A quick test on an IFR 2975 showed the P25 modulation fidelity was way too high (around 25%), so I figured an alignment was in order...
Both the Reference Modulation Compensation and TX Deviation Gain parameters required significant adjustment to get the P25 modulation fidelity down to 2% or so.
The Reference Modulation Compensation adjustment was fairly sraightforward, but the instructions for the TX Deviation Gain adjustment left a little to be desired...
Basically, for the TX Deviation Gain adjustment, the station transmits what sounds like a 1 kHz tone on four test frequencies, and you're supposed to measure the deviation of this signal, and report what you're seeing to the Quantar via the CPS.
The instructions don't mention if the deviation value is supposed to be a peak or average. On the IFR 2975, you have choices of PEAK, AVERAGE, POSITIVE PEAK, and NEGATIVE PEAK. On the R2670, you get choices of Normal, Peak Average, and RMS Average. From trial and error, I found the Normal mode on the R2670 and the PEAK mode on the 2975 gave the values that the Quantar would be happy with...
In addition, the CPS help file states "Make sure the communication analyzer (R2000 series) is set to Wideband mode". The older RSS manual written for the R2001 makes no mention of this requirement. Fortunately I had access to an R2670, where the "wideband mode" is +/-100 kHz, and narrow is +/-5 kHz. The closest IF bandwidth choices on the IFR 2975 to the R2670's "wideband" are 60 kHz and 200 kHz. I found the 60 kHz setting on the IFR produced values similar to the R2670's.
Finally, the CPS help file states:
The instructions say to start with the first test frequency, and then do 2, 3, and 4. I ended up doing the tests out of order, otherwise the CPS wouldn't accept my changes. This didn't seem to cause any problems...The minimum for the first frequency field and third frequency field is 1.82 kHz. The minimum values for the second frequency field and fourth frequency field depend on the values entered in the first and third fields, respectively. The maximum frequency for all four fields is 10.00 kHz.
So, my questions:
Do the Quantar exciter modules always need to be realigned after a repair? As far as I know, we received the same exciter back, so I thought the tuning values wouldn't need to be changed, or this would have been done at the repair depot.
What realignments need to be done when an exciter is moved from one station to another (same band/range)?
Are the tuning values stored in the exciter or station control module?