Need of Sub nanosecond pulse generating circuit using RF transistors

Hi all,
I am now involved in a COMB GENERATOR design. It is going to used as a reference radiator. So anyone kindly provide me a circuit of a sub-nanosecond pulse generator using RF transistors along with the values of the components used. Eagerly waiting for reply...
Thank you

karthikselvan said:

Hi all,
I am now involved in a COMB GENERATOR design. It is going to used as a reference radiator. So anyone kindly provide me a circuit of a sub-nanosecond pulse generator using RF transistors along with the values of the components used. Eagerly waiting for reply...
Thank you

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As "sub-nanosecond" means that the frequency region is above 1 GHz, you will need a really good "RF" transistor capable to operate at several GHz.

Comb generators, on the other hand, are typically not used in pulse mode: they are connected to RF signal generator delivering quite high power (depends on the comb device specifications, 13 to 27 dBm). A comb generator typically utilizes a step-recovery varactor diode that can generate harmonics.

Find a good RF power transistor capable to generate ~100 mW at > 1 GHz, some recent MMICs from Mini-Circuits, Hittite and RFMD are good for it. Pulsing such power is a bit tricky, I would recommend to search for UWB circuits studied in many papers recently. Many years ago I used a 74F04 logic gates as good harmonic generators with a comb spectrum extended over 1 GHz.

If you need a reference radiator, I can recommend to use noise radiators. I made many for free by using the e-b diodes of a RF SMD transistor as avalanche-diode noise generators mounted in a dipole or a waveguide. Their typical excess noise ratio (ENR) is >30 dB, up to ~ 13 GHz, and can be easily pulsed, too. Instead of a comb-line spectrum they radiate a wideband continuous noise spectrum.

Hi,
Thanks for the reply. Instead of continuous noise emitter, i need a pulse train having <1ns pulse width with amplitude of 20 Volts at 50 ohms load(antenna). Could you help me on that.

What's the intended repetition frequency?

For 20V level, I would primarly think of a transmission line pulse generator with avalanche transistor generator. I have achieved rise- and fall times of 150 - 200 ps with selected exemplars of Motorola 2N708.

20 volts is a lot. You might get that with a big step recovery diode. But, I agree, it would be easy to do that with a transmission line charged to 20 v that was physically 300 pS long. If you used two fets to connect the line, one to charge it, and the other to discharge it...you might get up to 10 MHz of rep rate, maybe higher if you really smoked it with some sort of GaN power fets.

Please guide me a suitable RF transistor for my COMB GENERATOR design. I'm going to use RF transistor as a switch, which has to accept a narrow pulses of width 1-12 ns and with amplitude of around 1-5V. On occurance of these pulses, the RF transistor have to switch ON and have to produce same pulse at 50 ohms output but with amplitude of around 20-25V.

On whole, i need a amplitude level shifter from 1-5V to 20-25V, without change in pulse width at 50 ohms output.

Can i use RF transistor as a switch for this application. If so then please suggest me a suitable one and the circuit for using RF transistor as a switch for this application.

By requesting an output stage that reproduces the input pulse width, you are narrowing down the range of available solutions considerably. Microwave transistors are not primarly designed for switching applications and rarely specified for it. The straightforward way from a RF engineering viewpoint would be to refer to a wideband amplifier. The below linked 1 - 1000 MHz 6 W amplifier is at least placed in the intended range.



Strictly spoken, your specification is incomplete, because you didn't tell a rise time respectively bandwidth specification. The comb generator application suggests that the amplifier should be rather specified in frequency than in time domain.