LF Crystal Tester: OLED Noise vs. Log Amp

Having installed a cheap USB isolator to remove some obvious 60 Hz interference, the 100 Hz OLED refresh noise definitely stands out:

Log amp - xtal amp - OLED noise
Log amp – xtal amp – OLED noise

The bottom trace comes from the 100× = 40 dB MAX4255 amplifier boosting the crystal output to a useful level. The fuzz on the waveform is actually the desired (off resonance) 60 kHz signal at maybe 30 mVpp, so the input is 300 µVpp.

The worst part of the OLED noise looks like 100 mVpp, for about 1 mVpp at the crystal output, call it +10 dB over the desired signal. Some high-pass filtering would help, but it’s easier to just shut the display off while measuring the crystal.

The top trace is the log amp output at (allegedly) 24 mV/dBV. The input bandwidth obviously extends way too low, as it’s neatly demodulating the input signal: the peaks correspond to both the positive and negative signal levels, so reducing the 1 µF input coupling caps will be in order.

In between those 100 Hz groups, the input signal shines through to the log amp output at the V1 cursor. The peak noise rises 290 mV above that, so the log amp thinks it’s 12 dB higher. Pretty close to my guesstimated 10 dB, methinks.

So, turning off the OLED should help a lot, which is feasible in this situation. If you must run the display while caring deeply about signal quality, you must devote considerably more attention to circuit construction quality.

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