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Dual sine wave generator with variable frequency and 90 degree phase difference

Now that I think of it, there IS an analog solution: the old CA3080 transconductance amp application notes had the key item, I think it was ICAN-6668. National Semiconductor LM13700 data sheet, fig. 17, is similar.

Basicallly, it's a phase-shift oscillator using four amplifiers with four matched capacitive loads on the outputs. One tunes the frequency by modulating the output impedance of the transconductance amplifiers (all four amps tuned with a matched variable current). Your adjustment pot feeds two resistors (the dual amps are matched, or it would be four resistors) to the four program pins.

Dual OTA amps are still available (LM13700), though not the '3080' (so sad, RIP). The oscillator with three sections rings at 60 degree phase shift; you want to use four sections, 45 degrees shift on each, to get sine and cosine outputs. Remember, an odd number of sections have to be inverting amplifiers!

(sorry about reposting, I muffed the reference)

Reply to
whit3rd
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Hmmm..did not think of a 4-stage phase shift oscillator; would seem to be an ideal solution. My scheme (SwitcherCad): Version 4 SHEET 1 880 680 WIRE 144 -144 -192 -144 WIRE -272 -64 -304 -64 WIRE -192 -64 -192 -144 WIRE -192 -64 -208 -64 WIRE -176 -64 -192 -64 WIRE -64 -64 -96 -64 WIRE -304 -32 -304 -64 WIRE -144 -32 -208 -32 WIRE -144 -16 -144 -32 WIRE -192 0 -192 -64 WIRE -176 0 -192 0 WIRE -64 16 -64 -64 WIRE -64 16 -112 16 WIRE -176 32 -192 32 WIRE -144 64 -144 48 WIRE -144 64 -240 64 WIRE -288 96 -320 96 WIRE -192 96 -192 32 WIRE -192 96 -208 96 WIRE -176 96 -192 96 WIRE -64 96 -64 16 WIRE -64 96 -96 96 WIRE -320 128 -320 96 WIRE -192 192 -192 96 WIRE 144 192 -192 192 FLAG -320 128 0 FLAG -304 -32 0 FLAG -208 -32 P15 FLAG -240 64 N15 SYMBOL Opamps\\\\LT1001 -144 -48 R0 WINDOW 3 -139 58 Left 0 SYMATTR InstName U1 SYMBOL res -192 -48 R270 WINDOW 0 37 21 VTop 0 WINDOW 3 48 102 VBottom 0 SYMATTR InstName R1 SYMATTR Value 100K SYMBOL res -80 80 R90 WINDOW 0 6 21 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 100K SYMBOL res -192 80 R90 WINDOW 0 52 90 VBottom 0 WINDOW 3 36 36 VTop 0 SYMATTR InstName R3 SYMATTR Value 100 SYMBOL cap -272 -48 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 54 -10 VBottom 0 SYMATTR InstName C1 SYMATTR Value 1µ TEXT -192 224 Left 0 !.tran 0 12m 2m .1m startup TEXT -368 176 Left 0 !V1 P15 0 15V TEXT -368 208 Left 0 !V2 0 N15 15V TEXT 0 -112 Left 0 ;N001 TEXT 0 168 Left 0 ;N003 Look at nodes 1 & 3. This config oscillates near 1KHz.

Reply to
Robert Baer

The Analog Devices DDS chips include a couple that will generate quadrature outputs - the AD9958 would do the job

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though it is on the expensive side and much faster than you need.

Winfield Hill's web-site had a scheme for using two of their cheaper DDs parts to generate sine waves in quadrature, but I've not seen it recently, and Google can't seem to find it.

-- Bill Sloman, Nijmegen

Reply to
bill.sloman

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