TK-90
28
■
Protection Circuit
1) Current Protection
Even with a load whose VSWR is relatively in good condi-
tion, there may be cases where the consumption current in-
creases. This is the protection circuit which detects such cur-
rent and makes sure the current consumption is less than the
specified amount. It includes the circuit which detects the
current of the Final unit and the circuit which detects the cur-
rent of the drive step and prevents currents that exceed a
certain level.
The final side detects the voltage difference occurring at
both ends of R3, the current detection resistance, and the
drive step detects the voltage difference occurring at R1.
When the difference becomes a certain value or larger at the
TX-RX unit op-amp (IC361, IC362), the IF amplifier (Q186)
gain is controlled using the ALC circuit and excess current is
suppressed.
2) SWR Protection
When there is a problem with the antenna load imped-
ance, the transmission circuit may become damaged. By
lowering the power according to the VSWR deterioration,
this circuit is protected from damaging devices.
The reflected wave detection voltage VSR responding to
the reflected power exceeds a certain value at the final unit.
By using the function to lower the ALC circuit gain, the re-
flected power is controlled so as to not to be yielded again.
When the VSR for the op-amp (IC362) reversed input and
the SWR protection controlling voltage (PRO) for forward in-
put are connected, and when the VSR which exceeds the
PRO occurs, this op-amp output is also connected to the ALC
circuit and is controlled by lowering the IF amplifier (Q186)
gain.
■
Attached Circuit
1) High-boost Circuit
Because the SSB modulation frequency characteristics
are flat, the signal is transmitted with the frequency charac-
teristics of the microphone itself. Due to their structure, wa-
terproof microphone’s high pitch sound tends to be lowered.
To correct this, a high-boost circuit for emphasizing high pitch
sound is included.
The microphone amplifier (IC605) gain and frequency
characteristics are switched at Q607 and Q608. When high-
boost is ON, high-gain yields 3dB octave low-cut characteris-
tics. By combining this with the IF filter for SSB (XF252), a
modulation frequency of 2~2.5kHz is emphasized.
Thus, the practical sound quality difference from non-wa-
terproof microphones is lessened.
2) VOX Circuit with Anti-VOX
The signal from the microphone passes through the mi-
crophone amplifier (IC605) and DA converter IC (IC554), is
input to the VOX amplifier (IC604), is tested for VOX sensitiv-
ity at IC554, passes through the buffer amplifier (IC603), is
detected by the detector (D604), and changes to a DC level.
The received signal coming in from the speakers is ampli-
fied to an appropriate level at the anti VOX amplifier (IC605),
is tested for anti VOX sensitivity at the DA converter IC
(IC554), passes through buffer amplifier (IC603), is detected
by the detector (D603), and changes to a DC level.
CIRCUIT DESCRIPTION /
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