Quadrature Oscillator

On Tuesday, May 13, 2014 3:58:36 PM UTC-4, Jim Thompson wrote:
> On Tue, 13 May 2014 12:50:50 -0700 (PDT), George Herold
> 
> <gherold@teachspin.com> wrote:

> >> Yep, the Q is so high it takes a bare twiddle to make it go.
> >OK.. From my limited oscillator experience it will depend on how good
> >the cap is.  (good, as in low dissipation.)

> >> 
> 
> >> I threw this together some 8 years ago to demonstrate the gyrator
> >> band-pass effect.  IRL I used an AGC loop.  (An accurate clipper work
> >> also.)
> >>
> >> You can get really low distortion with good OpAmps.  I built one for a
> >> Sperry Flight Systems synchronous detect at 400Hz... 0.001% harmonic
> >> distortion :-)
> 
> >> 
> 
> If you run the numbers you'll find that dissipation factor (in active
> filters) is only a problem at low frequencies.

Hmm OK... I'll have to think about that.  (I'm not even sure the dissipation factor is one number, it probably changes with frequency.)  
I've got an SRS LCR meter, but I don't really believe the dissipation numbers when they get low... Sometimes it gives me a negative number for instance.

I did just do a simple Wein bridge oscillator, I used NPO ceramics,
someone in production subbed in a film cap. And that caused it to stop working at the highest frequencies  (80kHz.)  

George H. 
> 
> See...
> 
> <http://www.analog-innovations.com/SED/StateVariableFilter(P+1).pdf>
> 
> for how I cured (in the early '70's) the dissipation factor problem in
> telephone-signaling low-frequency filters.
> 
> 		
> 
>                                         ...Jim Thompson
> 
> -- 
> 
> | James E.Thompson                                 |    mens     |
> 
> | Analog Innovations                               |     et      |
> 
> | Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
> 
> | San Tan Valley, AZ 85142   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.

On Tuesday, May 13, 2014 4:15:54 PM UTC-4, John Silverman wrote:
> Tim Wescott <tim@seemywebsite.really> wrote:

> > I'd be tempted to integrate-integrate-invert.  It would take three op-
> 
> > amps, but you may have a better chance to get each section working 
> > correctly.  Of course you'd need the obligatory teeny bit of lag 
> > somewhere, to make the oscillation start.
> 
> > 
> 
> 
> 
> This is getting way too complicated. Analog circuits drift and can never
> guarantee quadrature. A simple 74HC74 will. See 
> http://www.wb5rvz.com/sdr/ensemble_rx_ii_vhf/04_div.htm
Hmm that might work.  (Well I'll have to figure it out first :^)
Does it keep a fixed phase reference to the input LO?  

Thanks,
George H.

On Tuesday, May 13, 2014 4:45:01 PM UTC-4, whit3rd wrote:
> On Tuesday, May 13, 2014 1:15:54 PM UTC-7, John Silverman wrote:
> 
> > This is getting way too complicated. Analog circuits drift and can never 
> > guarantee quadrature. A simple 74HC74 will. See 
> > 
> > http://www.wb5rvz.com/sdr/ensemble_rx_ii_vhf/04_div.ht
> 
> There's ways to make an analog signal lock-in to a digital
> quadrature clock, of course.
> There's also a switched-capacitor solution, see figure 15
> 
> <http://www.ti.com/lit/an/snoa572c/snoa572c.pdf>

OK, But I don't think that will go anywhere near 1 MHz.
> 
> And, if one wishes to go old-school, an LC oscillator
> puts out sin(wt) voltage on the capacitor, and cos(wt) 
> current through the inductor; you can make a sense
> coil lightly coupled to the inductor to pick off the second
> phase.  If Q is high, it needs no correction; if Q is low,
> you can make a weighted sum of the two signals that is
> tunable to get closer to 90.00 degrees phase, and
> a simple multiplicative mixer makes a DC signal of the
> error, to guide  the tuning.

OK thanks, 
George H.

