GRE PSR500/600 I/Q Test Points

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Mike_G_D

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That information would be interesting to me too! However, I am beginning to suspect that it does not use an I/Q demodulator and instead uses the more common, primitive, and less effective method of demodulating digital signals - using a simple FM discriminator output with a form of data slicer. That may explain partly why linear simulcast distortion causes such havoc on these radios. It's suitable for C4FM in non-heavily phase distorted cases but not so much when faced with QPSK variants wherein information is in the amplitude and phase of the signals.

I am interested in what you find out!

-Mike
 

mancow

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As you mentioned, the simple FM demod is how the PRO96 appears to work, according to the schematic.

The Motorola units use an abacus chip and actually digitize the baseband for analysis.
 

Mike_G_D

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As you mentioned, the simple FM demod is how the PRO96 appears to work, according to the schematic.

The Motorola units use an abacus chip and actually digitize the baseband for analysis.
I'm not familiar with the "abacus chip" you mentioned but, otherwise, that sounds like the method I am familiar with from working on PI/4 DQPSK modulated TDMA cellular units back in the day. In fact, I had never heard of a "data slicer" at all until I got heavily into these hobby forums. I was always under the impression that I/Q modulators/demodulators were the primary method of dealing with complex digital modulation signals.

The way I remember it, on the receive end, I think I recall that we took the downconverted IF ran it through an I/Q demod IC and then A/D'ed the I and Q signals. They were then processed by DSP engines for equalization, error correction, and demod (voice data extraction, etc.); however, they might have applied the equalization to the digitized final IF before I and Q demod (which would seem to require an extra A/D and, possibly, D/A step...?). It's been quite awhile and I mostly worked on the transmit side at the time.

I would imagine that the operation of the 500 in terms of how it demodulates digital signals is probably unchanged from that of the 96 (at least in the fundamental sense). That's actually a shame but I guess it's cheaper than using the I/Q demod method. The thing is, it seems to me that with cellular phones using it pretty ubiquitously (at least they were when I worked on them) one would think it to be fairly cheap to implement in a scanner?!?

Personally, I would like to see them move away from FM discriminator efforts and toward true I/Q demod if they haven't already done so, especially given the move toward increasingly complex digital signals by the industry. Using true I/Q raw output rather than a tapped discriminator would also really benefit those working on open source multi-mode digital voice demodulating/decoding software;-)!

-Mike
 
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Gadgetman728

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That was the most interesting piece of scanner Tech I'll never understand :) Man was it good for you too?

Just kidding, any time I THINK I know about scanners, I come here and read it from the experts, that REALLY know radios & keep improving the hobby .
 
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SCPD

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Create your own I/Q test points.

Pull the IF signal and feed it into a Soft Rock to produce the I and Q outputs. Assuming the GRE has a 10.7 Mhz IF available (don't know and too lazy to check) the 30 meter version of the Soft Rock could be "bent" from 10.25 Mhz over to 10.7.

This is pure speculation - I have not tried this BUT the unit is less that $20 USD.

KB9YIG.com - Your Source for SoftRock Radio Kits
 

Willcoele

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Demod Tap

I was able to tap the FM demodulator on my GRE-600. I up loaded oscilloscope images to the GRE-600 Yahoo group in the Files/Misc folder. I don't understand the P25 modulation scheme but it appears to be data pulses that are superimposed on a sine wave. When I display it on my HP 8924 in "O" scope mode I can see the data bit pattern. The bits are positive and negative going. It reminds me of the alternate mark inversion [AMI] that the telephone company uses to transmit a T1 carrier. I assume the slicer extracts the bits when they reach a certain threshold. I taped the demod at the output of a gain op-amp where the bits reached a maximum level and "flat topped" in both directions.
 
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