Function Generator Sinewave distortion?

[Bill Bowden]

I have an old function generator that has 6 pots to adjust the shape of the
sinewave for minimal distortion I don't have a distortion analyser so I was
thinking of using a DIY sinewave oscillator through a filter to arrive at a
good reference and then compare the waveforms on a scope in the add mode and
adjust the pots so they cancel out indicating minimum distortion. Problem
is, I can't figure out how to synchronize the 2 waveforms so they both are
at exactly the same frequency, 180 out of phase and they cross zero at the
same time.

How do I set this up so I can calibrate the function generator for minimum
distortion?


..



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[Phil Allison]

Bill Bowden wrote:

I have an old function generator that has 6 pots to adjust the shape of the
sinewave for minimal distortion I don't have a distortion analyser so I was
thinking of using a DIY sinewave oscillator through a filter to arrive at a
good reference and then compare the waveforms on a scope in the add mode and
adjust the pots so they cancel out indicating minimum distortion. Problem
is, I can't figure out how to synchronize the 2 waveforms so they both are
at exactly the same frequency, 180 out of phase and they cross zero at the
same time.

** That is the main problem with that dopey scheme.

Ideas:

1. Use twin-T notch filter to remove the fundamental so you can see all the harmonics on a scope. Piece of cake to make one.

2. Use the FFT function on a DSO to look at the harmonics.

3. Use both above together.

4. Set the generator to about 500Hz and use your ears - it is usually easy enough to put the speaker in spot that almost nulls the fundamental where you are sitting making the harmonics more obvious.



..... Phil

 

[Ralph Mowery]

"Bill Bowden" <bperryb@bowdenshobbycircuits.info> wrote in message
news:mddmot$2off$1@adenine.netfront.net...

I have an old function generator that has 6 pots to adjust the shape of the
sinewave for minimal distortion I don't have a distortion analyser so I
was thinking of using a DIY sinewave oscillator through a filter to arrive
at a good reference and then compare the waveforms on a scope in the add
mode and adjust the pots so they cancel out indicating minimum distortion.
Problem is, I can't figure out how to synchronize the 2 waveforms so they
both are at exactly the same frequency, 180 out of phase and they cross
zero at the same time.

How do I set this up so I can calibrate the function generator for minimum
distortion?

Can't help; with that part, but what would hapen if you just built a very
sharp filter to filter out the sine wave ?
Then looked at what ever was left.

 

"If there's a bunch of pots, likely they are generating a triangle wave
(easy to do)"

Are you thinking Wavetek ? I lik those. They claim to do 0.5 % harmonic distortion. I think tha tis some damn good engineeriong myself. It is not just diodes, it is also using the knee of the beta curve of amost common bas stages. I have a Wavetek 111, and I thihnk for being back in the 1960s, the thing is fantastic. for what they diud at the time I tink they were like Tektronix. Not as complex but just as anal about certain things.

" and then using a set of diodes to "sculpt" it into a
>sinewave. At the best of times, it will not be a perfect sinewave"

The Wavetek 1XX series used three stages. Emitter fed by the triangle through the diodes, each biased a bit differently. Totally push pull, positive and negative sides matched to a "T". Those pots adjusted the parts of a sine wave that the circuit acted upon, and it acted upon it. Wavetek found a way to make it a bit better. It hadd negative current feedback and brought the low current hfe of the transistors into play. It has it just coming into conductance, and nulling it. there is a set of precision resistors that if not right, you would NEVER be able to align it down to 0.5 % harmonic distortion. Ever.

My buddy has a 103 that has a distortyd sine output. I found it to have a DC offset From the triangle output to the sine convertor. This is consistent with the fact that his "sine" is compressed at the top and ponty at the bottom. I temporarily fixed it after gettijng aggeavated working on it by putting a blocking capacitor between the triangle generator and the sine convertor. It worked fine but he decided to be anal and get it workin right so he took it apart and now we have a basketful. He frigged someting up and now it won't even oscillate.

That's OK. It is his. He did it. No problem.

Anyway, I was never impressed by generators tha tneed AGC and all that to stay flat response. And thought I REALLY respect GOOD analog design, when it comes to this, digital is probably good enough now. Although I will not sel my old Wavetek, thsat doesn't mean I don't want something else.

See how we are ?

 

"Isn't that how a distortion analyzer is made? I dont' recall needing an
oscillator. "

Some have it included, others do not. And some that do have it included give you the option of using an external oscillator.

 

[Phil Allison]

Michael Black wrote:

Phil Allison wrote:

1. Use twin-T notch filter to remove the fundamental so you can see all
the harmonics on a scope. Piece of cake to make one.

Isn't that how a distortion analyzer is made? I dont' recall needing an
oscillator.

