Maintenance Manual

MAINTENANCE MANUAL
RT-1601 RECEIVER-TRANSMITTER
815 Broadhollow Road 631-755-7000
Farmingdale, N.Y. 11735 FAX [phone]
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where selection of either the modulator short pulse (0.2 usec) or the modulator long pulse (2.35
usec) is selected.
Pulse width selection is made depending on the operation mode selected on the radar indicator.
Modulator long pulse is selected for modes using a 200-Hz trigger; modulator short pulse is
selected for modes using an 1536 Hz trigger. With either modulator pulse, a negative-going, 330-
volt, 110-ampere pulse is applied to the pulse transformer. The pulse transformer supplies the
magnetron with a 5.5 kV, 4.5 A, negative-going pulse that causes the magnetron to oscillate at its
9375 MHz resonant frequency. A 10-kW nominal peak power RF output pulse is generated by the
magnetron and is directed through the four-port, E-plane circulator to the radar antenna.
B. Receiver Function
The R-T unit employs a dual-conversion, superheterodyne receiver. Signals received by the radar
antenna are routed through the four-port, E-plane circulator, and the TR limiter to a alanced mixer.
A TR limiter is a microwave switch used to prevent damage to the receiver crystals, from the
magnetron oscillator high power microwave pulses or from other radars in the vicinity. The
balanced mixer combines the echo signal with a signal from the first local oscillator (operating at
9401.3 MHz) to produce a difference frequency of 26.3-MHz. After amplification in the preamplifier
stage, the 26.3-MHz signal is mixed with a 36.6-MHz signal from the second local oscillator to
produce a 10.3-MHz I-f signal.
A bandwidth switch selects either a 2-MHz bandwidth for search-mode operation or a 0.5-MHz
bandwidth for weather-mode operation. The 10.3-MHz logarithmic-gain amplifier following the
bandwidth switch provides for a great variance (approximately 50 dB) in signal levels usable by the
R-T unit. Following the logarithmic-gain amplifier, is the video detector and buffer. The detected,
buffered video (in the search 1 mode) is acted upon by the FTC circuit (fast time constant) and it is
used to remove noise caused by the sea clutter. In the A/D converter following the FTC circuit the
amplitude of the detected video signal is compared to three preset dc thresholds. A two-bit logic
code tells the radar indicator the highest threshold level exceeded by the echo return. The two-bit
codes for the four thresholds, in increasing level, are 00, 01, 11, and 10.
C. AFC Function
After the system trigger, while the magnetron is oscillating, the first local oscillator frequency must
be adjusted to ensure a correct i-f echo-return signal. The AFC-AGC trigger circuit turns on the
AFC gate, and the I-f signal from the preamplifier is supplied to the AFC mixer. Using the 36.6-
MHz signal from the second local oscillator, the mixer converts the 26.3-MHz I-f signal to a 10.3
MHz second I-f. The discriminator following the 10.3-MHz buffer checks the I-f frequency. If the
second I-f is exactly 10.3-MHz the AFC circuits remain stabilized. However, if the second I-f is not
exactly 10.3 MHz, the discriminator produces a deviation voltage. The deviation voltage is
amplified, integrated, buffered, and applied to the varactor in the first local oscillator to adjust the
local oscillator frequency. An inverse-logarithmic-gain buffer amplifier is used between the
integrator and the first local oscillator to cancel the logarithmic voltage-to-frequency characteristic of