4-13
TR RECOVERY TIME
The time required for tr recovery is determined by the time taken by the tr switch (tube) to deionize
after each transmitter pulse. It is usually defined as the time required for the receiver to return to within 6
dB of normal sensitivity after the end of the transmitter pulse. However, some manufacturers use the time
required for the sensitivity to return to within 3 dB of normal sensitivity. Tr recovery time is a factor that
limits the minimum range of a radar because the radar receiver is unable to receive until the tr switch is
deionized. In various radars, the recovery time may differ from less than 1 microsecond to about 20
microseconds.
The primary function of the tr switch is to protect the sensitive crystal detectors from the powerful
transmitter pulse. Even the best tr switches allow some power to leak through; but when the switch is
functioning properly, leakage power is so small that it does not damage the crystal. However, the useful
life of a tr tube is limited because the amount of leakage to the receiver increases with use.
To ensure efficient performance, some technicians make a policy of replacing the tr tube after a
certain number of hours of use. A better practice is to measure the tr recovery time at frequent intervals
and make a graph or chart. A graph or chart will immediately disclose any change in performance. Figure
4-10 shows how the recovery time is correlated with leakage power. Note that the end of the useful life of
the tr tube is indicated by an increase in recovery time.
Figure 4-10.—Tr recovery time versus leakage power.
This method of checking the condition of a tr tube is reliable because recovery time increases before
leakage power becomes excessive. In practice, a tr tube is replaced when any sharp increase in recovery
time becomes apparent.
Ambient temperature also has an effect on recovery time. The colder a tr tube, the greater its
recovery time. When tests are conducted under widely varying temperature conditions, this effect must be
considered.
One method you can use in testing a tr tube is to measure the KEEP-ALIVE current. This current
keeps the tr tube partially ionized, which makes the firing more instantaneous and thus helps protect the
