User's Manual

Doc. No.
Rev.
Date
1.00
2011-12-08
Damm Cellular Systems A/S, Denmark
TETRAFLEX
®
V7.5 MANUAL - TX Combiner for BS41x
TetraFlex® 7.5 Manual 4-11
The TX signal enters the port on the lower circulator and follows the green line. It enters the
first circulator, follows the arrow and leaves it again in the first-coming port. The same
happens in the second circulator. The TX signal passes the circulator with an insertion loss of
around 0.5dB.
Any signal entering the upper port in the reverse direction will follow the red line. This could
be the transmitter signal reflected at the cavity filter or at the antenna, or a signal from
another transmitter.
Most of the reverse signal will be dissipated in the big high-power resistor connected to the
first port. A diode detector connected to the resistor makes it possible to monitor the size of
the reverse signal.
Some of the signal will be reflected at the high-power resistor and follow the red line to the
low-power resistor on the next circulator, where most of the remaining signal will be
dissipated. However a small fraction will be reflected and end up at the TX input port. The
total rejection in the reverse direction will normally be higher than 50dB.
The size of the reverse signal can be monitored either by a voltmeter attached directly to the
Test Point or via the O&M interface and the TR41x or the TCC411. The test point is used for
adjustment of manually tuned combiners. The TR41x will also monitor the level on the TP
and generate an alarm if the reverse signal becomes too high.
Note, that when the Base Station runs without antenna or with the cavity filter tuned to a
wrong frequency, all the reflected power ends up in the high power resistor, which is
designed to dissipate the total transmitted power continuously. The PA of the TR41x will
always look into a perfect load as long as the cable is correctly connected between the
TR41x and the circulator.
4.2.2.2 Cavity Filter
From the Circulator the TX signal enters the Cavity filter to combine the TX signals
selectively. For the 400MHz range the cavity is a ¼ lambda TEM resonator with an unloaded
Q of about 10.000. For the 800MHz range the cavity is a ¾ lambda TEM resonator with an
unloaded Q of about 13.000.
The cavity is fully temperature compensated, and is tuned to the center frequency of the
transmitter either automatically with a stepper motor or optionally manually by hand.