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Old 25th Oct 2006, 2:24 am   #1
Top Cap
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Default Home Brew Wobulator - Design sought

Hi All, I keep seeing references to a Wobulator and I must admit it is something I have often wanted myself. I tried building the simple 2 transistor types and though they worked fairly well I was never happy with them. I could never decide where the lower and upper frequency limits were let alone the centre point. It is way past my bedtime now but I had a brainstorm (at my age! I think I will have to change my brand of coffee ) and put together the attached ideas. If it works I will be able to preset the DAC to min, centre and max, adjusting/scaling the VCO to give a nice sweep through the I.F. and have suitable markers appear on the scope. I put it up here so you can hopefully pick it to bits, let me know how it won't work or perhaps suggest things like a suitable FET VCO that can operate on any I.F. including 10.7 MHz. Probably using at least two switched VCO's.
I reckon it best to build the digital/triangular wave part first then experiment with different VCO circuits later. Maybe we can all get together to produce a fairly good instrument that others can builld for themselves.
If however you happen to have a circuit already with these features then it may save me a lot of experimental time. I want to use the DAC rather than a waveform generator for the triangular waveform so that I can easily control its DC output to the VCO for setting up purposes, by presetting the counters as required. I also think it better to gently sweep the signal back and forth rather than drive with a sawtooth and abruptly start at the lowest frequency for the next sweep or am I getting too pedantic?
Cheers
Les G4CNH
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Old 25th Oct 2006, 8:22 am   #2
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Default Re: Home Brew Wobulator - Design sought

I too would be a bit warey of sudden step changes but its just a feeling as opposed to any grounding in science.

If any project was crying out for a PIC micro then this is one ! Can't help feeling that the box labled "Decode" might contain quite a bit of logic. Also some means for fixing the DAC at particular points would be very useful so LF point etc could be accurately calibrated.

TTFN,
Jon

PS : You could simplify your design by using an (audio) CD to generate the waveforms for the sweep circuit, one channel for the VCO and the other to generate the bright-up pulses. Just a thought.

Last edited by Duke_Nukem; 25th Oct 2006 at 8:31 am. Reason: Forgot the PS. Look, its early in the morning, ok !?
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Old 25th Oct 2006, 9:01 am   #3
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Default Re: Home Brew Wobulator - Design sought

Just for a moment I thought this was going to be a design using direct digital synthesis (DDS). Then I noticed the separate VCO.

I'm sure that Kat would have some ideas on getting a VGA card to do a DDS wobbulator. A lot simpler than using it to make modulated video. Then use a PC card with an ADC (maybe even a sound card though it would need to go down to DC) to capture the output of the radio etc under test and display it on the computer monitor.

The merit of using a sawtooth sweep is that it's easy to ignore or blank the flyback. If you use bidirectional sweep (sine or triangle) then you will get 2 horizontally displaced traces due to low frequency phase shifts.
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Old 25th Oct 2006, 12:03 pm   #4
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Default Re: Home Brew Wobulator - Design sought

I would prefer to see a DDS chip, driven by a PIC - Theoretically you could then make a wobbulator that covers DC to 200Mhz - I think that would be fairly simple (says he with zero design experience)

As far as I know you can do waveform shaping as part of the DDS chip, so might be worth looking into....

Cheers
Sean
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Old 25th Oct 2006, 12:13 pm   #5
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Default Re: Home Brew Wobulator - Design sought

Quote:
Originally Posted by m1ecy View Post
As far as I know you can do waveform shaping as part of the DDS chip, so might be worth looking into.
Try googling "dds chip" to find a few possible parts. I remember looking at an old Harris part once. This had the digital oscillator, sine shaper and DAC all on the chip. I've no idea of the part number or even who owns Harris semi these days.

If you use a DDS chip then you'll want a second low frequency DAC to drive the X plates of the scope. This can be driven from the PIC or other microcontroller. Even a simple pulse width modulator DAC might be adequate.
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Old 25th Oct 2006, 3:48 pm   #6
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Default Re: Home Brew Wobulator - Design sought

Hi,
Quote:
Originally Posted by ppppenguin View Post
I'm sure that Kat would have some ideas on getting a VGA card to do a DDS wobbulator.
I have indeed, brought about by a) not having a wobbulator and b) having a pile of unwell Hacker radios. Mater artium necessitas...

