Introducing The QA480

Post by Matt, July 2020

The QA480 will be a limited-run (~25 units) product for those doing hard-core R&D testing on advanced products that are at the limits of what their current test setup can achieve. 

As with everything in this world, increasing performance carries with it a growing price tag. Roughly, to improve your THD+N test capability by 10 dB generally requires you spend about 10X more money as shown in the graph below:

Point A on the graph above is the QA401 ($450). Point B on the graph above is a state-of-the-art tester with an analog option ($30k)--and there are a range of options in between. Make no mistake--the $30k price tag gets you a lot more than just better THD+N. But the graph should illustrate the point that a few extra dB of performance costs, and it costs a lot. The QA480 is a way to cheat this, with the biggest cheat coming from the fact that it's a fixed frequency device. 

Usually, we're conditioned to believe that digital is better. But that is turned on its ear here--the highest performance analyzers are hitting their numbers using mostly analog circuits.

Today, you'd be doing really well to achieve -110 dB THD+N using a state of the art DAC and ADC chipset and a host of external circuits (and lots of relays) to keep the ADC and DAC fed at their optimum point. If you need better than -110 dB THD+N, you aren't going to generally achieve it with digital converters. Or at least with an unaided DAC. To push beyond -110 dB THD+N you will need to use a discrete oscillator driving into your DUT. The discrete oscillator built from modern parts can generate a large amplitude sine wave with harmonics at -150 dBc. This is far beyond what any DAC can hope to accomplish on its own. 

So, the first takeaway here is that a discrete analog oscillator is required to hit the spectral purity you need to approach the -120 dB THD+N figure. 

Once you've pushed the ultra pure sine wave into your DUT, you need to measure it to see how the DUT might have distorted it. Again, if you just pump that signal into an ADC, you'll be disappointed because the ADC, especially at higher levels, will introduce its own distortions.

Below you can see a graph of THD+N versus input level for the AK5397 (taken from the AK5397 EVM data sheet) which is the ADC used in the QA401. Normally, what you see in a THD+N measurement is a ratio. In those measurements, the tone level is compared to the noise and distortion level.  Below you are seeing a level measurement, which means what you are seeing is the level of the noise and distortion (Y axis) for a given input level (X axis)

In the plot above, take a look at Point A. This indicates to us that when the input level is 0 dBFS (read from the X axis), the THD+N level will be -93 dB below that. We can convert that to a ratio by 0 - -93 = -93 dB THD+N. At point B, we're incoming at a -20 dBFS level and the level of the THD+N is -115. So, the THD+N ratio is -20 - -115 = -95 THD+N. Clearly, there is no point on the graph where you'll get -120. At a single point of -5 dBFS you could see around -115, and this translates to -110. The story is mostly similar for other converters, with the usual trade offs you'd expect.

Something interesting happens, however, if you use external analog processing (a notch filter) to reduce the amplitude of the fundamental while disturbing the harmonics as little possible. For example, if you have an incoming 1 kHz signal at 0 dBFS, but you use a notch filter to knock that down 50 dB, then you'd expect your ADC to see an input of -50 dBFS, and this would give a level of THD+N of -115 dB. Your ratio in this case would be -50 - -115 = -65 dB THD+N. But in fact we know the 1 kHz is actually 50 dB higher, so we can add that 50 dB to the -65 dB figure and we get -120 dB. 

And suddenly, what was pretty routine is now exceptional. The magic comes from the notch. And fixed notches are cheap and easy to build. Tunable notches (and generators)--the kind you find on the $30K analyzers, are not cheap and easy to build. And again, that's the "cheat" here--the fixed frequency makes this much easier.

Typical Test Connections

The connection diagram of the QA480 and QA401 for testing a generic device under test (DUT) appears as follows. Note that if your DUT is a single modern opamp, you'd still not be able to measure the opamp's performance using this setup. That's because a modern opamp can exhibit THD+N levels (20 kHz bandwidth) approaching -135 dB THD+N. And that means you'd need a test setup with roughly -145 THD+N capability (and since those don't exist, opamp makers have to rely on distortion magnifying circuits to measure THD and THD+N--see Section 8.3 of the OPA1612 spec for an example). But this test setup will readily let you measure the differences in capacitor quality, for example, among very esoteric capacitors.  

