Tektronix 1720 User Manual

Theory of Operation
1720/1721
4–7
Burst
demod
Phase
control
Burst
gate
Phase
detector
Loop filter
Subcarrier
oscillator
Burst ref
VCO
Burst amp
Figure 4-2: Block diagram of the Phase Lock Loop.
The heart of the Subcarrier Regenerator is a phase lock loop. See Figure 4-2.
The Subcarrier Oscillator (VCXO) is a voltage-controlled oscillator that freeruns
near the reference subcarrier frequency. The Burst Demodulator is a mixer that
detects phase differences between the reference input and the Subcarrier
Oscillator during burst time. The difference output is an error signal proportion-
al to the phase difference detected.
The error signal drives the Phase Lock Control, which is a low-pass filter to
remove high-frequency ac components in the error signal. The filter has two
bandwidths, a wide one to search for the unlocked signals, and a narrow one to
maintain stable phase lock once the signal has been captured.
The Phase Lock Control loop filter completes the loop by controlling the
Subcarrier Oscillator. If the input reference changes, the oscillator will follow.
For the 1721, the PAL Phase Lock Control block samples every burst.
U889 is a Quad CMOS switch that selects the appropriate input signal for the
Sync Separator and the Subcarrier Regenerator. In normal operation, both the
sync and subcarrier sources follow the front-panel Reference switch and are
driven by A or B when Internal is selected, or by the EXT REF input when
External is selected. When J696 is in the 2 and 3 position, the Sync Separator is
always driven by the A or B input, even when External Reference is selected.
This mode should be used if CW Subcarrier is used as the Reference for a
composite video input signal. It will ensure that the clamp pulses are synchro-
nous with the incoming video.
The two remaining switches in U889 allow the reference signal to pass to the
Subcarrier Regenerator only during burst time. In NTSC, with W986 installed,
the two switches both close during every back porch, allowing every burst to
pass to the two phase detectors. In PAL, with W987 installed and U986
removed, the Loop Phase Detector, U646, receives every burst and the Quadra-
ture Phase Detector, U854, receives every other burst.
Reference Switch

Tektronix 1720 Specifications

General

Brand	Tektronix
Model	1720
Category	Measuring Instruments
Language	English