On Tuesday, May 13, 2014 9:12:24 PM UTC-4, John Silverman wrote:
> Tim Wescott <tim@seemywebsite.really> wrote:
> 
> 
> 
> > On Tue, 13 May 2014 20:15:54 +0000, John Silverman wrote:
> 
> 
> 
> >> This is getting way too complicated. Analog circuits drift and can
> 
> >> never guarantee quadrature. A simple 74HC74 will. See
> 
> >> 
> 
> >> http://www.wb5rvz.com/sdr/ensemble_rx_ii_vhf/04_div.htm
> 
> > 
> 
> > If George can deal with square waves, yes, that's going to work way 
> > better than an analog solution.
> > 
> > If he needs sine waves, by the time he's gotten the harmonics beaten
> > out of that he'll be back to depending on analog components for his
> > precision. 
> 
> 
> I dunno why he needs quadrature signals for a lockin, but presumably they 
> are for I-Q. Mixers work better with square waves.

Oh I wanted to do these CV measurments, so put an AC voltage across a cap and
measure the out of phase current.  
I can do this other ways, But I was thinking if I started with a source that had both I and Q, then it might be easier.  

And yeah I think it will all get turned into square waves to turn switches or something on and off.  So your D-flip flop circuit may do the trick.  

George H.
> 
> 
> 
> Also, the 74HC74 guarantees symmetry, which reduces the even harmonics. 
> 
> Analog methods can drift, so you never know what you have.

On Tuesday, May 13, 2014 10:45:08 PM UTC-4, Bill Sloman wrote:
> On Wednesday, 14 May 2014 00:36:36 UTC+10, George Herold  wrote:
>=20
> > Quadrature oscillator.
>=20
> > Hi all,  As part of the CV measurements (from a previous post)=20
> > I'm thinking about a 1 MHz lockin.
>  > (Maybe a different thread to talk about how to do the switching.)=20
>  > I was wondering if I could make a quadrature oscillator from opamps.
>=20
>=20
>  Here something that might be interesting
http://www.janascard.cz/PDF/An%20ultra%20low%20distortion%20oscillator%20wi=
th%20THD%20below%20-140%20dB.pdf
>=20
Got it... That looks a lot like what J. Williams did (AN43??) =20
He (JW) beat on the Wein bridge till he got distortion down below the origi=
nal=20
HP200.  It was in one of his Electronics compendiums also. =20
(Well worth a read if you haven't already.)

George H.
>=20
> The LME49710 is much too slow to be interesting at 1MHz - it's amazing di=
stortion performance stops being amazing above a few kHz - but the amplitud=
e control scheme might be worth looking at.
>=20
>=20
>=20
> I've played around (only in LTSpice) with using an AD734 to provide the v=
ariable part of the amplitude-controlling feedback for a 17kHz (100,000 rad=
ian per second) oscillator. At 1MHz you might look at an AD834.
>=20
>=20
>=20
> My amplitude control scheme looks a bit different - e-mail me if you want=
 to look at it (since I'm thinking about publishing it if I ever get around=
 to putting together and testing a real circuit).
>=20
>=20
>=20
> Using an AD825 or some other fast, low-distortion part to provide the bul=
k of the gain (tweaked so that the AD834 is nominally doing nothing, and as=
  likely to provide negative feedback as positive as the component values d=
rift) is an attractive way to go.=20
>=20
>=20
>=20
> Even "low distortion" four quadrant multipliers like the AD734 and AD834 =
have much higher levels of distortion than regular, low distortion op amps,=
 but if you only use them to tweak the loop gain the amount of extra distor=
tion they feed in is small - verging on negligible if you spend heavily on =
stable passive components.
>=20
>=20
>=20
> --=20
>=20
> Bill Sloman, Sydney

On Thursday, 15 May 2014 02:41:55 UTC+10, George Herold  wrote:
> On Tuesday, May 13, 2014 10:45:08 PM UTC-4, Bill Sloman wrote:=20
> > On Wednesday, 14 May 2014 00:36:36 UTC+10, George Herold  wrote:
> > =20
> > > Quadrature oscillator.=20
> > =20
> > > Hi all,  As part of the CV measurements (from a previous post)=20
> > > I'm thinking about a 1 MHz lockin.
> >  > (Maybe a different thread to talk about how to do the switching.)=20
> >  > I was wondering if I could make a quadrature oscillator from opamps
> >
> >  Here something that might be interesting
>=20
> http://www.janascard.cz/PDF/An%20ultra%20low%20distortion%20oscillator%20=
with%20THD%20below%20-140%20dB.pdf=20
> > =20
> Got it... That looks a lot like what J. Williams did (AN43??) =20
>=20
> He (JW) beat on the Wein bridge till he got distortion down below the ori=
ginal HP200.  It was in one of his Electronics compendiums also. =20
>=20
> (Well worth a read if you haven't already.)