More thoughts on this once I've had some more thoughts; I'll probably find the tuits once I've got a pile of working radios in need of alignment.

Regards, Kat
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Old 25th Oct 2006, 5:56 pm   #7
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Default Re: Home Brew Wobulator - Design sought

Hi

Notwithstanding the desire to have a cutting edge DDS wobbulator, there are several designs about which use function generator chips as the VCO for up to 470kHz IF.

The Eddystone User Group magazine published a design entitled "A low cost IF sweep generator" by Joe LeKostag K9LY. It uses an LM3900 analog ramp generator and an XR2206 function generator chip. It covers 85kHz, 100khz and 455-470kHz.

One I recently discovered on an American web site is available here.
www.pavekmuseum.org/Wobbulator.html

It uses a MAX038 function generator for the IF frequency and a discrete component/op amp ramp generator for the fairly slow 120ms sweep. The all important marker signal is generated by a crystal controlled digital timebase which generates a 10ms gate pulse. This is used to gate the swept frequency into a digital counter and to trigger a 1 ms bright-up for the 'scope z axis. The counter displays the frequency at the centre of the sweep (presumably an average over the 10ms gate time) and the marker bright-up can be switched to be on the left, centre or right of the centre frequency shown on the counter display.

I have been experimenting using a Wavetek model 166 50Mhz pulse/function generator as a self contained 10.7MHz wobbulator, with a Heathkit IG-37 borrowed from Igranic as a source of 5.35Mhz marker pulses. I have had some success, but the sweep start and stop frequencies have to be carefully monitored with a digital counter because of drift in the Wavetek (a long warm-up helps) and loosely coupling the marker oscillator to the IF of the D.U.T. does not give as narrow a spike as I had hoped. It does serve however to mark the centre frequency quite clearly though.

Ron

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Old 25th Oct 2006, 11:11 pm   #8
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Default Re: Home Brew Wobulator - Design sought

On the DOS idea the AD9856 from analog devices seems simple enough. How fast does the sweep need to be and how many steps and will the steps generate harmonics that might mess it up?
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Old 26th Oct 2006, 7:22 am   #9
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Default Re: Home Brew Wobulator - Design sought

The AD9856 should be able to sweep at almost any speed and harmonics should not be a problem. If you go too fast then you could cause errors if the DUT has a very narrow bandwidth or very steep roll off.

I haven't looked at the data sheet in detail so I don't know how easy it is to use. It's primarily a quadrature modulator with built in DDS so the sweep function (the coefficient that increments the accumulator in the DDS) may not be the easiest thing to control as most applications will probably leave the DDS at the same frequency for long-ish periods.
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Old 26th Oct 2006, 1:29 pm   #10
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Default Re: Home Brew Wobulator - Design sought

As it is a Vintage wireless site.. How about the method used in the old Samwell and Hutton Wobulator. I had the dubious honour of having to align quite a few TVs with this monster as an apprentice, mostly Murphy V310s and V430s I think.
The Swept oscillator consisted of a variable capacitor driven from a syncronous motor. Simplicity itself. The sweep time was fixed I recall and locked to the mains. The unit had a switch on the front panel for the motor which had to be turned on and off to get the rise in frequency going from left to right otherwise the display was reversed.

Fortuately I have a digital Spectrum Analyser with Tracking generator for the purpose now but If it was me needing a wobbulator for aligning IF stages I think I would do this...
Make up a VCO covering the desired range.
Plot out the voltage frequency characteristics.
An 8 bit DAC with some smoothing on the output would probably be adequate in which case an R2R network directly on a PIC would suffice. With the characteristics of the VCO known draw up a table to linearise the VCO sweep.
The PIC can also output a trigger signal to sync a scope for display.
Make up a couple of fixed marker Oscillators or use a Signal generator for markers.

It's easy to overcomplicate a piece to test kit when it often only needs to be fit for purpose.. I do however like the DDS approach... Good luck with it..

