Work on the 3rd edition of The Lie Behind the Lie Detectoris nearly completed, and a public release is anticipated in August 2003.
George,
Maybe I've missed where you previously discussed this, but what are the differences between the 2nd and 3rd editions? Just curious. :)
Thanks,
PK
It is a relatively minor update compared to the 2nd edition. Details will be posted when the new edition goes on-line.
I just wanted to point out that if the baseline on tests is raised, then the outcome can be deemed useless.
As an example, if everyone appeared to lie on a lie detector, would that not be the norm? The suspicion would revert to those that pass since they are the outer 3 sigma of the population.
Where am I going with this? I would like to see developments in high tech countermeasures that remove the human element from the equation.
Having said that, let's state that the polygraph is in essence, an electrical conductivity test with electronic bio monitors in place. With that in mind, it is possible to "jam" electronics in numerous ways both local and remotely.
If anyone out there has a thought along this line, please reply. I think we could make a fortune in totally disabling the polygraph system by EMF and such. Remember that it's not our responsibility that the equipment malfunctions, right?
Just rambling on,
AH
You have two pneumo tubes around your chest to monitor respiration rate. There are two gizmos attached to two fingertips to monitor GSR. Bloodpressure cuff used to monitor blood pressure and pulse rate.
These feed into a little box that is nothing more than a transducer. What is a transducer? A device to convert mechanical input into electrical output (http://wombat.doc.ic.ac.uk/foldoc/foldoc.cgi?transducer). The electrical output goes into the computer. The software on the computer (like Polyscore), takes the signals and converts to the rather "scientific" displays, As Seen on TV, no less.
How to counter that process using EMF? Well, there is always the nifty side-effect of an atomic bomb, EMP, that would be sure to create the effect you are looking for. Downside: a lot of people with really, really bad sunburns (think Linda Hamilton in Terminator 2).
Being a movie buff, you may be thinking of that device used in Ocean's Eleven, the one used to shut down the entire electric grid for a city. Upside: the test is stopped. Downside: your mother may be in surgery.
Okay, so let's scale down the weapons a bit. Some form of transmission that would jumble the input into the computer would do the trick. Where to aim it, how to aim it, how powerful, how big, where do you use it from, what about side effects on other computer users? Once that is discovered, all it takes is a shielded cable to protect the transducer/computer connection. Just like the way your cable TV cable uses a shielded jacket so outside/interferring signals are not allowed to affect the cable transmission.
Got your work cut out for you, don't you?
Not really, I am an engineer, mostly digital and not analog as in electro-disturbance techniques. My thought is simple (as most of mine are), use a Tesla coil device. They can be very small to quite large you know. Come on guys, I'm serious. It can be done. Don't let people drive your life and will.... nor explain that "it cannot be done, your doomed" syndrome.
I have numerous patents in high tech and publications. Not speaking off the cuff here, looking for really ingenious people who can make it happen.
For profit? You bet, can you think of a better reason to deliver freedom and make money for those that have been "marked" by society as Godless and without hope. Find a need and fill it.
Gee, I would do it for free if I was wealthy philanthropist, but not the case here. Just want to help those that have been wronged by the technology that I help create and now regret.
Thoughts?
Regards,
AH
Quote from: aldo_huxley on Aug 07, 2003, 10:02 PMNot really, I am an engineer, mostly digital and not analog as in electro-disturbance techniques. My thought is simple (as most of mine are), use a Tesla coil device. They can be very small to quite large you know. Come on guys, I'm serious. It can be done. Don't let people drive your life and will.... nor explain that "it cannot be done, your doomed" syndrome.
Telsa Coil? I don't think ionizing the air locally or even the fairly high RF field would have any effect on a polygraph. High RF fields are pretty ubiquitous these days and most instruments are designed to neither emit nor respond to significant levels of EM fields. None of the polygraph sensors are intrinsically sensitive to EM fields.
-Marty
Try out a Tesla coil in your house with your computer on. Proof is in the pudding so to speak. Another approach is to use a 13.56MHz RF generator. Unshielded unit will cease up any CPU or MPU within a 150 foot radius.
Just thinking out load.
