LTRDump & Data Dicer Questions & Help

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nickde

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I have built the "data dicer" designed by Eric Cottrell and I am running it with LTRdump.

It seems to be decoding intermittantly, everyonce and a while I will get a good LTR decode LCN and TGID. I am also getting some messages decoded with the neighbor frequency (passport ?). But most of the time when I am hearing activity on the channel there is either "unknow" messages with all zeros in the fields or nothing.

I have scoped thur the circuit and most of the time when there is no activity on the channel (I see discriminator "white noise" at the input to the data dicer) there is a constant square wave at the output (pin 14) of the comparator/rs-232 driver. The wave form seems to change when there is activity on the channel. The output level is just less then 5v (4.5) on the positive or negivite peaks (about 9 v pp). Is this enough output level? I am feeding the output of the slicer into the CTS input even though the diagram says "rs-232 data output" which I take to mean RD input.

I am thinking that I might need a level shifter to increase the level going into the com port.

I have tried the normal/invert and it seems to work best on the invert position. I think that I have good RF signal it is very clean at the input, when there is activity on the channel.

Thanks for any help or ideas.

Nickde
 

Forts

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Maybe it's because you built a dicer instead of a slicer? Or maybe yours dices and slices? Sorry, just being a smartass... it's actually called a data slicer, not a dicer. Have you tried it with any other programs like DOS Trunker or ETrunker? (Not Unitrunker as I believe the serial inputs are no longer an option). If you have any trunked Motorola or EDACS systems in your area, try your slicer on them and see what kind of performance you get. LTR is a bit of an odd duck to deal with, and decoding can be tricky.
 

nickde

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Oh yah, didn't mention that I am using a LM324 Quad Op amp instead of the MC33174 that the diagram wanted. It has a potentional defferance of 14 vdc across V+ and V-.
 

wa8pyr

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nickde said:
Oh yah, didn't mention that I am using a LM324 Quad Op amp instead of the MC33174 that the diagram wanted..
That's likely part of your problem. Don't have the specs in front of me, but IIRC the LM324 has a higher noise figure than the MC33174. With complex subaudible data like that you want as little noise as possible.
 

nickde

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Trying other op amps

I had a MC3403 in my parts bag, so I tried it, no differance. Let me know about the noise figures, if I need to I will get one ordered up.

Thanks

Nickde
 

EricCottrell

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nickde said:
I have built the "data dicer" designed by Eric Cottrell and I am running it with LTRdump.

It seems to be decoding intermittantly, everyonce and a while I will get a good LTR decode LCN and TGID. I am also getting some messages decoded with the neighbor frequency (passport ?). But most of the time when I am hearing activity on the channel there is either "unknow" messages with all zeros in the fields or nothing.

I have scoped thur the circuit and most of the time when there is no activity on the channel (I see discriminator "white noise" at the input to the data dicer) there is a constant square wave at the output (pin 14) of the comparator/rs-232 driver. The wave form seems to change when there is activity on the channel. The output level is just less then 5v (4.5) on the positive or negivite peaks (about 9 v pp). Is this enough output level? I am feeding the output of the slicer into the CTS input even though the diagram says "rs-232 data output" which I take to mean RD input.

I am thinking that I might need a level shifter to increase the level going into the com port.

I have tried the normal/invert and it seems to work best on the invert position. I think that I have good RF signal it is very clean at the input, when there is activity on the channel.

Thanks for any help or ideas.

Nickde
Hello,

The Dicer part is my attempt at humor. A Dicer is a Slicer with a low pass filter.

The "rs-232 data output" can be used for CTS or DCD or whatever input pin is supported. I prefer DCD because it gets away from the handshaking on the port.

The reason for the M33174 is from information I read about data slicers. The author said the M33174 has enough high-frequency response to do 9600 baud but not enough to cause problems.

I found on some receivers that a DC level built up on the input to the dicer so I put a 1 megohm resistor between the input and ground. If you measure the voltage at the input and it suddenly works again then this is the problem.

It is also important to have both positive and negative voltages connected to the "power supply". Trunker and LTRTrunk will set the signal lines mentioned to provide the positive and negative voltages.

Also if you are in Filter Bypass Mode (Motorola/EDACS), you can decode LTR but any voice will kill the decoding.

73 Eric
 
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