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Audio Discriminator Tap Hardware Specs

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Coverted

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I have looked through all threads that discuss the best hardware to use for a discriminator tap. I think from what I have read and seen that using a 10uF Capacitor and a 10k resistor will do the job nicely. After calling my local electronics supplier and asking for the parts needed I was stumped when I was asked what wattage of 10uF Cap i needed and what voltage of 10k resistor. After looking through the Pro 106 Service Manual to try and discern the proper specs I concluded Its to complicated for me. Can anyone help me with the specs. I am going to make the discriminator Tap that is shown in the wiki for the Pro 106 However it only shows the pictures with no specific information. In the picture it shows a 47k Res However in reading further I believe a 10k Res is the way to go. Any feedback would be greatly appreciated. Thankyou Steve
 

Thayne

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It sounds like the guy who asked you about the wattage for the cap and voltage for the resistor got his question(s) backwards, but for a Disc tap the lowest rated ones available will work fine; like a 1/2 watt resistor or a 10 or 16 v Cap. The voltage & wattage for a tap is very small.
 

Coverted

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So a "1/2 watt 10k Resistor" and a "10v or 16v 10uF Capacitor". I seen a few references to a 10uF Tantalum. Is this a better quality capacitor?
 

SCPD

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The 10k ohm resistor may or may not work well for you. There's no one magic value that works for all computer / radio combinations. These components are cheap so you can buy and try several different values. 1/2 and 1/4 watt will do fine.

For capacitors, the 10 uF value is probably bigger than needed (but causes no harm). Tantalum isn't necessary. The lower voltage capacitors are smaller and cheaper. I'll second Thayne's suggested voltage ratings.
 

Cruiseomatic

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Do all taps require the 10 uF cap or just some? Looked at the wiki for the pro-164 and there was no mention of installing a cap. Just making sure.
Thanks.
 

DaveH

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Just a quick note, I earlier had no success with UniTrunker P25 decoding, using
recent and "classic" versions, using different tapped receivers, two different PCs
with different sound cards.

Just picked up an Acer netbook which uses Realtek audio. After watching the P25
input signal with UniScope in "scope" mode could see it was overloading. Then using
an audio cable with inline volume control, adjusted the level, and I get some useful decoding
(finally); no capacitor. The level adjust is touchy, and for some reason decoding VHF systems
barely works, but on an 850MHz system I get fair to good decoding, enough to log
useful results. This is with a tapped PRO-2004; I've got two others (BC890, PRO-2022)
still to try.

Dave
 

Cruiseomatic

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Better question, Can I put both a cap AND the resistor on just for good measure? Could it hurt anything? I did a "beta" test and it still works with the 4.7 uF cap in place. Reason for the 4.7 is some say 10 uF and others say a 2.2 uF because of the different radio's however they all do the same thing. I went with one in the middle. I currently only have the resistor in place.
 

Mike_G_D

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The resistor is for keeping the load (PC audio input) from possibly loading down the source (tapped radio's discriminator) while the cap is primarily for DC blocking - keeping any DC voltage present on either side of the tap (source or load) from interacting with the rest of the circuit. Ideally, you only want to pass and deal with the audio ac signal so you don't want to pass any DC. Some discriminator taps on some radios may have DC present and some PC audio cards might not deal with it well; so, to be safe, a DC blocking cap is a good idea as far as blocking DC is concerned. The non-ideal problem with the cap, however, is that it creates a "filter" together with the source and load impedances and any other series impedance such as a series resistor. This filter effect may help you or hurt you depending on what type of signal you are dealing with. Ideally, to yield completely raw discriminator output across the whole audio range (including sub-audible signals) you don't want any filtering (in the tap circuit) so as to allow the software to have full access to all possible discriminator output. In some cases, though, filtering can help but then you should really know exactly what the load and source impedances are so as to calculate it properly. The series cap in a basic discriminator tap is not really intended as a true filter with unknown source and load impedances, etc., but some filtering will occur. To keep that to a minimum, ideally a larger value is used, but the non-ideal characteristics of common electrolytic caps will cause them to become inductive at high values. This is not usually a problem at audio frequencies, depending, again, on the source and load impedances, but it should be kept in mind.

For most discriminator taps, a value of 1uF to 10uF is fine and is not terribly critical. You could get a few caps with values spread throughout that range and try them each to see which gives you the best signal for your software. Ranges in the 1uF to 2.2uF range can also be accomplished using ceramic caps which are inherently non-polarized and so can be inserted in the line without regard to DC polarity unlike common electrolytics which have a polarity. The most positive DC voltage point should have the "+" terminal, of course, usually that being the source. There are non-polarized electrolytics as well and there are special very wide range DC blocks if you want to spend the money and have the widest range possible with no DC passage (Mini-Circuits has them). Generally, if you want to pass very low frequencies as clearly as possible then you want as large a value cap as practical or no cap at all (or else a special DC block with wide range). The higher the frequencies needed the less the cap will have an effect until the non-ideal characteristics begin to manifest themselves.

In many cases, using no cap and even no resistor works fine. I do this with my old Radio Shack PRO-2052. But, to be safe, especially when dealing with an unknown source and load it might be best to at least put a resistor inline (4.7K to 10K ohms is common here, as you have seen). You can always measure how much DC might be present at the discriminator tap of your radio with a multimeter - if you see a steady DC bias with no signal present then, well, you know it is there. Whether or not the DC will be a problem depends on your particular PC sound card/audio input circuit design.

-Mike
 
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