04 Transmission; If Circuit For Clean And Stable 200 W Output Power; Fet Final Amplifier Circuit - Kenwood TS-990S In-Depth Manual

04 TRANSMISSION

IF Circuit for Clean and Stable 200 W Output Power

A signal modulated and multifariously processed in the DSP is fed as the 24 kHz transmit first IF signal by a D/A
converter, and a mixer IC converts it to 10.695 MHz. The second IF signal of 10.695 MHz passes through an IF filter with
a 6 kHz bandwidth at which undesired frequency components outside the band are attenuated, and is then amplified.
Next, the signal goes through a gain control circuit that corrects the gain difference by band, enters a TX/RX mixer,
and is then converted to the 73.095 MHz 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 to be a stable transmit power, and
enters a mixer circuit for conversion to the desired transmit frequency. If there is no key‑down during operation in CW
mode, the amplifier gain is lowered. As described above, fine gain control in compliance with the situation implements
low‑noise and high‑quality transmit signals. 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 TS‑990S final amplifier employs the well‑reputed MOS FET VRF150MP (Pch 300 W) and is configured with a push‑
pull system. The FET, supplied by Microsemi Corporation, is paired, with two VRF150s of the same rank and carefully
chosen by the Vth characteristics. By connecting FETs of matched characteristics in the push‑pull system, fine IMD
characteristics and high stability can be achieved. The drive amplifier employs a MOS FET RD100HHF1 and the pre‑
drive amplifier employs a MOS FET RD06HHF1.
The final amplifier circuit is configured with 50 V power and enables continuous operation with low distortion and stability.
The Transmit IMD Characteristics (14.2 MHz, 200 W output power) and Transmit Spurious Response (14.2 MHz, 200 W
output power) graphs show the IMD characteristics and harmonic spurious response. The graphs prove superb anti‑
distortion characteristics and clean output power.
Fig. 28 Transmit IMD Characteristics
(14.2 MHz, 200 W output power)
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Fig. 27 Final Devices and Final Amplifier
Fig. 29 Transmit Spurious Response
(14.2 MHz, 200 W output power)