4 Signal Path Vids in less than 2 weeks? The gods have heard my cries!

You are a gem for the hardcore electronics enthusiastic... your new videos always makes me smiling...please make a video on how you achieve all these high skilled stuffs... Thank you so much Sir...

Amazing piece of kit, nice troubleshooting and an easy problem to repair. The biggest flex in this video in my opinion is the little "Bench 8" sticker on the table, I wish I had as much benchspace. ;)

Great video, great explanations as always! I'm pleasantly surprised how much this makes sense given that I took the relevant college classes 40+ years ago and have worked exclusively in the digital domain since then.

A very good video as per. I really like the SRS equipment UIs and industrial design. The notch method of THD measurement seems so obvious... once it's explained. Thank you for doing so.

3458A is also very capable to measure THD, with floor below -110dB at 1kHz or there about. There are some examples available with GPIB code for this application.

YIKES!!! It's not often we see TSP playing down near MY end of the frequency spectrum (i.e. Almost DC) We didn't even see a 1mm, $20,000.00 connecting cable being used! Are you feeling alright Shariar?

Cool video. I'm not familiar with lock-in amps, so I'll have to watch that other video but using a notch and adding gain seems like an intuitive solution to get the measure. I haven't worked with a lot of analog equipment, but I have written phase accumulators and worked with SIMD math libraries for audio synthesis before. Cool to see a practical DDS application for lab use.

Repair, repair and repair again! Shariar, thanks soooo much for your repair videos, I really like those! Waiting for the DSA to be added to your lab!

Great Video. Although, -110dB THD equals 0.0003%. I think there's an error on your first picture.. 0.000002% THD would be -154dB, which no generator is able to achieve. I did AC-Susceptibility measurements / Harmonic Analysis on Hi - Tc - Superconductors about 30 years ago. Other scientists stacked several LI amplifiers to collect the higher harmonics simultaneously. Very costly.. Instead, I built a Lo THD FFT system using the ultra-linear HP3458A, which I ordered in 1989, i.e. one of the first ones. With 100kHz / 16bit I achieved -96dB SNR / THD, using a Krohn Hite 4400A analog low distortion generator with specified 0.001% THD. At 50kHz, 18bit resolution, I could demonstrate even -108dB THD, @ 195Hz, if I remember correctly. PTB and I think also NIST still use the HP3458A for making ultra precise and low distortion analysis on AC signals, i.e. AC power measurements. You need to synchronize the sampling frequency with the internal oscillator of the HP3458A, so there is a modification to output the internal 10MHz via an optocoupler. The Black Edition 3458A has / had mysterious non equipped pads on its A/D PCB, which could be assembled with a few components for that purpose. 😎

Nice repair!

As always, learned something new, very much enjoyed this episode.

Really nice video. The main contributor to the THD is the second harmonics alone. 20 x log(1.4/0.9e-6) -9 = 114.8 dB. The rest of them didn't sum up to anything interesting. But they could have...

shahriar to this audio frequency pos: "i find your lack of gigahertz disturbing"

So glad the repair wasn't just another power supply, but alas just an opto . . . You gotta get that rental place to send you another one that they can't fix. But still enjoyable to see how you verified it.

interessting application of a locking amp. great video as usual. I own a EG&G Instruments 7260 DSP which should also be able to do such measurement. Now placing the DS360 on the wish list ...

Really cool video, and a creative way to measure the THD. I am not as familiar with the lock in amplifiers. I am just wondering if it is still possible that the THD may be worse on other ranges and frequencies of the DS360. Perhaps the signal path used for the 1.4V 775hz does not go through the resistors which reached high temperatures?

That source is impressive!

Good stuff,thanks.

Very interesting Video! Thank you!

Again lucky man :) (only one optocoupler) Great video! THX

You can use pro audio stuff for high dynamic range at these frequencies. Lately ES chips like the ES9822pro improved the landscape, resulting in devices measuring around 125 to 135dB THD with a noise floor better than 150dB (depending on the device YMMV). And of course it's trivial to connect these to a PC for FFT and see the entire spectrum.

Maybe review SR560 and SR570 and their NIM/SIM modules? Femto has some good amplifiers. Lecroy has a preamplifier.

I'm astounded that the critical filter was on a breadboard ... and still specs were met!!!

Awesome signal generator, which is almost always used in calibrations (just like the 3458). Which dynamic signal analyzer are you getting, also a Stanford like the SR785? That is an absolute banger, I still don't know if there are modern analyzers which can do a real sweep (with autoranging mid-sweep) which are _that_ fast.