AH
Aldo,
One agency that I tested for made me leave my briefcase in another room.
They said that it was necessary because an applicant might attempt to disrupt the 'test' through electronic means.
I think that they were just paranoid that I might sneak in a tape recorder (which I wish I had).
I think that if your were caught trying to disrupt the process you could kiss that agency goodbye forever and it probably wouldn't look good on other applications.
Who says I will have anything on me? A fairly higher power system can disrupt a whole floor of a building if the structural frame has steel girders (they act as antennae with an RF generator). The system can be placed anywhere in proximity of the office including nearby offices, restrooms, etc. as far as this is concerned. Of course, a decent power outlet is required. Take notice that if installed and left on, the polygraph office will be rendered useless and it could drive them at least from this local, or worse out of business, therefore I do not suggest such actions. This is just an exercise in theory. Much the same as what I got in trouble for, not reality, just thinking. Take what I say as "fantasy" thinking, LOL.
AH
Quote from: suethem on Aug 08, 2003, 11:21 PMAldo,
One agency that I tested for made me leave my briefcase in another room.
They said that it was necessary because an applicant might attempt to disrupt the 'test' through electronic means.
I think that they were just paranoid that I might sneak in a tape recorder (which I wish I had).
I think your hypothesis was most likely correct. Hey, what's another polygrapher's lie. BTW, in some states it would be legal to tape an exam without all parties being informed, in others it isn't. Tripp learned all about that.
-Marty
Quote from: aldo_huxley on Aug 08, 2003, 11:38 PMWho says I will have anything on me? A fairly higher power system can disrupt a whole floor of a building if the structural frame has steel girders (they act as antennae with an RF generator). The system can be placed anywhere in proximity of the office including nearby offices, restrooms, etc. as far as this is concerned. Of course, a decent power outlet is required. Take notice that if installed and left on, the polygraph office will be rendered useless and it could drive them at least from this local, or worse out of business, therefore I do not suggest such actions. This is just an exercise in theory. Much the same as what I got in trouble for, not reality, just thinking. Take what I say as "fantasy" thinking, LOL.
AH
AH,
Near field effects from a Tesla operating at 13.56MHz dominate only out to about 20 ft or so. After that a Tesla is pretty much the same as a conventional RF source albeit operating at whatever the ERP happens to be. (Telsa's, like all shortened antennas, are not efficient). There is also some modulation and spurious high freqs from the HV arcing's non-linear effects.
You are quite correct that, inside a metal structure, you will get some amplification due to standing waves, depending on the construction.
I stand by my statement that most polygraph equipment of recent design would not be affected by "normal" Tesla's at 150 feet.
-Marty
Sorry, but I did not make myself clear. Tesla coil was just and idea, the 13.56 MHz generator I was speaking of is a linear design. We had a near disaster due to 13.56 MHz system which had RF leakage affecting a system about 150 feet away where the CPU was "frozen" as were the sensors of a hot acid bath and the critical temperature was almost reached before I used the old fashioned method of a mercury thermometer. Gee, I ramble. Funny thing, the leakage and effect was due to 1 screw missing from a cooling fan. I think you can image what you could do intentionally if you so desire. To clear things up a bit, the generator is 1KW design.
AH
AH,
If you somehow manage to disrupt the workings of the polygraph, no doubt it will be readily apparent to the polygrapher. So he'll just reschedule your exam for another day.
So how many times do you suppose it would take for the testing agency to figure out that the machine only goes t_ts-up when you are around?
Better to work to eliminate the machine entirely, rather than seek some mechanical means to disrupt it.
If you turn on your system at a time your not scheduled, leave it on till dommsday so to speak, they cannot do there job, EVER!
No connection to you, they are rendered helpless.
Think about that, yea takes effor and planning, but it can be done.
Last post on this subject. I just wanted to throw it out there.
Later,
AH
Quote from: aldo_huxley on Aug 09, 2003, 06:33 PMSorry, but I did not make myself clear. Tesla coil was just and idea, the 13.56 MHz generator I was speaking of is a linear design. We had a near disaster due to 13.56 MHz system which had RF leakage affecting a system about 150 feet away where the CPU was "frozen" as were the sensors of a hot acid bath and the critical temperature was almost reached before I used the old fashioned method of a mercury thermometer. Gee, I ramble. Funny thing, the leakage and effect was due to 1 screw missing from a cooling fan. I think you can image what you could do intentionally if you so desire. To clear things up a bit, the generator is 1KW design.
