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2-(4) Auto Tuning Hold Circuit
Referring to the block diagram Fig. 7, the Auto Tuning Hold circuit operates in the following way:
\/7
Antenna
ee
Recovered
Audio
Discriminator
Switching
e
Tuning
Circuit
Detector
Time
Constant
Circuit
Error Voltage Detector
When the tuner has been tuned to a desired station, the discriminator delivers a detection signal which swings tuning meter.
The
tuned-state
detecter
switches
to "hold'"' with a delay determined
by the time constant circuit, and it turns DC
amplifier on. This amplifies the error voltage generated
from the discriminator's output. The bandwidth
of the error
voltage will be limited by the detector. The low pass filter will pass the DC component to drive the local oscillator. Thus
the auto tuning hold circuit not only keeps the tuner from drifting out of tune but facilitates tuning operation. Fig. 8
compares the operating characteristics of ordinary tuning, conventional AFC and Tuning Hold circuit operations.
(Vy
Hold
Indicator
Local Oscillator
Low Pass Filter
Fig. 7
Receiving
Frequency
Change width
in response to signal level.
Tuning Hold
ee ca
(Ideally Locked)
> RE
ae
Conventional AFC
-"
—___
3.
Dial Pointer Movement
Center of Station is Here.
Ordinary Tuning
(Receiving frequency and
dial movement
are coincide.)
Fig. 8
When the circuit is activated continuously, it is somewhat difficult to tune in precisely to a desired frequency. The auto
tuning hold circuit is automatically activated one moment after precise tuning is completed. When detuning the tuner,
the tuning hold circuit will be deactivated. The operational feeling of tuning is therefore that of ordinary tuning.
T-X5
No. 2468
_5_
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