In this post I would like to share our experience in making comb generators. Comb generator is basically a periodic sharp pulse generator. There are a number of methods to generate pulses, such as using step recovery diode (SRD), tunnel diode, non linear transmission line (NLTL), etc. However, according to this paper we found another simple and clever trick to generate short pulses, i.e. by exploiting the asynchronous reset feature of D Flip-Flop. We have made 3 types of D Flip-Flop based pulse generators using different logic families:
- 74LS74. Low power Schottky TTL device. It has typically maximum operating clock frequency at 25 MHz.
- 74ACT74. Advanced CMOS with TTL compatibility device. The maximum operating frequency is at 210 MHz.
- MC100EP31. Emitter Coupled Logic (ECL) device. Maximum frequency is 3 GHz.
The device’s maximum frequency is related to its rise time and fall time capability. Higher maximum frequency means faster rise/fall time. As for our pulse generator, faster edge transition will result in shorter pulse. Longest to shortest generated pulse is in order: 74LS74 –> 74ACT74 –> MC100EP31.
All three pulses shown above are generated periodically at certain frequencies. So when viewed from frequency domain measured using spectrum analyzer, they look like this:
Frequency spectrum of 74LS74 and 74ACT74 may look the same. However if we extend the frequency measurement range, it will be evident that the spectrum content of 74ACT74 is higher than that of 74LS74. This is because the pulse of 74ACT74 is shorter than that of 74LS74.
More details about the circuit diagram, how it works, and also deeper analysis can be found in our paper:
- Yoppy, Haryo Dwi Prananto. “Comb Generator for Verification of Terminal Disturbance Voltages Measurement According to CISPR 14-1”. Annual Meeting on Testing and Quality (AMTeQ), 2015, pp. 143-151.
- Yoppy, M. Imam Sudrajat. “Design of Radiated Comb Generator Using Single-Ended Positive Emitter Coupled Logic (PECL) D Flip-Flop”. Progress in Electromagnetics Research Letters, Vol. 70, September 2017, pp. 67-73.