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Bypassing the Discriminator Circuit?

This forum is for the discussions targeted at converting various models of Motorola equipment to operate in the 900MHz Amateur Band.

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dapaq2
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Bypassing the Discriminator Circuit?

Postby dapaq2 » Sat May 10, 2008 8:12 pm

Group,

Ive looked around but did not see anything covering this. I am looking for detailed instructions on how to by pass the discriminator circuit in most motorola radios, specifically the Motorola Maxtrac's and GM300's. Is there any documentation of anykind either here on this site or anywhere else that shows me what needs to be done to accomplish bypassing the discriminator circuit?

Thanks much

Doug

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Re: Bypassing the Discriminator Circuit?

Postby PETNRDX » Sun May 11, 2008 2:05 pm

What do you mean by "bypass the discriminator circuit"?
Give us a better idea what you are trying to do, maybe we can answer...

dapaq2
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Re: Bypassing the Discriminator Circuit?

Postby dapaq2 » Mon May 12, 2008 10:53 am

I need to access the UNFILTERED RAW audio before it goes through the descriminator circuit of the radio. Kind like from what I understand that they do with Packet Data radios?

Doug

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Re: Bypassing the Discriminator Circuit?

Postby PETNRDX » Mon May 12, 2008 11:27 am

Thats what I thot you might want.
You are not bypassing the discriminator, you want to bypass the DE-EMPHASIS.
Common misteak.
On the GM300, Maxtracs, Radius, etc. Most of the time FLAT audio is one of the choices coming out the rear accessory plug when you put Jumper (551 I think) in the correct position.
I am pretty sure JU 551 in postion A gives FLAT (not De-emphasised) audio out pin 11.
Position B is squelched and de-demphasized.
I know this gets mentioned a lot, but I will say it again anyway, depending on what "type" of audio you need, keep in mind that this "FLAT" audio will have been Pre-Emphsized by the radio that transmitted it.
So, depending on how you use this audio, you need to make sure it doen's go into another transmitter and get Pre-Emp again without something De-Emp.
If heard that before, just ignore...
But, seems to come up a lot.
BTW, on an old 5 pin logic board I think you have to get FLAT audio direct off a jumper inside the radio.

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Re: Bypassing the Discriminator Circuit?

Postby dapaq2 » Mon May 12, 2008 12:44 pm

Maybe this little bit of info I found on the net will nelp explain what I am trying to accomplish...

"Almost all radio receivers, scanners included, are designed for voice communications. Voice is pretty narrow-band, consisting of frequencies between 300 and 3000 Hz. This is not the full range of voice, but it's good enough that the Bell System determined that using just this band of frequencies to pass voice communications would be sufficient for 99.999% of all users. Most FM radio receivers use a low-pass filter, located just after a function called the discriminator, to remove higher frequencies that the ear would interpret as noise. It's this filter that is a problem for data communications. The "discriminator tap" is a hack on a radio receiver that allows us to get at the audio information before the low-pass filter, and therefore get more frequency data that would normally be necessary for standard voice communications. The discriminator tap is a way to bypass the low-pass filter and get the wide-bandwidth signal for our signal processing purposes. It means that you'll have to open the case of a radio receiver, solder in a couple of components, and obviously void the warrantly on anything that you just bought."


I would like to know if anyone here has ever done this to commercial two way radios. It would be nice to start an archive covering commerical two way radios instead of just scanners. Anyone have any experience with this that would be willing to share their knowledge on how to do this?

Thanks much

Doug

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Re: Bypassing the Discriminator Circuit?

Postby kf4sqb » Wed May 14, 2008 3:21 pm

Doug, that's pretty much what PETNRDX just gave you instructions for doing. What you want for data use, such as packet, is direct, unfiltered, discriminator audio. On Motorola Radius series radios, such as the GM300 and Maxtrac, this can be accessed from the rear acc. plug if the radio has the 16 pin acc. plug. If you bypass the discriminator, you will have raw RF, not raw audio. The discriminator is what "demodulates" the signal, or recovers the useful information (voice or data) from the RF carrier.
kf4sqb "at" wetsnet "dot" com



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Re: Bypassing the Discriminator Circuit?

Postby WB6DGN » Wed May 14, 2008 9:38 pm

located just after a function called the discriminator,


Well, for starters, a discriminator is a CIRCUIT, not a function (though, hopefully it DOES function), and is one of several FM detectors commonly used in communications grade FM transceivers. In the radios mentioned above, "bypassing the discriminator" wouldn't accomplish much as none of those radios use a discriminator as the detector; all of the radios mentioned above use a "quadrature detector" which, among other things does not require preceding limiter stages to reduce susceptibility to AM type (primarily impulse) noise. It is also a more stable circuit and usually results in better audio recovery for the "newer" narrow-band systems. As was stated several times earlier, it is not necessary to reinvent the wheel, flat (un-deemphasized) and unsquelched audio is already available at the accessory coneector in most of the radios mentioned. This is what you need for data communications.
Tom

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Re: Bypassing the Discriminator Circuit?