Hello, great video. But why was it necessary to remove the fundamental when using a lock-in analyzer (LIA)? I understand that removing the fundamental is mandatory when measuring the signal energy with wide bandwidth instruments such as AC millivoltmeters. This was done that way in the late 1960s with the HP distortion meters such as the HP 334A. Otherwise the energy in the fundamental (which we do not want to measure) would totally dominate the measured value. But a LIA is the opposite of a wide bandwidth instrument. As you explained in one of your videos, it has an ultra-narrow bandwidth down to a few ten millihertz. So I would have assumed that it can measure the harmonics correctly even if the fundamental is present. The frequency distance between the 2. harmonic and the fundamental ist 775 Hz, much more than a LIA needs for totally ignoring the fundamental. Is this assumption wrong? Greetings and keep up the good work.

Love the thumbnail!

XLR output says a lot on its own.

Great video. At first after the 2015 measurement I expected you to just go with a notch filter and a DSA, which would be the obvious method, but using the lock-in amp to do the measurements was SO much more interesting! 👍

Thanks. I always learn what i did not know,yes?

Very nice , I bought my self brand new AP555 30k$. It measures down to -120dB THD. It has very nice Software .

PS I'd LOVE to have one of those.

In the audio range one can hack together quite impressive test gear, but my favourite piece of audio test gear is easily my Quantasylum QA403. Good enough for the vast majority of tasks and if a sub-ppm THD source is needed there are always fixed frequency wien-bridge oscillators, notch-filters etc. I was going to post a few interesting links but KZfaq won't let me.

Isn't just beautiful when you make something work again. I would be worried a bit about those opto-couplers forward current being maybe to high? Why would otherwise the led go bad? Or maybe the type of signal going in being to "sharp" rise time...

always thumb up

PS. Did the heating of the power amp section sort itself out? I'm not sur if you mentioned it?

do you konw about the "E1DA Cosmos ADC" Projekt it promises THD measurements of -116dB

I have a Wavetek model 112, it's not as good as this instrument. On serious note I want to adjust it but the pots are buried away and you need a riser card. However the PCB connectors are a weird gold flashed individual soldered pins that work like two scissor's, they interlock perpendicular to each other in a 'scissor action' as you push the card down onto a 'motherboard' below. I can't find them anywhere, am I making any sense ? or have you or your fan's below heard of such an animal please ? or even know where I might find such adapters ? If I could source the pins I would make my own riser and one for anybody who wants one !......cheers

I’d love to see the circuit diagram of the power amplifier output stages for them to be such a low distortion. Most power amplifiers seem to manage .01% distortion. I thought it would be fascinating to see how they drop this by such a huge amount annd before before the attenuator.

You can use good audio interface card to measure thd in < 48 kHz < 2 Vpp range signals

Would be useful to show the signal fundamental and harmonics in a Spectrum Analyzer with a very low noise floor?

The game of, is the pin stuck in the via? Or is there more solder. Frustrating looking at the board lamination composition and just giving a sigh, "going to be one of those boards" thankfully the lifted trace is part of an isolation circuit and not a high frequency matching circuit. Optocouplers are amazing but surprising when expensive lab equipment are brought down by burned out diods.

I wonder what settings the Keithley used - THD+n ? or just THD ?

Wouldn't the power amplifier (without additional filtering) at the end of the chain increase the THD again? I don't understand why there aren't filters after the power stage to reduce the distortions introduced by the non-linearities of the amplifier.

Your title 0.000002% seems to be a typo. -113dB is about 0.0002%. Forgot to multiply by 100? thd_percent = 100*10^(thd_db/20). I usually calibrate these by direct reading on NI PXI5922 and get something around that number. But it's actually about limit of 5922.

Is there a name for the method described here using the lock-in amp to measure the THD?

half a million times smaller! I can hear that 2nd harmonic because I'm a bat.

My Wavetek model 296 is now cowering in SHAME!!!

With the RTO, which i'm most familiar with, it's possible to apply a pretty deep digital filter on the first harmonic, and i've been able to measure down to 5 μV signals with it, using the math channel and zero clamping, to get rid of drift and 1/f noise...50-100x averaging gaveme 10,000:1 dynamic range, and, we tested that against the fft, and we were able to get 'direct' measurements that are about as dep as the fft. Nice setup you made...btw, some of those resistors that i have came from a ballistics computer, which had a huge diode /resistor / capacitor / inductor signal processor, which even had temeperature control heater resistors on the board...that group of resistors near the output reminded me of that ....they spared no expense in those military computers inn the 1960's!

-114dBc is not 0,000002% THD. It's "only" 0,0002% ...

Then I get jealous. Mine just pops up. :＜0.05 %, 10 Hz to 20 kHz, 1 Vpp

That thing must've gave itself a lobotomy!

I have to say you had a bit of luck finding the fault that fast.... Come on admit it ha ha. I have to say, I could have been there all day.