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Motorola - IF Offset Xtal - a question

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g8tzl2004
Posts: 1114
Joined: Wed Sep 08, 2004 10:13 am

Motorola - IF Offset Xtal - a question

Postby g8tzl2004 » Thu Mar 06, 2014 12:29 pm

As crystals tend to drift over time, how are you meant to tweak the IF Offset Xtal back onto frequency?

With the reference osc Xtal, you can warp it back onto frequency by using the tuner. Sometimes its over 2KHz off frequency, especially on UHF radios.

But what happens if the IF Xtal has drifted. How is this warped back onto frequency?

Thanks

RFguy
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Joined: Wed Dec 21, 2005 6:17 am

Re: Motorola - IF Offset Xtal - a question

Postby RFguy » Thu Mar 06, 2014 12:36 pm

You don't. if it goes out of spec, you replace it.

The big difference is the multiplier. The reference osc. is multiplied many times, along with the freq. error. A small error in the reference Osc shows as a large difference at the final freq.

An IF is not multiplied, so the error is much smaller. typically not enough to notice.

g8tzl2004
Posts: 1114
Joined: Wed Sep 08, 2004 10:13 am

Re: Motorola - IF Offset Xtal - a question

Postby g8tzl2004 » Thu Mar 06, 2014 2:26 pm

Thanks RFGuy.

Do you replace many IF Xtals?

I have a few UHF Jedi MTS2000's where I am sure the IF Xtal is slightly off freq. The ref osc Xtal is spot on but the RX signal appears to be distorted and slightly off freq on very weak signals compared to other identical MTS2000's. If I set the ref osc so that is it a few KHz's off frequency (on the affected radios), the RX signal improves...its like adjusting the ref osc is compensating for the IF Xtal being off freq...although adjusting the ref osc causes the TX to then be a few KHz's off freq.

I've searched the archives and have not found much mention of issues with IF Xtals...but I think a good few of my Jedi's are affected.

I zero beated the IF frequency and definitely noticed a difference between the IF freq of a "good" Jedi vs a "bad" Jedi. I need to check my notes on the actual size of the difference. But do you have any idea how much error is acceptable? I think the IF Xtal was about 2 KHz off freq so I guess this will have a big effect on weak signals.

Thanks

CircleBat
Posts: 170
Joined: Mon Oct 08, 2007 3:11 pm

Re: Motorola - IF Offset Xtal - a question

Postby CircleBat » Thu Mar 06, 2014 6:19 pm

Common problem with IF crystals failing on these radios...just replace them

Circlebat

AEC
No Longer Registered
Posts: 1889
Joined: Wed Dec 22, 2004 7:56 pm

Re: Motorola - IF Offset Xtal - a question

Postby AEC » Mon Mar 24, 2014 2:56 pm

The IF Xtals are easy to replace, and if an error as great as 2Kc is apparent on the fundamental, it MUST be replaced, as the end freq. error will be far greater with the operating frequency of the channel you are using, and the RX suffers even more.

Just as with those infamous Maxtrac second oscillator rocks, the RX goes out first, and if you have no AF, it is a certain sign the 2nd osc. is kaput.

That rock is the same as those old SP50 portables as well, so if you have a bad 'Trac or SP50, swap the rocks and run.
You can try to get around the error if you are willing to suffer with reduced RX, and reprogramming the radio to offset the offset, which is dumb to begin with.

And xtal that shows a sizable error in its fundamental, should be replaced. It makes good sense, and is the proper method of repair.
And of course, run a realignment afterwards, just to make sure the new rock didn't alter the initial adjustments.

Will
Posts: 6823
Joined: Tue Sep 04, 2001 4:00 pm

Re: Motorola - IF Offset Xtal - a question

Postby Will » Tue Mar 25, 2014 11:29 am

IF Offset Xtal, they are talking about the second oscillator crystal.

If the second osc Xtal is off, or is dead, then it gets replaced. Common problem in Maxtracs, and Gm300's.


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