Using a similar circuit to the LF block converter, I've made a Schumann Resonance Converter that moves the near-DC signals up to around 2 kHz for an ordinary soundcard. Here's the maiden run plot:
It seems to work! The image is from Spectrum Lab with an offset of 2.048 kHz (the L.O. frequency). The 60 Hz is reduced by an unusual bridged-T notch filter that uses a little audio transformer as the inductor. I discovered that the inductance of such a transformer can be varied quite a bit with a very tiny current (or even a weak magnet). Only 100 uA of DC current will tune the filter many 10's of Hz! Here's a snapshot of the schematic so far:
Circuit Description: Starting on the left, the antenna should be a fairly long vertical mounted high in the air, away from trees. The lead-in cable will have significant capacitance, perhaps as much as 1000 pF, and that's fine. (In fact, you might want to add a shunt capacitor if the hum is too big for the FET follower. If that's the case, you might also try increasing the 220 pF capacitor, to, say, 470 pF, but that's starting to eat into the gain at the higher Schumann resonance frequencies.) The first 62 megohm bleeds charge off the antenna and the neon bulb or other gas discharge device limits the voltage. Those components are on my RF connector, not in the photo. The .01 uF helps to isolate the JFET from the voltage and it should have a high breakdown voltage, just in case. The second pair of 62 megohm resistors bias the JFET gate to V/2, to give the source plenty of room to swing, since there's likely to be a big AC hum signal. Those big resistors could be comprised of 3, 22 megohms in series. The 10 megohm is in the signal path, but its noise is well below the atmospheric noise, and it works with the 220 pF to roll off the frequency response above about 70 Hz. The 1k is there to keep the JFET stable, regardless of the type chosen. The FET is very lightly biased by the 56k resistor and some current flowing in the transformer to conserve power for battery operation. The source of the JFET drives the bridged-T notch filter. The selected capacitor (indicated as two) resonates the transformer at 60 Hz, and the value will depend on the transformer. The 50k pot sets the depth of the null and that value may require some experimentation with other transformers. The 500k pot and 100k resistor cause current to flow in the transformer, changing its inductance for tuning the center frequency. The differential amplifier performs the mixing, with the current sink transistor turning a small current on and off, about 200 uA (there should be a 2 volt square wave at 2 kHz on the 10k emitter resistor). The pot on the far right balances the two transistors so that they are turning on and off with equal duty-cycles, causing the 2 kHz to be canceled in the output transformer.
The two transformers are tiny 10k : 250 ohm (or something like that) types that I have in quantity. The winding ratio isn't important. It's a fairly low-Q set of values, and it might be interesting to try one of those telecom transformers that have about 10 H inductance. The required resonating capacitor will be significantly higher, near 1 uF.
I'm using the 20 dB microphone boost option in my soundcard menu and I selected a resistor to connect across the output to kill the gain a little. A shunt resistor across the output is a fine way to control the gain. Set the gain so that the time domain display has a large signal, but isn't clipping anywhere.
The total current drain is only 300 uA, so this can run on a lantern battery for years. Using a battery means the audio output transformer will completely isolate the grounds. It would probably run on a molded supply just fine.
The null potentiometer (on the right in the photo) is adjusted to minimize the carrier bleed-through at 2048 Hz. Try adjusting that for minimum signal on the time domain display with the input shorted.
Tuning the 60 Hz notch is tricky. My favorite way is to use a selective level meter tuned to 2108 Hz (2048 + 60) with a 20 Hz bandwidth, but you can also use Spectrum Lab with the spectrum scrolling pretty fast, say 300 mS. First adjust both pots to minimize the waveform on the time domain scope, then fine-tune both for minimum 60 Hz, Take your time; let the display settle between adjustments. You can get dips on the order of 30 dB or more, but it won't last as the line frequency wanders. The only purpose is to knock the 60 Hz down to avoid overloading the mixer stage, so a deep notch isn't necessary.
The low 2.048 kHz L.O. frequency means that Argo can view the results:
This project is on hold until Jupiter comes up at night again near the end of the year.
These days I'm looking at ways to pick up emissions from the Jovian system. Going against tradition, I've opted to try to make two small loop antennas do the job, instead of the typical long dipole. The antennas are much like the Hula-Loop antenna, only built for outdoors. In use, the two loops are arranged in a line and spaced 1/2 wavelength apart (25 feet for 20 MHz). This spacing and alignment causes the signals from the antennas due to vertically polarized signals along the antennas' line to be out of phase, and easily canceled simply by adding the signals. Alternatively, one antenna can be flipped around to reverse the phase and a differential amplifier can be used to subtract the signals, with the same cancellation occurring. Signals along the other horizontal axis are already ignored by the loops. The result is a pattern that points straight up. Signals from above will exhibit the same phase in both antennas. Here's a couple of very, very preliminary schematics:
This is the schematic for each antenna. This part seems to work fine and may not change much. The schematic below is a possible receiver that has not been tested, except for the two variable-voltage circuits at the top. Those adjust the voltage to the antenna amplifiers for fine-tuning the gain. The phase is also fine-tuned by slightly changing the frequency of one loop to get minimal response to terrestrial signals. I've gotten from 6 to 10 dB of noise reduction. And, if the two in-phase signals add the way they should, that really means over 9 dB improvement over a single loop!
