W
Winfield Hill
- Jan 1, 1970
- 0
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.
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.