TM9100 Service Manual Circuit Descriptions 65© Tait Electronics Limited August 20053.1 Transmitter CircuitryIntroduction For a block diagram of the transmitter circuitry, refer to Figure 3.2.The transmitter circuitry is different for the 50W/40W radios and the 25Wradios, and the different bands.Exciter With the 50W/40W radio, the discrete-component exciter is designed forspecific bands (UHF or VHF). It is made up of Q3501, Q3502, and Q3505,which amplify the signal provided by the frequency synthesizer from its levelof 7 to 10dBm up to 24dBm for the frequency bands 136 to 174MHz and400 to 520MHz.With the 25W radio, the broadband exciter is a common element in all thebands, as it operates across all frequencies from 66 to 940MHz. It is madeup of Q300 and Q303, which amplify the signal provided by the frequencysynthesizer from its level of 7 to 10dBm up to 24.5dBm for the frequencyband from 66 to 530MHz and slightly less than this for the bands covering530 to 940MHz.The exciter operates in full saturation, thereby maintaining a constantoutput power independent of the varying input power level supplied by thesynthesizer.Power Amplifier The power amplifier comprises the driver amplifier Q306 and two paralleledfinal devices Q309 and Q310.With the 50W/40W radio, the signal from the exciter is amplified by Q306to a power level of approximately 2W (VHF) using a PD55003 and about3W (UHF) using a PD55008. The resulting signal is then amplified byQ309 and Q310 to produce a typical output power of 90W at 155MHz and65W 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 aband-dependent pi-attenuator and is amplified by Q306. The resultingsignal is then amplified a second time by Q309 and Q310 to produce atypical 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 usedto attenuate unwanted harmonic frequencies.Power Control Loop Calibration is used to adjust the power control loop, thus setting the outputof the transmitter to one of four preferred power levels:■ 10, 15, 25, and 50 watts (VHF), and10, 15, 20, and 40 watts (UHF) for 50W/40W radios■ 1, 5, 12 and 25 watts (all bands) for 25W radios