The receiver portion is untested and will require that I rotate one antenna 180 degrees. Looking at it, I need to add termination resistors for the cables. I'd probably eliminate the 1000 uH chokes and bypass the emitters of the darlingtons to ground with .01 uF caps. That would terminate the cables with close to 75 ohms (68 plus a little from the darlington emitter). The receiver is a direct conversion type with a low-level audio output that will need further amplification and processing. Eventually, I want to make a circuit that will "collect" pops and pulses in windows of time in some fashion, perhaps a peak/hold or an integrator. That way, the storms can be displayed on a slow plot, showing several hours of data.
I've been thinking about the circuit above and I've decided that it would be better to have a 1 MHz tuned transformer in the collectors of the differential amplifier, followed by a 1 MHz filter. That would drive another amplifier before driving a detector. The reason to not go directly to baseband is that it would be easy to set the bandwidth of the filter to 50 or 100 kHz, before the detector, to get a higher receive bandwidth.
For now, I've been using an ordinary power combiner feeding a selective level meter for testing the antennas (at 20.1 MHZ). I transmit the audio all over the house with the "Unfair Radio Transmitter." The antenna phasing trick does seem to work. I can get up to 10 dB reduction in background noise during the day, and that's not counting the fact that two antennas should increase the signal strength significantly. Here's a photo of the antennas and receiver in an empty room above a garage:
The camera was inside one loop, facing the other loop. They're slightly out of alignment to reduce a local interference source, but that's gone now, so they're nice and straight, in a near north/south alignment.
There was an "Io B" period on 11/25/2011 that I seemed to have received with this setup. Keep in mind that virtually all of my experiments give "false positives" at the beginning! This will be no exception, I suspect. Most of the time, the receiver outputs a clean white noise sound, but this racket occurred during the expected window of time, and faded right on schedule - seemed real to me. (Well, I think I heard that "noise" again and it might be splatter from a strange-sounding teletype transmission of some sort - not sure yet.) Fine-tuning one antenna could peak the signal or completely null it out, so the cancellation technique does work. Here's a terrible little video where I disconnect one antenna from the power combiner to show how much louder the background noise is without cancellation. Unfortunately, the battery in my camera died right at the end, so you only get a fraction of a second of sound with the second antenna disconnected.
I'm pleased that these electrically small loops seem to work indoors! But the room is a good distance from the electronics in the house, it's a second story above a garage, and there's little metal in the walls.
It was pointed out to me that Jupiter will be invisible to this type of receiver for much of next year. It will be high in the sky at the same time as the sun. It will be late next year before the conditions are good again. So, I'll probably use this setup with the power combiner and selective level meter to determine whether the antennas really work and wait to build the stand-alone receiver for a while.
Below is a ratiometric voltage-to-frequency converter I'm developing. Here's a list of advantages:
More measurement resolution is easily had by simply counting longer (4.5 digits in 1 second and 5.5 digits in 10 seconds for a 10 kHz V/F), or by using a high resolution period measurement or fractional cycle measurement. The circuit below will work well up to about 30 kHz.
Ratiometric response rejects excitation voltage variations when using bridges or voltage dividers. Simply run the V/F and sensor on the same voltage.
The signal is averaged over the full measurement interval. That's more important than it sounds in noisy environments!
Open-loop design gives very low jitter at low input levels, giving steady readings and making high-resolution period measurements possible.
Frequency output can be directly plotted using FFT software, such as Spectrum Lab.
Dirt cheap! I recently bought dirt for my garden, and I can attest that this circuit is significantly cheaper than dirt. In fact, most things are cheaper than dirt.
Linearity adjustment can remove some non-linearity from the source as well as the reset non-linearity.
Not shown is that a transistor may be added to pull pin 2 down for an external sync, greatly reducing the tendency for the last digit to jump around in simple counter applications. The gate signal simply turns on the 2N4403 until the counter is ready to count again, making the circuit start at the same point for each count period.
The top schematic is the main unit, and it uses an ordinary quad comparator (LM339). The bottom schematic uses a "rail-to-rail" op-amp as an optional buffer. The buffer isn't always necessary, but the linearity resistor on the converter's input does source a little current to the input that might cause a problem for very high impedance signal sources.
