50 kHz VCO w/sine output

It'll be rough to get the high impedances and low voltage swings you need for the Wien bridge oscillator to work either with varactors or with a suggestion that I've made previously of using JFETs as variable resistors.

So: a high frequency VCO to a DDS chip, or a fixed-frequency reference to a DDS that you write a frequency to with a micro, or (if you are for some reason insisting on "analog only") a VCO feeding a mixer, beating against a crystal oscillator.

I'd look to the DDS solution, or see if you can make a suitable block with a PIC or similar, a good DAC, and some code.

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www.wescottdesign.com

This is the TV dinner version, costs a bit more but you just rip open, microwave, and eat:

Z5U as a VCO works but it's tough to get the distortion low enough, since you want a nice sine wave and not a crummy one I assume.

If it has to be cheap there's this secret trick:

Let the VCO run on a much higher and more practical frequency. Let's say 2030kHz to 2100kHz, for example. That can easily be done with a varicap, a transistor, some R, C, and an inductor. Very cheap. Then mix that with a 2MHz clock. The result will be these three spectra:

30kHz-100kHZ 2030kHz-2100kHz (40dB down or better if using a DBM) 4030kHz-4100kHz plus some harmonics.

A very simple lowpass that cuts off at a few hundred kHZ can fish out

30kHz-100kHz and ... bingo.

If you search around you might find a radio chip that does most of nearly all of this. Depends a bit on what supply voltages you have available. Something like this maybe:

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Regards, Joerg

http://www.analogconsultants.com/

You could do a varactor-tuned LC oscillator, but a 3:1 range is pushing things.

Some classic integrator-based function generator thing wouldn't be hard, with a sine shaper downstream. There are chips to do this, and some probably run up to 100K.

John

Workable, of course, but not exactly a VCO anymore... and there's the Nyquist filtering to do afterward.

Not a great choice, because the Wein bridge requires MATCHED capacitors, tuned together, for best performance. It can be done with a few trimmers (probably easiest to trim the offset + gain of the control voltages). Then, there's the level translation problem (the capacitors in a Wien bridge don't have one end grounded).

Good sinewave LC VCOs in narrow ranges are easier, and a 4.0 MHz sine source and 4.030 to 4.100 MHz sine VCO can be mixed down to get you what you want.

uff

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Z5U

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Hi Tim, I've got a spice simulation with just diode limiting for the varible gain. ~1.2 Vp-p on the output. I couldn't find any varactors in LTspice so I just used a diode and added some more C in parallel. The AC voltage across the 'varactor' is less than a volt. By playing around with the gain control I can probably get reduce that by a factor of two. Is that too much?

Yeah DDS from a micro looks like the 'best' path. It's just outside my present comfort zone and so hard to predict how long it will take.

George H.

e

Yeah the dang square root of C.... well maybe only a 2:1 range would be enough.

Yeah the triangle wave/ integrator is nice, but the sine shaper is a bitch... and ugly! We got some of the old ICL8038 chips, I'll have to fire one of those up too.

George H.

f

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Thanks, I only did a quick scan... To use it as a VCO I'd have to write new frequencies via the SPI interface.

Hmm, I'm not so sure. What we really want to do. (I think.....) is drive the piss out of a VCO so we can see all the various side bands. Crank it up to where you see the fundamental dissapear...(The first zero of the Bessel function) And even higher... watch the first sideband go away... Anyway I think this is what we want, I'll have to check. So I'm not sure I care too much about a crummy sine wave. Maybe the first few harmonics down by 40dB is OK.

Yeah I thought of that too. Cheap is not really important. But I'd really like parts that are still going to be around in ten years.

Nice, thanks. I usually just go to minicircuits for this type of stuff.

George H.

Not sure what granularity you need but there are also digital potentiometers to vary the "R" part.

Switching caps in and our can be done via 75HC4051 or CD4051 if you need more volts. One of those plus 8 caps would give you 256 frequencies, two plus 16 caps would (well, in theory) give you 65536 frequencies.

Search for this:

"" plus "sine wave generator"

and

"" plus "wave digital filters" (or "WDF")

That can point you to resources where most of the coding has already been done and you may only have to do some adaptations. WDFs are method to do this with really cheap uC that don't come with a HW multiplier on board. But you do need a DAC. In a pinch maybe a "poor man's DAC" using a port register plus eight resistors R2R-style.

[...]

--
Regards, Joerg

http://www.analogconsultants.com/

Hi Whit3rd, Did you look at my schematic? I seem to have it running in LTspice.

Ah, good. This was my first suggestion on how to get what was wanted. It was rejected. But perhaps I should push it a bit more!

George H.

Yep, it's gonna be digital. Eeuw, yuck ...

Then by all means. But then you could also use a NE555 :-)

That chip should be around for a while because it's used in so many gizmos. Arrow has 60,000 in stock, those are usually the signs I look for before deciding. They wouldn't do this if it had become a dud.

--
Regards, Joerg

http://www.analogconsultants.com/

The 3rd harmonic of 30kHz is lower than 100kHz. I think you need a clean sine wave.

Well, heck -- if you're going that route just get a crystal oscillator, a regular VCO, and a mixer. Filter the piss (well, the harmonics) out of the oscillator that you feed into the "RF" side of the mixer (so you get a clean sine wave out), and go.

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www.wescottdesign.com

It's what I'd push, if I couldn't convince people to use a DDS or if there were some overriding systems reason why a DDS wouldn't do. You probably don't want two LC VCO's -- one crystal oscillator and one VCO is probably the way to go. If your frequency range were narrower you could use one crystal oscillator and one VXCO -- but it isn't, so you have to do with what you have.

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www.wescottdesign.com

I'll second that. Maybe we should make some signs and start a virtual picket line. Need a nice chant though. "Rah-rah-rah, os-ci-llate, down-con-vert, rah-rah-rah". Oh, and drums, of course :-)

[...]

--
Regards, Joerg

http://www.analogconsultants.com/

It's on its way to jelly-bean status. The 602 is also available, and it's been around since at least the 1980's. It's just a really useful, easy to use building block if you don't need "mini-circuits" quality.

--
www.wescottdesign.com

Sort of like our college math club football cheer:

Secant tangent Cosine Sine

3.14159 e to the x y to the rho Beer in the end zone Go team go.

Jim

Cool!

--
Regards, Joerg

http://www.analogconsultants.com/

you can make a suitable block

You can still get low-nanofarad Y5V caps in small sizes, and those plus a boost converter are about the best things going right now for high kilohertz stuff. All the MVAM-style varactors are long gone.

I'd probably be wanting to use a tri-wave oscillator with a sine shaper. (You don't have to use the crappy diode ones--you can use a tanh to roll over the peaks and then subtract a bit of the original sine, as we discussed a year or so back. I posted a suggestion at

, but it's a much older technique.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

Victoriana!

m

It looked OK, but (1) tuning voltage of 0 to 30V is typical to get the full C range of a varicap (and that's gonna go outside most op amp toleranc= e), and (2) the amplitude-stabilizing part of a Wien oscillator will get added in, presumably? It has to oscillate at low V to keep the varicaps in their linear range, so they don't frequency-multiply. Varicap diodes are STILL diodes, they'll rectify if you let the signal get as big as the bias.