how to fix LSM issues without buying a new scanner. :lol:
-AZ
It is possible (in theory) to modify a scanner to fix the simulcast distortion problem in a proper way.
Before even considering this for a particular unit it would be necessary to confirm clean oscillation with low phase noise for all of the LO's in the scanner - in practice it's usually only necessary to check the first LO (e.g. using a good UHF receiver, in CW mode, tuned near the frequency of the first LO, with a pickup loop antenna placed near the scanner under test). If the tone is raspy or jumpy, forget about it.
Next it's necessary to check the scanner's final IF filter (usually a ceramic filter with a part number such as CFx-455x made by MuRata or Rubycon etc) for excessive group delay distortion. This problem can be worsened if the target signal is not exactly tuned in the center of the passband of this filter.
The cause of the simulcast distortion problem in scanners is known and has been isolated to the FM demodulator stage (aka PLL/discriminator). When CQPSK with additive simulcast distortion is applied to any FM demodulator (or discriminator etc). the resulting mangled signal cannot be properly decoded and the correct signal can no longer be recovered.
The solution is to demodulate the signal as (D)QPSK, not as C4FM. This must be done at IF , i.e., *prior to* the FM demodulator (see previous paragraph), with the result that the simulcast distortion is removed (there is a limit to how much, which applies just as well to "real" user radios in the system).
So the steps to modify our hypothetical scanner would be
* tap the last IF (either immediately before or after the C.F., see above)
* demodulate the signal (typically 455 KC) using the [D]ifferential QPSK method
* use the result of the previous step to C4FM-modulate an FM carrier signal (at 455 KC)
* apply this regenerated "IF" signal to the next stage after where we interrupted the signal flow (first step above)
As an exercise, for extra credit:
* allow the above mod to be switched out, when non-P25 signals are received
* perform FEC (forward error correction) based on soft-decision information from the DQPSK symbols
Practially speaking, we should be demanding scanners that handle this correctly. All it takes is IF DSP - already a very common feature in ham equipment. UPMan - are you listening ?
73
Max
) the best solution to this problem has been to put up a Yagi antenna and position it so that signals are only coming from ONE site. Once you do that, it comes in very very well. Not quite perfect, but considering the fact that you can buy a decent Yagi for less than 50 or build one yourself for less than 20, good enough.
