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T-X5
JVC
has already succeeded in developing the high performance PLL (Phase Locked
Loop) discriminator that is scarcely
affected by variations in environment or in aging. The tuner incorporating the PLL discriminator has been made available
to the general! public as a proposal of a course for development of tuner circuit. The recent development of the PTL (Phase
Tracking
Loop)
discriminator
pursues
this prescribed
course.
The
PTL
discriminator,
incorporated
within the T-X5,
functions as described below.
Phase
F
90° Phase
Phase T.
Shifter
Filter
Recovered Audio
FM
IF Signal
Fig. 1
The loop formed by the phase comparator, loop filter and phase tracking filter is called the PHASE TRACKING
LOOP
(PTL).
The phase-tracking response of the loop to the input signal is improved as the loop gain is raised. When the delay of the
response is reduced ultimately to zero, the circuit becomes the Phase Locked Loop (PLL).
As the loop gain is reduced, on the contrary, the delay of the response to phase changes of the input signal increases. When
the phase tracking loop becomes insensitive to phase changes, the circuit operates as the Quadrature discriminator. These
different configurations are shown in Fig. 2.
Lock Range
IF Input
Output
a
ge
Voltage C. OSC.
]
Capture
Range
Phase Comp.
Loop Filter
Tracking
(PTL)
Range
Phase T. Filter
'Quadrature )
[nd
Tuning
Point
;
y
ore
Fig. 2
Thus the PTL discriminator, taking the middle of the road between the PLL and Quadrature, can be converted to any of
the discriminator by varying its loop gain.
And
the PTL has a advantage other than the PLL discriminator, which
is no oscillator incorporated, so that no noise
source
is included
in the loop and no affection of its harmonics to the preceding stage of the tuner. The level of noise
entering the phase comparator will be reduced by making the resonance curve of the tracking filter steep. Stable and high
performance can be obtained by optimizing the loop gain.
The features of the PTL discriminator summarized below:
*High signal to noise ratio and low distortion.
Since the linearity of discrimination
(differential
gain characteristics)
is determined
by the loop response, this
discriminator can operate with high S/N and low distortion
in a wide range merely because its tracking filter is
suitably designed.
*Stable FM demodulation
The feedback loop secures stable operation over a long period, maintaining a well tracking condition by absorbing
variations due to changing temperature and aging.
*Signal to noise ratio improved for weak input
The FM threshold
level is improved
because of its feedback operation, and the S/N is high even when the input
signal is weak.
*Excellent interference rejection
The combination of the loop filter and tracking filter is excellent in eliminating interference.
*Excellent AM suppression
The capability of AM suppression is proportional to the response of loop. AM components generated by interference
or multipath distortion will be suppressed.
No. 2468
=
oe
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