New Mixer - Kenwood TS-990S In-Depth Manual

RECEPTION 03
The "Frequency Response" figure shows an example of typical Pre‑selector Frequency Amplitude characteristics.
Fig. 11 Frequency Response
Pressing the [P.SEL] key to turn the pre‑selector on makes the tuning filter waveform‑shaped by the relay operation
logic configured as the factory default, following the frequency displayed at the main band reception. Pressing and
holding the [P.SEL] key shifts the center frequency of the tuning filter up to ±20 steps. For instance, when a strong
adjacent interfering signal suppresses the desired signal, shifting the center frequency in the opposite direction of
the interfering signals attenuates the level of the interfering signals instead of attenuating the desired signal, thus it
effectively reduces interference as much as possible.

New Mixer

The first mixer was a part of the block on the main band receiver and was examined the longest. Double balanced
mixers with conventional J‑FET implementation have been unable to fully satisfy the required IP3 characteristics.
To satisfy the requirement, a mixer called the "H‑mode mixer", which employs the double balanced grounded switch
type that is different from conventional types, is employed for the first mixer. The circuit of the mixer can turn the GND
terminating ends on the signaling paths On or Off, to minimize the mixer distortion.
For the switch elements, bus switches are used and turned on/off only by low‑power local oscillator signals. For the RF
input and IF output of the mixer, large balun transformers with distortion resistance characteristics are used, and the
winding methods and materials of the cores with different magnetic permeability are carefully chosen and examined.
Furthermore, after the mixer, a duplexer circuit is placed to stabilize the impedance of the roofing filters in a wider band.
L59
+2.5V
IC7
RF L55
1st IF
8.248MHz
x4
L60
IC8
+2.5V
MLO1
8.278~51.752MHz
Fig. 12 Double Balanced Grounded Switch Type Mixer
9