| AM Modulation Monitor
Calling this circuit a "Modulation Monitor" might be a bit of a misnomer to those who are expecting peak detection LEDs and a large analog meter to show positive and negative modulation. Instead, this simple circuit allows the user to hear what their transmitted signal sounds like, so it is a different kind of modulation monitor. I was inspired to build one of these after hearing numerous hams ask me how they sounded. Rather than take my word (or anyone else's) the user can hear their own audio from the mic through to the transmitter output.
Circuit Description:
Simplicity is important for two reasons. First, everything used should be able to be found in a parts drawer or at Radio Shack. Second, the output needs to be as literal as possible with as few components that could color the sound as possible. The 100uH choke and 180pF capacitor are used to help isolate the RF section from the rest of the circuit although I couldn't measure any difference in perfomance. Purists will want them there, I guess.
An RF sample needs to come from the transmitter output. I used a coaxial T connector, then made a connector with R1 to feed an RG58 (or any short coax). The other end connects to the Mod Monitor. I found that a 1.5k Ohm resistor works well for a 50 watt carrier level. Some experimentation might be needed since R1 isolates the sample line with a high impedance, so attenuation will vary with cable length. Toroid pickups work well also, especially for matching to long cable runs. Here's a great reference for RF sample circuits. The goal is to measure between 1.5 and 2.0 volts at TP. If you want to get fancy and install a meter, use a 1mA (fs) meter and set the input level for half scale.
The RF signal rectified by the diodes creates enough voltage to keep them properly biased for good audio reproduction, however, if you omit the meter (M) then make sure that the 4.7k resistor goes between TP and ground otherwise the DC bias voltage won't be there. I purposely chose to us a 1N914 as D1 and D2 over the popular 1N34. Not only are they easier to find, but their inferior rectification at low voltages ends up being a benefit for reducing distortion when receiver audio is mixed in to the circuit.
Diode polarity was chosen to keep the audio output in phase with the transmitted audio so that positive modulation is represented by a corresponding positive swing in audio output voltage. This is important when connecting an oscilloscope to the output to view the waveform and when listening on headphones (as long as the amplifier input and output is in phase as well).
The 10k pot and both 2.2k isolation resistors are only needed if you want to blend in audio from your receiver speaker output. This is very handy of you want to wear headphones and hear yourself when you transmit. The receiver audio is then heard when you unkey. The 10k pot is used to make the levels even so you can leave your speaker volume control where you like it. Another use for this is to record a QSO so you get everyone through your receiver and your audio comes through from this circuit. Pretty cool, I think. The 2.2k resistor in series with the 10uF capacitor is used to isolate the receiver audio from the diodes which would cause distortion to audio fed into this circuit from the receiver.
The output level is not sufficient to drive headphones, but it should be more than enough to feed directly into an oscilloscope, computer soundcard, or cassette deck for recording. You will need a small audio amplifier to bring it up to headphone or loudspeaker levels. The circuit was designed like that for better audio quality (distortion occurs at high output levels) and for flexibility for those who might want to add audio gear inline like a limiter or equalizer.
Always make sure the input control is turned down when you key up for the first time. Then slowly bring up the input until you measure around 1.75 (not critical) volts at TP or midscale on the meter if you installed one.
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