User's Manual

85894_M&R_TA7650C_A 2-4
The signal then the first amplifier, Q6. Q6 is autobiased by the circuit consisting of Q3 and
Q1. The gain in this stage is also adjustable by altering the bias of D1. This is part of the
overall gain control in the PA.
Then the signal is amplified further in the pre-driver, Q6. This is a MOSFET type transistor,
and it is auto-biased by Q2 and Q4. The output is taken trough a 4:1 impedance transformer
and fed to the input of the driver transistor Q8. Q108 is also of MOSFET type.
Q8 drives the output stage Q5 trough a single ended 4:1 impedance transformer, and a
9:1 balanced impedance transformer. Q111 is a “gemini” transistor set which operates as a
push-pull amplifier. The output is impedance transformed and converted to single ended by
transformer T2.
All RF stages are enclosed in a shielded box to avoid excessive RF interference on the
surrounding circuitry.
PIN switch
From the output of the power amplifier the signal is fed to the T/R switch which is realised by
using PIN diodes. In transmit mode D7 and D6 are forward biased, and the RF then passes
through D7 and trough the antenna filter. In receive mode both D6 and D7 are unbiased, and
the antenna signal flows through the directional coupler and antenna-filter, trough L6 and out
on the Rx port PL5.
Antenna filter and directional coupler
In transmit mode, after passing trough the T/R switch the output signal passes trough the
lowpass filter consisting of L7/L8/L9/L10/L11 and C67/C68/C69/C70/C71/C72. The low
pass filter ensures excellent attenuation of the harmonics from the output stage.
A directional coupler is the last element in the signal path. It plays a vital role in the
modulation process, and is a critical part for obtaining good modulation characteristics.
Both forward and reflected power is detected in the directional coupler. D9 rectifies the
reflected power and this voltage is used in the BITE and to detect poor impedance matching
of the output. The detected forward power is passed trough the attenuator consisting of R81,
R82 and R83 and passed to the modulator module through PL7, where it is demodulated by
the quadrature demodulator (se modulator module).
Voltage regulators and control circuits
IC1 is the fan control circuit. It reads the voltage from temperature sensor IC4. IC4 senses the
temperature of the cooling tunnel and the speed of the fan is regulated according to the
measured temperature. The fan starts at approx. 40°C.
The main supply for the RF amplifier is regulated with IC17B and Q9. Q9 is a very low