Very cool! Absolutely enjoy the vintage Tek scope and nixie tube equipment, very classy! Thanks for the great video!

Thanks for the CT-1 data, Jim. However, it seems to me that if you had placed the CT-1 on the high side of the crystal, at the output of the LTC1440 instead of the input, stray capacitance would become irrelevant.

I see that you are post-filtering the RMS/DC converter. With a 10uF averaging cap and 32kHz in, this is unnecessary. Was this part of the circuit an already-built leftover from other work?

The Tektronix CT-1 probe does indeed have less than one pF of stray capacitance. The older datasheets specify this, and are in general, more complete. Please see this copy, taken from the 1978 Tek catalog. It lists .6pF introduced capacitance.

Each crystal manufacturer specifies a maximum power dissipation in the crystal, typically 1uW. We have found that if you stay below half this figure most crystals become reasonably insensitive to stray capacitance. A guideline is 2 to 5 ppm/pF load capacitance introduced shift. Note that this figure is for load capacitance and not stray capacitance introduced by the measurement. Typically, we introduced small, controlled amounts of capacitance at the measurement point to determine measurement induced shift.

I think this will answer your questions, but anyone should feel free to contact me at LTC for a complete Tektronix datasheet or further discussion. I am on extension 3730.

1. At the beginning of the video Mr. Williams makes a strong point that it is extremely important to measure the crystal current while minimizing the introduction of stray capacitance into the circuit. A limit of 1 pF is mentioned. I can't imagine how the Tektronix CT-1 probe could have 1 pF or less stray capacitance, but I've never used one. The probe spec sheet does not make reference to the probe's capacitance; only to its leakage and magnetizing inductances and insertion impedance at different frequencies.

2. Although a very small current was indeed measured using Mr. Williams' circuit, there was no quantitative description indicating by how much an improper amount of (small) current, would change the crystal frequency, or more correctly "the crystal plus its opamp" operating frequency.

So, in summary, my questions are:

How much stray capacitance (less than 1 pf ??) was introduced by the current probe, and

How much did the frequency change as a function of operation current.

I suppose a simple proof of "low enough capacitance" would be to measure how much the circuit's output frequency changed when the probe was removed completely from the circuit.

I’d also like to learn exactly how to modify the crystal oscillator op amp circuit, to ensure that the proper value of 'very small' crystal operating current would be obtained.