Home Brew Wobulator - Design sought

[Phil G4SPZ]

Originally Posted by joe

I have a circuit for a valve wobbulator in a Radio Constructor, Oct 1960. If anyone is interested I can scan the article & send it.

Joe, that would be very useful, thanks. I'll PM you with my e-mail address.

Phil

[Mike Phelan]

The ETI project by Raymond Haigh that, er, (d'oh - forgot!) kindly sent to me looks promising; it does use a varicap and a couple of JFETs, and more to the point, will cover 465khz and 10.7mhz.

One thing that might improve it would be to use the Z-axis as a bright-up marker rather than beating it - probably another tuneable generator like the main one, and an ex-Superdrug frequency counter, or for nicer appearance, the counter and 74 series chips from the Pavek link above.
The varicaps mentioned are still available, as, I think, the Toko coils are.

Or the marker could be a programmable divider and Xtal osc like a 74HC4013? Wonder how far that would go up to? Maybe use harmonics?
No, probably overkill we have a counter.

Maybe a switchable "detector" diode as well especially for FM when we are not displaying an S-curve.

The Jason design that Brian mentioned, IIRC, used a Fireball tuner (beloved by Jason) - are any of these still about; I assume it would have to be a TV/FM one like in the Fergy 506, and wobbly would be for Bands I / II / III only and not 465KHz?

I'm really just thinking aloud here, but all comments gratefully received ...

[Kat Manton]

Hi,

I found this while re-reading some of my magazine collection. It's from the August 1967 issue of Short Wave Magazine, I thought it might be of interest.

X input is from the X output of the 'scope, oscillator is swept around 5Mc/s and mixed with the output of another generator to produce the desired RF output. It could form the basis of something, I'm tempted to play with this circuit a bit myself.

Cheers, Kat

[Top Cap]

That's a neat idea Kat and I might use a similar scheme once I get a controllable ramp sorted out. I have built quite a few circuits using the X output ramp waveform of the scope to drive the oscillator but always finish longing for some way of setting the centre frequency. With a controlled ramp one can set the VCO to the lower, centre and upper points on the scope screen. At present I have constucted a good master timebase from a 20MHz temperature controlled xtal that gives 10Mhzs down to 1Hz symetrical square waves. I hope to drive a comb generator with these outputs to make an oscilloscope calibration tool. In addition, I have managed a good ramp waveform using an AD622 found in my junk box. I tried a DAC0800 but had bad luck in trying to get a unipolar output, I kept blowing up following op-Amps. The 622 gives a unipolar output and I am just using 8 bits of its possible 12. Using presettable counters as a drive for it, I can set the ramp to 25%, 50% or 75% so that the resulting steady DC can be used to set the planned VCO at the right frequencies for each step. Presently working on the frequency counter part of the project but finding it hard to produce a synchronised full 10ms gate pulse whilst sweeping. I may have to go over to using a dual set-up, i.e. accurate 10ms gate for non-sweep measurement and a mono-stable produced gate pulse whilst sweeping though it will not be as accurate but then I wonder if this sampling while sweeping will really be required. At least the timebase board is built and fully working. Les

[boiss]

In the past I have used modified Varicap VHF radio, V/UHF TV and satellite tuners as swept signal sources using the amplified sweep output of my scope. While I have never used these for IF alignment they may well be good enough seeing that the I/P bandwidth must always be greater than the IF bandwidth.
Mike.

[Chris55000]

Hi!

I've got the PW design based on the SVC1 GEC Varicap diode, plus a home-brew unit (not working & not my design!!!) based on Denco Coils (blue), what looks like a JFET-based RF unit, a plug-in lump of vero with 741s & things on it, more vero with a buffer amp and two radio-type (valve-sized!!!) tuning capacitors!!!

I'm still dissecting this home-made unit & I'll post plans as soon as I've made sense of it!!!

Heathkit used a Transductor attached to the RF tuning coils in their HFW-1 unit but trying to duplicate this at home is likely to be almost impossible unless you could get the right iron/ferrite core material!!!

Chris Williams

[oldeurope]

I build an (AM) Transduktor Wobulator with colour TV Transduktors and coils.
Works fine see pics attached.

Kind regards,
Darius

[MichaelR]

Just a point about the design which will be important and that is please make sure that you can have variable sweep times, the narrower the filter is that is going to be swept then the slower you need to sweep otherwise you will get a distorted response.

Mike

[oldeurope]

Originally Posted by MichaelR

Just a point about the design which will be important and that is please make sure that you can have variable sweep times, the narrower the filter is that is going to be swept then the slower you need to sweep otherwise you will get a distorted response.

Mike

Hi Mike,
the pot with the switch on the left without nob (Wobbelgeschwindigkeit) makes the
sweep time adjustable.

Darius

[XTC]

Darius, that's a very nice looking piece of work.

Pete.

[XTC]

There are all sorts of ways of tackling the problem of building a wobbulator, depending on the range and level of sophistication you need.

You can build a controllable sweep circuit and use it to control a VCO which could be a function generator chip, or a function generator instrument. The best function generator chip was the MAX038, which is now out of production and seems very hard to get. Most of the others such as the XR2206, can't do much more than 1MHz, if that, and their amplitude drops off as they get close to their limit.

See http://www.cappels.org/dproj/functsw...nctionswp.html for this approach.

You can build a sweep generator which sweeps a single frequency and mix it with a signal from another sig gen and get a pair of output frequencies, one of which will be the one you want.

See http://users.tpg.com.au/ldbutler/sweepgen.htm

The Cossor Ganging Oscillator also used this approach, and the manual available here

http://www.thevalvepage.com/servdata/testeq/cossor.htm

is a good read - if you are interested in wobbulators.

Pete.

[MichaelR]

Originally Posted by oldeurope

Hi Mike,
the pot with the switch on the left without nob (Wobbelgeschwindigkeit) makes the
sweep time adjustable.

Darius

Thank you Darius,my German is not very good as you would expect , as Pete has commented (XTC) that is a nice piece of kit you have produced.

Mike

[oldeurope]

Hello,

thanks Pete and Mike,

Mike, normally I do the labelling in English.
But often I lend this apparatus to German enthusiasts who don't understand the international labelling.

Today I am using a home brew MAX038 Wobulator as XTC mentioned above and the Hameg HM5011 spectrum analyzer with its tracking generator, mostly.

Kind regards,
Darius