For testing a PC-connected DAC it would be similar. Note that the DAC would be required to generate its own sine, but there are plenty of applications (such as Audacity) that can generate very high quality sine waves in just about any format you might need. The benefit of a DAC is that you can slightly tune the sine frequency to "ride" anywhere you wish on the notch.

A state of the art audio DACs (the kind you hook to your PC to listen to music) today are approaching -120 dB THD+N at a single point. You'd need a test setup with roughly -130 dB THD+N to accurately measure that, which isn't possible at any price. But the setup below would let you verify if the numbers were in the ballpark. 

Background Reading

Before going into more detail on the hardware, there are a few additional sources of info that might be helpful on background. These include a discussion of the circuit lineage, performance improvements from each iteration, layout, shielding, etc. 

1) Blog post: Notch Filters

2) Forum post: Notch Filter Hardware RevC

3) Forum post: Notch Filter Hardware RevD

The QA480 pieces (oscillator, notch and attenuator) are definitely not new ideas. What is new is that it's in a single box, powered by USB, can deliver monster drive levels (+18 dBV) and has a 63-step, 1 dB relay-based attenuator, to make the pristine signal useful at a variety of levels. And it integrates nicely with the QA401 software to de-warp the notch effects and let you look at a waveform as if none of the signal processing magic were present.

The Hardware

The near-final front-panel appears as follows:

On the left side is the output from the analog oscillator. This is fixed at 1 kHz, plus minus a few Hz that come from component tolerances.

On the right side you can see the input and output of the notch. Internally, there's a relay that allows the notch to be bypassed so that you can do quick sanity checks on your setup without re-plugging cables. 

The LINK led indicates you are power and connected to the QA480 application. The HI GAIN OUTPUT means your max output is +18 dBV instead of +6 dBV. In this higher gain mode your performance should still be quite good (THD of -130 to -138), but it won't be quite as good in the low-gain setting (THD of -138 to -148). Finally, the NOTCH BYPASS LED reminds you that the notch has been bypassed.

QA480 User Interface

The user interface on the QA480 is shown below. The left side controls the oscillator amplitude, and the right side controls the notch. There will be more cleanup on the attenuator settings. Note that the indicated level of +6 dBV is rough. It will likely range from 5 to 7 dB depending on tuning tolerance. 

You can see the final output range of the oscillator will range from +18 to -57 dBV. The lower range (+12 dB disabled) will be the highest performing range. 

For the notch controls, there will also be unit to unit variations in the depth of the notch as well as the center frequency of the notch. A particular unit might have an oscillator frequency of 998 Hz and a notch frequency of 1001 Hz. Or an oscillator frequency of 1001.7 Hzand a notch frequency of 1000.2 Hz. It's the job of the QA401 software to de-tangle these differences. 

There might be an option to allow the notch to be nudged slightly higher or lower. That is the Up/Down "Notch Center" control. It's still TBD to see if that works reliably or not. But it will be likely present (and fallow) in the first build.

Oscillator Performance

Let's take a quick look at the QA480 oscillator going directly into the QA401. In the plot below, you can see the amplitude is 5.26 dBV. There are substantial 2H and 3H products, but that is because the QA401 is about 0.5 dB away from the overload point. We'll solve that momentarily.

Checking the frequency with the Wow and Flutter visualizer shows no real wandering, and a frequency of 999.688 Hz. We'll need this info later when we export the notch.

Notch Performance

A problem you can read about in the previous posts (linked above) above was 1 and 2 kHz leaking into the notch. This manifested as 2 kHz products appearing in the output of the notch while the notch input was shorted. Initially this was though to be radiated, but even after full shielding was added there was no change. The ultimate mechanism appears to have been due to regulator pumping. That is, as the 1 kHz oscillator traced out the sine wave, the instantaneous current required by the oscillator would rise and fall. There is a spec on regulators called "Load Regulation" that indicates how the output voltage changes in response to a given load. These numbers are usually very small--under 1% over a 150 mA change in current. But in a high performance circuit the change in LDO output voltage is seen by the notch opamps as a power supply disturbance. The OPA1656 has about 90 dB of PSRR on the negative rails at 1 kHz. In the end, the regulator output was wiggling in response to the current demands of the oscillator, and that wiggle was being impressed on the output of the notch due to insufficient PSRR. Normally, this isn't a problem. But when you are looking for products 150 dB below the fundamental, it matters. 

