Transmitter Power Amplifier (PA) 5-25W
5C.3-20 Theory of Operation
8.3 IF IC (U2201)
The first IF signal from the crystal filters feeds the IF IC (U2201) at pin 6. Within the IF IC the
21.4MHz first IF signal mixes with the second local oscillator (LO) at 20.945MHz to the second IF at
455 kHz. The second LO uses the external crystal Y2211. The second IF signal is amplified and
filtered by two external ceramic filters (FL2201, FL2202). 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 (U2201) 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
interpreted by the microprocessor (U0101 pin 44) and in addition 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 three stage amplifier used to amplify the output from the exciter to the
radio transmit level. It consists of the following stages in the line-up. The first (Q2511) is a bipolar
stage that is controlled via the PA control line (line PWR CNTL). It is followed by a MOS FET stage
(Q2521) and a final bipolar stage (Q2531).
Devices Q2511 and Q2521 are surface mounted. Bipolar Transistor Q2531 is directly attached to the
heat sink.
9.1 Power Controlled Stage
The first stage (Q2511) amplifies the RF signal from the VCO (line EXCITER PA) and controls the
output power of the PA. The output power of the transistor Q2511 is proportional to its collector
current which is adjusted by a voltage controlled current source consisting of Q2641, Q2642 and
Q2643. The current of the whole stage is drawn from the RX-TX Switch through coil L2652.
Transistor Q2643 , controlled by the microprocessor via signal K9V1, switches the current source on
in transmit mode and off in receive mode.
The collector current of Q2511, drawn via L2641, causes a voltage drop across the resistors R2645
and R2646. Transistor Q2641 adjusts the voltage drop across R2644 controlled through the PA
control line (PWR CNTL). The current source Q2642 adjusts the collector current of Q2511 by
modifying its base voltage via R2601-R2603 until the voltage drop across R2645 and R2646 plus
V
BE
(0.6V) equals the voltage drop across R2644. If the voltage of PWR CNTL is raised, the base
voltage of Q2641 will also rise causing more current to flow to the collector of Q2641 and a higher
voltage drop across R2644. This in turn results in more current driven into the base of Q2511 by
Q2642 so that the current of Q2511 is increased. The collector current settles when the voltage over
the series configuration of R2645 and R2646 plus V
BE
of Q2642 equals the voltage over R2644. By
controlling the output power of Q2511 and in turn the input power of the following stages the ALC
loop is able to regulate the output power of the transmitter.
In receive mode the PA control line (PWR CNTL) is at ground level and switches off the collector
current of Q2641 which in turn switches off the current source transistor Q2642 and the RF
transistor Q2511.
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