E = (CV^2) / 2
A 1uF cap at 10V = (1e-6 * 10^2) / 2 = 1e-6 * 100 / 2 = 50 microjoules
Change the capacitance to 0.5uF. The voltage goes to 20V for the same charge:
(0.5e-6 * 20^2) / 2 = 100 microjoules
Voltage wins.
I have a dim recollection of you posting a discussion of some sort of distributed shock line using varactors, or something like that.
How about first doing a voltage to frequency conversion and then doing the phase noise measurement ?
Put a capacitance diode into a crystal oscillator (VXCO) and feed the voltage to be tested to the capacitance diode. Multiply the oscillator frequency by multiple frequency stages (to say 1 GHz) to broadening the phase deviation. Using an other clean crystal oscillator to heterodyne down the multiplied VXCO frequency down to a lower more easily managed frequency (say 1 MHz) and measure the large phase noise deviation with a spectrum analyser. ?
Seismologists, perhaps?
CH
Gerhard Hoffmann wrote: =====================
** What planet are you from ???Nobody wanted to measure noise at 15mHz. That number merely results from my suggested input RC values.
10Mohms is a standard voltmeter impedance. 1uF is a small, readily available *low leakage* cap.Get real.
...... Phil
You're putting in energy by pulling the plates apart against electrostatic attraction.
Jeroen Belleman
** Was waiting for someone here to post that *pedantic* twaddle .
..... Phil
Steve Wilson wrote: ...>
Why not use a battery?
-- Uwe Bonnes snipped-for-privacy@elektron.ikp.physik.tu-darmstadt.de
Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt
--------- Tel. 06151 1623569 ------- Fax. 06151 1623305 --------- (Phone also available during "mobile work")
Oh, get lost!
Jeroen Belleman
Uwe Bonnes wrote: ============== Steve Wilson
** FYI: Batteries are chemical noise generators, including rechargeable types.I own bench gear with high input impedance and gain of 90dB with -3dB response to 100kHz to measure such things.
If I need to.
...... Phil
Sure, I have too. In his frequency range and noise levels there's no reason not to. But a dedicated noise tester really needs to be able to go below 0.1 Hz IME. Lots of voltage references specify noise in that range, and below a few hertz the tempco of the capacitor can easily be a serious limitation.
The effect is so large--like 1 mV/K on a 5V supply-- that you don't have to slew the temperature of the inside of the cap. Just the outer foil is enough to worry about, and that changes surprisingly fast with air currents and so on.
Cheers
Phil Hobbs
If all you're attaching is a mic and some speakers, that's true. In an instrument that 16 nV/sqrt(Hz) could easily degrade the noise floor by
25 dB. (Discrete circuitry is de rigueur for the lowest noise applications, and generally doesn't have much PS rejection.)Cheers
Phil Hobbs
Getting within 30 dB of there will be a challenge with that circuit.
Cheers
Phil Hobbs
Charge is conserved always, and CV is conserved if the charge has no place to leak to. If epsilon is nonlinear with voltage, you have to do an integral to get Q, but film caps are pretty good that way, and anyway this is a one-figure argument.
Yes, but not 1/2 CV**2.
It is, but you have to do mechanical work against the E field to get them apart, which is where the increase in 1/2 CV**2 comes from.
Cheers
Phil Hobbs
Good ones are very good, apart from temperature drift. See this paper from NIST:
Cheers
Phil Hobbs
That could work in principle. The difficulty is coupling the desired voltage to the varactor while preserving the desired high frequencies.
Hyper-abrupt varicaps used to be available that gave a fairly linear capacitance change vs voltage over a limited range. Normal junction varicaps are still available but are highly non-linear. You would have to know the approximate voltage of the source to know where the varicap is operating.
Multiple varactors could be run in parallel to reduce thermal noise.
However, the intent of the direct-coupled preamplifier is to add sections in parallel to obtain noise values less than 1nV/root(Hz). The LT1028 has a typical corner frequency of 3.5 Hz, which is hard to beat.
I'm not sure if a varactor bank in a low noise oscillator could reach those levels. Probably not.
The 1/f flicker noise of a vco would probably turn out to be the limiting factor.
But thanks for the suggestion!
Nice thanks. I have seen vibration effects in batteries. Mostly from air vibrations... Batteries were used to bias photo-conductive detectors (Scanning FFT spectrometer) Signals were in the audio range, and loud talking when taking data would go right into the signal. (Everyone in the lab would tell me to shut up! :^) George H.
There are various mitigating options, such as enclosing the preamplifier in a shielded box to minimize RFI and stray air currents, immersing the preamplifier in mineral oil to minimize temperature fluctuations, etc.
The biggest problem is the 3.5 Hz corner frequency of the LT1028. This limits the usable low frequency response of the preamp to 1 Hz or so. Temperature fluctuations of a shielded capacitor are probably well below this, so they will not contribute to the measurement.
Long term drift is automatically cancelled by the feedback of the op amp. Again, the low frequency cutoff of the preamp minimizes long term effects.
The typical 1/f corner frequency of the LT1028 is 3.5 Hz. This limits the usable low frequency response of the preamp to 1Hz or so. Multiple sections can be added in parallel to reduce noise, but this quickly runs into practical limits of 1/sqrt(N).
The value of the preamp is measuring sources that have greater flicker noise than the preamp. Examples are low noise power supplies, zener diode noise, IR LED noise, etc.
This approach is used in some instruments for extremely low noise measurements. The batteries have to be charged, but the charging connection is removed in order to make measurements.
The low noise supply is intended to power low noise crystal oscillators in GPSDOs or other applications where continuous operation is needed.
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