Elecraft KDSP2 Manual - Manual (Page 42)

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Circuit Description
The KDSP2 is an interface board that adapts the DSPx Module for internal operation in an Elecraft K2
Transceiver.
Please refer to the schematic for the following discussion.
KDSP2
Audio Path
Differential audio from the K2 Product Detector is supplied to the KDSP2 via 5-pin connector P1, pins 1
and 2. The audio is AC-coupled to the DSPx module inputs via capacitors C11 and C15. Resistors R15
and R16, along with capacitor C9, provide a simple low-pass filter to help suppress IF and RF feedthrough.
Diodes D5 and D6 act as a limiter to prevent excessively strong signals from overloading the DSP section.
These diodes will normally only provide limiting if the K2’s AGC is OFF and a very strong signal is
received. With AGC ON, the diodes should never conduct.
After processing, the differential audio is made available from the DSPx and is applied to capacitors C10
and C14. These couple the signal through muting transistors Q5 and Q6 to the K2 at P2, a 3-pin connector.
R17 and R18 provide bias from filtered +5VDC to simulate the source normally seen by the K2 audio
system.
+5V (VCC5), filtered by R13, C13 and C16, is used for the bias source.
Q5 and Q6 are N-channel JFETs with a low pinchoff voltage. Under normal operation, R14 biases the
gates of these transistors to the 5-volt bias source, and the transistors are on, providing an audio path. The
audio may be muted if transistor Q7 is turned on by applying a positive voltage to R19. When on, Q7 pulls
the gates of Q5 and Q6 low, turning them off and muting the audio signal. When the voltage to R19is
returned to 0, Q7 is turned off and capacitor C8 charges through R14. This charging action turns on the
JFETs over a period of several tens of milliseconds, to ensure there is no “pop” or other disturbing artifact
as a result of un-muting.
Power System
Incoming +10 to +15 VDC is applied through R12 and C2 to U2, an LM340T linear regulator. This is a
linear regulator so R14 shares the load with U2. A switching regulator might introduce noise into the K2 -
the AGC IF is on the control board and is a wideband, untuned amplifier looking at a nominal 150 kHz
signal. The maximum load current on the 5 volt regulator is about 70 mA. The output of U2 is filtered by
C1 as well as various 0.1 uF capacitors distributed throughout the KDSP2.
VCC5 is further filtered for the audio bias, described above.