COUNTERS 



accuracies of ±10 parts per million, and the latter of 2 or 3 parts. Tuning 

 fork frequencies are commonly in the high audio range; quartz crystals of 

 the most familiar variety are intended for radio-frequency work and 

 oscillate at upwards of 100 kc/s, but special low-frequency crystals are 

 obtainable which generate oscillations in the range 4-20 kc/s. A single 

 accurately known source of frequency is valuable but not very convenient; 

 usually the utility of the apparatus is extended by using the master frequency 

 to control a chain of frequency-divider circuits, so that timing pips are 

 available with a range of intervals — Figure 41.5 may be regarded as typical. 



Figure 41.5 



Timer units along these lines, using multi-vibrator frequency dividers, have 

 been described by Attew^ and by Dickinson^. The use of phantastron 

 dividers is discussed by Moody and Williams'. 



A disadvantage of the frequency-divider circuit is the possibility of the 

 division ratio 'slipping'. For extreme reliability the oscillator may be 

 followed by a chain of 'counter' circuits. As counters are to be the subject 

 of the next section it is not proposed to discuss them here, beyond mentioning 

 that these devices count either on a 'scale of two' or on a 'scale of ten'. 

 The reader will appreciate that whilst an oscillator followed by a-i-10 

 frequency divider, and an oscillator followed by a scale of ten counter, 

 effectively do the same thing, the counter and the divider circuit are quite 

 different. Whilst the latter is an oscillator synchronized to a drive at its 

 tenth harmonic, the former is a circuit which is quite passive until the next 

 drive pulse arrives. Timers employing frequency division may slip and 

 make small mistakes which go unnoticed for a time. Timers employing 

 counters either work properly or make errors so gross that they are recog- 

 nizable at once. An excellent timer unit employing Dekatron counter 

 circuits and a 10 kc/s crystal has been published by Kay*. The author has 

 built this circuit and found it very easy to get going; he has no hesitation 

 in recommending it strongly. 



COUNTERS 



For low counting rates, up to about 10 per second, the 'Post Office Counter' 

 for registering subscribers' calls at the exchange is suitable and convenient 



43 645 



