Pls recommend a signal generator with external pll input

[Bxaresearch]

This is what was communicated to me by the article author:

You need calibrated notches of known frequency response.

My colleague did the actual distortion measurement.

He used RC notches.

 

To improve a source, you notch out its harmonics.

Then you notch out the fundamental after the DUT, so the spectrum analyzer doesn’t distort.

 

But in this case, we were measuring the source, so all you have to do is notch the fundamental, and know how much the notch loses at the 2nd and 3rd harmonics.

 

But these days, I think an SRS SR1 is good enough to get down below 120dB.

 

[jbb]

Sorry for delay... was ill over weekend.

So, I'm still trying to understand what the exact phase is required for.  Thinking about what's been in the thread so far, it seems like the plan is to:

• begin with excellent 1 kHz sine wave
• pass through some Device Under Test
• generate an n*1 kHz signal for use as a Local Oscillator (LO)
• mix the DUT output Vout with the LO
• low pass filter mixer output down to DC, i.e. select the nth harmonic
• measure said DC signal with multimeter to determine harmonic amplitude

Is that right?

If so, I can see why phase control is required; should the LO phase happen be 90 degrees away from the harmonic out of the DUT, you'll get zero.

With that in mind, how about this idea?

• begin with excellent 1 kHz sine wave
• pass through some Device Under Test
• generate an (n*1 kHz) - 55 Hz signal for use as a Local Oscillator (LO)
• mix the DUT output Vout with the LO
• band pass filter mixer output to select the 55 Hz output, i.e. select the nth harmonic
• measure said AC signal with multimeter to determine harmonic amplitude

Helpfully, you no longer need to fuss about DC offsets.  And you might be able to place the frequency of interest a bit higher than the 1/f noise corner of whatever post-mixer amplifiers you need.

I'm not certain that 55 Hz is exactly the right frequency offset here; I wanted to pick one that isn't an integer division of 1 kHz (to help mixer rejection) and lies between the 50 Hz and 60 Hz calibration points of the multimeter. You'd need to do some mixer harmonic analysis to check out all the mixer harmonic products.

I think in this case that you might be able to get away with an LO that isn't frequency locked to the 1 kHz tone, i.e. use 2 benchtop generators without clock synch.

[1audio]

This is an instrument used to "precisely" calibrate some of the lowest residual analyzers. http://www.shibasoku.com/download/avc/ac12b_e.pdf  When dealing with really low distortion all manner of little annoyances become challenges. The caps and resistors can limit the achievable distortion.
I would start with several of Victors oscillators, inexpensive and state of the art for distortion, typically -150 dB for all the harmonics. https://www.diyaudio.com/community/members/vicnic.19397/ 

The Shibasoku 725 uses a phase locked sampling system to measure harmonics and reject noise. Its pretty trick. And this software does something similar https ://www.data-odyssey.nl/Diana.html 

Also some power quality analyzers give level and phase of harmonics. They can be a big issue in high power transmission systems.  This may be of interest https://vitrek.com/pa910-high-accuracy-multi-channel-harmonic-power-analyzer/ Or at least you may be able to find out how they calibrate it.

[Bxaresearch]

Sorry for delay... was ill over weekend.

So, I'm still trying to understand what the exact phase is required for.  Thinking about what's been in the thread so far, it seems like the plan is to:

• begin with excellent 1 kHz sine wave
• pass through some Device Under Test
• generate an n*1 kHz signal for use as a Local Oscillator (LO)
• mix the DUT output Vout with the LO
• low pass filter mixer output down to DC, i.e. select the nth harmonic
• measure said DC signal with multimeter to determine harmonic amplitude

Is that right?

If so, I can see why phase control is required; should the LO phase happen be 90 degrees away from the harmonic out of the DUT, you'll get zero.

With that in mind, how about this idea?