Yep, 1kw of rf there can easily lock up an ad hoc lab setup. I have had lab kludges lock-up from ESD and leaking RF from an unbalanced ham xmtr. Not so though on a properly shielded design which commercial equip tends to have to pass for FCC and ECMA standards. Modern stuff is surprisingly resistant to high RF fields though this is not true of stuff designed 30 years ago and a lot of poly's have been around a long time.
-Marty
and if you load it through the AC line?
Maybe you should take the tin foil off your head. :D
Your probably correct, took the foil off; tried out a high power Tesla and WHAM! East coast went dark......
And now for something completely diffent...............
:)
Quote from: aldo_huxley on Aug 12, 2003, 03:06 AMand if you load it through the AC line?
RF loads into the AC lines in 2 ways, common mode and differential. The latter, won't be a problem as the impedance mismatch with a Tesla is huge. The former is typically shunted to gnd by capacitance in various devices along the path.
-Marty
Gee Marty, methinks we got away from the suject here. Sorry everone!
So..........If we realistically look at a problem there is always a solution. I have been gining the Tesla as an example, but looking for real feed back as how to do it, not "it can't be done". I was always told can't can't do anything.
Anyway, other approaches to jamming can include harmonic amplification( as in h of n(x) = A of n cos(n omega t+thea of n) are the harmonics, let's see how about a 25ns 500V P-P spike in the line? on and on......
any EE's out there?
Aldo
giving, sorry typo.......
Aldo
Sorry Marty, I find that you are an EE. I'm an engineer producing 95nm gates curently running 37 GHz technology. How about you?
Aldo
Quote from: aldo_huxley on Aug 19, 2003, 02:13 AMSorry Marty, I find that you are an EE. I'm an engineer producing 95nm gates curently running 37 GHz technology. How about you?
Aldo
That's nice. How does that relate to Tesla's disrupting polygraphs at 13 MHZ? I seem to have off put you. Sorry. Why not use a real name and not an alias? Perhaps you been subject to polygraphs? They sure have their downsides.
-Marty
No relationship between Tesla coils and linear 13.56MHz amps, no experience with polys to date.
I guess my main point is that polygraphs are sensitive pieces of equipment and should inherently be subject to EMI if purposely designed to do just that. Of course $$$ is always a driving force to any new innovation.
Electronic jamming is not new, nor confined to specific areas.
Search the following:
Skin Conductance and Cardiac Responses to Masked Presentations of Fear-Relevant and
-Irrelevant Conditioned Stimuli
Changes in human skin conductance are frequently used as an indirect measure of a subject's cognitive effort, emotional arousal, or level of attention. Recording skin conductance responses (SCRs) is useful during
"Psychologists have been trying to identify universal patterns of physiological response since the early 1900s, but without success. We believe that the lesson to be learned there is that there are no such universal patterns," said Smith.
Galvanic Skin Response is a measure of the skin's conductance between two electrodes. Electrodes are small metal plates that apply a safe, imperceptibly tiny voltage across the skin. The electrodes are typically attached to the subject's fingers or toes using electrode cuffs (as shown on the left electrode in the diagram) or to any part of the body using a silver-Chloride electrode patch such as that shown on the EMG. To measure the resistance, a small voltage is applied to the skin and the skin's current conduction is measured.
Skin conductance is considered to be a function of the sweat gland activity and the skin's pore size. An individual's baseline skin conductance will vary for many reasons, including gender, diet, skin type and situation. Sweat gland activity is controlled in part by the sympathetic nervous system. When a subject is startled or experiences anxiety, there will be a fast increase in the skin's conductance (a period of seconds) due to increased activity in the sweat glands (unless the glands are saturated with sweat.)
