CRYSTAL OSCILLATORS

Evening Post, Volume CX, Issue 104, 29 October 1925, Page 14

CRYSTAL OSCILLATORS

A NEW USE FOB QUARTZ.

A considerable amount of interest has" been aroused recently, especially in amateur circles, by the discovery that certain crystals can be used in conjunction with valves for tho generation of oscillations and fov the accurate maintenance of oscillations of dcu'nito frequency. The two crystals most mentioned arc tourmaline, which is expensive, and rock quartz (or rock crystal), which is not. These crystals have nothing in common with those, usoil ar; detectors; the ];tttor are slightly conductive, whereas tho cryrtliils under discussion -arc very high-grade insulators. The crystals do not themselves generate oscillations.

it has, long been known that such crystals, when mechanically strained in certain ways, display electric charges, whifh in sotno instances arc of very liigli poie-uliiil. Like niaiiy oilier physical, peculiarities, this phenomenon is rcvoi'sibli!, and tho application of electrical potentials to such crystals in tin; proper way causes them tn change, their form.

CJuarU is ;l rigid ;ind exceedingly ol;iati'; snaa't.-ini-.u, ho that the iipplicunoii nl rv potential ty It produces only a very small change-in magnitude, and its recovery, when the charge is removed, is very rapid. The firaalior the crystal, the more rapid the recovery. It mn'y Ijc easily realised, therefore, that if the change of form duu to the application of potentiii.l resiUts in reducing the potential, and the consequont recovery of form can be made to restore the potential, the proccna will become, in certain conditions, periodic; and by making the crystal small enough, a very liigli-i'i\quency change of form can be brought about. Owing to the physical characteristic of quartz, tho frequency obtained with a %'tven. crystal is remarkably constant.

In a rii'li'i circuit, a quartz crystal i:-: UHOf 1 in n. very simple way. The fip|i;ini)us is simply a valve with a tuned pl;ite-cir<-uil. bill' with neither coil nor condenser in the grid circuit. These arc rcphicvl by a pair of small metal pilules, insulated from nnc nnol.her. between ivhich tlic quartz crystr>l can l>c placed; .it rests upon oneRiid is covered, without being tonche<l, by'the other. By careful ttmiEg

of the plate circuit, an adjustment can :be found at which the system goes' into persistent oscillation of fixed frequency. This frequency is determined by the height of tho crystal measured vertically between the -plates. A crystal one millimetre thick causes oscillations of a frequency of 3,000,000 _ cycles ap : I proximately, corresponding to'a wave length of 100 metres. ' A crystal one centimetre thick will give a wave of approximately 1000 metres. By using a block-shaped crystal of which the three measurements are different, three different fundamental wave lengths are obtained.- „v .

'; An important feature of the crystal ose'illator'is that the wave generated is a very impure one; that is, the fundamental is accompanied by a number of harmonics, any_ one of which can be picked out. Consequently" a single crystal, with its triple fundamental, can be embodied in an apparatus which will enable a wave meter to be very iuUy calibrated. The fundamental of 1000 metres, for example, gives the following harmonics (to'the nearest metre): 500, 333, 250, 167, 143/125, 111, 100, 01, 83, and so 0n.,;:.™-.' k,-,.--:-.y^

Outside the "testing laboratory or the experimenting room, however, the crystal control has already found practical application by being used to provide constant frequency in the carrier wave of broadcast transmission. It cannot be applied to the powerful apparatus of the transmitter itself, but can readily be used to coutrol .a, master oscillation which in turn controls the power output.