50 kHz VCO w/sine output

As I posted to the sample-hold thread by mistake:

Using a CD4017 and some resistors will get rid of all harmonics up to the ninth, so a fixed LPF will make a nice sine wave with 3:1 tunability.

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

Here is the evaluation software for the 9850/9851 DDS chips. You can install it and look at it without the DDS chip connected. One of the $13 modules, a couple of 74HCT574 octal latch ICs and a few other parts will let you connect it to a standard parallel printer port. The

72 MHz lowpass filter is on the module, along with the 125 MHz oscillator module. That software has the interface schematic.

It was released in 2003. We were already using the AD9850 early in

2001.

has versions for older versions of Windows.

--
You can't have a sense of humor, if you have no sense.

Do it with the exact data...

...Jim Thompson

--
                  [On the Road, in New York]

| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

ors

Thanks Frank, If I go the varactor route I'll just bread board it.

George H.

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Hi MeO, The product will be doing all sorts of Fourier analysis. At the moment I think the reason for the VCO is only to look at sidebands*. And mostly sidebnads at a high modulation index... So I'd like to modulate it with a (say) 1kHz sine wave and see sidebands out to maybe the 7th to 10th harmonic. I'm not sure how to turn this into a resolution spec.

George H.

*It would be nice if we had some other 'physics' to do with the VCO. Then I might have some 'real' specs.

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Hi Phil, CD4017? A decade counter. Summing different 'fractions' into an opamp to get a 'stepped' sine wave? Color me confused.

George H.

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Thanks Jim, I saved it. I've got so many ways to 'skin this cat' now. I don't think that I'll be using a varactor. (at least for the first try.)

George H.

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It's the same as what I posted, except Phil's gone one better and merged the counter and decoder into one device:

CD4017 .-------.

10x | | clk --|> 0 |---[R0]---. | | | | 1 |---[R1]---+ .-----. | | | | | | 2 |---[R2]---+---| LPF |----- | | | | | | 3 |---[R3]---+ '-----' | | | | 4 |---[R4]---+ ~~~ ~~~ ~~~ ~~~ ~~~ | | '-------'

where energizing R0-9 in succession produces a full sine-wave cycle.

I was going to suggest generating a quarter or half cycle with the resistors, then using analog steering and extra logic to invert and reverse as needed. That boosts resolution, but Phil's sure is simpler.

-- Cheers, James Arthur

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George,

So do I understand correctly, you need a variable sine source that tunes in frequency from 30Khz to 100KHz; this will serve as the LO in a system that generates a modulated output?

So my question re resolution, how course a step size do you need the variable sine source to move in? Is it 1Hz, 100Hz, 1KHz, 10KHz, etc?

Then of course the next question is, what is your requirement on frequency accuracy? For example say you want to set the system to

30KHz, if it=92s off by 60KHz is that an issue? If it wonders around by +/- 30KHz once set, is that an issue?

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Ahh OK, I think I see. I've got to make some of the numbers give a negative voltage... or else let the 'sine wave' have a DC offset and AC couple it.

All fun stuff. Thanks

george H.

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Hey MeO, thanks for all your effort here! (I'm feeling guilty, wasting too much of everyones time already.) I've got a minicircuits order in, and I'll play around with the pieces and report back. After that I'll have a better idea of how this approach will work.

George H.

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Good luck George,

I won't beat a dead horse, but the DDS really sounds like it was designed for your app.

Have a look at there app notes if you already haven't. Depending on the part, great freq resolution, modulation capability and crystal stability.

Regards

quoted text -

Voltage tunable makes it tricky. Can you use range switching and a FET as a variable resistor? ...Jim Thompson

--
                  [On the Road, in New York]

| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

It's not just a decade counter--it's a decade counter _with_one-of-10_outputs_--the poor man's sequence generator. Wire the outputs together via weighted resistors. You have 10 samples per cycle, so you can get rid of the second through ninth harmonics. If you don't mind adding a couple of open-collector or tri-state buffer parts(*), the resistors just go as

R_N = 1/sin((N+1/2) pi/10).

Pretty simple. If you want elegance, you can save the buffers and just form a weighted sum of all of the conductances--the math isn't hard, because it's one-of-10.

4017s are wonderful for bodging together some little piece of test gear that needs good timing accuracy.

Cheers

Phil Hobbs

(*) You know the trick for getting open-collector behaviour out of tri-state parts, right? You ground the inputs, and put the logic signal into tri-state line. You don't get as many per package, but they come in faster logic families.

--
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

Gee, i currently have a real good use for something like that. I would like to see your answer.

?-)

Download the tek FG02 to see how to do it.

tek/fg502/

Wow, that's really rococo. Four diode bridges to make a breakpoint amp!

I like the diode bridge tri wave generator.

And the real problem with those 500-series plugins is that they die if you hot-plug them.

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

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No problem MeO, A DDS design ....someday...

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Thanks Phil, James A. set me straight. Sitting in the hospital yesterday, waiting for my Mum to get out from knee surgery, I realized you can do a simple little 90 degree phase shift too.

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Nah, I hardly know any good tricks. But thanks, I looked it up in AoE.

I was thinking of just summing current at the inverting input of an opamp, so I wouldn't need the open collectro trick... That's assuming that a digital 'zero' isn't too far from analog ground.

George H.

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