After a startle, the skin's conductance will decrease naturally due to reabsorption. There is a saturation to the effect: when the duct of the sweat gland fills there is no longer a possibility of further increasing skin conductance. Excess sweat pours out of the duct. Sweat gland activity increases the skin's capacity to conduct the current passing through it and changes in the skin conductance reflect changes in the level of arousal in the sympathetic nervous system.
The electromyographic sensors measure the electromyographic activity of the muscle (the electrical activity produced by a muscle when it is being contracted), amplify the signal and send it to the encoder. In the encoder, a band pass filter is applied to the signal. For all our experiments, the sensor has used the 0-400 microvolt range and the 20-500 Hz filter, which is the most commonly used position.
Most people cannot mentally control their skin conductivity response in any kind of rapid and fine-tuned way. The response can take seconds to arise and longer to decay. However, you can experiment with thinking of exciting or calming thoughts and watch how your galvactivator responds. Once you have dialed it to your proper baseline, the galvactivator should be sensitive enough to respond to internally-generated imagery in a broad way.
The startle reflex is a characteristic spike in the galvanicskin response that usually occurs 1-3 seconds after the onset of a startleprobe or a novel stimulus. Typically, a startle probe refers to a loudnoise, bright light, or puff of air designed to "startle" or suddenly divert the attention of the subject. A similar spike in conductivity can be generated when a subject "orients" to a new stimulus -- a person suddenly standing next to you, for example.
Can someone tell if I am lying?
Not with the galvactivator, though the galvanic skin response is one of the measures commonly used as part of a lie detecting apparatus. However, detection of deception requires a thorough experimental setting which does not compare to everyday's use of the galvactivator.
More later, Nuff to chew on.
Aldo
Quote from: aldo_huxley on Aug 21, 2003, 02:29 AM
I guess my main point is that polygraphs are sensitive pieces of equipment and should inherently be subject to EMI if purposely designed to do just that. Of course $$$ is always a driving force to any new innovation.
Electronic jamming is not new, nor confined to specific areas.
The polygraph, including the galvanic skin resistance component, is not intrinsically vulnerable to electronic jamming measurement though each instrument may have defects that produce specific sensitivities. They would not be consistent from one design to another however due to the above.
-Marty
So how many manufactures are they? What components are common? Just curious. Mainly interested in the type of sensors and IC's utilized.
Oh yes, a friend of mine is replacing a cap in an old stereo system. Question posed is which type is best, electrolytic, mylar, or ceramic? The former is best for filtering but I'm not sure about it's general effect on the sound. Any ideas?
Gee, went off the subject.......
So you think the EMI protection within the unit exceeds the ability to disrupt it's functionality?
On the subject of conductivity sensors, what if you went to a poly with a tinge unit on?
Aldo
The oldest analog polygraphs are the least sensitive to external EMI. They didn't have any electronics except for the GSR which was typically passive, Wheatstone bridge based. The ones most sensitive were likely designed 30 years ago or so using electronic transducers (strain gauges) requiring significant amplification.. The types of capacitors make little difference, rather pcb layout and capacitor lead lengths (series inductance) are the important factors. Newer designs would have much more resistance as a side effect of stringent emitted EMI regulations.
As for the amplifier, resistors and capacitors in an RIAA equalization network should be highly accurate (or batch selected) and polystyrene types are quite good there. Low memory, linear, low leakage. For EMI, ceramic caps are normally best, available inline and with low series L.
-Marty
Quote from: Marty on Aug 22, 2003, 05:06 PM
The polygraph, including the galvanic skin resistance component, is not intrinsically vulnerable to electronic jamming measurement though each instrument may have defects that produce specific sensitivities. They would not be consistent from one design to another however due to the above.
-Marty
Actually, I think it stands to reason that the GSR instrument would be the most vulnerable to externally-generated electric fields. It's likely that those fields would need to be fairly strong and low frequency, though.
In my experience, the wires connecting the GSR (ohmmeter) to the fingers are not shielded, which would probably be the weak point for messing with the recorded signal.
Skeptic
Quote from: aldo_huxley on Aug 23, 2003, 09:48 PMSo how many manufactures are they? What components are common? Just curious. Mainly interested in the type of sensors and IC's utilized.
