03 Transmission Circuit; If Circuit For Clean And Stable 100 W Output Power; Fet Final Amplifier Circuit - Kenwood TS-890S In-Depth Manual

03 TRANSMISSION CIRCUIT

IF Circuit for Clean and Stable 100 W Output Power

A signal modulated and multifariously processed in the DSP is fed as the 24 kHz (modes other than FM) or 36 kHz (FM
mode) transmit first IF signal by a D/A converter, which is then converted by a diode mixer used for both transmission
and reception to 8.248 MHz. The second IF signal of 8.248 MHz passes through an IF filter with a 6 kHz (modes other
than FM) or 15 kHz (FM mode) bandwidth at which undesired frequency components outside the band are attenuated,
after which it is being amplified.
Next, the signal goes through a gain control circuit that corrects the gain difference by band, enters the mixer IC, and
is then converted to a 71.752 MHz or 41.752 MHz (European model 70 MHz band only) third IF signal. After that, the
signal passes through a gain control circuit that corrects the signal gain to the required level for the specified transmit
power. The signal then passes through a filter that eliminates unwanted spurious components and an ALC circuit that
controls the signal for stable transmit output, and enters a mixer circuit for conversion to the desired transmit frequency.
Also, fine gain control is performed according to the situation, such as lowering the amplifier gain when there is no key-
down during operation in the CW mode. Processing in this way enables low-noise and high-quality transmit signals to be
obtained.
The signal converted to the desired transmit frequency passes through a bandpass filter to eliminate spurious signals,
so as not to generate an interfering signal outside the transmit bandwidth, and is amplified to a certain level and sent
to the final circuit. The drive signal generated through the process is also available from the DRV terminal (if the DRV
output is enabled).

FET Final Amplifier Circuit

The final amplifier uses two MOS FET RD100HHF1 (Pch 176.5 W) manufactured by Mitsubishi and is configured with a
push-pull system. Meanwhile, MOS FET RD16HHF1 is employed for the drive amplifier and MOS FET RD06HHF1 for
the pre-drive amplifier. Optimization of matching is performed to achieve a 13.8 V final circuit with low-distortion signal.
Coupled with heat dissipation by the large die-cast, stable continuous operation can be achieved.
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