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Record discriminator output on Quantar

This forum is for discussions regarding System Infrastructure and Related Equipment. This includes but is not limited to repeaters, base stations, consoles, voters, Voice over IP, system design and implementation, and other related topics.

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SpeakRFMan
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Posts: 3
Joined: Mon May 04, 2020 7:26 pm

Record discriminator output on Quantar

Postby SpeakRFMan » 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

Karl NVW
Posts: 153
Joined: Wed Sep 03, 2003 7:13 am

Re: Record discriminator output on Quantar

Postby Karl NVW » Wed May 06, 2020 11:30 am

Any reason you can't record your signal from the wireline remote output? Audio at that point will be squelched when the carrier goes away and will have the PL/DPL signalling removed. It's also run through a de-emphasis filter to provide a true reproduction of what went into the transmitter at the other end of the hop.
Karl - WA8NVW AFA5VB
SHARES + NCS

Jim202
Posts: 3571
Joined: Sun Sep 09, 2001 4:00 pm

Re: Record discriminator output on Quantar

Postby Jim202 » Wed May 06, 2020 4:20 pm

I always use a 1.0 mfd capacitor, non polarized in series with the audio output you have selected on pin 50. This will take the DC bias voltage off of the line and your computer will work OK with it then.

SpeakRFMan
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Posts: 3
Joined: Mon May 04, 2020 7:26 pm

Re: Record discriminator output on Quantar

Postby SpeakRFMan » Thu May 07, 2020 6:42 am

Karl,
I tried the wireline, but as you said, it filters and removes PL. I'm trying to record whatever is coming in the antenna when there are no active transmitters. I'm hunting power line noise.

Jim,
That's interesting. I didn't have a 1mfd cap handy so I used a 0.47 mfd one. However, I used it in conjunction with a shunt resistor to achieve a high pass filter. I think I set my values to get me a corner frequency of 20 Hz or something like that. Perhaps the Quantar RX Aux pin doesn't take too kindly to filters, and that's why the it faded away... Next time I'm in the lab, I'll try it with just the cap.

Thanks

SpeakRFMan
New User
Posts: 3
Joined: Mon May 04, 2020 7:26 pm

Re: Record discriminator output on Quantar

Postby SpeakRFMan » Thu May 07, 2020 6:01 pm

Update: Ugh... I'm an idiot. The salvaged telco connector I was using was pre-wired (incorrectly and for another application). The RX Aux pin was crossed-wired to the PTT line, so when I grounded (connected the signal and ground pins to the mic jack) it pulled the PTT line down and keyed the radio... Once I fixed the problem, everything worked like a charm. The cap removed the offset and all was right in the world...

My question stands... does anyone know why is there a DC offset on the RX aux audio port?

Dario

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kcbooboo
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Joined: Wed May 01, 2002 9:03 am

Re: Record discriminator output on Quantar

Postby kcbooboo » Sat May 09, 2020 12:16 pm

Most Motorola equipment uses op-amps in the audio stages and run them from a single positive power supply. This means all the audio signals run with a DC bias of around +4.8VDC. That's just the way they were designed and you have to live with it. You should always use a coupling capacitor when dealing with audio in to or out of any station. I typically use a 10uF 16V cap with the positive end towards the Motorola equipment.

Bob M.


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