Discriminator tap voltage

[wyldman]

I have tapped a few radios,and I have a few that were already tapped.Some of the radios seem to decode data better than others,so I'm wondering if the taps were done correctly.

Most of the tap info online suggests using a 10K resistor inline,and a few suggest adding a small capacitor between the tap and ground.

I read some stuff in the Wiki about measuring the AC output voltage to determine the correct resistor.Does anyone have any better info regarding output voltage and the correct resistor to choose ? How do I know if I need a cap,and how to select it ?

Also,does anyone know the correct output voltage AFTER the resistor ? If I know this,I can test my tapped units I already have without opening them up to get at the chip.A lot of them are handhelds,and the chip is a PITA to get at.

I have measured a few,at my output jack,and seem to be getting anywhere from 0.1 to as high as 1.0 volts AC on different units.Some of them seem to fluctuate when traffic is heard,or the squelch is open.Which is the correct way to check the output voltage ? Squelch closed,with no signal ? Squelch open ? or when there is traffic on that frequency ?

 

[rescuecomm]

The output of the descriminatior should be zero with no signal if the cans are tuned correctly. To measure the voltage, you will need a signal with audio on it to have something to measure.

Bob

 

[wyldman]

I'm no expert,but I'm pretty sure there should be some output,even if the radio is not recieving anything.There must be,because the decoding programs will register when when connected,even when there is nothing being recieved.

I tested one radio,which decodes well,and there is about .250 V AC when it is sitting idle,squelch closed,and not receiving anything.When it recieves something,the voltage drops down to .125 V or so,and then recovers to .150 until the transmission ends,and it goes back to .250 V.

Is the purpose of the cap to prevent the voltage fluctuation when recieving (the drop to .125 V then back up to .150 V),or is it used as some kind of filter ?

 

[N_Jay]

The discriminator output should exist with or without the squelch, unless it is not truly a "discriminator output" and is actually a later stage after the squelch gate.

The output should be highest with no signal and go down as the carrier quiets the receiver and go up with the audio signal.

The signal is AC, but typically has a DC bias voltage.

The capacitor is to block the DC bias.

The resistor is to prevent loading the discriminator.

I would use a very small capacitor and a 100K resistor and follow that with a MOSFET input op-amp with the gain set to give me at least 1V P-P on no signal.

 

[wyldman]

Thanks for the info,I'm starting to understand now how it works.You description of the output voltage fluctutation makes sense,and that's what I'm seeing on most of them.

Most of the tapped radios I have don'y use a cap,and seem to decode stuff just fine.Would adding one hurt ? Any safe type or value to use ? Can I check DC bias with a regular DVOM ?

I don't think I'm going to get fancy with any op-amps,but could you explain an easier way to check the final output voltage after the resistor and\or cap ? Should it be as high as 1.0V AC ?

BTW what is 1V P-P ?

If i know what the final output should be,I should be able to fine tune the resistor needed from that.

 

[rescue161]

Vpp is Volts Peak to Peak.

What you are reading on your multi-meter is Vrms, which is Root Mean Square and will be read less than Vpp. Vpp is basically what you are seeing on an oscilloscope where you see the AC sine wave going positive and then negative. Your house voltage is basically 340 Vpp. Take that and multiply it by .3535 and you'll come up with about 120 Vrms. Half of Vpp is Vp, which would be approximately 170. In Vp, you are only looking at one side of the alteration (sine wave), be it positive or negative. If you know the Vp, then multiply it by .707 and you'll come up with the same Vrms value of 120.

Since the tap is putting out AC, putting a cap in-line with it will not hurt the performance. Not to get too complex, you can basically say that AC will pass through a capacitor, but DC will not. The cap is actually charging and discharging when the signal goes positve and negative which makes it look like it's passing the AC signal. DC only stays either positive or negative and doesn't fluctuate, so the cap isn't told to charge/discharge with DC, so DC voltage isn't "passed" through the cap. In all actuallity, nothing is passed through the cap...

 

[wyldman]

So based on that formula,my AC output should be around .350 V RMS,with no audio,which I can measure with the meter.

So when you suggest putting a cap inline ? along with the resistor is series ? or run it off the tap to ground ? I have seen it done both ways.

Would a 10-uF tantalum capacitor be a good choice ?

Thanks for all the help.