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 NXP Semiconductors

  • Crystal Controlled Time Base Generator

  • Created: Nov 26, 2014

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This project is a time base generator that provides high precision and stable output that is used to count the frequency with high resolution. The device uses a crystal oscillator, 74HC4060D and 74HC4017D ICs that produce a sawtooth waves. The 74HCT4060 is 4-stage ripple-carry counter/dividers and oscillators with three oscillator terminals (RS, RTC and CTC), ten buffered outputs (Q3 to Q9 and Q11 to Q13) and an overriding asynchronous master reset (MR). In addition, the 74HC4017D IC is a 5‑stage Johnson decade counter with 10 decoded outputs, two clock inputs and an overriding asynchronous master reset input.

A digital frequency counter needs a time-base generator to count the frequency with high resolution. Normally, a crystal-based oscillator with divider IC chain or a similar circuit in the form of an ASIC (application-specific IC) is used for time-base generation. The circuit is an accurate time-base generation using the readily available 3.5795MHz crystal commonly used in telecommunication equipment. The 3.5795MHz crystal is used in conjunction with a 74HC4060D based crystal oscillator-cum-divider (IC1). The crystal frequency is divided by 512 by IC1, which is further divided by 7 by 74HC4017D (IC2). IC2 is reset as soon as its Q7 output goes high. Thus the crystal frequency has output of around 998.8 Hz. This frequency can be trimmed to exactly 1 kHz with the help of trimmer capacitor VC1 as shown in the figure. The 1kHz signal can be further divided using decade counters to generate the required time period. To generate required gate for use in a frequency counter circuit, the final oscillator output needs to be followed by a toggle flip-flop. When applied to a toggle flip-flop, it will generate gates with 1-sec 'on' period and 1-sec 'off' period.

Crystal controlled time base generator is an electronic circuit that generates a varying voltage to produce a particular waveform. Time base generators are commonly used in radar system to determine range to a target, by comparing the current location along the time base to the time of arrival of radio echoes. Moreover, the device is also used in analog television systems that has two time bases, one for deflecting the beam horizontally in a rapid movement, and another pulling it down the screen 60 times per second.Furthermore, oscilloscopes often have several time bases, but these may be more flexible function generators that are able to produce many waveforms as well as a simple time base.