modulation percentage testing with an audio oscillator

[KK4RF]

Gentlemen:
     I have a Hallicrafters HT-40 novice-type  rig which has controlled-carrier type screen modulation.  I would like to be able to feed an audio oscillator into the pre-amp and then adjust my mic gain so as to then check my modulation percentage on my SB-614 station monitor. My question is: what audio voltage level peak to peak coming from the oscillator would correspond to the audio voltage coming from my D-104 microphone ( unamplified microphone?)  I made a twin-T oscillator and can adjust the peak-to-peak output down to about 5 millivolts. Would this be an appropriate level or should it be higher? My audio frequency is about 800 hertz. Is this adequate for testing or should I have a higher frequency? I want to be sure I'm adjusting the mic gain on this to optimize my modulation percentage without overdoing it. Thanks for opinions.   ---Marty, KK4RF---

[W4AMV]

I had a similar question testing a Johnson Ranger. 5 mV is about right and to be sure, double check the D104 or similar mics wrt their p-p or rms values with normal speech. I used an ol HP200 audio osc. and padded it with a pot. Consider running your Twin-T into an OP-AMP for additional gain, then POT attenuate and follow with a 2nd OP-AMP as a voltage follower. Now you have a nice test set.

Hope that helps, Alam

[Steve - K4HX]

Connect the D-104 to the scope and check the level.

[KK4RF]

Alam and Steve,
     Both excellent points. The 1956 ARRL Handbook suggests crystal mic elements have an output voltage of about 0.03 volts (30 millivolts.) They didn't say whether this was P-P or if rms. The manual for the HT-40 says the mic  input needs 0.004 Volts at a minimum (4 millivolts.) Will give the mic a test on the oscilloscope later today. Thanks again, Gentlemen. ---Marty, KK4RF---

[Jim, W5JO]

Hook the oscillator to the transmitter and turn up the gain until you reach rated current on the transmitter.  Then measure the voltage out of the oscillator with your scope on the grid of the first audio amp to see what it is.  Then hook the D104 to the transmitter and see what current the D 104 prduces.  You can leave the scope (10:1) across the grid of the first audio to see if it approximates the voltage from the audio oscillator.

The comparison will tell you if your mic element is ok or not and what is necessary for complete modulation. 

[KK4RF]

Jim,
     Thanks for the ideas. I measured my D-104 voltage output into my o-scope and with normal speech got levels of between 30 and 60 millivolts, this with the probe at 1x, not the 10x. Anyway, I measured the millivolts on my oscillator that produced 100 % modulation, about 40 millivolts. Interestin tinkering. I noticed that with the microphone that certain consonants produce quite a spike in the scope tracing, to levels  of 70 and higher. Also, you don't need to be all that close to the microphone to get quite a response. Anyway, thanks to all who replied and I'll print out these responses and keep them with my HT-40 manual. Thanks again,   Marty,  KK4RF

[Jim, W5JO]

Watch where you use the 1:1 probe, it can load a circuit. 

[KK4RF]

Jim,
     Thanks for that bit of info. I did not know that. Thanks again. ---Marty, KK4RF---

[W7TFO]

It is fun to see just how high you can go, freq-wise.  Be careful, as anything past 7500 Hz > 80% mod and you'll find the yield points of modulator parts, RF insulators, caps, eTc.

73DG