Denis
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Old 28th Oct 2006, 6:21 am   #11
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Default Re: Home Brew Wobulator - Design sought

Hi Everyone, sorry for the delay in getting back but my head is still swimming from calculating all the DAC addresses and clock frequencies. I apologise if this post is a bit lengthy but it is necessary.
I was very pleased to receive all the suggestions and all very interesting. To Jon, yes it would be possible to do that with a CD but I think you would still have to add things, it's a pity that one could not get a swept 455kHz off it at the same time but of course this is impossible. If it was you could select a CD track for each I.F. frequency! All noted Jeff on the phase shifts, I had not taken into account the possibility of that happening, worse of course if the VCO was not rock solid apart from the ramp. Sean, I would love to get into PICs but I am hopeless at assembly language and have yet to acquire something to allow me to enter Hex, yep I know, HEX! but I prefer that.
Maybe someone on the Forum can apply my latest ideas to a PIC.
If I had a good workshop Denis then I may very well have gone the route you suggest with a motor driven pot and capacitor, what I am trying to do with my circuit is emulate just that but with the benefit of being able to effectively stop the motor at 25% points across the crt screen. Kat you are amazing and highlights the difference between software and hardware engineers, my hat is off to you as you really know your stuff on the PC scene. I suppose I was trying to design/build something more or less from the junk box here as cash is too tight to get really ambitious. Ron directed me to an american design that had some really good points and I have tried to fit them into my preliminary design, I do like the 10ms gating at centre and I was going to directly decode the DAC address to do this but I reckon using a PROM clocked in parallel saves a lot of decoding chips and latches. With it I can produce the 10ms gate pulses direct and the 2ms centred markers at the addresses shown on the attached block diagram. I am hoping a simple summing amp can marry the DAC output to a variable voltage that sets the range. My biggest problem is finding a 41kHz clock to drive the whole thing so that I get the right timings, using a 3.6864 quartz gives 40.960 kHz resulting in a freq counter error of around 500Hz, only 1% but an error all the same. The junk box has provided a 9.955 MHz rock which divided by 243 gives me 300 odd Hertz error which I can live with. Probably would be better to buy a rock later with the ideal frequency of 9.963MHz. The basic idea of the instrument is to force the DAC to 1024 and add to its output with the SET freq pot to give say 4kHz down from the I.F., the frequency counter gate input could be selected using the same switch. The DAC is then forced to 3072 and its output used to set the VCO up 4kHz using the other pot with the fine pot centred. Centre frequency at 2048 should then be automatically at 455 kHz or whatever. Adjusting the +/- fine control enables you to look at frequencies at specific 3dB points as in the American design.The MAX038, yes I had already looked at this and I will probably have ago at making the VCO with this and I have an AD667 in the junk box for the DAC. The display and counters will be junk box as well and like most of my projects, the most cash will be spent on a decent case for the finished item. Lots of breadboarding to do but at least it will keep me busy during the dark winter nights .
Les
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Old 29th Oct 2006, 12:54 pm   #12
Phil G4SPZ
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Lightbulb Re: Home Brew Wobulator - Design sought

I've been reading this thread with some interest... I'm hoping to make a useable wobbulator by applying a frequency sweep to a circuit based on a superhet receiver local oscillator - valve, of course - taking the ramp waveform from the 'scope X-output, and applying a proportion of it to a varactor diode across the oscillator tuning capacitor. I saw a similar principle used many years ago in an old PW article, long since lost of course. I'm only looking to shift the oscillator up and down a few kc/s, and I've done this with ham VFO circuits quite easily using varactor diodes. I don't need great calibration accuracy as I've got a DFM.

If my sums are correct, to cover IFs from 85 to 470 kc/s I'll probably need two switched bands. No interest in 10.7 Mc/s IFs here yet...

If anyone knows of a more authoritative details or an article describing the technique, please advise. I'd like to make this equipment out of old valve radio parts from the junk box if possible, a bit like Les (only cheaper!)

