Filter
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Theory of Operation
1780R-Series Service Manual
3–13
Only one signal is switched through the saturating transistor at a time, although
Flat, Luminance, and Chroma may be time multiplexed.
The luminance filter has a 1 MHz bandwidth and a transmission zero at the
subcarrier frequency. It has been optimized for best transient response. This filter
has performance identical to the luminance filter used in the Tektronix 1480 and
1485 Waveform Monitors.
The chrominance filter is a bandpass filter centered on the chrominance
subcarrier. It is a Lerner-type filter, exhibiting arithmetic symmetry and linear
phase across its bandwidth for excellent phase transient response.
The low-pass filter is an eighth order Bessel-type filter consisting of four second
order active sections.
The differentiated step filter is a high-pass filter that acts on the output of the
luminance filter. It is a differentiator, or slope detector. The signal from the
luminance filter has uniform luminance rise times, so that the differentiated step
filter can generate pulses mostly dependent on step amplitude.
The Auxiliary Input, when combined with the Auxiliary Output, provides a
means for connecting an external filter to the 1780R-Series. This is a 75 W input
with 1.5 dB gain to compensate for loss in the applied filter. Of course, AUX IN
can also be used as another input to the 1780R-Series.
Difference Amplifier. U935 is a high bandwidth operational amplifier connected as
a difference amplifier with a gain of 2. This is where the subtraction occurs for
the front-panel A-B1, B1-B2, or B2-B3 selections. U935 drives all of the internal
filter circuits. Q1032 is a current source that inserts an offset into the Pix Mon
video for an intensified display during line select operation. Q937 performs the
same function for the picture monitor display on the Vectorscope CRT.
Filters and Filter Switching. The emitters of all the switches are at 0 volts DC. All
of the switching transistors, except the selected filter switch, have –1.7 volts on
the base to keep them off. When a switch is off, the diode connected to the
switching transistor collector is biased on to provide a low impedance signal path
to ground. A TTL high at the base resistor network turns on the transistor to
provide a low impedance signal path from collector to emitter.
Luminance Filter. The filtered signal is picked off by transistors Q847 and Q939
for C/Y measurements and picture monitor display. The picture monitor display
also has the line select strobe added to it through Q937.
Low-Pass Filter. This filter is an eighth order Bessel-type filter with four second
order active sections using operational amplifiers in U1046. Bandwidth is about
300 kHz, and transient response is optimized by adjusting C951.
Circuit Theory