Why Do My Discriminator Taps Always Stop Working Eventually?

prcguy

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Yes it can. Discriminator taps in scanners have been used since the 1960s or so to feed tone decoders and various other audio band devices. You would gently couple to that with a low value capacitor to get the frequency response you need without loading down the circuit.

Then at some point digital modes come along and people try and connect new digital decoder stuff using old methods and in some cases it just won't work reliably unless you actively buffer and isolate the discriminator without loosing its frequency response down to DC to preserve digital 1's and 0s and the perfect square waves that go with them, which were created by FSK type modulation.

That's not what's being done here though. The scanner discriminator tap isn't expecting anything to be connected to it, so ignoring DC bias or overloading the circuit can cause the scanner to quit working when you connect to the discriminator.
 

Ubbe

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If a receiver or device is designed for you to connect right to its discriminator then it probably has a DC offset adjustment circuit which would be diddled to zero out DC offsets. I would not expect scanner receivers or other consumer grade things to have this.
Whistlers TRX scanner has a function called IF Out that uses the earphone jack that gives a direct signal from the DSP that are super clean and have perfect waveforms. When feed to DSD+ it decodes signals that are almost burried in noise. My Line In on the PC has options to ignore DC offset so it probably have near DC capability.

When decoding weather baloons the program I use for that signalling type actually still decodes when I only hear noise. That IF Out function alone makes it wortwhile to have a TRX scanner.

/Ubbe
 

slicerwizard

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If DC blocking caps inhibited P25 decode, nobody would be able to use unitrunker or similar programs without specialized ADC circuits that passed DC and allowed manual adjustment of the DC offset.
You continually wave away the fact that poor cap choices increase signal distortions and that when those are added to other signal issues, such as weak/noisy signals, decoding is worse than it needs to be. Broken headlights work fine during the day, so no need to fix them, eh?
 

jonwienke

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If decode errors are 0, I wouldn't call it "broken". YMMV.
 

KA1RBI

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without loosing its frequency response down to DC to preserve digital 1's and 0s and the perfect square waves that go with them, which were created by FSK type modulation.
No. The entire family of FSK4 modulations (P25, NXDN, DMR, YSF, .....) include a Raised Cosine filter. In transmitters the 1's and 0's are thus filtered and shaped prior to the signal being applied to the direct modulator input port of the FM transmitter (and, if the disctap is working well, its output is identical). The filtered / shaped waveform is nowhere close to a square wave.

Also, a separate strike against the square wave theory is that square waves have harmonics out to ... theoretically, forever (Gibbs phenomenon). After modulation, such signals would occupy a lot of RF spectrum!

Anyway, there are other disadvantages to using a scanner and FM disctap that I haven't seen mentioned....

1. The cheap discriminators can introduce a type of distortion known as group delay. This can show up in the three "eyes" on the datascope (eye pattern diagram) where the uppermost eye is displaced left or right with respect to the middle and bottom one, and/or the height of the top and bottom eyes may differ noticeably.

2. Scanners may not allow you to tune the exact frequency of the station you want, perhaps only allowing you to tune to within, say, +/- 2.5 KHz of the actual frequency. This issue and the previous one can interact.

3. The IF filter width of the scanner may be too wide with respect to the desired station. As might be expected this seems to cause the most trouble with NXDN48 in a packed band.... And (again) it interacts with any tuning offsets caused by point #2...

prcguy's basic point is correct in that many of the basic scanner designs were fully cooked well before 9,600 bps digital came on scene. A lot of pain was suffered by listeners of LSM systems (on every model of scanner prior to the SDS) as they learned this lesson the hard way...

Max
 

jonwienke

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No. The entire family of FSK4 modulations (P25, NXDN, DMR, YSF, .....) include a Raised Cosine filter. In transmitters the 1's and 0's are thus filtered and shaped prior to the signal being applied to the direct modulator input port of the FM transmitter (and, if the disctap is working well, its output is identical). The filtered / shaped waveform is nowhere close to a square wave.

Also, a separate strike against the square wave theory is that square waves have harmonics out to ... theoretically, forever (Gibbs phenomenon). After modulation, such signals would occupy a lot of RF spectrum!
Thank you. It's worth noting this is the reason overmodulated signals splatter outside the intended channel boundaries. The signal clipping generates unwanted square or square-ish waveforms which have components outside the allowed channel bandwidth.
 
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Okay, so getting back to this as I have time. I'm looking to order the parts and am confused about the capacitor. Mouser has gobs of stuff and not sure what to get. If it's supposed to be polarized like in slicerwizard's link, then what kind should I get?

Most of the ones on Mouser look like boxes and I thought capacitors are supposed to be shaped like a little disk or a tube:

https://www.mouser.com/Passive-Components/Capacitors/_/N-5g7r?keyword=10uf
 

jonwienke

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If you get a polarized type it will be smaller, but you have to make sure there is enough of a DC offset in the circuit to prevent it from ever being charged with reverse polarity, or you will damage it.

A non-polarized capacitor doesn't care about polarity; you just have to make sure peak voltage doesn't exceed its' maximum rating.
 
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So is there any benefit to using a polarized cap? I'm not sure what a DC offset is or how to tell if I have that. I guess a non-polarized one could be a little easier to use since you can just slap it on either way.
 

RFI-EMI-GUY

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So is there any benefit to using a polarized cap? I'm not sure what a DC offset is or how to tell if I have that. I guess a non-polarized one could be a little easier to use since you can just slap it on either way.
If you use a polarized vs a non-polarized, you need to pay close attention to the voltage potential and polarity when installing it.
 
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So either one would work OK , polarized or non-polarized? I'd rather get non-polarized if it makes no difference. I think they're called ceramics.
 

jonwienke

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Polarized caps tend to be a little smaller than non-polarized, all else equal. I would go non-polar, the convenience of not having to worry about DC offset is worth the little bit of extra bulk.
 

majoco

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If I was to connect my tap into a microphone input on my laptop, I would use a non-polarised capacitor. Microphone inputs often have a voltage on them to supply power to a capacitor microphone which has an FET amplifier after the capsule. If you can select that input to be be either a line or mic input in software, it probably just changes the gain - it may not remove the voltage. True line inputs will just have a resistive input but it won't care if it was connected to a polarised electrolytic or a non-polarised plastic capacitor, probably just a 1µf would do.
 

Ubbe

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I agree, or perhaps you agree with me. Use a non-polarized cap. There are no cheramic types to have at uF values but looks like big electrolyt types. They are always used in loudspeaker filters, but you only need it to handle 12volt as max. The lower the voltage rating the smaller the size.

/Ubbe

If I was to connect my tap into a microphone input on my laptop, I would use a non-polarised capacitor. Microphone inputs often have a voltage on them to supply power to a capacitor microphone which has an FET amplifier after the capsule.
The only thing that can fail are the capacitor. Put a new one in and perhaps try it polarized the other way. Some audio inputs on a PC are made for powering capacitor mic's and might output up to 5volts and the discriminator tap probably have lower voltage than that.

Or use a non polarized cap. They are much bigger but shouldn't be a problem.
 

ArloG

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It's probably been mentioned but tantalum capacitors are very sensitive to reverse polarity.
Any circuit with AC or negative reference DC riding on it needs a non polarized decoupling capacitor.
 

slicerwizard

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True line inputs will just have a resistive input but it won't care if it was connected to a polarised electrolytic or a non-polarised plastic capacitor, probably just a 1µf would do.
Oh sure, drive a line input through a 1uF cap. Who needs low frequency response?
 
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