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6.2
VCXO measurements in the radio calibration in EFRA
There are three measurements and one calibration, concerning VCXO, in the radio calibration in
EFRA.
The measurements are:
1. VCXO Control at DAC 00 Hex
2. VCXO Control at DAC 3FF Hex
3. VCXO Control Range.
These three measurements control the adjustment range, in ppm, of the crystal.
The measurement is performed as follow:
1. The transmitter is started in static mode at any channel and the VCXO value 00 Hex.
2. The output frequency of the transmitter is measured.
3. The adjustment range in ppm for DAC 00 Hex is measured according to the formula
below:
The adjustment range, in ppm = (The measured frequency – the frequency of the channel) *
1000000 / the frequency of the channel
E.g. channel 699.
The frequency of the channel l: 1747.6 MHz
The measured frequency: 1747.4 MHz
(1747.6 – 1747.4) * 1000000 / 1747.6 = - 114 ppm
The abbreviation ppm means "parts per million", i.e. 1 Hz divergence per MHz of the output
frequency of the transmitter.
Meaning that a difference of one ppm at the middle channel of the transmitter (1747.6 MHz)
gives a frequency divergence of 1747.6 Hz.
The VCXO value changes to 3FF and the frequency of the transmitter is measured again.
The adjustment range are calculated the same, but the result should be positive.
The VCXO Control Range is calculated from the values from the two measurements above. You
check the adjustment range for the values between 00 and 3FF Hex.
The measuring of the adjustment range is important to verify that the reference frequency can be
controlled enough up and down.
In Calibration VCXO the 13 MHz crystal is being trimmed at channel 570.
By sending the DAC value 200 Hex and compare the received frequency to the one for channel
570, an offset is calculated.
This offset is used in an algorithm to establish the value for the DAC for the TCXO.
The calibrated VCXO value is somewhere in the middle of 00 and 3FF Hex.
4/00021-3/FEA 209 544/18 B
Trouble Shooting Guide, Advanced
28(121)
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