Chapter 5
Theory of Operation
5-1. Introduction
See Figure 5-3, MICOM-3 block diagram.
This section provides a functional description of the MICOM-3 radio, in the following order:
• Transmitter
• Receiver
• Synthesizer
• Electronic boards.
5-2. Transmitter
Incoming audio signal is fed directly to the DSP processor U6042 after amplification and filtration.
The DSP algorithm is translating the incoming audio signal to a stream of I and Q bits (Inphase and
Quadrature). This I/Q representation of the audio signal is transferred to the Digital SSB modulator
U6000 where a digital quadrature mixing is performed. The digital signal is then transferred to Sigma-
Delta D/A which converts the digital signal to an analog form. The result is the "classic" IF SSB signal
with a suppressed carrier at 1.05 MHz and "natural" suppression of the unwanted sideband. Since the
quadrature mixing is performed digitally, there are no amplitude and phase inaccuracies which may
degrade the unwanted carrier and unwanted sideband suppression. The only limiting factor is the
performance of the Hilbert transform, performed by the DSP. Since this performance is controllable,
the algorithm is designed to achieve unwanted sideband attenuation of at least 60 dB.
The 1.05 MHz IF signal is converted to 45.1 MHz IF signal at mixer U1108 and then amplified by
Q1006. The crystal filter stage cuts out unwanted mixing products and suppresses spurious signals.
This unit consists of crystal units FL1 and FL2, centered at 45.1 MHz.
The last conversion stage of the transmission path, implemented by mixer U1000, converts the signal
to the final transmission frequency.
The transmission signal is then amplified by the exciter stage Q1004, Q1005 and then transferred to
the High Power board, where it first passes the ALC attenuator Q39 and is amplified by the pre-driver
amplifier, which consists of Q2-Q6.
The Power stage consists of two transistor pairs acting as two cascaded push-pull stages the driver
stage (Q32 and Q33) is connected to the final stage (Q34 and Q35) via transformer T5. The output of
the power amplifier is transferred to a 50 Ω impedance by transformer T3.
Maximum output power is 125W PEP (Peak Envelope Power) and average at transmission duty cycle
of 1 to 4. Output power can be preprogrammed to one of 4 possible levels: 25W, 50W, 62.5W and
Cat. No. 6886859J01 5-1
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