** THD analysers use one or more notch filters to remove the fundamental frequency and then measure what is left. Most allow that to be read a percentage of the original signal and be viewed on a scope.

Most are purely manual while more expensive models adjust the notch frequency automatically.

My home brew, Wien Bridge notch filter job has three fixed frequencies ( 66, 998 and 5280 Hz) plus two 10 turn WW pots to trim the notch frequency and depth. With a little patience, it can resolve down to 0.002%.


.... Phil

 

[Michael Black]

On Fri, 6 Mar 2015, Phil Allison wrote:

Bill Bowden wrote:

I have an old function generator that has 6 pots to adjust the shape of the
sinewave for minimal distortion I don't have a distortion analyser so I was
thinking of using a DIY sinewave oscillator through a filter to arrive at a
good reference and then compare the waveforms on a scope in the add mode and
adjust the pots so they cancel out indicating minimum distortion. Problem
is, I can't figure out how to synchronize the 2 waveforms so they both are
at exactly the same frequency, 180 out of phase and they cross zero at the
same time.

** That is the main problem with that dopey scheme.

Ideas:

1. Use twin-T notch filter to remove the fundamental so you can see all
the harmonics on a scope. Piece of cake to make one.

Isn't that how a distortion analyzer is made? I dont' recall needing an
oscillator.

Michael

2. Use the FFT function on a DSO to look at the harmonics.

3. Use both above together.

4. Set the generator to about 500Hz and use your ears - it is usually easy enough to put the speaker in spot that almost nulls the fundamental where you are sitting making the harmonics more obvious.



.... Phil

 

[Michael Black]

On Fri, 6 Mar 2015, Bill Bowden wrote:

I have an old function generator that has 6 pots to adjust the shape of the
sinewave for minimal distortion I don't have a distortion analyser so I was
thinking of using a DIY sinewave oscillator through a filter to arrive at a
good reference and then compare the waveforms on a scope in the add mode and
adjust the pots so they cancel out indicating minimum distortion. Problem
is, I can't figure out how to synchronize the 2 waveforms so they both are
at exactly the same frequency, 180 out of phase and they cross zero at the
same time.

How do I set this up so I can calibrate the function generator for minimum
distortion?

If there's a bunch of pots, likely they are generating a triangle wave
(easy to do) and then using a set of diodes to "sculpt" it into a
sinewave. At the best of times, it will not be a perfect sinewave.

HOw much fussing depends on how much you need the sinewave, and even then,
I think there will be more distortion than if you set out with an
oscillator that generated a sinewave directly. Function generators are for
general purpose, an oscillator with a wide range of frequency and
different output waveforms. But if you need a good sinewave, you look
elsewhere.

Maybe stuff about the 8038 would give some ideas about adjusting the
sinewave. It generated a triangle and then used diode-resistor shaping
network, but there was little external adjustment of the waveform. But
the old articles might have given some hints on getting a better sinewave.

As someone pointed out, if you need a good sinewave and it's a relatively
fixed frequency, you could just pass it through a low pass filter, likely
clear up things relatively well.

Michael

 

[Helmut Wabnig]

On Fri, 6 Mar 2015 18:11:40 -0800, "Bill Bowden"
<bperryb@bowdenshobbycircuits.info> wrote:

I have an old function generator that has 6 pots to adjust the shape of the
sinewave for minimal distortion I don't have a distortion analyser so I was
thinking of using a DIY sinewave oscillator through a filter to arrive at a
good reference and then compare the waveforms on a scope in the add mode and
adjust the pots so they cancel out indicating minimum distortion. Problem
is, I can't figure out how to synchronize the 2 waveforms so they both are
at exactly the same frequency, 180 out of phase and they cross zero at the
same time.

How do I set this up so I can calibrate the function generator for minimum
distortion?

Since you are writing this on a PC,
you already have the instrument of your dreams.

Connect the "sinus" to the sound card,
run a free FFT software and - bingo!

w.

 

[RobertMacy]

On Sat, 07 Mar 2015 00:01:00 -0700, Helmut Wabnig --- -.dotat>
<"<hwabnig"@.-> wrote:

On Fri, 6 Mar 2015 18:11:40 -0800, "Bill Bowden"
bperryb@bowdenshobbycircuits.info> wrote:

I have an old function generator that has 6 pots to adjust the shape of
the
sinewave for minimal distortion I don't have a distortion analyser so
I was
thinking of using a DIY sinewave oscillator through a filter to arrive
at a
good reference and then compare the waveforms on a scope in the add
mode and
adjust the pots so they cancel out indicating minimum distortion.
Problem
is, I can't figure out how to synchronize the 2 waveforms so they both
are
at exactly the same frequency, 180 out of phase and they cross zero at
the
same time.

