Journal notes, Corrected NBLNA PCB, Active Filter
GELLER Labs "Backyard Science"
Thoughts on a proton precession magnetometer design - a Proton Magnetometer Project. Build an Earth's field magnetometer.
The FDM MAGNETOMETER1 project is a low cost high performance proton magnetometer (a digital magnetometer) kit under development for universities and amateur scientists to be able to accurately measure and monitor changes in the Earth's total magnetic F field and to observe geomagnetic storms. Magnetic storms can cause large excursions in the field and are of concern to interests ranging from electrical power grids, radio communications, and satellite operations, to aurora watchers and amateur radio operators.
1 Filter Diagonalization Method "FDM" (harmonic inversion), see Jan 21 and Jan 23 entries, based on: Vladimir A. Mandelshtam, Howard S. Taylor, Harmonic inversion of time signals and its applications, Journal of Chemical Physics (1997), Volume 107, Issue 17, 1997, Pages 6756-6769
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I created a new Proton Magnetometer Group at Yahoo groups for those interested in discussing proton magnetometers with an emphasis on Earth's field measurements. I will try to keep it as open as possible without getting attacked by spam. I will also try first with no review of posts, let's see what happens. Please keep it friendly and professional. Use of real names is preferred, possibly required in the future.
September, 2009
Project Documentation (very early stages)
Journal Notes:
Monday, August 30 , 2010
I posted some notes on the second order active filter on the analog board at the Proton Magnetometer Group. Since then I noticed this very interesting active filter calculator. Here is a PDF of the results for the filter we are using. Active filter design has come a long way since the nomographs (sample nomograph, a now very outdated method of graphical determination of component values) of Hilburn and Johnson's 1973 Manual of Active Filter Design Design! Our Second-Order Multiple-Feedback Band-Pass Filter comes from Hilburn, page 100.
An error has been reported on the analog printed circuit board. Pin 5 of the SSM2019 (input stage), the local common reference, was inadvertantly left floating on the first PCB prototype board (it was correctly connected on the hand wired prototype). The error has been corrected (pin 5 to local ground plane via a thermal relief) and the prototype board will need to be re-evaluated. It is unclear why the "mystery circuit" (with pin 5 floating) worked at all. There will probably be some small gain changes in other stages. Such corrections are greatly appreciated, many thanks! The current corrected version of the narrow band low noise amplifier (NBLNA) is Ver. 0.91. All seems well this evening: PDF, TXT. I will completely re-test the analog board tomorrow.
Tuesday, August 31, 2010
NBLNA Board Characterization (retesting the corrected board)
GAIN: [588,000] Gain of the newly configured NBLNA Board (some feedback resistors were changed for very slightly higher gains of individual stages) was measured using a hp 3325B synthesizer with a 10 mV output at 2290 Hz into a Kay 837 attenuator set to 69.5 dB attenuation for an actual input voltage of about 3.4 uV. Voltages (except for the estimated 3.4 uV input) were measured with an Agilent 34410A DMM. The NBLNA input was configured with two 50 ohm resistors (one from each input) to common. The signal was applied across one 50 ohm resistor (simulating the output of only one of the counter-wound coils (only one of the counter-wound coils is powered and has the NMR liquid sample). 50 ohms is used for proper termination of the hp 3325B and the Kay 837 attenuator, all calibrated for a 50 ohm load (powered coil R including the 100' cable is around 6 ohms). The gain of the second stage was below the ratio of the resistors for a non-inverting stage (1+Rf/Rg) becuase of some roll off in gain (at 2290 Hz) caused by the 1 nF capacitor which could be made a little higher (roll-off higher, C smaller) now that the feedback resistor is 2.87k. The overall gain, based on output voltage over input voltage (2.0 V / 3.4 uV) was measured to be about 588,000. Using the gain measurements of individual stages, the overall gain was found to be about 582,000 agreement to about 1%. Here is a PDF of the gain testing notes: PDF.
EFFECTIVE NOISE BANDWIDTH: [182 Hz] A test setup sweeps the hp 3325B through a range of frequency, measures the output with the Agilent 34410A, and computes the EFNBW: PDF This number might be adjusted during noise testing which follows, the first run was probably made over too narrow a bandwidth to achieve an accurate effNBW number. yes, the first run was over too narrow a bandwidth (1,250 Hz to 4,500 Hz). A far more accurate run (consistant with the Johnson noise test data) was made over a wider bandwidth (300 Hz to 20 kHz) with more points (300): PDF. It turns out that very high Q (narrow band) amplifiers need both a significant number of data points (e.g. 300), as well as a wide enough frequency sweep to achieve accurate effective noise bandwidth results. The number of data points is less important for relatively wide bandwidth filters, such as the wide band filter that we used in our JCan (Johnson noise measurement) experiment.
NOISE FLOOR: [1.8 nV/rt-Hz] PDF. In this measurement, output voltages are measured for various input resistors connected across the NBLNA input terminals (bias current and the common connection is made through the existing 10 kohm Rs to common). Assuming the validity of the known Johnson thermal noise equation, gain and effective noise bandwidth are reconciled to match the theoretical noise curve while accounting for input bias Rs and temperature. Once the gain, effNBW, and noise with a shorted input is determined, the noise floor can be calculated. Once the gain of a low noise amplifier is determined, slewing the effNBW figure as a variable until the measured curve overlaps the theoretical Johnson curve on a properly constructed worksheet can be an easy and effective method for measuring the effNBW of an amplifier.
Relatively quiet magnetogram this evening, the repaired analog board seems to be doing fine (small step changes are parked vehicles arriving (depressed field) and departing (increased field)). PDF, TXT, sample spectra, log spectra.
Project Documentation (very early stages)
QUESTIONS/COMMENTS/notice of typos, etc. send email to joegeller at gellerlabs dot com
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