C4FM vs CQPSK - Scan Behavior

[steve108]

BLUF: Why can I receive a P25 channel regardless of the modulation type (C4FM vs CQPSK) but while scanning the radio will only land on this channel if the modulation is set to CQPSK?

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I have an Astro Spectra that mostly gets used as a scanner for conventional channels, a mix of NFM and P25. It has worked flawlessly for years.

Last weekend I noticed that the radio would no longer land on a specific P25 channel when scanning. If I tune it to that channel manually, it will still receive it and it sounds fine. I did not make any changes to my setup. The squelch is just digital csq, but I did double check with op25 and the NAC has not changed.

I did some experimenting and noticed that my XTS2500, which is programmed the same way, has the same problem.

The only thing I can find that fixes this was to change the modulation type from C4FM to CQPSK. After this change both radios will now land on that P25 channel while scanning.

A few questions:

1. Is there a way to know what the right modulation setting should be for my other P25 channels, other than trial and error?
   1. I tried looking at the eye plot on op25, and to me it still looks like C4FM (based on this), but I may be wrong about that and I'm not sure if this is even valid way to test. I tried op25 with the demodulator set to both fsk4 and cqpsk - the eye plot looks the same and the audio sounds the same to me.
   2. Do I just set everything to CQPSK at this point? Any downside to that?
2. Can anyone explain why this happens? Obviously the system owner changed something, but why won't scan land on this channel despite the radio still being able to receive and demodulate it?

OP25 eye plots attached. Labeled fsk4 or cqpsk depending on the "-D" setting in op25

Thanks

 

[KevinC]

Just set everything to CQPSK. You "may" lose a little over 1 dB, but the receiver will decode better up to that point.

 

[jeepsandradios]

I know in the XTL/XTS line there was an MTN for this in scan at one point. Had to upgrade lots of radios. Would miss alot in scan. Shut off scan and it would RX all day long. I dont recall the version but thousands of radios got touched....

 

[NVAGVUP]

As to why, CQPSK is used by simulcast infrastructure. (TX). Stand alone RF sites use C4FM TX modulation.

On paper, it shouldn't matter much (Per KevinC- 1 db among friends is minimal) But if there was an issue with old firmware, as jeeps mention there may have been an MTN to fix issue.

If you have the resources and want to update firmware, that would likely fix issue.

Or you can simply change to CQPSK in CPS and live on.

 

[steve108]

Thank you everyone!

 

[wgbecks]

OP25 eye plots attached. Labeled fsk4 or cqpsk depending on the "-D" setting in op25

Steve,

The suggestion you received to set everything to CQPSK is sound advice and will always work regardless of the actual modulation
in use. However, the "Eye" (Datascope) patterns you posted are both that of CQPSK Linear Simulcast Modulation (LSM).

Left: C4FM (FSK4) RIght: CQPSK (LSM)

 

[petnrdx]

They look like a C4FM and an LSM to me.
Why to you think the left one is not C4FM?

 

[wgbecks]

They look like a C4FM and an LSM to me.
Why to you think the left one is not C4FM?

Per Post #6, continuous 4-level FM (C4FM) "non-linear" modulation produces very distinctive diamond shaped eye patterns representative
of the standard two-bit (00,01,10,11) symbols with frequency deviations of +/- 600Hz and +/-1800Hz.

 

[merlin]

I have never seen a difference between C4FM and CQPSK. They are basically the same.
CPS defauts to CQPSK, so that is where I leave it. Never had any scanning issues.

 

[petnrdx]

Having done this for many years, I can tell you that the trace on the left is exactly what I see (and look for) on my Anritsu S412E
when analyzing P25 Phase I in basic C4FM. The trace on the right is definitely CQPSK (or LSM) and that trace is similar to what I would be looking for on the Anritsu. The main difference being that my displayed trace(s) would have two "center" open "windows"
where the dwell time is more open.

 

[petnrdx]

C4FM Above