50 kHz VCO w/sine output

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Yeah searching for varactor's lots of them were/ are OBSELETE. sigh.

PIN photdiodes have C's that change by factors of 4 or 5. But that seems like a spendy solution.

Hmmm, Thanks Phil, I'd forgotten about that. (I had no need for it at the time.) How do I get a tanh function out of a diff pair. (Yeah, I'll try google too.)

George H.

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Yeah but the steam punk crowd love our stuff. So that's a feature, and not a bug.

Optial pumping was on boing boing a few years ago.

Teaching apparatus it's about 2/3 of the way dwon the page.

George H.

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A couple of words of caution if you decide to go the down-converter route.

I missed your final amplitude requirements, but I would caution you if you decide to use the 602 part and require an output approaching 0dBm or better. This really is a low power part with a weak IIP3.

Even though you have a single tone at the input, over driving parts such as these can cause funny distortion effects.

Re VCO: I doubt very seriously that you could use a free running VCO. The damn thing will just be doing to much of the =93wautsi=94. So plan on lock=92d VCO.

I would recommend, as others have suggested, a DDS system.

You might consider getting an eval board from AD. They use to pass those things out like candy. But in this economy, who knows. The big advantage of the eval board is that they come with control software that can be run off the pc.

Hope this helps.

lem

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Thanks Whit3rd, For the tuning range I might have to move the circuit 'ground' around a bit. But, i'm probably stuck once the once the varactor C's get below 10pF or so... stray capacitance will eat up any changes.

For gain control I just had back to back diodes and a series resistor across the feed back R.

Here's the LTspice file, Sorry about the LT1354, but I don't do much spicing and have problems getting the right .include files.

If you change parameters and it has trouble starting then goose R2 lower a bit.

George H.

Version 4 SHEET 1 948 1108 WIRE 352 -16 288 -16 WIRE -16 16 -48 16 WIRE 288 16 288 -16 WIRE 288 16 112 16 WIRE 416 16 416 -16 WIRE 480 16 416 16 WIRE -48 48 -48 16 WIRE 288 48 288 16 WIRE 352 48 288 48 WIRE 416 48 416 16 WIRE -16 128 -16 16 WIRE 0 128 -16 128 WIRE 112 128 112 16 WIRE 112 128 80 128 WIRE 176 128 112 128 WIRE 560 128 560 16 WIRE 560 128 256 128 WIRE 720 128 560 128 WIRE 112 224 112 128 WIRE 128 224 112 224 WIRE 720 240 720 128 WIRE 720 240 192 240 WIRE -48 256 -96 256 WIRE 32 256 -48 256 WIRE 112 256 32 256 WIRE 128 256 112 256 WIRE -48 272 -48 256 WIRE 32 272 32 256 WIRE 512 272 400 272 WIRE 624 272 512 272 WIRE 512 304 512 272 WIRE -288 320 -288 304 WIRE -176 320 -176 304 WIRE -96 336 -96 320 WIRE -48 336 -96 336 WIRE 400 352 400 272 WIRE 400 352 384 352 WIRE 32 368 32 352 WIRE 112 368 112 256 WIRE 128 368 112 368 WIRE 208 368 192 368 WIRE 304 368 288 368 WIRE 320 368 304 368 WIRE 624 368 624 272 WIRE 624 368 608 368 WIRE 400 384 384 384 WIRE 416 384 400 384 WIRE 512 384 496 384 WIRE 544 384 512 384 WIRE 624 400 608 400 WIRE 720 400 720 240 WIRE 720 400 704 400 WIRE -288 416 -288 400 WIRE -176 416 -176 400 WIRE 112 448 112 368 WIRE 192 448 192 368 WIRE 192 448 176 448 WIRE -48 464 -48 336 WIRE 16 464 -48 464 WIRE 304 480 304 368 WIRE 400 480 400 384 WIRE 400 480 384 480 WIRE 512 480 512 384 WIRE 544 480 512 480 WIRE 624 480 624 400 WIRE -48 496 -48 464 WIRE 288 512 224 512 WIRE 96 528 96 464 WIRE 160 528 96 528 WIRE 256 544 224 544 WIRE 288 560 288 512 WIRE 352 560 288 560 WIRE 464 560 352 560 WIRE 624 560 624 480 WIRE 624 560 544 560 WIRE -48 608 -48 560 WIRE 96 624 96 528 WIRE 256 624 256 544 WIRE 256 624 96 624 WIRE 352 656 352 640 FLAG -288 304 +V FLAG -176 416 0 FLAG -288 416 0 FLAG -176 304 -V FLAG 32 368 0 FLAG -48 608 0 FLAG 512 304 0 FLAG 352 656 0 FLAG -48 48 0 FLAG 160 208 +V FLAG 160 272 -V SYMBOL voltage -288 304 R0 SYMATTR InstName V1 SYMATTR Value 15 SYMBOL voltage -176 304 R0 SYMATTR InstName V2 SYMATTR Value -15 SYMBOL res 272 112 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 10K SYMBOL res 96 112 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 4.6K SYMBOL res 304 352 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 100K SYMBOL res 16 368 M180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R4 SYMATTR Value 100K SYMBOL cap 192 352 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C1 SYMATTR Value 20pF SYMBOL cap -64 272 R0 SYMATTR InstName C2 SYMATTR Value 20pF SYMBOL res 720 384 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R6 SYMATTR Value 10k SYMBOL res 640 464 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R7 SYMATTR Value 10k SYMBOL res 512 368 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R8 SYMATTR Value 10k SYMBOL res 400 464 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R9 SYMATTR Value 10k SYMBOL cap -64 496 R0 SYMATTR InstName C4 SYMATTR Value 10=B5f SYMBOL res 112 448 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R10 SYMATTR Value 10 SYMBOL voltage 352 544 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 WINDOW 0 31 36 Left 2 SYMATTR InstName V3 SYMATTR Value 5 SYMBOL res 560 544 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R11 SYMATTR Value 10k SYMBOL OPAMPS\\OPAMP 576 448 R180 SYMATTR InstName U1 SYMBOL OPAMPS\\OPAMP 352 432 R180 SYMATTR InstName U3 SYMBOL OPAMPS\\OPAMP 192 592 R180 SYMATTR InstName U4 SYMBOL Opamps\\LT1354 160 176 R0 SYMATTR InstName U2 SYMBOL diode 416 32 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D2 SYMBOL diode 352 0 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D4 SYMBOL res 576 0 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R5 SYMATTR Value 10K SYMBOL diode -112 256 R0 SYMATTR InstName D5 SYMBOL diode 112 464 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D6 TEXT 168 784 Left 2 !.tran 0 10ms 0 5u startup TEXT 512 752 Left 2 !.include opamp.sub

