Split an audio signal

[upand_at_them]

I want to test out two radio packet decoders I've built.

Can I just split the audio output, or do I need to amplify it or isolate the circuits? The decoders are the same hardware design (just changes in software), with the signal AC coupled via a small cap and then feeding a zero-crossing detector (PIC comparator).

Mike

 

[Sceadwian]

For such passive splitting you'll only need a couple resistors 50-200ohm on the signal and ground line of each input from the original source, the input impedance of a PIC is quiet high so it shouldn't effect your received signal on either chip. The resistors are there simply to balance the possible signal to each chip to make sure it's share equally. You could probably get away with higher resistor values but it will form a voltage divider with the PIC at some point and act as a low pass filter with the native capacitance.

 

[Nigel Goodwin]

No need for any resistors - just run two wires to the two inputs.

You only need resistors when it's two outputs feeding one input.

 

[Sceadwian]

Watch the harmonic effects Nigel when the sampling rates are perfectly synchronized.

 

[upand_at_them]

I have resistors after the caps, like this:

 

[ronsimpson]

upand_at_them,
I do not like your AC circuit. If the signal is larger than 0.6 volts it will load the source. The positive half of the signal will work OK, the negative half will turn on the bottom diode. This will mess up the AC coupling.

Use the 10k to ground and a second 10k to VCC. This way the signal will center around 1/2 VCC.

 

[upand_at_them]

Wasn't my design. But it does work very well. I'm only working on different software decoding.

Edit: I tried it with the extra resistor and it didn't work at all. I'll stick with the original.

 

[ronsimpson]

With your "zero-crossing detector (PIC comparator)" what voltage is the comparator set to. (ref. voltage)

 

[upand_at_them]

It's not using the internal Vref. It's using both comparator inputs. Vin+ is the signal, and Vin- is tied to ground.

 

[ronsimpson]

What is the audio lever? Volts P-P, peak, rms any measurement?

 

[upand_at_them]

Where I keep the volume I measured it to be about 0.1V P-P. If I turn it up to almost half (which I don't) it's still only 0.4V P-P. It's the speaker jack to a handheld transceiver.

 

[ronsimpson]

That is good. If you audio is below 0.6V pk then the diodes will stay off.
I thought the audio was volts and then the comparing should be done at 1/2 supply to keep the diodes off.