Electronics > RF, Microwave, Ham Radio

cheap design for a fast keying transmitter (microwave)

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CopperCone:
So in order to test the PlutoSDR POI, I think that what I need is something like an oscillator hooked up to a switch.

I wanted to key a transmitter at various intervals for various duty cycle (easy to control with a pulse generator), but I need some kind of fast RF switch.

I'm under the impression that I should be looking at PIN diodes but I am not sure. I would say a power handling capability of -40dBm is fine.

Are these a good choice?

I have a old spectrum analyzer that's way beyond economical repair, I thought I might be able to harvest some parts from it. I am however reluctant to take apart the attenuator. It has a YIG oscillator too. Are there any parts in a typical SA that could handle this requirement? (it goes to 3GHz)

I wanted to try keying duration's of 10nS, 100nS, 1uS, 10uS and 100uS. I would like to use a signal of at least 1GHz, but being able to do multiple frequencies up to 6GHz would be good to see how uniform it is.

Or, are there designs for some kind of oscillators that lock really quick, so I can just use a regular transistor to apply power? The problem with this is that I wouldent actually know how long it oscillates for, because I don't have a fast oscilloscope.. so the design would have to be well proven.

randsl:
What sort of frequency accuracy you need?

If you need really accurate frequency, I don't think switching the oscillator is not a good solution. Normally, if you use a PLL to generate a really accurate frequency, the lock in time is typically us or ms range. So switching the oscillator is not a solution if you need ns, 10 us pulse with very accurate frequency.

However, if you don't care about the frequency (and transients) and just need a RF pulse which is closer to a certain GHz value, you can use a single transistor oscillator with turning on/off with short pulses with the required duration. I've done it with 24 GHz frequency and 2-3 nS pulse on time.

CopperCone:
so these oscillators start up very fast?

I was worried because once you flip the switch, the oscillator needs some time to start up before its truly on... I was not sure what that would be.

I guess it needs to be within 30MHz or so.

Any particular circuits in mind?

randsl:
if you use a single transistor oscillator, with no varicap or DROs, yes it does, but the frequency is not accurate as it is a free running oscillator & it's subject to voltage transients during the start-up, die-down... There's a small delay typically in ns scale. but these ones are very fast ...

If you need to be within 30 MHz closer, then this is not an option I guess...

I'm unable to provide you the exact one which I used for 24 GHz as that one is related to a my workplace. However, it's vary similar to the topology used in this research paper : https://www.researchgate.net/publication/269040235_Multifrequency_Oscillator-Type_Active_Printed_Antenna_Using_Chaotic_Colpitts_Oscillator

If you switch on/off with a microcontroller pin (if you need < 10 ns, you may need special pulse gen. circuit), which can provide sufficient current typically (10 - 30 mA) it works well.

randsl:
If you use a vericap or dielectric resonator, then the frequency accuracy is getting bit better than this method. However, startup time slows down..  but again, that depends on what you need. If you need slower keying, and no need of very accurate freq. could be a solution as well..

If you need faster keying, and accurate frequency need to consider a different solution...

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