XTL 5000 Lowest Power Issue

[ricksplacer]

Has anyone been exposed to this issue raised by a Motorola representative?

I have a
XTL5000 Mobile
Mid Power
Type II Trunking UHF Band
Remote Control Head

The system is designed for portable coverage and I have reason to set the Mobile to its lowest power output, 4W, matching the portables.
The data sheet states the lowest power setting is 4 W.
The FCC Type Acceptance Test data says the lowest power setting is 4 W.
The CPS R13.00.01 says the lowest RADIO WIDE Transmit Power Levels are 4W Low and 4W high both Freq Range A and B, High Power not lower than Low Power.

Yet the Motorola rep says don't program the radios to 4 W with no real explanation other than "we don't like to program the radios to the lowest power"

Is there a technical explanation?

Thanks

[N4KVE]

Apparently the radios don't like to go too low in power, because they don't operate efficiently if they are set too low. I'd love to set my 100 watt MCS 2000 to 20 watts when I hit the low power button, but I've been told by tech's that I shouldn't set it lower than 60 watts. Spurious emisions will result. My 15 watt Spectra will go to 4 watts, but a 40 watt radio won't be happy at 4 watts. GARY N4KVE

[ricksplacer]

Thanks Gary.

The part that I don't understand is:

The radio passed FCC Type Acceptance down to 4W.
CPS allows programming down to 4W.

If the critter emits spurs I'd have thought it would not have been Type Accepted at 4 W power
And CPS would only permit configuration at power levels with acceptable performance.
I've never seen Motorola cut corners where they would leave the company exposed for FCC performance issues.

Rick

[wavetar]

You shouldn't have any issues setting it to 4 watts. It's true that earlier programmable Motorola radios such as Maxtrac/GM300 would let you set the power down to a couple of watts or less, but they tended to become spurrious below 10 watts. Motorola's answer was to come out with the lower powered 10 watt (maximum) unit, which would go down to 1-watt with no issues. They continued the trend in later models...you need a CDM to go down to 1-watt, get the 1-25 watt model. If you want high power, get the 20-45 (or 50) watt model...it won't go below 20. This is why Gary's 100 watt MCS2000 can't go very low...there are other models for that. If the CPS lets you set your XTL to 4 watts, I have every confidence it will work fine.

[tvsjr]

I disagree. A PA is designed to be efficient through a certain range. Go plot supply current vs. power out - you'll find that supply current at 4 watts is likely the same as at 10 watts. All that extra power goes somewhere - heat. Heat kills PAs.

Moto markets the XTL5000 as 10-35, 10-40, 10-50 watt radios (for mid power) and 50-110 for high power. I wouldn't drop below 10 watts.

[wavetar]

I disagree. A PA is designed to be efficient through a certain range. Go plot supply current vs. power out - you'll find that supply current at 4 watts is likely the same as at 10 watts. All that extra power goes somewhere - heat. Heat kills PAs.

Moto markets the XTL5000 as 10-35, 10-40, 10-50 watt radios (for mid power) and 50-110 for high power. I wouldn't drop below 10 watts.

Good point as well. I'll have to do so when I find the time, for curiosity's sake. Or the original poster could let us know, if he has a way of measuring current draw while getting the power set.

[ricksplacer]

While I haven't measured the current draw I did go back to the Motorola test data on file with the FCC. I can't say how the efficiency at low power stresses components vs. high power but at 484.975 MHz the XTL5000 is ~55% Efficient at full power, 46% Efficient at mid power while 22% Efficient at low power. Of course there's a lot more waste heat at full power but less on a percentage basis. For the Efficiency Calculation, I backed off the standby current (0.85 A) figuring that's the power to run the non-transmitter radio functions. Here is the Motorola data on file.

RF Power Output Data

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Frequency, MHz 484.975              Power             Supply Voltage, V            Current, A[/b]
Measured High Power Level           54.0W                 13.6                             8.02
Measured Medium Power Level         27.0W                 13.6                             5.15
Measured Low Power Level            4.00W                 13.6                             2.18

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Frequency, MHz 508.375              Power             Supply Voltage, V            Current, A[/b]
Measured High Power Level           48.0W                 13.6                             7.68
Measured Medium Power Level         24.0W                 13.6                             5.06
Measured Low Power Level            4.00W                 13.6                             2.16

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Frequency, MHz 519.375              Power             Supply Voltage, V            Current, A[/b]
Measured High Power Level           30.0W                 13.6                             5.78
Measured Medium Power Level         15.0W                 13.6                             4.13
Measured Low Power Level            4.00W                 13.6                             2.15

FCC Link