Postby WB6DGN » Wed May 14, 2008 10:25 pm

I would like to know if anyone here has ever done this to commercial two way radios...


Yeaaaahhh! Again, as stated earlier, most commercial radio manufacturers have done it for you. This type audio is used to support several options that are commonly used in commercial systems, so the need for flat (unfiltered, un-deemphasized, they all describe the same thing in this case) audio is common.

Most FM radio receivers use a low-pass filter...


NO! COMMUNICATION GRADE RADIO RECEIVERS use a BANDPASS filter that rolls off all low frequencies, beginning at 300 cycles and below, and high frequencies, beginning at 3000 cycles and above, at a 6db/octave rate. This is referred to as the "standard deemphasis characteristic" and is commonly used on FM systems ranging from broadcast (yes, your "MUSIC" radio has the same de-emphasis/pre-emphasis CHARACTERISTIC as your two way radio, BUT with different terminal frequencies).

Bell System determined that using just this band of frequencies to pass voice communications would be sufficient for 99.999% of all users


Once again, NOT EXACTLY! Bell Labs. determined that 90+% of [b]all voice energy[b] fell within the band of frequencies between 300 and 3000 cycles! Splitting hairs? Not exactly. The difference in meaning is the difference between night and day. For example, if your transmitter had 90% of its energy in spurious emissions, it may be SUFFICIENT to communicate, but it definitely would not make the FCC feel all warm and fuzzy; nor would it win friends with those on neighboring frequencies.

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Re: Bypassing the Discriminator Circuit?

Postby mike m » Thu May 15, 2008 2:17 pm

Actually in FPGA code for a SDR receiver a discriminator implemented in software is a mathematical function so he is correct in saying a discriminator function.

Sorry I've been working on a Avionics SDR receiver design for the last 8 months and I can do an FM discriminator in FPGA code in my sleep now.


Mike

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Re: Bypassing the Discriminator Circuit?

Postby WB6DGN » Thu May 15, 2008 4:13 pm

Mike,
Interesting, but I just don't think about SDR very often. I would imagine that the deemphasis characteristic could be integrated directly into the detector in that case rather than being a separate network following the detector.
Just curious though; where is NBFM used in avionics? Communication still uses AM on both VHF and UHF doesn't it? Can't think of a system that uses FM but my avionics experience goes WAY back; they were still working on tin cans and a string!
Tom

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Re: Bypassing the Discriminator Circuit?

Postby WB6DGN » Fri May 16, 2008 5:46 am

Actually in FPGA code for a SDR receiver a discriminator implemented in software is a mathematical function so he is correct in saying a discriminator function.


Not in the context of the question. None of the radios mentioned was an SDR, therefore a CIRCUIT is what was being discussed.

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Wowbagger
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Re: Bypassing the Discriminator Circuit?

Postby Wowbagger » Fri May 16, 2008 6:00 am

kb5dpe wrote:
Actually in FPGA code for a SDR receiver a discriminator implemented in software is a mathematical function so he is correct in saying a discriminator function.


Not in the context of the question. None of the radios mentioned was an SDR, therefore a CIRCUIT is what was being discussed.


Sorry, but I disagree - the idea of calling a circuit element a "function" has been around for longer than most of the posters on this board. What do you think "op-amp" is a shortcut for? "Operational Amplifier" - an amplifier that can be used to carry out a mathematical operation or function. (and BTW: op-amps have been around MUCH longer than ICs - indeed, the very first op-amp was a plug-in tube based model.)

Now, a given function can be embodied in a circuit, or a block of code, or a collection of gates in an FPGA, or a mechanical element, or ....

But it STILL is a "function".
This is my opinion, not Aeroflex's.

I WILL NOT give you proprietary information. I make too much money to jeopardize my job.

I AM NOT the Service department: You want official info, manuals, service info, parts, calibration, etc., contact Aeroflex directly, please.

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Re: Bypassing the Discriminator Circuit?

Postby Max-trac » Fri May 16, 2008 6:27 am

Here is link to find the jumpers.
One thing to note is the flat audio you most likely want is unsquelched, ie white noise present when no signal, which may be a problem if you are mixing audio.

http://www.repeater-builder.com/maxtrac/maxtrac-logic-board-jumpers.html

And can we post pics from the service manual without upsetting Moto???

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Re: Bypassing the Discriminator Circuit?

Postby WB6DGN » Sun May 18, 2008 4:59 pm

What do you think "op-amp" is a shortcut for? "Operational Amplifier" - an amplifier that can be used to carry out a mathematical operation or function. (and BTW: op-amps have been around MUCH longer than ICs - indeed, the very first op-amp was a plug-in tube based model.)


Speaking of opamps (WHO was speaking of opamps?) some may find the following article interesting. It is by Bob Pease in the current issue of ELECTRONIC DESIGN in his regular column. With all the usual disclaimers; it is a good mag., lots of interesting articles.
http://electronicdesign.com/Articles/Ar ... 18732.html

Tom


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