| RMS Ratio Monitor
This circuit displays the difference in RMS energy between the positive and negative halves of an audio waveform.
Circuit Description:
The RMS Ratio Monitor is basically a dual half-wave power supply driven by audio. As long as the waveform contains symmetric energy, such as found in a clean sinewave, the two outputs will produce the same voltage and the "center 0" meter will not move. Any asymmetry will cause one output to be greater than the other abd the meter will deflect to the side that contains the greater voltage. The meter shown to the right of the schematic came from an FM broadcast monitor, so the scale doesn't apply for this circuit (but it works fine for demonstrations). I'm actually not sure what the proper scale should be anyway!
The meter is calibrated with a sinewave by adjusting the 1k balancing trimmer so that the meter reads center scale (no deflection). The sensitivity of the circuit depends on the driving voltage (volume). The input transformer can be anything for proper circuit matching as long as the output has a center tap and is relatively broadband. For best linearity, the output impedance should be chosen based on the load provided by the balancing trimmer and meter. I found 1k to work well.
AM broadcast stations usually show only a slight deflection, indicating that the audio energy is similar between the positive and negative halves of their modulated waveform. This makes sense as they are limited to -100% to +125% by FCC rules. However, the meter may show dramatic differences between amateur radio stations using consumer processors, negative only clipping, and out-of-phase microphones (or any combination of these).
Keep in mind that this circuit *does not* indicate peak modulation since there is no reference to the carrier, but it does show changes in RMS energy, thus it is an RMS Ratio meter.
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