I'm thinking about the possibility of using high-Q 80MHz crystals in a sensitive electric-field detector.* You know, consider the usual amplifier in a high-performance crystal oscillator, but without ANY feedback path, and further modified so the amplifier doesn't excite the crystal, not even a little.
Looking at crystal models, the loss-resistance element may be on the order of 10 ohms, which implies a Johnson noise density on the order of 0.4nV per root-Hz. So an optimum kT-sensitive amplifier would need a similar noise level. Hmm, that could imply a rather large JFET with excessively-high capacitances. Certainly the JFET will be part of the crystal's tuning capacitance, but I'm probably going to be limited to say 10pF or less. A 2sk146 JFET has 1nV noise, but has 40pF of capacitance. A 2sk152 has 1.8nV with 8pF, that's getting closer. But 1.8nV is 4.5 times kT for a 10-ohm crystal...
Maybe a low-noise BJT amplifier would be better... I have some 2sd786 transistors, which state 0.55nV on the datasheet, for Ic = 10mA. Oops, then r_e = 2.5-ohms and with a beta of 500 Zin would be only 1.2k, not good enough to avoid loading down a sensor with Q = 20,000. BJT base-current noise would be another issue. Hah, JFETs suffer from a bit of current noise at RF frequencies, according to AoE, but no doubt much less than a BJT amplifier.
Another issue, fundamental vs overtone mode crystals. I've read that fundamental-mode crystals have much lower Qs than overtone mode, e.g., Corning lists a 4:1 improvement for 3rd versus 1st.
- Don't ask about the application just now. It's a bit exotic, the same experiment that's getting the 10kV 1us precision ramp.