AR 10013130, Operating And Service Manual

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040609
Delay, RF Rise and Fall time and the Pulse Input
The beam is turned on and off in response to a TTL-level input (typically from an external Pulse
Generator) applied to the Pulse Input Connector. A positive level (logical 1) turns on the beam. If RF
had been applied to the TWTA input prior to the positive (logical 1) TTL input, the RF output would
reach 90% of its final value within 300ns (Delay) with an observed RF Rise time of up to 30ns (10% to
90%). (See Figure 1.)
300ns
max.
300ns
max.
TTL Input
t,t < 30ns
r f
RF Input
RF Output
30 µs
max
Figure1: CW RF Input
After this initial delay in enabling the RF output, the RF output level will respond to the level of the RF
input. As with a CW amplifier, the subsequent RF rise and fall times are inversely related to the RF
bandwidth of the specific TWTA, with typical rise and fall times in the low- or sub-nanosecond range.
If the RF input remains and the TTL level goes low (logical 0), then the RF output would reach 10% of its
prior "on" value within 300ns (Delay). The observed RF Fall time would be less than 30ns (10 to 90%).
(See Figure 1.) Using the TTL input to modulate the RF output may result in some small distortion of the
output pulse width (usually shrinkage), as compared to the TTL-level input pulse width, and therefore is
not recommended for producing pulses lasting less than 0.2 microseconds, or for applications where the
pulse width must be accurately preserved. Alternately, the RF input can be turned off prior to the end of
the TTL pulse to obtain a fast and well-defined RF fall time.
To obtain well-defined RF timing while minimizing power consumption, the RF and TTL-level inputs
should be timed as shown in Figure 2:
0.3-1µs
TTL Input
RF Input
RF Output
(short delay between RF input and RF output typically a few ns)
(recommended range) 0.3-1µs
Figure 2: Pulse RF Input