This documents some follow-up to the last post on this topic (here). REVD of the hardware is back and has been built. There were two big changes made: The first was to restructure the lineup so that the 1 kHz oscillator was followed by a 0/12 dB gain stage, and that is followed by the attenuator. That is reflected in the app/block diagram below:
The second big effort was to completely shield the different sections. The photo below shows a board without shields installed.
With shields installed, the board appears as follows:
The shields were laser cut from 0.5mm 0.02" Nickel Silver (H02 1/2 hard) from OshCut. They are a bit heavy and too expensive for production, but solderability is fantastic. Ideally for shielding below 10 or 20 MHz you’d want something with more steel.
The shields ship flat can be bent into shape:
In the end, the shielding doesn’t seem to have made much of a difference. The 2H might even be a little worse coming out of the notch with the input shorted:
The risky change was moving the amp ahead of the attenuator. This is because it required an extra opamp inserted in the signal path. SPICE suggested it probably wasn’t an issue, and in the end it appears SPICE was correct.
The oscillator center frequency on this build was reported at 996.26 Hz.
Sweeping the notch yields the following. Note that the notch depth is almost 70 dB. This is better than the unit tested on RevC, but not due to any changes. It’s just the luck of the draw given part tolerances.
We can export the notch and get some key metrics on the notch performance. Note that at the fundamental we’re getting 54 dB of attenuation.
With the notch loaded correction loaded into the User Weighting, we can look at the oscillator output through the notch with the correction applied at roughly 6 dBV. Note our output is indicated at 17.17 dBV. This is because the notch has 12 dB of gain built-in to the output stage (see block diagram above)
Normalizing and upping the FFT size to 256K and 5 averages gives a THD figure of nearly -150 dB. Now, it’s not clear why this should be any better or worse than the RevC hardware. The layout is a bit cleaner, and there are shields, but I’d be inclined to believe it’s more part tolerances than anything else. But more study would be needed.
And engaging the +12 dB amp on the oscillator output yields the best-case THD+N
That’s all for now!