I'm not sure how many manufacturers there are, but Lafayette Instrument Co. is one of the biggest suppliers of polygraph instrumentation:
http://www.lafayetteinstrument.com (http://www.lafayetteinstrument.com)
QuoteOh yes, a friend of mine is replacing a cap in an old stereo system. Question posed is which type is best, electrolytic, mylar, or ceramic? The former is best for filtering but I'm not sure about it's general effect on the sound. Any ideas?
Was the cap in the audio path to begin with, or in the power supply? In general, I would replace it with something close to what you took out.
For the record, electrolytics aren't necessarily "the best for filtering". Again, it depends: upon the frequency range of noise you're looking to filter, the noise amplitude, and how much you're looking to spend. Usually for power-supply-pin bypassing, you'll find both electrolytic and ceramic in parallel to cover a larger frequency range for noise filtering (most electrolytics are pretty much useless by the time you get into the tens-to-hundreds of kilohertz range).
QuoteGee, went off the subject.......
So you think the EMI protection within the unit exceeds the ability to disrupt it's functionality?
Absolutely not. But it all depends upon the strength of the disruption practially available, of course.
I would imagine some sort of high-voltage spark-gap device would likely induce significant noise in the GSR channel, at least. Whether the noise-induced nature would be obvious is another question.
It would be an interesting experiment to do, and could be tested using a common multimeter, unshielded leads and spade connectors strapped to the index and ring fingers using velcro straps (yes, I made my own crude GSR meter).
Skeptic
Quote from: Skeptic on Aug 24, 2003, 02:10 AM
Actually, I think it stands to reason that the GSR instrument would be the most vulnerable to externally-generated electric fields. It's likely that those fields would need to be fairly strong and low frequency, though.
In my experience, the wires connecting the GSR (ohmmeter) to the fingers are not shielded, which would probably be the weak point for messing with the recorded signal.
Skeptic
Transducers used in the BP cuff produce relatively small signals and would be most sensitive. The failure mechanism is rectification or other non linear interations with jamming RF. Shutting down a control CPU is best done with wide spectrum impulse noise.
Jamming with EM fields in the area of interest (in band) would require huge B or E fields. Given the wavelength here, all effects are near field. I would give B fields the biggest chance. Any effect would require B fields so large that they would vibrate metal of any significant cross section. In band E fields would not show since GSR would couple poorly (cap impedance is quite high compared to skin Z)
-Marty
Quote from: Marty on Aug 24, 2003, 06:04 AM
Transducers used in the BP cuff produce relatively small signals and would be most sensitive. The failure mechanism is rectification or other non linear interations with jamming RF. Shutting down a control CPU is best done with wide spectrum impulse noise.
The thing is (in my experience, at least with Lafayette instrumentation) the transducers are actually in a box on the table, with pneumatic tubing running from the box to the cuff. I would bet the box is EMI shielded, at least.
And where would rectification come in? I'd think the transducers would be DC coupled, given the signals of interest...
QuoteJamming with EM fields in the area of interest (in band) would require huge B or E fields. Given the wavelength here, all effects are near field. I would give B fields the biggest chance. Any effect would require B fields so large that they would vibrate metal of any significant cross section. In band E fields would not show since GSR would couple poorly (cap impedance is quite high compared to skin Z)
-Marty
I'm not sure this sounds right to me, Marty. Although my EM education is at the undergrad level and RFI is hardly an area of my EE specialty, it seems to me that the long leads to the GSR measurement amplifiers would be good at picking up strong electric fields. And I would think that the input impedance to the amplifier would be on the order of skin Z, for best power coupling.
Do you or anyone else have a schematic for a typical GSR meter? Are we talking a voltage source with a series current measurement resistor and instrumentation amplifiers, are things still done with a bridge, or some other mechanism?
Skeptic
Food for thought. I know this is an MRI paper, hence the relationship to Tesla type fields, but anyone that has to go to this much trouble to prpoerly detect the human resistance response........got to be a fire in the somke somewhere......