The solution to the above is a dedicated LDO for the notch and for the oscillator, and this has fixed the 2H appearing in the notch output. Ultimately, that was also limiting the measurements on previous hardware. A plot of the notch spectrum with the notch input shorted is shown below. This is nearly a 15 dB improvement on 2H compared to the REVD hardware--it's completely gone in this build. The fundamental has also been knocked down by nearly 20 dB. Note the 20 to 20k RMS is 107.6 dBV. Input referenced, this is about -120 dBV. Not bad for having two CMOS opamps in the signal path. 

Sweeping the notch yields the following plot. The sweep amplitude was -12 dB, and the notch has 12 dB of gain so we see the 0 dB level at 100 Hz. Note that when sweeping a notch for export, it's very important to use turn of all smoothing and use the largest FFT (256K). This will let you see the true depth of the notch.

When we export the notch, we need to specify the frequency of the oscillator. We measured that above using the wow and flutter tool. Below, you can see where the export tool has examined the characteristics of the notch and measured the attenuation of the notch on the oscillator at 1H, 2H, 3H, etc. This will be used later to automatically "unwarp" the amplitude distortions caused by the notch.

Now, we can route the oscillator output to the notch input, and then route the notch output into the QA401. The plot of that measurement is shown below. Remember, the oscillator output is nominally 6 dB, and here we see it at -55 dB, meaning it's been suppressed 61 dB by the notch. But remember the +12 dB gain on the notch. So the actual suppression is closer to 73 dB. And that coincides with the -72.19 dB we see in the fundamental export box above.  

 

 With the de-warping applied via the User Weighting (where we import the previously exported notch sweep), we get the following:

Normalized to 0 dB, we get the following: The 2H is at -146 and the 3H is at -153. What appears to be a harmonic at 4 kHz is mostly likely not a harmonic as its frequency is higher than would be expected.

Anticipated Performance

Right now, I think the following are probably reasonable targets (+/- 2 dB)

 Osc Level (dB) Atten Gain (dB) Output Level (dB) THD (dB) THD+N (dB)
6 0 6 -143 -122
6 -5 1 -138 -118
6 -10 -4 -129 -114
6 -15 -9 -129 -108
6 -20 -14 -123 -104

 

At the lower output levels, the noise floor is the limiting factor. 

QA480 Availability

The QA480 is going to be a limited run because it requires a fair bit of hand-tuning and frankly I'm not sure what that means. All the units I've tuned to date have been 5-10 minutes per unit and I think I've got it down, but I don't have a good idea on what the yield will be. Nor do I want to be spending all day tuning. Early on I eliminated pots because I don't trust them in the long term. But that means tuning requires part swapping. All of our other products are "no tune" designs that simply require an automated test to run to verify operation.

This is also an expensive product to make. The QA401 has 60 unique parts and 261 placements.  The QA480 has 68 unique parts and 200 placements. All the resistors in the signal path are thin-film 0.1% 25 ppm, which add up when used in such low quantities. 

Assembly is done in the US, and will probably run $60 per board given the volume and part count. That cost is just the cost to assemble and doesn't include the parts, the case, etc.

Normally, this would be a $349 product for us. But given the niche demand, uncertainty and limited volumes, the first run will be $239 + shipping. In return for the discount, my hope is that there will be some customer leniency on any rough edges related to performance as we jointly learn about this product. That's going to be the nature of the beast given the tolerances on notch and oscillator. 

If you receive and decide you don't like the product, the 15-day "no questions asked" policy always applies. If you are in the US and return the product at your expense, you'd be out $10 or so for shipping. If you are outside the US, the return costs would be higher and you'd have to weight that.

Final schematics for the oscillator, AGC and notch will be made available once completed. This would let you tune for another frequency, let you try other caps, etc. Firmware, PC source code or schematics for the other sections will not be available. 

Stay tuned for more info. If you've sent a mail to the sales alias with QA480 in the subject line, you should get a mail once the link to purchase goes live. 

Thanks, Matt

PS. This post can be discussed on the forum at the link HERE.

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