Radio Bygones Wobbulator

[mpegjohn]

I have just built a RB wobbulator. and was surprised to find that the waveform output from the unit, is not a nice sine wave at all. It is full of even harmonics.

Has anyone who built one of these units, looked at the output with a scope? If so can they confirm that their waveforms are non sinusoidal either.

I think it is perfectly OK, as the harmonic is well outside an IF passband, but I would like to have confirmation that I have the same results as everyone else.

Also, on the top range I get drop outs every now and again as I tune, anyone else had this?

Many thanks,
John.

[Ricardo]

John,

Have a look at post #16 in the following thread.

https://www.vintage-radio.net/forum/...463#post261463

Do you recognise the waveforms?


Regards,

Richard

[mpegjohn]

Thank you Richard,

That is just what I have. Must be OK then!!

[Herald1360]

Unless some form of ALC is used an oscillator will increase in output until the limiting resulting from hitting the limits of the possible anode or collector excursions effectively reduce its loop gain to unity. The resulting distorted waveform looks 'orrible and would be disastrous as a transmitter output, but as noted here doesn't matter much for relatively narrow sweeping wobbulator. A decent low pass filter would clean it up quite well but a lot of them would be needed to cover a decent operating range for the centre frequency.

[Bazz4CQJ]

I've never owned a wobbulator, but building one is on the "to do" list for the winter. Is it the case that the "ideal" waveform would be a sinewave, but it looks as if it is not at all critical?

I heard about this particular design before, but this was the first time I'd looked the circuit up. Having foolishly disposed of my old traditional scope some time ago, I am left with usb scopes which do not have trigger output, so I'm going to have include a trigger circuit in whatever design I finally go with, and make the usb scope trigger on the leading edge....

B

[Herald1360]

The "ideal" waveform for any signal generator is a sinewave, but producing a clean one over a wide frequency range isn't trivial- look at some of the serious professional sig gen circuits like Marconi TF144H or HP606 series- loadsa bottles compared with amateur gear or even typical radio shop equipment. In many cases, the distortion doesn't really matter as long as you know which frequency you're really using. Some cheaper sig gens even used 2nd harmonic from the next lower frequency range as the VHF output range at the top end.

[David G4EBT]

The waveform I got on my R.F. Haigh Radio Bygones wobbulator design is shown in the attached pic. (The wobbulator front panel layout differs from the original RB layout).

Hope that's of interest.

[Bazz4CQJ]

Yes, thanks for those comments; seems like that's how the circuit perfoms and it has no detriment on its function .

B

[bc312]

I have contemplated building this design, but like Bazz my two scopes do not have a trigger output. Has anyone built a ramp generator that would provide the necessary output for this design, and could be built into the same case.

[Ricardo]

Mike,

I had the same problem, in that my scope could not provide a suitable timebase output.

I built a Timebase Generator from an article by P. R. Arthur that was published in Radio & Electronics Constructor, April 1976.

See post #3 in the following thread for a link to the article.

https://www.vintage-radio.net/forum/...ad.php?t=52696

It is simple to construct and works well with the wobbulator.


Best regards,

Richard

[Bazz4CQJ]

If you spend a bit of time on Google, you can find a number of interesting wobbulators, some with their own ramp generator, some not.

The 555 is a popular (cheap and easy) solution, but it seems that many designs produce a saw-tooth rather than a ramp waveform. Attached is a copy of one article I found which claims to give a ramp. I've not yet tried it.

B

[David G4EBT]

I've attached the waveform generator circuit that I used, which was kindly passed to me by forum member Colin Armstrong, though I've no idea from whence it originated. As will be seen, it uses a 555 IC but when I built it, try as I might, I couldn't get it to work. Colin had a look at it for me and found that for whatever reason, it wasn't happy with a standard 555 but it works fine with a 7555 - the CMOS version. I've attached a pic of the ramp waveform that it generates, though I've referred to it as a 'saw-tooth generator'. The PCB can be seen mounted vertically at the end of the main PCB.

There is no voltage regulator shown on the circuit, but as I powered both the wobby and the waveform generator from 2 x 9V batteries in series, I fitted a 12V regulator on the PCB. Though the PCB overlay shows a preset pot, I brought the leads out to a normal pot on the front panel with an amended front panel to accommodate that extra pot (expertly drawn by Colin Armstrong, as was the PCB layout).

I've never actually used the wobby as I've never had need to, but one day I'll have to hook it up to a radio, if only out of curiosity!

Hope that's of interest.

[Bazz4CQJ]

@David; thanks for that post; looks as if detailed the circuitry around a 555 may need to be a little more refined to deliver the goods. I never recognised the difference between sawtooth and ramp until I started looking at wobs, but I suspect that the nearest thing to the pure ramp-form is the best way for a wob. Yours looks pretty good; Haigh MkII ?

B

[bc312]

Thanks all for the suggestions; some good ideas to read up in more detail and experiment with. I have all the components for the RB wobbulator design, just never got round to building it. A nice project for the coming months. Thanks again.

[bc312]

To the two David's - thanks for the caveat re the 555, duly noted.

[Bazz4CQJ]

If anyone wants to stay clear of 555s, and wants to consider circuits other than the Bygones unit, this circuit http://www.qsl.net/va3iul/Wobbulator...cerization.htm looks interesting.

If you fancy a really simple circuit, but using a 555, this is worth a look http://www.vk2zay.net/article/93.

I gather that long ago, wobulators had arrangement involving electric motors driving variable capacitors and other electromechanical arrangments; anyone got any links to such designs?

B

[AC/HL]

Two posts moved to a new thread: https://www.vintage-radio.net/forum/...d.php?t=108378