Transmitter Circuitry - Tait TM8100 mobiles Service Manual

3.1

Transmitter Circuitry

Introduction
>25W 25W
Exciter
>25W
25W
Power Amplifier
>25W
25W
TM8100/TM8200 Service Manual
© Tait Electronics Limited November 2007
For a block diagram of the transmitter circuitry, refer to
The transmitter circuitry is different for the 40W/50W radios, the
30W/35W radios (available for K5 band only), and the 25W radios, and the
different bands.
With the 40W/50W radio, the discrete-component exciter is designed for
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 30W/35W radio, the discrete-component exciter 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 26dBm.
With the 25W radio, the broadband exciter is a common element in all the
bands, as it operates across all frequencies from 66 to 530MHz. 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.
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
final devices Q309 and Q310.
With the 40W/50W 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 30W/35W radio, the signal from the exciter is amplified by Q306
to a power level of approximately 3W (VHF) using a PD55008.
The resulting signal is then amplified by Q309 and Q310 to produce a
typical output power of 45W, 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 40W when measured after the series capacitors
(C348, C349, C350) at the start of the directional coupler.
Figure 3.2.
Circuit Descriptions 67