• begin with excellent 1 kHz sine wave
• pass through some Device Under Test
• generate an (n*1 kHz) - 55 Hz signal for use as a Local Oscillator (LO)
• mix the DUT output Vout with the LO
• band pass filter mixer output to select the 55 Hz output, i.e. select the nth harmonic
• measure said AC signal with multimeter to determine harmonic amplitude

Helpfully, you no longer need to fuss about DC offsets.  And you might be able to place the frequency of interest a bit higher than the 1/f noise corner of whatever post-mixer amplifiers you need.

I'm not certain that 55 Hz is exactly the right frequency offset here; I wanted to pick one that isn't an integer division of 1 kHz (to help mixer rejection) and lies between the 50 Hz and 60 Hz calibration points of the multimeter. You'd need to do some mixer harmonic analysis to check out all the mixer harmonic products.

I think in this case that you might be able to get away with an LO that isn't frequency locked to the 1 kHz tone, i.e. use 2 benchtop generators without clock synch.

Sorry for the delay on my part. I was talking to some Analog Devices people (after reading the above mentioned article, beyond just the author himself) on this subject to see how the question of exceedingly low distortion and low amplitude is approached there. I am in no position to give an official run-down of what I found out, this is my distorted recollection, so take it as such.

What is special in the problem I'm considering is that the 1kHz sine wave is output at 1mV level and the distortion of the signal should be at least -130dB or preferably much lower. My current understanding is that there is no instrument that is able to generate and measure in this regime. Notice that all of the low distortion oscillators maintain the specified specification only at fairly high output level - 1V to 10V levels. One would think that attenuating this output is an easy task, however a resistive divider would quickly destroy the distortion specification of the oscillator. So what is done to address that is that the modestly clean sine wave generator, capable of outputting 2 channels that are phase locked is first attenuated and then harmonically filtered using proprietary circuits, until the wave is 1mV and less than -130dBc distortion. The other sine wave that is phase locked (but not exactly in phase at this point) is used to sync the lock-in amplifier. An additional step is to sent the said 1mV 1kHz signal through the DUT and then use another proprietary circuit to notch filter the fundamental (1kHz). The lock in amplifier can then be used to tune phase and frequency of each harmonic and accurately measure its amplitude.
That was my original plan, but I was lacking all of the proprietary aspects. So apparently my original question is answered by: no there is not a commercially made instrument that can accomplish all this out of the box. Pieces of the measurement chain must be built and care must be exercised during measurement and interpretation process.

[1audio]

130 dB below 1 mV? To avoid noise the impedances will be very low. I'm not sure how resistive dividers made with suitable quality resistors would add distortion. Metal film resistors added distortion is in the -160 dB range. You would need to notch the fundamental 40+ dB and then add an amp to boost the output of the notch to something a lock in amp can measure. You will also need to correct for the phase shifts from the notch filter. The DiAna program has a routine to do just that and will show the amplitude and phase of each harmonic. I suspect you will need an amp to get the levels high enough to process. This ADC and separate notch may be useful (and lots cheaper than a lock in amp) https://e1dashz.wixsite.com/index/cosmos

[tridac]

You ciould keep it simple and use audio transformer diode doublers, (full wave rectifier), or electronic split phase, with level detect / schmit trigger on the output. Low phase noise and guaranteed phase coherence with the input. Most likely less than $100 of parts. To get square wave, double frequency twice, then divide by two to get a symmetrical square wave output. So may ways to do that without costing the earth, or complexity...

[TimFox]

In the locked wein bridge article quoted above the author says:
The oscillator powered up fine, giving a nice sinusoidal 5.15-kHz output at several volts, and independent EDN DI5291 measurements showed the second- and third-harmonic distortion products to be lower than −120 dBc


What measurement does it refer to? I failed to google it and don't know what that means

That should mean that each of the second and third harmonic signals are more than 120 dB below the level of the fundamental signal.
This can be measured with a spectrum analyzer, easiest done if you first null out the fundamental with a distortion analyzer (e.g., -hp- 339A) to reduce the dynamic range required on the SA.
"dBc" means dB with respect to carrier.