Thus:
human skin conductance changes (SCRs)
Changes in human skin conductance are frequently used as an indirect measure of a subject's cognitive effort, emotional arousal, or level of attention. Recording skin conductance responses (SCRs) is useful during functional magnetic resonance imaging (fMRI) studies that hypothesize correlations between blood oxygen level dependent (BOLD) signals and anxiety levels or emotional responses. Because clinical MR scanners do not come equipped to monitor SCRs, one must independently acquire the apparatus to do so, and then adapt it for use in an MR scanner. The two main technical challenges are: a) obtaining SCR signals that are not corrupted by interference from the MR scanner's changing magnetic field gradients, and b) maintaining low background noise in the MR images. Recent studies reported monitoring SCRs during fMRI (1, 2), but the technical issues above are not addressed in detail. In addition, reference (1) reported that scanner-induced artifacts rendered SCR data useless in 4 out of 9 trials. Both studies reported normalized SCR data rather than amplitudes measured in S (S=1/), making it difficult to compare their SCR data to values reported in the literature. We therefore feel the need for an article that addresses the technical aspects of monitoring SCR on human subjects during fMRI. In this article we describe the construction, calibration, and testing of a versatile, low-cost system for monitoring SCRs in a clinical MR scanner. We show that the system suppressed scanner interference during fMRI to an acceptable level independently of the repetition time (TR) and slice orientation of the fMRI sequence. In addition, the presence of the SCR system inside the MR scanner did not adversely affect fMRI images as measured by the image background noise level.
SCR monitoring circuit: The design of our system was based on the frequency and amplitude characteristics of SCRs. The range of human SCR magnitudes is from SCRmin0.01 S to SCRmax 1 S (3,4). In order to ensure adequate resolution in measuring the smallest conductance responses, we required the SCR monitoring system to suppress interference during fMRI to a level , an order of magnitude below SCRmin
 =SCRmin/10=10-3 S
We hypothesized this could be accomplished by using a low-pass filter with cutoff frequency of 1 Hz (5), because SCR signals change relatively slowly compared to interference generated by the MR scanner. . Figure 2 shows the schematic diagram of the SCR monitoring circuit consisting of a Wheatstone bridge, a differential amplifier, and low-pass filter. The design of the bridge circuit was taken from Ref. 3, and a complete description of its use is given there. R1 is a 10-turn potentiometer, and all fixed resistors have 1% tolerance. The bridge output Vout (between points C and D of Fig. 2) is proportional to the change in the subject's skin conductance C4, as given by
(1)
(see Appendix) where C3=1/R3=5000 S, and Vin=0.488 V is the voltage from A to B, regulated by the LM113H reference diode. If one inserts the component values into Eqn. (1) and uses the lower limit of human SCR amplitudes C4=SCRmin, one finds an expected minimum bridge output voltage of Vmin=1 V. We adopt as a rule of thumb for determining adequate amplifier gain G that a minimum of 10 voltage steps Vres must be present over the lower limit of amplified output voltage GVmin, or GVmin/Vres10, yielding for G the criterion
(2)
We therefore calculated that the gain should satisfy G  610. We chose to use the differential amplifier chip AD624AD (Analog Devices, Norwood, MA) designed for a gain of 1000 (details are in Ref. 6). The amplifier output was fed to a low-pass filter with a 3 dB cutoff frequency of 1 Hz. We selected the 2-pole, Butterworth low-pass filter for its flat pass-band magnitude response and its moderately steep attenuation (-40 dB/decade) beyond the 3dB point. The filter circuit of Fig. 2 was designed using FilterProTM software for the universal filter circuit UAF42AP (Burr-Brown, Tucson, AZ), where R6=1.58 M and C=100 nF. NPO ceramic, silver mica, or metallized polycarbonate capacitors are recommended (6). Voltage supplies ranging from 6 to 18 V may provide power to the amplifier and filter circuits. The resistor R5 limits current through the LM113H, which maintains a constant voltage of 1.22 V, and satisfies
Cable shielding: Shielding the cable (shown schematically in Fig. 1) with braided copper sheath was another step we took to reduce scanner interference. This established a ground connection between the chassis of the electronics box, the custom-built aluminum door, and the scan room shielding, and reduced interference considerably. In our case shielding was particularly important because we put the 40 m data cable permanently in place over the scan room ceiling. The cable extended from the rear scan room door to the console room in front of the scan room. The length of the cable and its proximity to gradient power supplies made this step necessary.
