Radar System Engineering

SEC. 125] SECOND DETECTOR 449
This combined response curve has the same shape as that for a transitionally
coupled double-tuned circuit, so that the advantage of cascading
is preserved. In actual practice, a receiver that has a gain of 100 db with,
for example, six stages, will be approximately twice as wide if staggered
pairs are used as it would be if single-tuned circuits were used (Fig. 12.8).
The principle of stagger tuning can be carried further. Staggered
triples (three different frequencies) are in fairly common use, and staggered
n-uples are practicable for wideband amplifiers. The advantage
of stagger-tuned amplifiers lies in the fact that simple single-tuned circuits
are used throughout, a fact that makes for ease of manufacture and
servicing.
There are other schemes for obtaining large bandwidths. Feedback
pairsl and feedback chainsz have both been used fairly extensively.
More recently a scheme of stagger damping has been proposed. s
Before leaving the problem of i-f amplifier design, some mention of the
gain required will be necessary. To detect signals barely visible above
the noise it is necessary that the noise originating in the input circuit and
the first stage be amplified to a point where it is easily visible on the indicator.
How this gain will be split between the i-f and video amplifiers
depends upon such considerations as second-detector efficiency and the
complication involved in building high-gain video amplifiers. In most
receivers the i-f gain is enough to amplify noise to 1 or 2 volts, thus bringing
it into the linear region where diode detectors have maximum efficiency.
This requires a gain of around 106 times at the intermediate
frequency. Building an i-f amplifier with this gain while avoiding trouble
with regenerative feedback has been one of the most difficult problems in
radar receiver design. Ground current loops must be confined by careful
bypassing and grounding; power leads must be proper] y filtered; coils
must be wound and spaced intelligently; the shielding must be adequate.
Careful attention must be paid to all of these items if the over-all response
characteristic is to bear any resemblance to what is expected.
12.5. Second Detector.—The purpose of the second detector is to
produce a rectified voltage that is proportional to the amplitude of the
i-f waves. In most receivers it is important that this rectified voltage be
proportional to the first power of the i-f amplitude (linear detector).
However, in radar receivers higher powers are permissible so long as
reasonable efficiency is maintained. One of the simplest and most com-
I Bartelink et. al., “Flat Response Single Tuned I-f Amplifier,” GE Report, hfay 8,
1943.
2A. J. Ferguson, “The Theory of I-f Amplifier with Negative Feedback,” Canadian
National Research Council Radio Branch Report PRA-59.
s H. Wallman, “Stagger-damped Double-tuned Circuits,” RL Report h-o. 53s,
March 23, 1944,