LIMITING CONTROL

Radio Record, Volume I, Issue 47, 8 June 1928, Page 3

LIMITING CONTROL

COUPLE CONDENSERS, FUVERY effort is now made to reduce the number of controls on a set to a minimum, and this is particularly the case with the tuning controls, for it will be realised that the ideal set will only have one tuning control, and that so arranged that any particular reading on the dial will always correspond with a certain wavelength, so that the tuner can be calibrated, and any station instantly found. When a set consists merely of a detector valve, with or without a low frequency amplifier, there is no difficulty in limiting the tuning controls to one} in fact, no more is necessary; but when one or more stages of high frequency amplification is to be used the problem is different, for each stage has to be tuned. The only method of providing a single control is to couple the variable condensers together, so that they may be all turned by one knob at the same time. Bach condenser is connected in parallel with a coil to make the tuned circuit. It will be obvious that if the wave-length of each circuit is to be the same, and is to remain the same when all the condensers are rotated together, then the tuned circuits must

be exactly matched at the minimum setting of the condensers, and each condenser, when rotated, must increase the capacity of each circuit by the same amount for each degree of rotation, Matched Inductances. THIi first essential is to obtain coils with exactly matched inductances, for waye-length is made up of inductance and capacity, and as the inductance in each circuit is fixed, being almost all provided by the coil, and the capacity is the only variable, it will be obvious that it is first necessary to make the inductance of each coil identical, Assuming that each coil is matched, it will be seen that any difference in the wave-length of each circuit must be due to a difference in the capacity. The circuit coupled to the aerial will, for example, have some of the aerial capacity added to it,.and the other circuits will differ slightly, also due to the difference in the capacities of the Wiring, and slight discrepancies in the minimum capacities of each condenser. ‘The sum of all these capacities in each circuit is known as the minimum capacity, and is a non-variable quantity. To match the circuits, capacity should be added to those circuits with the least minimum capacity to make them equal to the one with the greatest amount. The capacities may be matched by placing small variable condensers in

>. parallel with each tuning condenser, except the aerial tuning condenser, and then adjusting each one until they synchronise aft every setting.