Circuit Notebook 84 - A Field Pulse for 'The Simplest Pattern Generator'

The CMOS 4060B (14 stage ripple-carry binary counter/divider) has been the basis of a number of waveform generator circuits. A version described by Terry Bond [1] in CQ-TV 138, page 34, as 'The Simplest Pattern Generator' has a minimum number of components and generates a TV line staircase waveform which is quite adequate for a quick check that ATV kit is working. A version of this circuit by John Cronk GW3MEO [2] was built into his 23cms ATV transceiver to provide a local signal source and was described by him in CQ-TV 181, page 74.

In essence, these circuits are the same as that shown in the top half of Fig.1. The first stages of the 4060 are used to make a 4MHz oscillator using a crystal or resonator. Outputs are available from each of the divider stages, Q3 to Q13. By taking the outputs from Q5, Q6 and Q7 and 'summing' these through resistors having a 4-2-1 weighting, a staircase waveform is generated. The lowest step provides an effective line sync pulse which is followed by seven equal steps up to peak white. The waveform is shown in Fig.2.

However, this signal does not contain a field synchronizing pulse, which may make it unsuitable for some purposes. The lower half of Fig.1. shows a simple field pulse generator which, when connected to the staircase generator, as shown, removes the pattern for about 3 lines forming a single broad pulse, as shown in Fig.3.

The field pulse generator consists of a free-running (50Hz or 60Hz) oscillator formed by two NAND gates (4011) followed by a pulse narrower circuit using the remaining two NAND gates and a transistor to couple to the staircase generator. RV1 is adjusted so that the bottom of the broad pulse and the line sync pulses are aligned. RV2 is adjusted to provide either a 50Hz or 60Hz field frequency.

This arrangement is an improvement on the original circuit and increases its usefulness. It is not suitable for feeding systems which require a full field sync signal to be present.

The sampling 'noise' on the waveforms shown in Figs 2 & 3 is due to using an inexpensive P.C. digital oscilloscope adaptor and is not present on the waveform. Honest!

Fifures

Fig.1. Staircase generator

Fig.2. Line waveform

Fig.3. Field waveform

References

[1] The Simplest Pattern Generator, Terry Bond, CQ-TV 138, p.34

[2] 23cms TV Transceiver, John Cronk GW3MEO, CQ-TV 181, p.74

[3] Digital-to-Analogue Converters, Don Lancaster, CMOS Cookbook, p.288

[4] Astable circuits using CMOS, gates Don Lancaster, CMOS Cookbook, p.225