From Hobby to Professional

A low cost comb generator for frequency synthesis applicartions

—

by

Alfred But
in

Introduction

A comb generator is a simple but useful piece of equipment. Most comb-generators rely on a Step-recovery-diode (SRD) to create very narrow pulses which are rich in harmonics. Unfortunately step recovery diodes are expensive and very fragile (easily damaged by ESD). Now they are hard to find. This post shows one low-cost alternative design that is easy to build and needs no SRD. Performance is somewhat compromised, but still meets the requirements of many applications.

Fig.1 Schematics of the comb generator
Fig.2 A 20MHz comb gen.

Theory of Operation

Refer to fig.1. U501A and associated components form an oscillator which is buffered by U501B. U501C and U501D is a power amplifier to drive D501 which is a PIN diode. L501, R503~506 provide DC return path.

This circuit is first-time-right and needs no tweaking, provided the following points are observed:

  1. Crystal (Y501) must be fundamental type; overtone crystals will not work. Other frequency crystals can be used, as long as it is fundamental. I have tried 50 MHz (highest fundamental crystal in my junk box) and it works just fine.

2. U501 is 74AC04 for best performance. It can be replaced with a 74HC04 with slightly loss in performance. Other logic families are not suitable.

Test Results

Refer to fig.3 and fig.4 for test results. Note that the actual output is 10dB higher because a 10dB attenuator is inserted (see fig.2). The power at 200MHz which is the 10th harmonics is -10.3dBm which is a highly useful level. The power of the 20th harmonics (400 MHz) is -14.8dBm, which is still a useful level.

There is a notch at 260 MHz (13th harmonics). This can be considered as an artifact. For 20th or higher harmonics, power drops off quickly. Anyway a PIN diode in no match to a SRD in terms of highest useful harmonics, but it is only a fraction of the cost.

Fig.3 Comb output up to 10th harmonics
Fig.4 Comb output up to 20th harmonics

Applications

A comb-gen is a useful frequency synthesizer building block. It provides the simplest way to obtain VHF/UHF/SHF signal sources with low phase noise.

Use band-pass filters to select the desired harmonics and amplify it to the needed level.

I will write some posts on the application. Do you have any project that needs high-order frequency multiplication?

Comments

Leave a comment