Discriminator output help

[scannerrail]

say if your receiving something and you plug your audio cable into the discriminator jack is it normal for it to cut the signal out so it's just barely audible? And when you remove it the signal comes back stronger?

What does that mean?

 

[bezking]

I have never added a tap to a scanner but I would fathom that the problem lays in either:

• You tapped the wrong place.
• You are using a bad cable
• Your computer is misconfigured.

 

[scannerrail]

I have never added a tap to a scanner but I would fathom that the problem lays in either:

• You tapped the wrong place.
  
  Unless someone made a mistake TP4 is the only place on the scanner that is to be tapped.
• You are using a bad cable
  hmmm radio shack cable...
• Your computer is misconfigured.
  
  maybe?

I might need to add a Cap along with the resistor.

 

[SCPD]

say if your receiving something and you plug your audio cable into the discriminator jack is it normal for it to cut the signal out so it's just barely audible? And when you remove it the signal comes back stronger?

That's normal. The radio's discriminator circuitry wasn't designed to drive an additional load. On top of that ... the signal impedance of the computer's sound input is likely much lower than the radio. The result is a drop in audio level when the disc. tap patch cable is plugged in to the computer. Unplug the cable and the sound level returns.

A cap or larger resistor value may help. You may also want to use a small zero gain buffer amplifier to provide the needed isolation between computer and radio.

-rick

 

[scannerrail]

That's normal. The radio's discriminator circuitry wasn't designed to drive an additional load. On top of that ... the signal impedance of the computer's sound input is likely much lower than the radio. The result is a drop in audio level when the disc. tap patch cable is plugged in to the computer. Unplug the cable and the sound level returns.

A cap or larger resistor value may help. You may also want to use a small zero gain buffer amplifier to provide the needed isolation between computer and radio.

-rick

Thanks, I was talking with a friend about this and he said the same as well try using a larger value for the resistor or put a 0.1µF disc-Capacitor inline as well.. well I think he means in-line..

I'm already just using a 10K Ohm resistor for the disc. tap now.

 

[kb2vxa]

RF, when a buffer is used it must have a high input impedance so it won't load down the discriminator output voltage. An FET is the ticket, an ordinary transistor has a base junction impedance of about 500 ohms which would only worsen the condition. This is why a high value series resistor is used to provide a high impedance load at the sacrifice of circuit gain where the sound card is concerned. That's why the mic input is used to make up for the loss and a series capacitor to block the bias voltage normally supplied to the electret element through the audio cable.

That just brought something to mind, if the cap is absent the discriminator diodes could be biased toward the "off" condition much like the DC volume control in certain types of high end audio amplifiers. The cap must be a non polarized type (not an electrolytic) such as a 0.01uF to 0.1uF ceramic or mylar.

 

[scannerrail]

RF, when a buffer is used it must have a high input impedance so it won't load down the discriminator output voltage. An FET is the ticket, an ordinary transistor has a base junction impedance of about 500 ohms which would only worsen the condition. This is why a high value series resistor is used to provide a high impedance load at the sacrifice of circuit gain where the sound card is concerned. That's why the mic input is used to make up for the loss and a series capacitor to block the bias voltage normally supplied to the electret element through the audio cable.

That just brought something to mind, if the cap is absent the discriminator diodes could be biased toward the "off" condition much like the DC volume control in certain types of high end audio amplifiers.

So what should I change the 10K ohm resistor to? I know I need a 0.1uf cap to go in series with that as well.

but another thing just thinking about it is with the other end of the cable not plugged into any of the PC's input ports just touching the metal ring around the jack you could hear the audio from the scanner speaker change.. Not too sure what that was all about but it's not as bad as having the other end plugged into a pc and touching the ring of the jack.