View Full Version : Simple am audio monitor and rf sampler

01-01-2012, 06:18 PM
This monitor works great will let you hear what you really sound like on the air provided the receiving end
has wide enough bandwidth to hear it also also works better patched through an amplifier it doesnt drive headphones all that well
if you dont need the scope output leave that half of the circuit out and just have a nice audio monitor diode can be just about
any germanium diode must be germanium


The East Coast Sound design is a good example (among many others) of an off-air monitor that will work fine: with one caveat

The circuit shown is good for monitoring an AM signal with around 16 watts of carrier:

I.e. 16 watts into 50 ohms corresponds to a peak rf voltage of 40 volts (at carrier), and 80 volts on 100% modulation peaks.

Even with the 2700 ohm resistor in series with the 100 uH choke (whose impedance will be somewhat unpredictable at frequencies above 1 MHz, but perhaps as high as 2k ohms at 3.885 MHz... depending upon what its self-resonant frequency is), there is probably too much reverse voltage across the diode if one were to use this circuit with a higher power transmitter.

Also, at higher frequencies (well past the self-resonant frequency of the 100 uH choke), the 2700 ohm resistor will dissipate about 1/50th of the transmitter's output power... so the 1 watt value will limit the carrier level as well.

I would suggest that one use the circuit shown with the following changes (again there are many alternative designs, but these changes will enable this circuit to handle the power levels):

1. Place another 2700 ohm 1 Watt resistor in parallel with the 100 uH choke. This will form a voltage divider with the existing 2700 ohm resistor of at most 1:2 (at most, 50% of the rf voltage will appear across this new 2700 ohm resistor)

2. For carrier power levels between 1 watt and 16 watts, leave the existing 2700 ohm, 1 watt resistor as is.

3. For carrier power levels between 10 watts and 160 watts, change the existing 2700 ohm resistor (not the new one you added in step 1. above) to a 10k ohm 2 watt resistor.

4. For carrier power levels between 100 watts and 375 watts, change the existing 2700 ohm resistor (not the new one you added in step 1. above) to a 27k ohm 2 watt resistor

Other comments:

The purpose of the 100 uH choke is to bypass any audio frequency voltage (not modulated rf) that may be at the output of your plate modulated transmitter.

The circuit would work even better (better fidelity in tracking your modulated envelope) of you changed the .01 uF capacitor to .001 uF, and changed the 1000 ohm resistor to 10,000 ohms. This will reduce the amount of current/charge that the diode has to deliver to increase the voltage across the capacitor... thereby reducing the distortion introduced on negative peaks due to the voltage drop across the diode.