Multi Channel Transmitter (MCTX) - Installation,
Commissioning and Maintenance
Page 7 of 16
VHF input to upconverter no modulation
VHF input to upconverter 100% modulation
The system has been designed to allow for up to 10 dB cable loss between the processor and the
up converter. To allow for cable loss of less than 10 dB an attenuator will be required in the FM
cable to compensate for the cable loss.
To select the correct attenuator value
measure the FM output of the processor,
the VHF signals should be -20 dBm ± 1
dB. Connect the cable and then measure
the signals at the up converter end.
Subtract the required up converter input
signal level of -30 dBm from the
measured value to obtain the value of the
attenuator. Check that the attenuator
value is correct by measuring again on
the output of the attenuator after
attaching it to the cable at the
upconverter end, which should be below
-30 dBm. This last check will verify that
the signal level is correct, if it is not then
there may be an impedance miss-match
in the cable. The input to the up
converter must be below -30 dBm. If it is
higher than -30 dB over modulation of
the up converter will occur and
harmonics and intermodulation will be
produced which will cause noise,
distortion and non compliance with the
standards.
Connect the attenuator and the FC cable
to the up converter. For the best
performance the attenuator must be at the
up converter end of the FM cable so that
it masks any impedance miss-match and
reduces any RF pick up in the cable run.
With the aerial connected check the radiated signal on your spectrum analyser using a second
aerial which is vertically polarised and is located 1 metre away from the transmitter aerial check
for out of band signals. The radiated signal level at this point should measure approximately -30
dBm. Measuring the RF signal adjacent to the transmitter aerial will produce a reading of about
-10 dB. The RF signal is attenuated by 6 dB as the distance from the transmitter is doubled or 20
dB as the distance increases by 10 times. At a distance of 100 metres then the signal will be
measured at approximately -60 dB. The loss from the receiver aerial (headphone lead) is 20 dB.
Therefore you can predict the service area of the system by adding the signal strength to the
receiver aerial loss, which in this example produces an RF signal input to the receiver of -80
dBm. These calculations are only a guide to what you should expect to receive, the measurements