How do I set this up so I can calibrate the function generator for
minimum
distortion?


Since you are writing this on a PC,
you already have the instrument of your dreams.

Connect the "sinus" to the sound card,
run a free FFT software and - bingo!

w.

Hear, hear!

The dynamic range is more than 120dB and most function generators using a
triangular wave will be around 60dB from memory.

Either use your soundcard and run the software yourself, or get FFT
programs.

I was impressed with the simplicity of the GUI done by Bob:

Bob Masta

DAQARTA v7.60
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
Frequency Counter, Pitch Track, Pitch-to-MIDI
FREE Signal Generator, DaqMusiq generator
Science with your sound card!

Notice he now has an LCR meter built-in! shows you Resr of caps.

 

[Bob Masta]

On Sat, 07 Mar 2015 04:41:36 -0700, RobertMacy
<robert.a.macy@gmail.com> wrote:

On Sat, 07 Mar 2015 00:01:00 -0700, Helmut Wabnig --- -.dotat
"<hwabnig"@.-> wrote:

On Fri, 6 Mar 2015 18:11:40 -0800, "Bill Bowden"
bperryb@bowdenshobbycircuits.info> wrote:

I have an old function generator that has 6 pots to adjust the shape of
the
sinewave for minimal distortion I don't have a distortion analyser so
I was
thinking of using a DIY sinewave oscillator through a filter to arrive
at a
good reference and then compare the waveforms on a scope in the add
mode and
adjust the pots so they cancel out indicating minimum distortion.
Problem
is, I can't figure out how to synchronize the 2 waveforms so they both
are
at exactly the same frequency, 180 out of phase and they cross zero at
the
same time.

How do I set this up so I can calibrate the function generator for
minimum
distortion?


Since you are writing this on a PC,
you already have the instrument of your dreams.

Connect the "sinus" to the sound card,
run a free FFT software and - bingo!

w.

Hear, hear!

The dynamic range is more than 120dB and most function generators using a
triangular wave will be around 60dB from memory.

Either use your soundcard and run the software yourself, or get FFT
programs.

I was impressed with the simplicity of the GUI done by Bob:

Bob Masta

DAQARTA v7.60
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
Frequency Counter, Pitch Track, Pitch-to-MIDI
FREE Signal Generator, DaqMusiq generator
Science with your sound card!

Notice he now has an LCR meter built-in! shows you Resr of caps.

Thanks for the plug! The OP can probably do his calibration
for free, before the 30-session / 30-day trial expires. (I
say "probably", because I don't know if there is a good
strategy for adjusting a multiple-diode wave shaper; I seem
to recall reading a *design* article back in the Bad Old
Days, though.)

Anyway, after the trial period, Daqarta's signal generator
keeps working, forever. (Plus a whole bunch of other stuff,
but not external signal *inputs*.) So even if he can't get
his old signal generator working he'll have a replacement
that not only produces lower sine distortion, but has a lot
of exotic modulation options. 2 channels are standard, with
up to 4 independent streams per channel. (Sneak preview:
Daqarta v8.00 will allow 8 independent output channels.)

One shortcoming of sound cards, however, is that they are
all AC-coupled so you can't get decent results below a few
Hz without circuit modification. (Some high-end cards seem
to claim DC coupling, but it's an "audiophile lie": They
just mean that there are no capacitors in the signal path.
They use a servo circuit instead, but still have an
AC-coupled high-pass response.)

Regarding sine-from-triangle generators, I think the average
function generator is around 1% (-40 dB). The "overdriven
differential pair with feedback" approach can do that
easily, and as Phil Allison notes it is easy to adjust by
ear. You have only 2 adjustments, and once you are close it
comes down to trading between 2nd harmonic and 3rd harmonic,
both pretty audible if your fundamental is in the 100-500 Hz
range. I have seen one (ancient) article where by careful
design and oven control of the differential pair, they got
down to -60 dB.

Best regards,




Bob Masta

DAQARTA v7.60
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
Frequency Counter, Pitch Track, Pitch-to-MIDI
FREE Signal Generator, DaqMusiq generator
Science with your sound card!

 

[Phil Allison]

Bill Bowden wrote:

Interesting ideas. So what twin-T, RC values would you recommend for a
notch frequency of 5Khz to observe distortion down to say 0.3%? .

** You do NOT need to *null* the fundamental to SEE the harmonics a scope - knocking it down by a factor of say 20 is enough.

Plenty of info on the www about Twin-Ts.

You will need one op-amp, 3x 6.8nF caps and 3 x 4.7kohm resistors.

