What RF signal generator comes to mind? UK

I'm not an accomplished electronics person.

I was told that if it was designed right you could make a (2.0-2.5Mhz) stabilized analogue VFO tune in very small increments, say 2 or 3 hz. (Electronically stablized by varacters).

Say you wanted to base such a VFO on an analogue RF signal generator, does anyone know what might be suitable?

I have been looking at what I beleive are stabilized analogue RF signal generators, but they are quite expensive, (HP 8640A and 8640b) so I thinks what I need is simply a good RF generator and I can add the stabilzation circuitry myself. It is important that the basic oscillator is quality in order to try for the 2 or 3 hz tuning steps.

Perhaps some of the older 1960's eqipment that is virtually obsolete would be okay because the oscillator itself is a quality item. Maybe some of the old Marconi stuff would do. Any peice of equipment come to mind? TIA.

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Richard
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I'm not entirely certain whether quality analogue RF generators are tuned with variable capacitors. I am assuming so. It's the tuning component that has to be high quality so the frequency does not jump several hertz when tuning, preventing say a 3 hz lock. I ask myself, can any analogue tuning component, like a variable capacitor, actually change frequency in the 1, 2, or 3 hertz range. Maybe for mechanical reasons it's not possible.

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Richard

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Yes, it's possible.. Your easiest approach will probably to put two or three smaller capacitors across the main tuning capacitor. Tune the main capacitor to approximately your desired frequency, then incrementally adjust the smaller caps to get to your final frequency. The exact values of these capacitors will, obviously, depend on the value of your main tuning element. I would suggest that you start with a capacitor of around 10% of the value of the main tuning element, and maybe another cap of 1% or 2% of the main tuning cap. All would be in parallel with the main tuning cap. A nasty side effect of this will be that you will upset the tuning range of the system. IOW, your frequency dial will no longer be accurate unless you redraw it.

If you want to use varicaps, then you would use the same theory.. just use varicaps with values that will allow you to get the frequency resolution that you want.

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Yes thanks, that sounds the proper way to deal with the problem. This means that the the bandspread capacitor is the key component, it is tasked with giving a smooth frequency adjustment, not the main tuning capacitor.

And in fact I have two small-value capacitors that came from a wobbbulator. They look to be very solidily built, I bet they would do. It's then just a matter of choosing a good VFO/Osc circuit that offers a "clean" output. I ether homebrew from scratch or try to get hold an oldish, cheap, but good RF signal generator that has a pure output/low noise. Then I "bolt on" a modern stabilizing circuit.

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Richard

What are you trying to accomplish? When I think 8640, I think microvolt outputs and heavy shielding so you can actually use microvolt outputs. How much FM can you stand on the signal? 2 Hz. steps sounds nice until you discover that you've got 2KHz of incidental FM. Once you start adding voltage sensitive components, you also have to add voltage stabilization components. Once you start adding circuitry to stabilize a piece of equipment that's basically unsutiable for the task, you're asking for grief.

The *best* commercial unit for the job is the one that pops un unexpectedly at a great price...and meets your minimum specs. If you go out looking for a specific item, you'll pay a LOT more.

All boils down to what you're trying to do with the result. mike

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