A low cost comb generator

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 SRDs are expensive, hard to find and very fragile. The above circuit and figure show one low-cost design that replace the SRD with a PIN diode. Performance is somewhat compromised, but still meets the requirements of many applications. The above photo shows an implementation using PCB. However it can be built using a breadboard, provided that lead length is kept to a minimum.

Theory of operation

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 the photos below. Note that the actual output is 10dB higher because a 10dB attenuator is inserted (see above photo). 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.

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.