Read it. It's much the same idea, but Linear Technology doesn't seem to hav=
e an equivalent of the LME49710 so he didn't do quite as well. I still like=
 the AD734 and AD834 as the loop-gain twiddling elements, but they aren't c=
heap. Jim Williams would not have used them because they are Analog Devices=
 parts, and Linear Technology doesn't seem to have any kind of decent multi=
plier. Thaler does - the THAT 2181, which can live with lower supply voltag=
es than the AD734.

--=20
Bill Sloman, Sydney
 Bill Sloman, Sydney

George Herold <gherold@teachspin.com> wrote:

> On Tuesday, May 13, 2014 9:12:24 PM UTC-4, John Silverman wrote:
 
>> > On Tue, 13 May 2014 20:15:54 +0000, John Silverman wrote:
>> >> This is getting way too complicated. Analog circuits drift and can
>> >> never guarantee quadrature. A simple 74HC74 will. See
 
>> >> http://www.wb5rvz.com/sdr/ensemble_rx_ii_vhf/04_div.htm
 
> Oh I wanted to do these CV measurments, so put an AC voltage across a
> cap and measure the out of phase current.  
> I can do this other ways, But I was thinking if I started with a
> source that had both I and Q, then it might be easier.  
 
> And yeah I think it will all get turned into square waves to turn
> switches or something on and off.  So your D-flip flop circuit may do
> the trick.  
 
> George H.

Yes, the idea is to feed a low level sine wave to the junction, then 
measure the in-phase and quadrature currents. Keithley has a brief 
article decribing some of the challenges of making the measurement at

http://www.keithley.com/data?asset=52523

An example of a CV measurement is shown on Page 3 of

<http://gato-docs.its.txstate.edu/cos-department-of-
physics/manuals/CVanalyzer/CVanalyzer.pdf>

The description reads:

  The block-diagram of the Keithley 590 CV analyzer is printed in Fig.
  3. The system can be used to measure the real and the imaginary part
  of the  impedance  at two different frequencies (ie 100  kHz,  and 1
  MHz). The impedance is determined by applying an AC voltage of 15 mV
  to the  sample and measuring the current that is in  phase  with the
  voltage and  the  current that is 90 degrees out of  phase  with the
  voltage.

The quadrature measurement requires square waves with low asymmetry, 
which the 74AC74 provides.

The trick will be to generate the 1MHz exitation sine wave and the 4X 
clock to the 74AC74. It needs to be in phase with the 1MHz sine wave.

Frequency doublers can be made using a limiter driving a tuned circuit. 
The tuned circuit frequency can be adjusted to vary the phase. Two 
doublers will give the needed 4X clock.

For broadband work, it would be desirable to minimize the number of tuned 
circuits needed. Wenzel shows how to make wideband doublers using 
switching diodes at

http://www.wenzel.com/pdffiles1/pdfs/diodedbl.pdf

A single tuned circuit at the output could be used to adjust the phase.

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

> On 05/13/2014 04:15 PM, John Silverman wrote:
>> This is getting way too complicated. Analog circuits drift and can
>> never guarantee quadrature. 
> 
> That isn't so.  As long as slew limits are observed, an integrator
> will give a very accurate 90 degree phase shift.  

You are correct. I made a simple comparison model in LTspice and tried
to break it. Nothing had much effect on the phase angle, except the op
amp GBW needs to be above 50 MHz, preferably 200 MHz or higher. I post
the LTspice files at the end. 

> Servoing the frequency to 
> make the amplitudes equal isn't very hard.  That makes the component 
> tolerances all come out as frequency shifts, and measuring the
> frequency accurately gets rid of all of it, for the purposes of
> capacitance measurement.

No need to go to that complexity. Simply adjusting the capacitance in
the op amp feedback will trim the amplitude. See the LTspice file at the
end. 

