Transmitter Circuitry; Exciter; Power Amplifier; Power Control Loop - Tait TM9100 Service Manual

3.1

Transmitter Circuitry

For a block diagram of the transmitter circuitry, refer to
Introduction
The transmitter circuitry is different for the 50W/40W radios and the 25W
radios, and the different bands.
With the 50W/40W radio, the discrete-component exciter is designed for

Exciter

specific bands (UHF or VHF). It is made up of Q3501, Q3502, and Q3505,
which amplify the signal provided by the frequency synthesizer from its level
of 7 to 10dBm up to 24dBm for the frequency bands 136 to 174MHz and
400 to 520MHz.
With the 25W radio, the broadband exciter is a common element in all the
bands, as it operates across all frequencies from 66 to 940MHz. It is made
up of Q300 and Q303, which amplify the signal provided by the frequency
synthesizer from its level of 7 to 10dBm up to 24.5dBm for the frequency
band from 66 to 530MHz and slightly less than this for the bands covering
530 to 940MHz.
The exciter operates in full saturation, thereby maintaining a constant
output power independent of the varying input power level supplied by the
synthesizer.
The power amplifier comprises the driver amplifier Q306 and two paralleled

Power Amplifier

final devices Q309 and Q310.
With the 50W/40W radio, the signal from the exciter is amplified by Q306
to a power level of approximately 2W (VHF) using a PD55003 and about
3W (UHF) using a PD55008. The resulting signal is then amplified by
Q309 and Q310 to produce a typical output power of 90W at 155MHz and
65W across the UHF band, when measured after the series capacitors
(C348, C349, C350) at the start of the directional coupler.
With the 25W radio, the 24.5dBm signal from the exciter is reduced by a
band-dependent pi-attenuator and is amplified by Q306. The resulting
signal is then amplified a second time by Q309 and Q310 to produce a
typical output power of 42W when measured after the series capacitors
(C348, C349, C350) at the start of the directional coupler.
The high-level RF signal passes via the directional coupler, the transmit-
receive PIN switch, and the LPF, through to the antenna. The LPF is used
to attenuate unwanted harmonic frequencies.
Calibration is used to adjust the power control loop, thus setting the output

Power Control Loop

of the transmitter to one of four preferred power levels:
TM9100 Service Manual
© Tait Electronics Limited August 2005
10, 15, 25, and 50 watts (VHF), and
10, 15, 20, and 40 watts (UHF) for 50W/40W radios
1, 5, 12 and 25 watts (all bands) for 25W radios
Figure 3.2.
Circuit Descriptions 65