Door: To prevent the SCR cable from picking up outside radio frequency (RF) interference, transferring the RF interference into the scan room, and degrading the quality of the functional MR images, we filtered the cable as it passed through the penetration ports on the custom-built door (Fig. 3(b)). The door itself was assembled from aluminum angle irons cut to the dimensions of the doorway, with allowance made on the sides and top for the addition of metal contact fingers (Lindgren, Glendale Heights, IL) compressed 70% of their width. The lower half of the door was covered with an aluminum plate 0.64 cm (0.25 in) thick, and the upper half with aluminum screen. Aluminum side handles were also added to make moving the door easier. A twin BNC connector was mounted to the aluminum plate (Amphenol #31-225) to mate with the SCR electrode cable on the immobilizer. Mounted to the aluminum plate was a filtered penetration port (Fig. 3(c)), consisting of an interior aluminum bulkhead through which two 400 Hz, low-pass, feed-through capacitors (Spectrum Control, Fairview, PA) were mounted.
Calibration: SCR amplitudes C4 are related to the amplified output voltage of the circuit by C4=-Vtotal  where = C3/(VinG) (see Appendix). Given that C3=5.00x10-3 S, Vin=0.488 V, and G=1000, one expects theoretically that =10.2 S/V. Further, because C3, Vin, and G are known to within 1% , one can show that  should have an uncertainty of 3%. We also determined  experimentally by sampling Vtotal while R4 was switched to the internal resistors 1 M and 0.1 M. Then =-C4/Vtotal was used to obtain
in good agreement with the expected value. We used the experimentally obtained  value, and programmed the data acquisition software to convert from volts to S. As a check of the system accuracy, we connected a precision resistor and potentiometer in series across the SCR leads and measured the difference between the maximum and minimum conductance. The difference in conductance was determined beforehand by digital multimeter to be 0.49 S. The calibrated SCR system yielded 0.46 S, in good agreement.
I see graphics is not the forte here. What methods are you guys use?
Aldo
Quote from: Skeptic on Aug 24, 2003, 04:20 PM
The thing is (in my experience, at least with Lafayette instrumentation) the transducers are actually in a box on the table, with pneumatic tubing running from the box to the cuff. I would bet the box is EMI shielded, at least.
And where would rectification come in? I'd think the transducers would be DC coupled, given the signals of interest...
Skeptic and AH:
I agree the box would be EMI shielded. In a commercial product the incoming leads would be shielded as well to prevent internal RF from escaping.
Rectification comes from rf on the bipolar amplifier inputs (most low noise, high sensitivity amps extant). They casue what lookes like an out of spec input EOS or IOS. Since it's at the inner most point on the loop, the offset gets directly amplified by the loop gain. I've seen significant effects from local FM radio on Analog Devices modules.
Quote
I'm not sure this sounds right to me, Marty. Although my EM education is at the undergrad level and RFI is hardly an area of my EE specialty, it seems to me that the long leads to the GSR measurement amplifiers would be good at picking up strong electric fields. And I would think that the input impedance to the amplifier would be on the order of skin Z, for best power coupling.
A rather simple way to look at it is this way. The capacitance between a 1m wire and a surface m^2 at a distance of 1m is going to be around 10^-12. Impedance at 1Hz is about 10^11. Assuming skin Z at 10^3, there would be a coupling reduction of the E field of approx 10^-8. Assuming a 1V DC operating point, a 1 Million volt, 1Hz E field would produce about a 1% modulation in measured skin Z. For impulse noise, anything more than a 1 pole filter would attenuate faster than the increased coupling at higher Hz's.
Out of band RF is a much bigger problem but easily contained, both by shielding and good bypass designs on the front end. The latter is typical of modern commercial designs but ad hoc lab work is far more variable - as the MRI-GSR descr. shows.
Wheatstone bridges have an advantage in reducing sensitivity to power supply variation and even working well with old, direct drive centered galvanometers. This has long been a non issue as high stability, cheap, voltage sources have been around 30 years or so.
-Marty