A quick calculation shows that it should be possible to do with an air coil.
I used this https://coil32.net/online-calculators/multilayer-coil-calculator.html
and approximate dimensions estimated from the pictures.
I used this https://coil32.net/online-calculators/multilayer-coil-calculator.html
and approximate dimensions estimated from the pictures.
@1audio Got your PM. I'll send you some samples today. Thanks!
My first series of tests on 15KΩ resistors used the 1984 CMF-55 10K matched to 0.01% in the bottom leg. I saw a little HD2 increase with all DUT so I changed strategy.
The bridge is driven by a ground-referred balanced output and and is recovered by a floating-input balanced "double-twin-T" (quad-T?) notch filter followed by a 5532 INA and cross-coupled THA1246 for CM rejection. The 1 kHz - and generator distortion - appear in common mode. In this situation the notch filter doesn't do much to reject the 1 Khz from the generator as most of the heavy-lifting is done by the CM stage.
When the bridge was all 10K the CM voltage pivoted around 0V. When the bridge was used with 15K in the top arms and 10K in the bottom the CM range shifted which gave rise to small amounts of HD2 in the INA. So I switched to an all 15K bridge and used the MBB0202 and YR1B as the reference resistors.
I couldn't find a combination of MBB/Y1RB that produced distortion. In this test there are 2X MBB and 2X Y1RB with the Vishay pair in diagonal legs.
No matter how I mixed and matched MBB0207 and Y1RB the results were consistent.
When a modern CMF-55 is introduced into the bridge things turn south.
Not good.
I also decided to test the theory that 4X "bad" resistors in the bridge would test great. This is 4X modern CMF-55.
They don't. The FFT looks like a pine forest.
If I were building a resistor tester using this method I think I'd make the bridge amplifier a THAT1510 or INA217 with the bridge connected directly to the bases and program the gain to be about 40 dB. With the THAT1510 input-referred CMRR would be about 90 dB. The INA217, which seems to have trimmed CM resistors it might be about 20 dB better. The bridge would supply bias current from the generator output or extra Rs at the tippy-top and bottom of the bridge pulled down to ground. That way CM rejection isn't affected by added bias Rs.
I'm enjoying this way too much LOL.
My first series of tests on 15KΩ resistors used the 1984 CMF-55 10K matched to 0.01% in the bottom leg. I saw a little HD2 increase with all DUT so I changed strategy.
The bridge is driven by a ground-referred balanced output and and is recovered by a floating-input balanced "double-twin-T" (quad-T?) notch filter followed by a 5532 INA and cross-coupled THA1246 for CM rejection. The 1 kHz - and generator distortion - appear in common mode. In this situation the notch filter doesn't do much to reject the 1 Khz from the generator as most of the heavy-lifting is done by the CM stage.
When the bridge was all 10K the CM voltage pivoted around 0V. When the bridge was used with 15K in the top arms and 10K in the bottom the CM range shifted which gave rise to small amounts of HD2 in the INA. So I switched to an all 15K bridge and used the MBB0202 and YR1B as the reference resistors.
I couldn't find a combination of MBB/Y1RB that produced distortion. In this test there are 2X MBB and 2X Y1RB with the Vishay pair in diagonal legs.
No matter how I mixed and matched MBB0207 and Y1RB the results were consistent.
When a modern CMF-55 is introduced into the bridge things turn south.
Not good.
I also decided to test the theory that 4X "bad" resistors in the bridge would test great. This is 4X modern CMF-55.
They don't. The FFT looks like a pine forest.
If I were building a resistor tester using this method I think I'd make the bridge amplifier a THAT1510 or INA217 with the bridge connected directly to the bases and program the gain to be about 40 dB. With the THAT1510 input-referred CMRR would be about 90 dB. The INA217, which seems to have trimmed CM resistors it might be about 20 dB better. The bridge would supply bias current from the generator output or extra Rs at the tippy-top and bottom of the bridge pulled down to ground. That way CM rejection isn't affected by added bias Rs.
I'm enjoying this way too much LOL.
Very, very informative. Thank you so much for your work!
Hi Demian,
That is a very cool instrument. I loved the USB plug for scale in the first picture. It had to be air core coils, nothing else would work. It also explains the 10KHz test frequency, balancing between HF effects and component size. A really well thought out instrument.
Hi Demian,
That is a very cool instrument. I loved the USB plug for scale in the first picture. It had to be air core coils, nothing else would work. It also explains the 10KHz test frequency, balancing between HF effects and component size. A really well thought out instrument.