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I didn't post any drive requirements. :^) But I was picturing it running into a mini-circits mixer... 7dBm on the LO (IIRC) and I assume something similar from the VCO, but at a few MHz I can certainly add some gain.

=A0So

Oh that sounds more complicated. I've only done a little RF stuff and most of that was 'cut and try', without really understanding what the 'F' I was doing.

Hmm and around I go again.

Thanks for the advice.

Your wife might object :-)

You could just use the SA612, figure 7 shows how to hook up LC to pins 6 and 9. You can do Colpitts and Hartley, the transistor would be behind those two pins with its base at pin 6.

As the next poster (me0223) wrote, this is a fairly wimpy chip. So don't push it. If you need more amplitude hang an opamp behind it. Or a small audio amplifier chip if you also need a low impedance output, many of those easily go up to 100kHz. Then you could generate several watts and scare the bats out of the rafters :-)

Been there ...

at

Diff pairs do tanh automatically--it's getting them to do anything else that takes work.

You just shove the tri wave between the bases, and subtract the collector currents, either with a current mirror or an op amp. If you don't mind an offset current of I_e/2, just connect one collector to the supply and come out the other one.

Cheers

Phil Hobbs

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Another way to smooth a triangle onto a sine is with linear segment breakpoints. One opamp and a mess of resistors and diodes can do that, but there are several ways.

Two breaks works pretty well. It can be done at high level, which is convenient.

I wonder how you'd find the optimum break formula. Or, in general, how one best approximates a function, over some range, with N linear segments.

John

If this is a low production volume system then you'll probably spend less $$ in the end to just buy a VCO from MiniCircuits. If it's high volume then you can spend more time engineering the VCO yourself and the parts will cost less.

What do the 'powers that be' expect? I spent years working at a company that was very "transistor averse" -- they were much happier with a square inch of board space and a $5.00 super-zoot op amp than they were with a quarter square inch occupied by a $0.25 transistor and a few resistors. One learned to do what was going to get through the design review in the end...

You can get a "AD9850 Module DDS Signal Generator with Circuit Diagram" in ebay for about $13 and demo software on the Analog Devices website. There is also VB source code & 80*51 source code availible for free.

I just bought a couple for a project I'm working on.

I don't know. CAN you? :)

Many years ago I wrote an optimization program for doing this for a nonlinear function with 'n' breakpoints. It would slide around the points from reasonable initial positions and optimize whatever error function you wanted (absolute error, error squared or whatever). These days you could probably do it with Excel's solver. I used it for fitting thermocouple curves.

Did it always converge to a sensible result? I can imagine all sorts of silly or chaotic behavior. This seems like an interesting problem.

I usually just eyeball it, for soft curves at least. I guess you want shorter segments on the curvier parts. For software fits, we just do some insane number of equally-spaced segments, like 64 or so.

For shaping a triangle into a sine, what should be optimized is the harmonic distortion, not the RMS error, I guess. Fiddling in Spice is probably as good a way as any.

John

It was a long time ago, but provided you started with a sensible start position I think it did.

I think it would be rather difficult to do in general, and I think I spent a bit of time looking at the general problem, but really I just wanted to calculate optimal precision resistor values for each input type so I could send the design to production and go have a beer. Plotting the nonlinearity and picking points manually was good enough to meet the spec.

The BFI method works fine.

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Well looking is OK, just no touching!

OK I've ordered some. These even come in DIP's!

George H.

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Yeah, low low volume. (say 20 per year) So it's cheaper to just buy something. But then I don't learn anything....

The lowest frequency VCO that mini circuits makes is 12-25 MHz. And higher frequencies for the peices with a smaller tuning range. It'd be nice to have something a bit lower. So maybe I can 'make my own' out of Joerg's SA612A part.

George H.

Thanks Michael, there's so many DDS chips. Which ones will still be around in ten years?

George H.

Nah, too old. My wife taught high school science for a few years while we were in Nashville. The school was 'on the wrong side of the tracks' or river in this case. (We lived on the wrong side of the river too, houses were cheaper.) On Friday nights teachers were encouraged to attend the football game. The best part was the half time show, lot's of drums, dancing and scantily clad drum majorettes shaking their bootie. (sp)

George H.

For approximating a sine wave, the harmonic distortion is identical to the RMS error, assuming there's no DC offset.

Cheers

Phil