F = 1 / (2.pi.R.C)

.... Phil

 

[Phil Allison]

Phil Allison wrote:

**Correction:

You will need an op-amp, 4 x 6.8nF caps and 4 x 4.7kohm resistors.

Two caps and two resistors have to be paralleled to get R/2 and 2C values.

F = 1 / (2.pi.R.C)



.... Phil

 

[Phil Allison]

Bill Bowden wrote:

"Phil Allison"

Phil Allison wrote:

**Correction:

You will need an op-amp, 4 x 6.8nF caps and 4 x 4.7kohm resistors.

Two caps and two resistors have to be paralleled to get R/2 and 2C values.

F = 1 / (2.pi.R.C)


Why the op-amp?

** It increases the Q factor and depth of the notch.

Results are pretty poor without it.



.... Phil

 

[Bill Bowden]

"Michael Black" <et472@ncf.ca> wrote in message
news:alpine.LNX.2.02.1503070024570.1140@darkstar.example.org...

On Fri, 6 Mar 2015, Bill Bowden wrote:

I have an old function generator that has 6 pots to adjust the shape of
the
sinewave for minimal distortion I don't have a distortion analyser so I
was
thinking of using a DIY sinewave oscillator through a filter to arrive
at a
good reference and then compare the waveforms on a scope in the add mode
and
adjust the pots so they cancel out indicating minimum distortion. Problem
is, I can't figure out how to synchronize the 2 waveforms so they both
are
at exactly the same frequency, 180 out of phase and they cross zero at
the
same time.

How do I set this up so I can calibrate the function generator for
minimum
distortion?

If there's a bunch of pots, likely they are generating a triangle wave
(easy to do) and then using a set of diodes to "sculpt" it into a
sinewave. At the best of times, it will not be a perfect sinewave.

HOw much fussing depends on how much you need the sinewave, and even then,
I think there will be more distortion than if you set out with an
oscillator that generated a sinewave directly. Function generators are for
general purpose, an oscillator with a wide range of frequency and
different output waveforms. But if you need a good sinewave, you look
elsewhere.

Maybe stuff about the 8038 would give some ideas about adjusting the
sinewave. It generated a triangle and then used diode-resistor shaping
network, but there was little external adjustment of the waveform. But
the old articles might have given some hints on getting a better sinewave.

As someone pointed out, if you need a good sinewave and it's a relatively
fixed frequency, you could just pass it through a low pass filter, likely
clear up things relatively well.

Michael

Yes, they use 15 resistors and 10 diodes. There are 3 voltage dividers of 5
resistors each and wierd values. The center divider has the triangle input
which is already calibrated for symetry and offset. 5 diodes on one side are
used for half the waveform and the other 5 diodes for the other half. There
are 4 pots to set the DC voltage at each end of the 2 dividers. And a couple
more pots for other minor adjustments. I think this thing does less than
0.3% distortion if set up right.





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[Bill Bowden]

"Phil Allison" <pallison49@gmail.com> wrote in message
news:d5fae599-9348-4ff4-8cd9-a1047858e72e@googlegroups.com...

Bill Bowden wrote:

I have an old function generator that has 6 pots to adjust the shape of
the
sinewave for minimal distortion I don't have a distortion analyser so I
was
thinking of using a DIY sinewave oscillator through a filter to arrive
at a
good reference and then compare the waveforms on a scope in the add mode
and
adjust the pots so they cancel out indicating minimum distortion. Problem
is, I can't figure out how to synchronize the 2 waveforms so they both
are
at exactly the same frequency, 180 out of phase and they cross zero at
the
same time.

** That is the main problem with that dopey scheme.

Ideas:

1. Use twin-T notch filter to remove the fundamental so you can see all
the harmonics on a scope. Piece of cake to make one.

2. Use the FFT function on a DSO to look at the harmonics.

3. Use both above together.

4. Set the generator to about 500Hz and use your ears - it is usually easy
enough to put the speaker in spot that almost nulls the fundamental where
you are sitting making the harmonics more obvious.

.... Phil

Interesting ideas. So what twin-T, RC values would you recommend for a
notch frequency of 5Khz to observe distortion down to say 0.3%? .
..



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[Bill Bowden]

"Phil Allison" <pallison49@gmail.com> wrote in message
news:df5566f6-3084-468b-9e65-3a1133168281@googlegroups.com...

Phil Allison wrote:

**Correction:

You will need an op-amp, 4 x 6.8nF caps and 4 x 4.7kohm resistors.

Two caps and two resistors have to be paralleled to get R/2 and 2C values.

F = 1 / (2.pi.R.C)



... Phil

Why the op-amp? This generator puts out 20 volts p-p open circuit, or 10
volts p-p into 50 ohms. Why not just feed the generator into a passive
twin-T notch filter and look at the other end of the filter?




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