USER’S MANUAL__________________________________________________________________
36 __________________________________________________________________ M211322EN-D
The linear amplifier is fed from the other half of the band pass filter split.
It may be preceded by a gain control circuit (IAGC), which adjusts the
instantaneous signal strength to fall within the limited dynamic range of
the linear amplifier. The amplitude and phase characteristics of the IAGC
attenuator must be calibrated so that the "I" and "Q" samples can be
corrected during processing.
The IF output from the linear amplifier is applied to a pair of mixers that
produce "I" and "Q". The mixer pair must have very symmetric phase and
gain characteristics. Each mixer must be supplied with an accurate 0
degree and 90 degree version of the COHO. The later is usually obtained
by sampling a portion of the transmitted pulse, and then phase locking a
COHO that continues to "ring" afterward. Phase locked COHOs of this sort
can be very troublesome. They often fail to lock properly, drift with age,
and fail to maintain coherence over the full unambiguous range.
The transmit burst that locks the COHO is also used by the AFC loop. The
AFC relies on a FM discriminator and low-pass filter to produce a
correction voltage that maintains a constant difference between the
magnetron frequency and the reference STALO frequency. The AFC
circuit is often troublesome to set and maintain. Also, since it operates
continuously, small phase errors are continually being introduced within
each coherent processing interval.
For the RVP900 digital receiver (see Figure 10 on page 35, bottom
portion), the only old parts that still remain are the microwave STALO
oscillator and the mixer that produces the transmit burst. The burst pulse
and the analog IF waveform are cabled directly into the IFDR on SMA
coax cables. These cables constitute the complete interface to the radar's
internal signals; no other connections are required within the receiver
cabinet.
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