in the "norm" mode. The first thing which must be set up is the calibration oscillator. Set the Cal. DC Offset control to 0. Set the Cal. Level adjustment to produce a few volts output. Adjust the Cal. Osc. Balance adjustment such that the oscillator "on time" (high) and "off time" (low) exactly the same. Now, set the Cal. Level adjustment for 0 output. Set the Carrier Level to the highest setting. Connect the oscilloscope to the point in the circuit which says "3 Vdc when normal carrier is applied", and set the oscilloscope to read 1 volt per division. With the monitor in the "norm" mode, adjust the Detector Bias adjustment to the point where the 1n34 rectifier diode just begins to conduct and voltage just measurable on the oscilloscope. Put the monitor into the "Cal" mode. Set the Cal. DC Offset so that 3Vdc is produced at the test point. Now connect the oscilloscope to the output of the DC Level Shift op-amp. Adjust the DC Level Set adjustment so that 0 (zero) volts DC is produced at the output of the level shift OP amp.

Reconnect the oscilloscope back to the input of the input of the previous stage (the main test point). Set the negative peak meter to monitor Carrier Level, and adjust the Carrier Level Meter Calibrate adjustment so that the negative peak meter reads 100% modulation. The carrier level is now established.

Set the Peak Flasher 100% Neg. Cal. adjustment to 0 V at the wiper of the pot. Set the Level Set Cal. adjustment for 3Vdc at the wiper of the pot. Increase the Cal Level (at the calibration oscillator) control until the low level of the square wave just about reaches 0 V. With the Peak Flasher Level Set control set at 100%, adjust the Peak Flasher 100% Neg. Cal. adjustment so the peak flasher just begins to flash. The peak flasher is now calibrated.

With the Negative Peak meter selector set to read Negative Peaks, set the Neg. Peak Cal. adjustment so the Negative Peak Meter reads 100% negative. With the Positive Peak Meter set to read negative peaks, ensure that both meters show the same reading. If it is necessary to change the Meter Cal. adjustment for the negative peak meter, readjust the Carrier Level Meter Calibrate adjustment, and the Neg. Peak Cal. adjustment. The negative peak meter is now calibrated.

Now, adjust the Cal. Level control so that the high level of the square wave reaches 6 V. Adjust the Pos. Peak Cal. adjustment so that the Positive Peak Meter reads 100% positive. The positive peak meter is now calibrated.

This completes the overall calibration of the modulation monitor.

If, under operation, the meters are "sluggish" (slow to respond), Increase the Meter Kick adjustments to about 5k, re-calibrate the positive and negative peak meters (see above). Conversely, if the meters show "overshoot" (easily observed if the negative peak meter reads greater than 100%), decrease the meter kick adjustments to about 3.5K, recalibrate the meters, and check the operation again. The meter kick adjustments are properly adjusted when the meters are very responsive, but do not show any overshoot.

The Audio Output provides a high quality output. An audio power amplifier should be added at this point in the circuit, if direct connection to headphones is desired, otherwise the output should be connected to a good quality, high fidelity audio amplifier.

Operation

If the output power of the transmitter under measurement is greater than about 50 watts, the monitor should be connected through some type of attenuation network to the transmitter output. A capacitive voltage divider is an easy and effective method of no-power-loss attenuation. If the carrier level is not set correctly, the monitor will not be accurate. The carrier level should be checked periodically under normal operation.

Note: The information presented herein may be freely used by individual Amateur Radio Operators for non-commercial, Amateur purposes. Use of this information, schematic diagrams or circuits presented herein, in part or full, for any commercial purpose whatsoever is prohibited without express, written permission of the author.