BCD436HP/BCD536HP: Adjusting receiver reference oscillator

[RFI-EMI-GUY]

I have had this receiver for a long time and would like to touch up the reference oscillator frequency. I do in fact have the necessary equipment, an HP8920B properly calibrated with HSO module. I have done this dozens of times on other equipment. Not a newbie.

Can somebody point me to the PC board layout where the trimmer is located, I guess I could just tear into it and look, but I would like to know for sure as I certainly don't want to turn the wrong trimmer...

Or is the TCXO controlled in software like other receivers? If so, how to set it?


Thanks in advance.

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[techman210]

You have identified what I think is the number one flaw in digital scanners.

There is no way for the user to “net” the scanner to a known frequency reference, such as a control channel.

Frequency accuracy is much more important in digital systems than analog. I would think a simple front panel discriminator voltage display and an ability to warp the oscillator +/- 1500 hz would be adequate.

We see late model Motorola equipment come in for “out of range” issues all the time, within the first year of service and it’s cured with a simple alignment.

If the batwing folks have trouble with oscillator aging, I would think the scanner folks are not much better.

When I looked inside my 996XT I saw no adjustment points- though I did not unsolder the shields on the boards.

My best guess is that either the oscillator is fixed, or it’s a “soft pot” adjusted by depot software.

 

[Ubbe]

Test first to see if it's off frequency. Use a low level RF signal like 1uV and overmodulate the signal, a 25KHz channel with a 6-7KHz modulated 1Khz testtone, and jog the RF frequency quickly up and down in 100Hz or 200Hz steps to find the middle where the distorsion are lowest when you're listening to the loudspeaker audio. One could first do this to the IF to rule out any IF frequency error but that are usually spot on and never any problem. Scanners seem to have much wider IF filters than commercial radios, probably to allow frequency drift during the scanners lifetime without needing alignment.

/Ubbe

 

[RFI-EMI-GUY]

It is the P25 I am concerned width. I could do the offset bandwidth test Ubbe suggests or sniff the LO.

I think it may be off frequency a bit and I would not be surprised because I have found public safety grade portable radios outside legal bounds for 800 MHz NPSPAC in just 18 months of aging.

There must be a soft Pot in there and some factory utility to tweak it.

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[techman210]

The other issue is a noisy DC supply. "Garbage in-Garbage out" rules apply here as with any other digital demodulator circuit.

 

[RFI-EMI-GUY]

The other issue is a noisy DC supply. "Garbage in-Garbage out" rules apply here as with any other digital demodulator circuit.

Hmm, I will have to try running on battery.

The possibilty of frequency drift is one that concerns me.

In general, I am not too pleased with the audio fidelity while monitoring the local P25 system. 20% of the transmissions are perfect, the rest are either garbled or are very overmodulated with bassey booming audio. I can hear the dispatchers asking for a repeat often as well. I don't think the system is very well optimized.

Today I am going to try some experiments with a directional antenna to rule out simulcast overlap.

When this system was analog, it was crystal clear with some very minor simulcast artifacts heard at my home monitoring location. I was using a Motorola Spectra and I had the secret sauce. I am not about to buy an APX8$$$$ or SDS200 to sort this out.






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[RFI-EMI-GUY]

Hmm;
looking at another post on this board about grabbing a discriminator tap (surprisingly easy) I looked at photos of the PCB and see no evidence of a reference oscillator or adjustment. There is a "can" (with a wire taped to top) adjacent to what appears to be the main receiver can . But I cannot tell if there is a trimmer hole. Certainly there must be an adjustment.