P25 and C4FM Analysis for Spike

Greetings Everyone,

In regards to future upgrades for Spike; C4FM Continuous 4 level FM modulation is a special type of 4FSK modulation for digital transmissions at 4800 baud and 2 bits per symbol, yielding 9600 bits per second total channel throughput. C4FM is utilized in the International APCO P25 Digital Voice Standard, which drives about 80% of local, regional and state communications … including Public Safety communications which must be aligned, calibrated and audited frequently.

A GREAT upgrade and addition to Spike would be the instantiation of a demod selection for C4FM, which would easily help the BB (and possibly SA series) fly off the shelves even faster.

I love my BB60A, and I love the recent upgrades to version 3.0.8 of Spike, with digital/quadrature demod methods. One of the reasons I almost went with the Tek RSA306 USB SA, is because SignalVu (the front-end application for their USB spectrum analyzer) does have a suite of P25/C4FM demod and analysis tools. But this could easily be added to Spike, with perhaps an Eye Diagram feature as well.

I believe thousands would vastly appreciate a P25/C4FM demod method to future upgrades of Spike, and it would make an awesome selling point, putting Spike and the BB series much more on par with the Tektronics RSA306 and SignalVu!

[Andrew]

Hello Rob,

Thank you for the feedback. We will be visiting ASK/FSK/MSK in the near future which will bring us much closer to your requested functionality. No time frame yet, but we are glad you are enjoying the BB60A in the meantime. Thanks for picking us over the RSA306!

Regards,
A.J.

• This reply was modified 10 years, 8 months ago by Andrew.

Thank you A.J., I look forward to seeing that functionality! 🙂

I see that 4FSK had been added to Spike 3.0.13. C4FM is only a short step away! Still patiently waiting 🙂

Btw, here’s a good presentation from Aeroflex on the standard and implementation of C4FM:

Aeroflex C4FM

Hi Rob,

Thanks for the follow up. With the 4FSK addition to the Spike software you should be able to analyze C4FM signals. My knowledge is cursory, forgive me if I am incorrect, but the C4FM spec is a 4FSK signal with 4.8k symbol rate, 1.8k deviation and root raised cosine filter. Our software is capable of measuring this signal. With our software you should be able to measure modulation fidelity, frequency offset, channel power, and ACPR. What other types of measurements are relevant to you for P25?

Regards,
A.J.

Hi A.J.,

Thank you for the reply. After checking out the interface again, all the necessary parameters for diagnosing P25 do indeed seem to be there, albeit slightly different nomenclatures for general (X)FSK.

RMS Deviation Error = RMS FSK Err(%)
Frequency Error = Carrier Offset
Symbol Deviation = Average Deviation

A Filter Alpha of 1 will work best with C4FM. The only thing which is lacking for complete analysis is the Symbol Distribution Table (reference in my video link). With the addition of that, it would be complete for P25 troubleshooting.

Great work so far, and very glad to have this added functionality in Spike!

Rob

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