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Old 16th Jan 2021, 4:38 pm   #1
Chris55000
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Default Wobbulator Project for my T & M Book?

Hi!

Apart from Raymond Haigh's design, can anyone point me in the direction of a reproducible design of Wobbulator designs suitable for 450–480 kHz and 9.0–12.0 MHz to feature in my T & M Book please?

I have just begun work on Chapter 84 which features Swept R.F. Generators or "Wobbulators, I have my Heathkit HFW–1 one and the Practical Wireless Design, but I'm looking for a reasonably easy to reproduce all solid–state design I can build to the above spec I can write up!

PS!

Just a thought, could I use coils from one of those cheap multi–band radios as a basis for something similar to the Haigh Design?

Chris Williams
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Old 16th Jan 2021, 6:43 pm   #2
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Default Re: Wobbulator Project for my T & M Book?

I think there are quite a few around, for example https://sites.google.com/site/linuxd...r-experimental.

I take it that you will be including the Haigh design; it does have some heritage.

B
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Old 16th Jan 2021, 6:58 pm   #3
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Default Re: Wobbulator Project for my T & M Book?

Hi!

Yes I will include the Haigh design but I can only write it up from a theoretical point of view as two of the Toko 10K adjustable coils specified appear to be completely unobtainable now!

There's a firm called LodestonePacific who claim they can remanufacture any original Toko Coil to the correct inductance and specification, but I dread to think who much they'd charge to make the ones needed for the R.H. Wobbulator, and I suspect the minimum quantity order would be steep as well!

Chris Williams
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Old 16th Jan 2021, 6:59 pm   #4
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Default Re: Wobbulator Project for my T & M Book?

Can I point you to my thread on building the Haigh where I suggest alternatives to the now unobtainable components of the original.
https://www.vintage-radio.net/forum/...d.php?t=172685

Peter
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Old 17th Jan 2021, 2:09 am   #5
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Default Re: Wobbulator Project for my T & M Book?

Replacements for 3 of the coils in the Haigh design are available from http://www.spectrumcomms.co.uk/Components.htm

The lowest frequency range might be possible with one of the above coils with a capacitor change or alternatively rewinding one of the coils to match (fiddly I know, but an option).
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Old 17th Jan 2021, 5:42 am   #6
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Default Re: Wobbulator Project for my T & M Book?

Adding capacitance will dilute the effect of the varactor diode and reduce the available frequency span.

The oscillator circuit for these things is fairly simple, and apart from the inductor, pretty cheap. Instead of switching inductors it would be easy to switch whole oscillators. More components, but you could optimise an oscillator for your choice of IF, and fiddle with it to get best linearity.

The tapped inductors could be wound on dust iron toroids. Their stability is decent, the turns counts won't be too bad, and you can pull a lop of wire out while you're winding for a tap, just twist the pull-out and solder up its length. Good cores are made by Micrometals inc. in the US, but are sold all over the place through amateur radio components traders as 'Amidon toroids' which was the name of an American distributor who set the American QRP world going on them. There are several grades and several sizes. Al value is the inductance of a 1-turn winding. What you get id Al times the number of turns squared.

The disadvantage is that the finished thing isn't adjustable. You can always add or remove turns to taste, though. Good enough to use for HF receiver VFOs.

Finally, professional-grade sweepers and wobulators tend to use a fixed centre frequency oscillator to which the sweep is applied, and then uses a variable frequency oscillator to mix it down to the wanted centre frequency.

By sweeping an oscillator whose centre frequency isn't going to be moved around, you can make a better job of getting it linear.

Super duper pro sweepers mix down the swept oscillator and stick it into a frequency discriminator. Pulse-count discriminators can cover big frequency ratios and can be fed back to the tuning voltage to linearise the sweep. This is called a Frequency Locked Loop. Makes a change from phase locked loops! An eaxample is the HP8601A 0-110MHz sweeper/sig gen.

David
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Old 17th Jan 2021, 10:51 am   #7
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Default Re: Wobbulator Project for my T & M Book?

