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Tx shoots high then corects itself quickly ?

Posted: Mon Feb 10, 2014 8:27 pm
by Satelite
Hello:
I have run into several Motorola mobiles in the past several years that when you first key up the transmit freq goes .5 kHz high and some went low but within a very very short time after keyed up would come back on freq.
I have seen this on maxtracs/radius m216/gm300 ect .
It never has been a complaint from the users as it corrects itself so quickly but I was wondering if it could be the Y151 14.4 tx crystal causing this ?
Has anyone else ran into this and what was your fix for this symptom ?
Thanx Satelite

Re: Tx shoots high then corects itself quickly ?

Posted: Tue Feb 11, 2014 3:10 am
by Jim202
Satelite wrote:Hello:
I have run into several Motorola mobiles in the past several years that when you first key up the transmit freq goes .5 kHz high and some went low but within a very very short time after keyed up would come back on freq.
I have seen this on maxtracs/radius m216/gm300 ect .
It never has been a complaint from the users as it corrects itself so quickly but I was wondering if it could be the Y151 14.4 tx crystal causing this ?
Has anyone else ran into this and what was your fix for this symptom ?
Thanx Satelite



It would help greatly if you pointed the finger at a specific radio family rather than generalized the question. Each radio model is different in the type of issue your talking about.

If your pointing the finger at the Maxtrac family, it might help to pull the control board out of the radio and re seat it. The pins that go from one board to the other can get a poor connection and cause the problem you talk about. Moving those pins will clean them and generally the problem will go away.

Jim

Re: Tx shoots high then corects itself quickly ?

Posted: Tue Feb 11, 2014 6:41 am
by Satelite
Hello :
I did point my finger at the radios that have done this.
I Have seen it happen on all of these models listed above.
It can be vhf or uhf doesn't seem to matter.
Yes I have seen the interconnect pins cause issues and have cleaned them to only see the issue still continue.
I could say a D43 16 pin 45 watt maxtrac but then it would be added info of I have seen this on the others too that someone would eventualy say - It would have been nice to know it was seen on other various models now and then.
This to me would indicate a common component in all the radios mentioned such as the 14.4 crystal.
I provided what I thought was important info to be able to get some info on this but if its not enough then well its not enough.
I will next time when I see it again check the 14.4 crystal to see if its causing it.
Just looking for any ones info and experience on this.
Satelite

Re: Tx shoots high then corects itself quickly ?

Posted: Tue Feb 11, 2014 10:53 am
by Al
Every radio, on keyup, has unique frequency characteristics. I won't go so far as to say that 0.5 khz is normal, but the "signature" that each transmitter produces has been used in the pre-MDC days as a way to identify a particular transmitter by analyzing the frequency characteristics at keyup remotely to match a signature with known signatures.

Re: Tx shoots high then corects itself quickly ?

Posted: Tue Feb 11, 2014 8:44 pm
by Bill_G
I would ask what you are using to measure the freq. When I monitor freq error on a service monitor, it's not unusual for it to take a second to settle down. When I look at it on a spectrum analyser, it's rock steady - power comes up at center, and doesn't vary. Only a really fast freq counter like an Agilent will show you the instantaneous freq error.

You may be observing two things - the normal time it takes a radio with a dual vco like the Maxtrac and GM series to settle, as well as the time it takes your monitor to settle on the measured error.

The Maxtrac / GM radios (along with many others) use two VCO's - one for rx and one for tx. After you hit ptt, the first thing that happens is the rcvr is shut down killing the VCO. Then the xmit VCO is powered up. Once it achieves lock (usually under 40mS), the cpu powers the rest of the xmitter. You should have full power in under 100mS from initial keyup. Perfectly acceptable for a voice call, but not so great for synchronous data. The transmitter cannot key / dekey quickly enough in it's timeslot, and then turn the receive back on to hear the system possibly return an ack.

Re: Tx shoots high then corects itself quickly ?

Posted: Wed Feb 12, 2014 2:49 am
by Jim202
I think I have to stand with Bill G on his comments about how are you measuring the frequency error. Unless your trying to use the discriminator output of an active receiver, I don't know of any method the average person can use to see a frequency error in that short of a time frame.

My second concern is that these older radios are not able to meet the narrow band requirements for normal use on say part 90 operation. So about the only place they could be used legally would be on the ham bands. As such, it's an I don't care on the ham bands. By the time this short time frame passes, your just starting to talk into the mic anyway and the radio has settled down on the correct frequency.

Think your trying to be concerned about an issue that you probably can do little about even if you could measure the error.

Jim

Re: Tx shoots high then corects itself quickly ?

Posted: Wed Feb 12, 2014 6:49 am
by Satelite
Hello:
Im seeing it on my service monitor and not all radios do this just a few now and then.
Its not very often I do see it maybe a couple a year.
True the maxtrac is not narrow band compliant but as you said its ham and a couple of murs vhf freqs along with the GMRS band that is still legal 25 kHz wide band yet today that's been on the bench.
As for the Radius M10 / M120 / GM300 they were made in 12.5 narrow band and are legal 12.5 nb from the factory for use yet today.
I do have customers using the narrow band factory radios in the Radius line still today.
Anyway it sounds like I have my answer on the tx freq swing so im satisfied and will not worry about it.
Just thought it was odd to see now and then and wondered if I could do anything bout it.
As I stated it did swing high or low but always corrected itself fast enough that the user never noticed and worked fine.
It wasn't a situation where I saw it on one radio only once but the radio did it every time.
Then next several radios would be steady on the tx so didn't appear to be a random thing but seems to be the just the one radios tx swing id see to not see it again for a long time on another radio.
I think just out of curiousity ill jot down the radios serial model numbers that do this tx swing and how much high or low it was doing and sit back and see if that radio comes in later on down the road and see if I can confirm its just that radio for kicks and giggles.
But as for trying to fix it now im not so concerned with bills info and id bet hes right sounds right and makes sense.
Thanx Satelite