Heterodyning of 3Y.A.

Radio Record, Volume II, Issue 6, 24 August 1928, Page 23

Heterodyning of 3Y.A.

HE Americans are experimenting with simultaneous brosdeast from several different stations at the same frequency (or on the same wave length). This has decided advantages in that much duplication of -programmes is saved, but it has a disadvantage in that the slightest variation i: the number of frequencies sent out by any station cause heterodyning and mushiness. AN example of heterodyning occirred recently when 3YA had to dtop two metres in wave-length (or, more accurately, increase its frequency six kilocycles per sec.) to avoid interference by 2GB. The normal: frequency at which 3YA should transmit is 98¢@ kilocycles per sec., while 2GB should transmit at 950 k.es, thus leaving a margin of 30 K.cs. To heterodyne at the proximity of this wave-length the stations must differ by 5 kilocycles (a note of 5000 frequencies) or less.’ It is evident, therefore, that one of these stations must be off its wave-length. Frequent testing at 3YA would indieate that 2GB is at fault, but there is hardly yet time to definitely determine this. If both of these stations were erystal controlled, variation of this dimension would be impossible. Herein lies the value of a erystal controlled transmitter. Without going into details, the principle of this type of oscillator consists in a smooth crystal of definite dimensions, within two metal plates, which are connected to a suitable circuit. This erystal has a natural frequency determined by its physical dimensions. These frequencies are true to within .00001 per cent. Arranged in a circuit, the minimum variation is about .005 per cent., while the maximum is .2 per cent. {[t can thus be seen that were these two stations erystal controlled their maximum variation would be 1.9 k.e. in the case of 2GB, and 1.96 k.ec. in case of 3YA, although in practice this variatiun would be much lower. Come. pare this with the 25 k.c. variation wich has led to heterodyning.