You have a microphone with voltage output of 0.5 to 5 millivolts (depending on h

Question

You have a microphone with voltage output of 0.5 to 5 millivolts (depending on how loudly you’re talking into it) and an output impedance of 10,000 ohms. Your “line-in” socket on a computer sound card accepts a maximum 1 volt signal at 600 ohm input resistance. You want to be able to set the amplification of that range of microphone outputs to the maximum input level with a gain knob. The microphone is on a balanced line (neither side is grounded), so you choose an amplifier that uses a differential input (rather than a ground-referenced one). Design an instrumentation amplifier with an appropriate gain range. Choose resistors except the gain resistor of about 10,000 ohms. Specify the variable resistor to have a range appropriate for the gain range you specified—you will need two components to do this. Do you need to adjust any of the component values or add other components to account for the microphone’s output resistance? For the sound card’s input resistance?

Explanation / Answer

1- Turn the receiver and signal generator on and allow them to warm up for 1 hour.
2- Put the receiver and generator on 4.8 mHz (or any other frequency of choice)
3- Set the initial generator output to 1v and modulation level 1kc, 30% but do not
connect the antenna input yet..
4- Set the R390A Function switch to "MGC," Bandwidth to 4kc, RF gain full CW,
BFO off.
5- Set the Line Meter range switch to -10 and carefully adjust the Line Gain control
for a -10db indication on the Line Level meter.
6- Watch the Line Level meter and peak the Ant Trim on receiver noise alone;
readjust the Line Gain control for a -10db indication on the Line Level meter. This
level is now indicating a relative level of the receiver noisefloor.
7- Connect the generator to the receiver antenna input (I use the "Balanced" input)
and adjust the Kilocycle Change control to center the output of the generator in the
receiver passband.
8- Adjust the RF output level of the generator to cause the Line Level meter on the
R390A to read "0."
9- Note the output of the generator in microvolts. This value is the 10db S/N + N
receiver sensitivity. It should be less than 0.5uv although the official specifications
of the receiver call for 3 microvolts.

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