Cavity Combiner - Theory Of Operation; Figure 157: Auto Tune Cavity Combiner - Motorola MTS LiTE Installation, Configuration And Basic Service Manual

6802800U74-AN
Chapter 7: Radio Frequency Distribution System
Minimum channel spacing of the TX channels is 150 kHz while the recommended channel spacing is
250 kHz. This limitation applies to all Cavity Combiners in all cabinets connected to the same transmit
antenna.

Figure 157: Auto Tune Cavity Combiner

7.3.6.1

Cavity Combiner - Theory of Operation

A minimum of 2 watts is needed at a cavity input. The ATCC will automatically tune in 40 seconds
maximum. For more detail, see the ATCC specification.
Once an RF signal greater than 2 watts is detected, the ATCC tunes the cavity and continuously keeps
it tuned over humidity, temperature and changing transmit frequency, so long as it does not sense one
of the following alarm conditions:
• Channel Spacing alarm
• VSWR alarm
• Failure to Tune alarm
Being tuned means that a cavity is within the insertion loss specification at the frequency of the applied
PI/4DQPSK or QAM4,16,64 signal that is within the average input power range specified above. Being
tuned also means that the cavity peak response is no greater than 25 kHz away from the TX carrier
center frequency. If the TX carrier does not change channel or average power level, the auto tune
algorithm will not initiate a re-tuning on its own which exceeds +/- 300 kHz from the carrier frequency.
The only exception occurs when the fine tune timer event happens. The fine tune timer is used to
compensate for large variations in humidity and is default set to 480 Minutes. The Cavity Combiner is
temperature compensated but large variations in humidity can de-tune the cavities up to 150 kHz with
the result of an increasing insertion loss.
When the fine tune timer event occurs, all cavities with RF applied will be re-tuned for maximum output
power of each TX carrier. The fine tune timer can be adjusted to compensate for fast humidity
variations; for instance if the MTS 4 is installed in outdoor sites without air-conditioning. The
recommended setting of the fine tune timer, if the MTS 4 is installed in a controlled environment, is 480
Minutes. For sites where the MTS 4 is exposed to more than +/- 20% variation in RH, the
recommended setting of the fine tune timer is 60-200 minutes depending on the speed of the variation.
Having a second cavity tune up and pass through the desired channel, the desired channels insertion
loss dips no more than 3 dB more than the max insertion spec for a period of 0.25 seconds. The cavity
tuning rate should be faster than 1 MHz per second.
284