This open-loop design relies on key components' stability for precision, but the lack of feedback gives it remarkably low jitter, especially at low input levels. The input signal is compared to a divided-down version of the power supply, so the frequency output is proportional to the ratio of the input voltage to the power supply voltage. This ratiometric response gives added stability when measuring voltage dividers, or bridges, even after amplification, including typical strain gauges, pressure sensors, and potentiometers. Since there is no feedback, it would be expected that the finite width of the reset pulse would cause the frequency to read low as the input voltage increases. That little error can be corrected by making the reference voltage track the input voltage slightly, which is accomplished by the linearity resistor. Mathematically, the correction is perfect and the linearity can be tweaked to better than 0.01% of span for a full-scale frequency of around 10 kHz.
The 1000 pF capacitor is critical, and should be an COG (NPO) or similarly stable capacitor. The MPSA18 connected to pin 1 is also critical and only very high gain types will work well.
It's hard to adjust the gain and linearity without a precision voltage switchbox, maybe with 1 volt steps from 0 to 5 VDC. The voltages can (probably should) be simply divided down from the power supply, thanks to the ratiometric response, but use high-precision or measured resistors for maximum accuracy. With care, the performance can be surprisingly good.
One could modify the input buffer circuit to output 2.5 to 5 volts for the desired input voltage range, causing the V/F to vary from, say 5 kHz to 10 kHz. That single octave range could be used with a spectrum analyzer program like Spectrum Lab to make a precise voltage chart recorder. The output of several V/Fs could be combined through fairly large resistors of different values to plot several variables on one chart. The different value resistors will present different amplitudes to Spectrum Lab, so each trace can be made a different color.
Here's briefly how it works: The first op-amp and MPSA18 sink current in proportion to the input voltage. The capacitor voltage ramps down at a rate determined by the input voltage. When the capacitor voltage drops below the reference voltage on pin 9, the comparators change state, and the PNP discharges the capacitor very rapidly. The discharge is so fast, that it is completed before the comparators reset. The 2N4403 has a very low saturation temperature coefficient when biased as shown, so it makes a stable reset switch, albeit with a little offset voltage which is calibrated out during initial adjustments. The reset pulse is quite short, so the last comparator is added to generate a square wave output.
This weekend, I'm working on a better system for posting real-time data. I think I have an adequate Visual Basic macro for Excel that will import data from an inexpensive Dataq device. The second graphic on the "live data" page shows the end product. Here's a list of what I'm doing so far:
A macro running in the Excel spreadsheet reads four channels of a Dataq DI148U every 100 seconds and places the data in four columns. (I modifed a macro I found on the Dataq site.) There are 864 rows, for a total of 24 hours. It also creates a fifth column with the current computer time. I'll send that spreadsheet to anyone who wants to give it a try. It's not too terribly difficult to get it working or to modify it for other Dataq devices. I can give limited advice, but I managed to stumble through it so you can, too. I have an aversion to software!
The spreadsheet generates the two graphs, one an x-y scatter plot of one channel (with connecting lines) and the other an ordinary line graph with three channels. So that the scatter plot looks sensible, I started the macro at 0:00, which is the beginning of the plot. Any sort of calculation or graphic may be performed or created with the data; it's just a spreadsheet, after all. It's pretty obvious what needs changing in the macro to modify the number of rows or columns, too.
Now it gets edgy: In order to get the image on the web page, I first use a "magnifier" written in AutoHotkey. It has a magnification of one, and only serves as a controllable window capable of "looking at" anything on the screen, even overlapping programs. I modified one that I found on the web by reducing the magnification to unity and by removing its transparency. I'll post that program, too. Then, I use Irfanview to capture the client area of the magnifier program every five minutes. Capturing the client area of this simple magnifier program results in a completely clean image with no "stuff" around the edges like scroll bars. Irfanview saves the captured image to a public dropbox.com folder on another local computer (it could be the same one). I use the URL of that image that dropbox.com supplies on my web page and they serve that portion of the page. This trick makes it easy to avoid extra security measures some web servers have that make it so difficult to automate uploads.
All of these steps are pretty easy, now that I've got the macro working. The hardest part for others who aren't programmers will be a couple of minor modifications to the macro to work with their Dataq device, basically making sure that the correct com port is set and the correct model number for the Dataq gadget is in the macro. Also there's an oddity with the baud rate that I can't explain but can work around. For some reason, changing the baud rate seems to change the sample rate, at least at the extremes. I set the baud rate in the macro to 9600 and I would do the same to the Dataq device, using the supplied configuration program. I think I didn't change the Dataq setting at first and it didn't seem to care. But it matters in the macro, at least for this computer. A mid-range baud rate seems to make the sample time entered give the correct delay. I'll leave that one for more software-oriented people to explain. I enter 0.01 Hz for the sample time to get 100 second samples. You might get fine results as-is.
I moved the solar flare stuff to: Solar Flare Project I'm basically waiting for the sun to become more active. I think a few M-class or X-class flares are needed to tell if the project has a prayer of working.