This allows the measurement to be made at a specific frequency, such as
1 MHz, which helps repeatability. 

> A simple 74HC74 will. See
>>
>> http://www.wb5rvz.com/sdr/ensemble_rx_ii_vhf/04_div.htm
> 
> Johnson counters are great in PLLs, where the square wave output is
> just what you want, but not much use for making quadrature sine
> oscillators (as others have noted).

The intended use is a CV analyzer. The quadrature signals are not
applied to the junction. They are needed to apply switching signals to
the in-phase and quadrature detectors. A CMOS switcher requires square
waves at logic level, which is naturally provided by the 74AC74. 

Sine waves would need to be converted square waves, which requires a
limiter. 

The switching signals need to have low even harmonic distortion. This
may be difficult to achieve using sine waves and a limiter, but it is a
natural result of dividing in a 74AC74. 

The integrator approach is single frequency, so different modules would
be needed to cover specific frequencies. The 74AC74 is inherently
broadband so a single ic can be used over a broad frequency range. 

The 74AC74 will clock at a minimum of 140 MHz. This would allow
measurements up to 35 MHz. It may be difficult to get an op amp
integrator to work at higher frequencies and still provide an accurate
90 degree phase shift. 

One difficulty with the 74AC74 approach is to get a 4X clock with the
proper phase aligned with the 1 MHz exitation signal. I proposed several
solutions in an earlier post. 

> Cheers
> 
> Phil Hobbs

Here are the LTspice files:

Version 4
SHEET 1 980 696
WIRE 192 80 144 80
WIRE 304 80 272 80
WIRE -32 160 -96 160
WIRE 0 160 -32 160
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WIRE -256 272 -256 256
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WIRE 304 592 304 480
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WIRE 160 608 144 608
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FLAG -256 160 +V
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John Silverman <Nospam@me.com> wrote:

> George Herold <gherold@teachspin.com> wrote:

>> And yeah I think it will all get turned into square waves to turn
>> switches or something on and off.  So your D-flip flop circuit may do
>> the trick.  
>  
>> George H.

Here's another approach. You need a low level sine wave to apply to the 
junction to maintain linearity. Perhaps 10mV to 100mV or so. This means 
you don't need a separate high level oscillator with low harmonic 
distortion, which may be difficult to achieve at MHz frequencies.

It means you can afford a simple resonant circuit on the 74AC74 output to 
generate the exitation sinewave. One advantage of the 74AC74 is the 
output square wave has very little even harmonics. This is desirable for 
the detection process. It also means very low even harmonics will be fed 
to the junction.

A simple LC tank with a Q of 40 appears to work very well. Setting XL to 
around 500 ohms seems to be a good starting point. Most commercial 
inductors can achieve a Q of 35 - 40 at these frequencies. You may even 
want a lower Q to reduce the sensitivity to changes in stray capacity.

You could have several tank circuits for different frequencies,  such as 
100KHz, 1MHz, 10MHz, and so on. You may need a small trim capacitor to 
set the phase angle to the exact value for each frequency.

Here's the LTspice files. You can adjust C2 and watch how it changes the 
phase of the sine wave. With a Q of 40, it is a fairly sensitive 
adjustment.

Version 4
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SYMATTR InstName A2
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WINDOW 3 23 53 Left 2
SYMATTR InstName C2
SYMATTR Value 299pf
SYMBOL ind 960 288 R0
SYMATTR InstName L1
SYMATTR Value 82&#4294967295;h
SYMATTR SpiceLine Rser=12
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TEXT 616 -240 Left 2 ;'I-Q Quadrature Generator With Sine Output
TEXT 984 -208 Left 2 !.options plotwinsize=0
TEXT 984 -184 Left 2 !.options nomarch
TEXT 1056 272 Left 2 ;Sine wave to buffer amp

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George Herold <gherold@teachspin.com> wrote:

> Quadrature oscialltor.
[...]
> I was wondering if I could make a quadrature oscillator from opamps.

This is probably a bit too elaborate for your requirements.  It uses PWM
control of the time constants and the amplitude.

http://www.poppyrecords.co.uk/other/images/MotorDriveInverter2.gif


-- 
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk