User Manual

1.RF Power Amplifier & Heatsink Assembly 30C1899G1 - G2 - G3:
Most LARCAN exciters produce their best linearity at or near their maximum rated
output levels, and often the overall system gain is sufficient to result in
overdrive of later stages of the transmitter. The transmitter or translator
lineup may therefore include an in-line attenuator between the exciter and the
preamplifier, to prevent overdrive from certain models of exciter-modulator.
This is especially true for the 10 watt system, because in our basic transmitter
family a 10 watt amplifier is simply a lightly driven 50 watt amplifier, and a 50
watt amplifier is likewise a lightly driven 250 watt output stage running without
its driver/IPA. (Our 250 watt transmitter lineup needs an additional IPA to get
enough drive to the identical final stage that is used in the 50 watt and 10 watt
lineups).
2.RF Preamplifier 10A1453G5 (Low Band) and 10A1453G4 (High Band):Figures 2 and 6.
This preamplifier design originally appeared in the aural/sound section of a dual
RF chain transmitter which operates two single RF chains in quadrature and
therefore requires phase and gain control of the input to each chain. The same
unpopulated circuit board is used for the present application, therefore has the
pads and holes for the components which performed the adjustment of RF gain and
phase in the parallel systems. In a single chain transmitter such as the present
one under discussion, there is no requirement for control of RF phase nor
consequently its components; they are therefore removed and wire jumpers
substituted.
In the Low Band preamp, the 50 Ù input cable is matched by C5, which uses the
inductance of the traces on the PC board to form a low pass matching network, and
fed to amplifier U2 whose output connects through a cable to the PA. U2 is a
linear amplifier designed originally for use as a wideband cable system trunk
amplifier. Cable amplifiers are nominally 75 ohms in and out, but the MHW6185 is
capable of a good match with 50 ohm source and load. C12 and the lead inductance
of a CA2885 when used as U2, perform output matching to 50 Ù. C12 is not
present in a board using an MHW6185 for U2. The gain of U2 is spec'd as 18 dB,
and allowing a few dB of losses, the gain of the Low Band preamp is 14 to 16 dB.
RF power FETs operated in High Band amplifiers exhibit about 6 dB less gain than
they do in Low Band, so an additional amplifier is necessary to make up the
difference. The High Band preamplifier therefore consists of two stages.
In the High Band preamp, the input is matched by C5 and the PC trace inductance
which together forms a low pass matching network, and the signal is fed to an
additional preamplifier stage U4, whose output appears as the input of U2, which
in turn feeds the board output cable. The specified gain of type MWA330 in the
U4 position is 6 dB, and a type MHW6185 or CA2885 (U2) is 18 dB. A few dB of
losses exist on the board, so the effective gain of the High Band preamp board
10A1453G4 is about 20 to 22 dB.
High Band preamps 10A1453G1 used in higher powered externally diplexed
transmitter aural service, have a type MWA130 instead of an MWA330 for U4.
Specified gain of an MWA130 is about 12 dB, but we don't recommend substitution
of U4 in any internally diplexed system due to overall linearity and intermod