Quote:
Originally Posted by Terry_VK5TM View Post
Replacements for 3 of the coils in the Haigh design are available from http://www.spectrumcomms.co.uk/Components.htm

The lowest frequency range might be possible with one of the above coils with a capacitor change or alternatively rewinding one of the coils to match (fiddly I know, but an option).
For my recent build I used a 455KHz if transformer with the capacitor removed (it breaks up easily with a pointed probe) for the lowest range inductor. It worked perfectly.

Peter
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Old 17th Jan 2021, 4:42 pm   #8
Bazz4CQJ
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Default Re: Wobbulator Project for my T & M Book?

I think that on a forum like this, the ancient and declining art of making an inductor is to be encouraged .

The idea that you have to have a commercially-made inductor is a quite recent thing. Sure, if they are available, why not, but the idea than an otherwise good design (not just the Haigh wob) should be abandoned for want of a few turns of wire is a bad sign of the times.

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Old 19th Jan 2021, 11:54 am   #9
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Default Re: Wobbulator Project for my T & M Book?

I built my own version of the sweep generator noted in Bazz4CQJ’s in post #2. I believe the design originates from a radio Amateur in Japan, JF10ZL, and versions written up by VK2ZAY in Australia and KDYSAS in the US all appear on the Internet.

One ‘555’ IC, two FETs and a couple of evenings soldering gave a handy little tester that does what is intended, -within limitations.

Selection of the fixed inductor/variable capacitor combination is critical, partly because the change in capacitance of the varicap diode causes different sweep widths at the top and bottom end of the main oscillator tuning range.

I wanted to be able to look at the behaviour of a band pass filter at 14MHz and a crystal filter at 10MHz, so aimed for a main tuning range of 8 to 16MHz, and while the main oscillator achieved this comfortably, I had difficulty finding a varicap diode that could ‘open up’ the particular sweep range that I needed to examine with adequate precision.

A camel is a horse that has been designed by a committee, so in amalgamating bits from other designs I might have tried to achieve too much from such a simple circuit, but the project certainly fulfils Chris55000’s specification of being reproducible. If I was to build a Mk2 version I would be less ambitious and aim for better performance over a narrower range.

There are some designs in ‘Test Equipment for the Radio Amateur’ published by the Radio Society of Great Britain that follow a modular approach with a separate ramp generator sweeping the range of an oscillator tailor-made for the frequencies of interest. These have been kept up to date through successive editions of the book so should still meet the reproducible requirement.

I wound the coils for my version of the sweep oscillator, [and also for an ancient Practical Wireless design that I wanted to build], -only simple toroids, but home-spun none the less,- and as Bazz4CQJ suggests in post #8, it’s not terribly difficult, even for a rank amateur like me. There are published designs for toroidal inductors to replace some of the unobtainium Toko coils, and many internet sources for inductor designs from scratch or using branded cores. I have found ‘Coil 64’ to be particularly friendly to use, [start off by looking at www.coil32.net ], and to meet the requirement of giving reproducible results.

Hope this helps

Jim M0GJD
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Old 19th Jan 2021, 3:12 pm   #10
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Default Re: Wobbulator Project for my T & M Book?

Apologies, I have not yet worked out how to Edit a previous posting, but I meant to add to the above message that there was also a very simple circuit for a fixed centre frequency Wobbulator published in 'Radcom' in November 1992.

Even simpler than the variable frequency ones noted in post#2, the design by G7IXK used one IC and one FET with a Toko Inductor to give a spot frequency in the middle of the 80m Amateur band, but within reason, component values could be selected to give a centre-of-sweep figure to suit other bands as well.

Old Radcom articles aren't freely available, but a Google search on the Author's call-sign reveals the relevant article on the wonderful American Radio History site. Perversely, however, only the first page is there and you have to hunt for the rest, so Chris55000 would need to contact the RSGB if the item was of interest and he wished it to be included in a different published format.


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