Transmitter frequency error

Joined
Sep 12, 2017
Messages
2
#1
I recently purchased a used but freshly calibrated service monitor and have been enjoying testing the radios I have around the house. I have not previously used a service monitor, so I'm learning as I go.

A local group gave me a pile of 20 year old vertex 400U handhelds to test.

FCC Part 90.213 says for mobile transmitters in the 421-512Mhz range, 12.5 kHz channel bandwidth must have a frequency stability of 2.5 ppm. If my calcs are correct, using an example frequency of 460Mhz, that would be +/- of 1150hz.

Most of these 20 year old radios are testing at around 600hz off frequency. Some are testing as high as 2100hz off.

I can't find a service manual for these radios, and the programming software I have for them does not have an alignment mode, like many of the newer versions of this series.

Any thoughts on acceptable tx frequency error, what limits to take them out of service absent ability to repair, is much appreciated. I just don't have any experience to know what is normal.


Thanks!
 
Joined
Jun 16, 2013
Messages
3,200
Location
Texas
#3
What you really need is a service manual with the alignment procedure. Your calculations are correct though, a frequency error of plus/minus 1.150 kHz is what would be allowable per Part 90. Not being familiar with that radio, it may require physical adjustment to correct the frequency error instead of just being able to do it in software. There may also be a flag option similar to that of the Vertex VXR-7000 which has to be added to get the software into an alignment/power adjust mode.

Now the real question, what kind of service monitor did you acquire?
 
Joined
Dec 22, 2013
Messages
2,733
#5
I have been refurbishing 450 MHz Motorola Systems Saber radios from the early to mid 90's. These have a high stability 2ppm oscillator, due to digital encryption. I am finding these radios all to be within 150 Hz. Which is about the limit of one soft pot step. I know that some may have been to the depot, perhaps in 2000. Still 18 years later, they are right on.

My experience with later model 800 MHz equipment, is that within 18 months they will drift high as the oscillator ages. Drifting enough to exceed the NPSPAC frequency limit. After they are, if reset, they will hold long term accuracy.

Sent from my SM-T350 using Tapatalk
 
Joined
Dec 22, 2013
Messages
2,733
#6
I recently purchased a used but freshly calibrated service monitor and have been enjoying testing the radios I have around the house. I have not previously used a service monitor, so I'm learning as I go.

A local group gave me a pile of 20 year old vertex 400U handhelds to test.

FCC Part 90.213 says for mobile transmitters in the 421-512Mhz range, 12.5 kHz channel bandwidth must have a frequency stability of 2.5 ppm. If my calcs are correct, using an example frequency of 460Mhz, that would be +/- of 1150hz.

Most of these 20 year old radios are testing at around 600hz off frequency. Some are testing as high as 2100hz off.

I can't find a service manual for these radios, and the programming software I have for them does not have an alignment mode, like many of the newer versions of this series.

Any thoughts on acceptable tx frequency error, what limits to take them out of service absent ability to repair, is much appreciated. I just don't have any experience to know what is normal.


Thanks!
For a $35 annual membership, it appears you can download all the vertex manuals you might need right here.

http://www.vertexradiogroup.net/membership-account/membership-levels/

Usually on older radios, there will be an TCXO , a small cube with a frequency like 14.4000 or 16.8000 or so stamped on it, and if you are lucky, a small hole for tuning or a pot adjacent to it for voltage tuning. If not it is a soft pot requiring tuning software.

Motorola made some radios, HT600 if I recall, that required a battery operated widget box with buttons to step the soft pot up and down in freq.

Sent from my SM-T350 using Tapatalk
 
Joined
Sep 12, 2017
Messages
2
#7
Thanks very much for the replies. Since I couldn't find an actual service manual with alignment procedures, I decided to call Motorola. Expecting to be told to pound sand, I actually spoke to very nice gentleman who sent me the service manual and a copy of the DOS based alignment tool. Who knew!

It does turn out there is a PLL reference trimmer pot under a label. Specs show freq stability as better than 2.5ppm and instructions are to tune it within 200hz.

MCore25: I bought an HP 8924C w/100W input option. It a bit of a boat anchor, but so far been very happy with it.
 
Joined
Dec 22, 2013
Messages
2,733
#12
Make darn sure your service monitor IS within calibration.
not hard to do for the reference oscillator if you can monitor a simulcast system!

Back in the day WWV at 10 MHz was good to zero beat against using my shortwave RX. You could tweak out the periodic phase reversals over several minutes and get it pretty close.
 
Joined
Feb 6, 2007
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175 DME, HEC 358° Radial
#13
not hard to do for the reference oscillator if you can monitor a simulcast system!

Back in the day WWV at 10 MHz was good to zero beat against using my shortwave RX. You could tweak out the periodic phase reversals over several minutes and get it pretty close.
Nowadays, you can buy a GPS disciplined reference for about $150 on eBay that will give you a 10 MHz source traceable to the U.S. Naval Observatory. Is 12 or 13 places to the right of the decimal point close enough for amateur work?
 
Joined
May 6, 2011
Messages
82
Location
Newport Beach, CA
#14
Make sure your Service Monitor is on-frequency after at least a 30 minute warm up, by using a local reference you know is phase-locked to a GPS Reference Oscillator. Ask around to see if your local Police or Sheriff has a simulcast system or some other system, like a paging company.

Or, buy your own GPS areference
 
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