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14. The Design of Delay Lines.
(i) General.- A considerable amount of work has been done on delay lines for R.D.F. purposes. On the whole our problems coincide with the R.D.F. problems but there are a few differences.
(a) Owing to the fact that there will be more than one tank used in the calculator the stability of the delay is of importance. In R.D.F. the delay is allowed to determine the recurrence frequency and the effects of variations in it are thereby eliminated.
(b) In R.D.F. it is required that the delayed signal should not differ from the undelayed by an error signal which is less than 60 dB (say) down on the signal proper. We are less difficult to please in this respect. We only require to be able to distinguish mark from space with a very high probability (e.g. at least 1-10-32). This requires a high signal to noise ratio, so far as the true random noise and the interference are concerned, but it does not require much as regards hum, frequency distortion and other factors producing unwanted signals of fairly constant amplitude.
Our main concerns then in designing a delay line will be:
(1) To ensure sufficient signal strength that noise does not cause serious effects.
(2) To eliminate or correct frequency and phase distortion sufficiently that we may correctly distinguish mark and space.
(3) To stabilise the delay to within say 0.2 pulse periods.
(4) To eliminate interference.
(5) To provide considerable storage capacity at small cost.
(6) To provide means for setting the crystals sufficiently nearly parallel.
The questions of noise and signal strength are treated in some detail in the following pages. It is found that there is plenty of power available unless either very long lines or very high frequencies are used. The elimination of interference is mainly a matter of shielding and is a very standard radio problem, which in our case is much less serious than usual. Various means have been found by the R.D.F. workers for setting the crystals. Some prefer to machine the whole delay line very accurately, others to provide means for moving the crystals through small angles, e.g. by bending the tank. All are satisfactory.
I list below a number of questions which must be answered in our design of delay lines. In order to fix ideas I have added the most probable answers in brackets after each question.
(1) What liquid should be used in the line? (Either mercury or a water-alcohol mixture.)
(2) Should we use a carrier? If so, of what frequency? (Yes, certainly use a carrier. Frequency should be about 10 Mc/s with water-alcohol mixture, but may be higher if desired when mercury is used).
(3) What should be the clock-pulse frequency? (1 Mc/s).
