Transmitter Power Amplifier (PA) 5-25W
4-18 Theory of Operation
8.3 IF IC (U5201)
The first IF signal from the crystal filters feeds the IF IC (U5201) at pin 6. Within the IF IC the
45.1MHz first IF signal mixes with the second local oscillator (LO) at 44.645MHz to the second IF at
455 kHz. The second LO uses the external crystal Y5211. The second IF signal is amplified and then
filtered by two external ceramic filters (FL5201, FL5202). Back in the IF IC the signal is demodulated
in a phase-lock detector and fed from IF IC pin 28 to the audio processing circuit ASFIC U0201
located in the controller section (line DET AUDIO).
The squelch circuit of the IF IC is not used. Instead the squelch circuit inside the audio processing
IC ASFIC (U0201) determines the squelch performance and sets the squelch threshold. The
detector output signal from IF IC (U5201) pin 28 (DET AUDIO) is fed to the ASFIC pin H7.
At IF IC pin 11 an RSSI signal is available with a dynamic range of 70 dB. The RSSI signal is used
by the ASFIC (U0201 pin G8) and after buffering by op-amp U0202-3 available at accessory
connector J0400-15.
9.0 Transmitter Power Amplifier (PA) 5-25W
The radio’s 5-25 W PA is a four stage amplifier used to amplify the output from the exciter to the
radio transmit level. It consists of four stages in the line-up. The first (Q8510) is a bipolar stage that is
controlled via the PA control line. It is followed by another bipolar stage (Q8520), a MOS FET stage
(Q8530, Q8531) and a final bipolar stage (Q8540).
Devices Q8510, Q8520,Q8530 and Q8531 are surface mounted. Bipolar Transistor Q8540 is directly
attached to the heat sink.
9.1 Power Controlled Stage
The first stage (Q8510) amplifies the RF signal from the VCO (line EXCITER PA) and controls the
output power of the PA. The output power of the transistor Q8510 is proportional to its collector
current which is adjusted by a voltage controlled current source consisting of Q8612 and Q8621.
The whole stage operates off the K9V1 source which is 9.1V in transmit mode and nearly 0V in
receive mode.
The collector current of Q8510 causes a voltage drop across the resistors R8623 and R8624.
Transistor Q8612 adjusts the voltage drop across R8621 through PA control line (PWR CNTL). The
current source Q8621 adjusts the collector current of Q8510 by modifying its base voltage until the
voltage drop across R8623 and R8624 plus VBE (0.6V) equals the voltage drop across R8621 plus
VBE (0.6V) of Q8611. If the voltage of PWR CNTL is raised, the base voltage of Q8612 will also rise
causing more current to flow to the collector of Q8612 and a higher voltage drop across R8621. This
in turn results in more current driven into the base of Q8510 by Q8621 so that the current of Q8510
is increased. The collector current settles when the voltage over the series configuration of R8623
and R8624 plus VBE of Q8621 equals the voltage over R8621 plus VBE (0.6V) of Q8611. By
controlling the output power of Q8510 and in turn the input power of the following stages the ALC
loop is able to regulate the output power of the transmitter. Q8611 is used for temperature
compensation of the PA output power.
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