Radar System Engineering

SEC. 16.19] DELA Y-LINE SIGNAL CIRCUITS 673
originating in the radar receiver. The high cost of delay-line input
power makes necessary the careful conservation of output signal. Maximum
signal-t~noise ratio is obtained with a short lead from the delay line
to the first amplifier grid. If the amplifier cannot be located within a few
inches of the delay line, a preamplifier is necessary. Low-noise input
circuits can profitably be used, but critically adjusted circuits should be
avoided, if possible, because of the difficulty of maintaining balance
between the delayed and undelayed channels.
The bandwidth of the amplifier must be sufficient to avoid serious
impairment of system resolution and the loss of system signal-to-noise. 1
In addition, the bandwidth of the delayed and undelayed channels must
be sufficient to make the interchannel balance noncritical. Bandpass
unbalance will have relatively little influence upon pulse shape if the
bandwidth of each split channel is perhaps twice that of the over-all
system.
Even though the bandwidth of these channels is large, it may still
be necessary to make an approximate compensation for two types of
unbalance. The smaller gain in the undelayed channel would ordinarily
result in a greater bandwidth. This channel should therefore be narrowed
to match the other by the addition of capacity to ground in the
low-level stages. The second type of unbalance arises from the frequency
dependence of attenuation -in the delay-line medium. The square-law
factor generally predominates. Compensation can be obtained by inserting
in the delayed channel an LC-circuit tuned to a frequency much higher
than the carrier frequency.
Amplitude cancellation to 1 per cent implies an extraordinary degree
of linearity in the two channels. The carrier level at the canceling diodes
must be well above the region of square-law diode response. The final i-f
stages must be conservatively operated. A first-order correction for
residual nonlinearity will result if the last two i-f stages of the two
channels are identical in every respect.
Gain adjustments should be made in low-level stages to avoid the
introduction of nonlinearity. A wider range of adjustment will be
required in the delayed channel because of possible variations in delay-line
attenuation. A first-order balance of transconductance changes due to
heater voltage fluctuations will result if the same number of amplifying
tubes is used in each channel. Pentodes with high mutual conductance
frequently exhibit small sudden changes in gain as a result of either heating
or vibration. The gains of the individual channels can be stabilized
through feedback of bias derived by detection of the carrier level. A
1In coherent MTI, the signal-to-noise ratio depends upon the over-all system
bandwidth and not in any special sense upon the bandwidth up to the detector.