Record discriminator output on Quantar
Posted: Tue May 05, 2020 7:45 am
Hi all,
I know I goofed something up. I just can't figure out what.
I want to record discriminator output from the RX Aux Audio pin (30) on the 50 pin telco connector on a Quantar base station. I've modified my wildcard table to turn on discriminator audio at the RX Aux Audio pin.
To my surprise the audio is sitting on a +4.3V DC offset with about a 1.8V pp swing at the max deviation I'm working with. Since I want to record the signal on a computer, I think to myself, "I can't have a DC offset". So I tried to build a simple RC high pass filter.
Before I pumped it into my computer, I connected the resistor to an o-scope. To my surprise, I watched the signal clear-as-day gradually flatten to nothing. Wait... what? I thought I used the wrong value capacitor, so I pulled out anther (smaller one) and tried that. With a smaller capacitor the signal decayed even faster. Wait... what?
At this point, I thought I was missing something relative to impedance matching and loading, so I connected pin 30 to ground through a few large-ish resistors. Based on the measurements I collected with the different resistors, I think I figured out that the output resistance of the RX Aux pin is about 10k-ohms.
At this point another weird thing happened. I inadvertently shorted the RX Aux pin to ground and the radio keyed up. More experiments and I found that when I pull more than 0.16 ma (160 micro amps) from pin 30, the radio keys up. Sigh...
And to add insult to injury. If I try to connect the audio to a pc, the microphone port (I have a combo port that can do mic and line in) it looks like a short to the radio and keys it up!
So... with all this said. Could someone enlighten me please? Why is the discriminator audio on a DC offset? What use is that?
Why does the radio key up when I short the RX Aux Audio pin? What is the best way to pull the audio I want and record it?
Thanks
Dario
I know I goofed something up. I just can't figure out what.
I want to record discriminator output from the RX Aux Audio pin (30) on the 50 pin telco connector on a Quantar base station. I've modified my wildcard table to turn on discriminator audio at the RX Aux Audio pin.
To my surprise the audio is sitting on a +4.3V DC offset with about a 1.8V pp swing at the max deviation I'm working with. Since I want to record the signal on a computer, I think to myself, "I can't have a DC offset". So I tried to build a simple RC high pass filter.
Before I pumped it into my computer, I connected the resistor to an o-scope. To my surprise, I watched the signal clear-as-day gradually flatten to nothing. Wait... what? I thought I used the wrong value capacitor, so I pulled out anther (smaller one) and tried that. With a smaller capacitor the signal decayed even faster. Wait... what?
At this point, I thought I was missing something relative to impedance matching and loading, so I connected pin 30 to ground through a few large-ish resistors. Based on the measurements I collected with the different resistors, I think I figured out that the output resistance of the RX Aux pin is about 10k-ohms.
At this point another weird thing happened. I inadvertently shorted the RX Aux pin to ground and the radio keyed up. More experiments and I found that when I pull more than 0.16 ma (160 micro amps) from pin 30, the radio keys up. Sigh...
And to add insult to injury. If I try to connect the audio to a pc, the microphone port (I have a combo port that can do mic and line in) it looks like a short to the radio and keys it up!
So... with all this said. Could someone enlighten me please? Why is the discriminator audio on a DC offset? What use is that?
Why does the radio key up when I short the RX Aux Audio pin? What is the best way to pull the audio I want and record it?
Thanks
Dario