Irregular Watt Meter readings...

[medic46]

Hello All, curious if anyone has come across this or may know why I am seeing this:

Testing power output of a Motorola M120 VHF Mobile, radio is connected to a Telewave 44A wattmeter. When I "toggle PTT" from the RSS the meter displays 45W, however on PTT from the local microphone the meter displays 31W.

Although the meter has not been calibrated in a few years I have tested it against numerous other portables and mobiles alike, all of which have shown readings consistant with their transmitter readings.

Any thoughts or help appreciated.

Take Care

[Wowbagger]

Silly questions:

Are you using the same power supply for both tests?

Are you sure the ground path from the radio to the power supply is good, and some ground current is not passing through the programming cable?

[medic46]

Thanks,

On # 1 - Yes same power supply

#2 - The ground for the PS has been tested and it known to be good. I guess I am unsure how to determine if the programming cable is allowing transient voltage.


Thank you - Tyler

[Al]

Is the frequency that you're keying the tx at with the RSS the same frequency that you're keying the tx at with the mic? If it's not, most likely there's a gain vs frequency slope in the PA or power levelling circuitry that hasn't been compensated for by doing a tx power alignment using the RSS.

[medic46]

Thanks, the freqs were different. Should is a drop in freq normal or is there another setting to check?

Thanks in advance...

Tyler

[wavetar]

The radio was aligned across it's entire frequency range at the factory, using 20 different frequency test points. The regular power adjustment in the RSS only aligns it at the specific test frequency, and lowers/increases all the other points the same amount. So, if the radio has been mis-aligned or has happened to have drifted, you'll see differing power levels at different frequencies. You can access the complete power level adjustments by going into the "replace PA" portion of the service menu if the M120 has it, I don't recall off-hand. If it doesn't, you'd have to go into the "replace RF or controller board" portion, which I don't recommend without a service manual procedure to follow. The first step involves entering specific numbers which are printed on the reference oscillator crystal and also hand written on a label on the RF board (if it hasn't been removed) as well as a voltage reading with a high-accuracy multimeter. If this isn't followed properly, the radio frequency error will drift all over the place across different temperature extremes.

Todd

[RKG]

Depending on the cables you've used, it is also possible that your meter readings are affected by the relationship between cable length and frequency.