Phil
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Old 30th Oct 2006, 1:32 am   #13
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Default Re: Home Brew Wobulator - Design sought

Hi Phil, I am complete with your sentiments about using bits from the junk box.
My endevour is to try same but with a bit more info available when using, e.g. getting a more defined bandwidth reading. With regard to my ideas I have abandoned the idea of using all 4096 address bits as going for just 4000 of them makes the clock frequency easier to find. A 1MHz clock divided down to 40kHz will give a step size of 25us and we can decode the addresses to give not only the 10ms counter gate pulse but also the 1ms markers plus the option of a 100us counter gate pulse. This will enable the counter to show 10.70 when aligning FM i.f.'s. Using a switch to re-select the 10ms gate will give us an over range and allow us to see the 10.7 MHz VCO down to 100Hz resolutiuon. i.e. a display of 7000. Nothing to stop us using a 6 digit counter though for a full readout at 10.7MHz. The markers will be at 3D4H to 3FCH, 7BCH to 7E4H and BA4H to BCCH. The 100ms gates are at 320H to 480H, 708H to 898H and AF0H to C80H. The 100us gates are at 3E6H to 3EAH, 7CEH to 7D2H and BB6H to BBAH. I am trying not to use programmable logic as a lot of folks, including me, have no hardware for programming. I could probably get a PROM done at work but otherwise they use large logic arrays there that program themselves on power up. Using discreet logic does give everyone a chance to build from bits that may be in the proverbial junk box, I have even considered the use of a diode array in this respect. Like anything I suppose, there are many ways to fullfill a particular design requirement and I am always looking for what I consider to be the easiest/cheapest option. I will let everyone know if I am successful or not but it will take me some time to make everything up. I hope to produce some kind of universal driver that can be applied to a number of different VCO's to cover various i.f.'s., each VCO having its own adjusted scaling circuit.
Les
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Old 30th Oct 2006, 11:39 pm   #14
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Default Re: Home Brew Wobulator - Design sought

Understood, Les. I'd like to use valves in my home made effort if possible, though! I played with 'scopes last night, looked at the timebase output of my Cossor 'scope on my other 'scope, and found a nice ramp waveform of 60 volts peak-peak, which through a suitable potentiometer chain should drive a varactor diode nicely

No time to play now (the working week has begun) but I'll tinker with this idea over the forthcoming Christmas break.

Regards

Phil
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Old 31st Oct 2006, 1:29 am   #15
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Default Re: Home Brew Wobulator - Design sought

Nobody seems to have mentioned this so far, but if the intention is to align an FM radio one solution would be to buy a cheap FM microtransmitter from eBay or wherever and feed it with a sine wave. The chips used in these things generate a very clean FM signal.

Have I missed something here?

Paul
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Old 31st Oct 2006, 11:06 am   #16
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Default Re: Home Brew Wobulator - Design sought

I seem to think that there was a simple design for a wobulator in the BVWS Bulletin a couple of years ago. AFAIK it was for 465KHz IF's and used a few 4000 series chips and some varicap diodes. The only problem was a couldn't find the diodes used in the design anywhere

Andy
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Old 31st Oct 2006, 12:19 pm   #17
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Default Re: Home Brew Wobulator - Design sought

A few paralled 1N4001 diodes reverse biased should do the trick on the BVWS published design.
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Old 31st Oct 2006, 1:14 pm   #18
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Default Re: Home Brew Wobulator - Design sought

Quote:
A few paralled 1N4001 diodes reverse biased should do the trick on the BVWS published design.
LEDs apparently work quite well as varicap diodes: http://www.hanssummers.com/radio/varicap/

Paul
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Old 31st Oct 2006, 1:21 pm   #19
Brian R Pateman
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Default Re: Home Brew Wobulator - Design sought

I have a simple (although solid state) design from RC.

It uses two BC109s or something similar, a UJT and a 556. Looks easy enough to knock up, probably and evening's work!

As I am scannerless at the moment I'll send a copy to anyone who PMs.

Regards,
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Old 31st Oct 2006, 1:49 pm   #20
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Default Re: Home Brew Wobulator - Design sought

I have a circuit for a valve wobbulator in a Radio Constructor, Oct 1960.
I think its really for TV use but I'm sure it can be adapted if anyone would rather use a hollow state system. If anyone is interested I can scan the article & send it.

As far as varicap diodes are concerned I read somewhere - possibly in this forum - that one of the main problems is the lack of linearity in the voltage/capacitance curve.

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