This is a very simple circuit allowing power input, attaching a diode for testing, and sampling both the curent and voltage across the diode under test, (DUT). I made a simple modification by adding a voltage divider, about 20:1 at the power input. The power supply can be connected either at the front across the divider, or after the divider in front of the 100k current limiting resistor for higher voltage readings. I get the input voltage from a modest 18V, 2A regulated DC power supply. This unit has both volt and amp controls with digital readouts, nice to use and dosnt take up much room on my desk. Here is a case where I recommend you look at getting a new unit. I looked long and hard on ebay, but found few bargins and few units with the same features. Still, the supply regulator was never designed to adjust the output to precise small values making it somewhat difficult/tedious to adjust. Adding a small rheostat to the test jig may aid for making fine adjustments on the voltage. Another piece of equipment you will need for this test is an amp meter. I use a nice small digital meter I found new on ebay for $15 shipping included. It reads amps to a lowest range of 200uA, more than enough precision for the task. For that price get two. The DVM is, of course, my trusty Keithley workhorse. Easy protocol as follows:
1) Set up the test with the diode and meters clipped to the jig.
2) Adjust the power supply until the reading on the DVM meter is where you plan to measure. Adjust the voltage (DVM readout) in steps from 0 to 1.0 Volt in 0.1 V increments and record the current (Amps) in a spreadsheet.
3) Repeat at each tenth volt recording while the spreadsheet makes a sweet graph.
Over a period of a few weeks I ran I-V characterization tests on a good number and variety of diodes, crystals and my two tubes to see how very thing stacks up. The following presents the resulting curves, diode photos, and some puzzling questions/conclusions I churned up in the process. I start off with the realization that when one orders diodes, it is by no means certain what one will get. This seems especially true for the supposedly ubiquitous 1N34A. Unless you see the part number actually on the diode, you probably need to test it to know what it really is. So, lets take a look!
Here we see a set of Germanium diodes and photos of the diodes under test. Immediately you should notice two sets of curves, what I call the "1N34A" set and the "1N270-277" set. In this analysis, the second set appears clearly superior in sensitivity and deservedly so. In the photos it is clear that one bunch of diodes I purchased as 1N34A were in fact better 1N277, the black band and thin gold contact wire as well as common curve unite these. "True" 1N34A's have a more robust contact wire. I also purchased a 1N270 and received pretty orange diodes with the number "37" stamped on it, a fairly robust contact wire and a curve that most closely resembles a 1N34A, go figure. Mercifully, I DID manage to get some bona-fide 1N34A's with the part number on the diode, as well as a single bona-fide ITT 1N270 diode with the part number. These are my base for comarison.
In addition to the standard Germanium Diodes tested above, I also have latched my hands on a few "other" germanium diodes for the fun of testing. These include two vintage Russian types, a D9E (in the 270 class) and a D18 (transitional between 270 and 1N34A). Additionally, I got a few FUZZ diodes popular, I suspect, with the guitar gadget crowd. These are larger packages in metal cases so I cannot see how the internal contact is made other that they are stated to be gold bonded point contact type. Clearly these are highly sensitive diodes in my 270 class, the OA5 looking best of the lot. Interestingly, I recently purchased from good Mr. Peebles a few of his "Holy Grail" ITT FO-215 diodes. My measured characteristic for one of these is a dead laydown on the gold-bonded OA5. From the photos, these are in different packaging with the FO-215 in a traditional glass casing. What makes the FO-215 so great is the fact that its resistance Rd is an interesting 150k ohm or so which matches very well with typical moderate Q tank curcuits. Interestingly, I find that an old Russian D18 diode has very similar characteristics to the FO-215 and so much also be placed into the "Holy Grail" class of germanium diode. Take your pick!
Spreadsheet for calculation of Germanium diode n and Is (modified from Mike Tuggle's spreadsheet)
The above spreadsheet is based on measurements of the diodes shown above. For the determination of Is and n, I chose at random two examples from my collection of various diodes, (all germanium in this case) and measured via a modified version of the methodology outlined by Ben Tongue and Mike Tuggle. For this work I have chosen to set Id2 about 0.5uA and Id1 about 1.0uA and then read the needed voltage. This is the inverse of the normal method but the justification is that for any diode regardless of forward voltage drop the measurement is made at the same part of the LOG I vs V characteristic. This allows comparison between all diodes. I have included the actual room temperature in the calculations in order to eliminate this variable as a source of doubt or error. All the measured parameters as well as the determined results are presented in order to facilate repeatability.
A final note on the plots above. You will have noticed that I sub-divide germanium diodes into a "1N34A" class and a "1N270-277" class. This is a result of looking at their I / V characteristics on a plot of 1 - 2V DC versus 0 - 10mA scale. At such a scale the slope of the characteristic for the 1N270 class remains much steeper than that for 1N34 type diodes. As this is not seen as a distinguishing feature at the small-signal scales looked at here, I judge the chief difference between the two types lies in the diode series resistance Rs. This impacts strong-signals but is of no concern for small-signal DX work. Just so you know.
Schottky diodes are very sensitive diodes that work excellently in crystal radios. Their construction and theory are different and I confess to not fully understanding these components. Still, from the characteristic curves, they are excellent! Again, note that you dont always get what you bargain for. Here I found what was supposed to be 1N34A's to be some sort of Schottky of unknown pedigree. The 1N5819, while having the lowest forward voltage drop, has a rather very high Junction Capacitance and will not perform well. Posts on the RaidoBoard Crystal Radio Forum however highly recommend the 1N5711 for crystal sets although the characteristic curve dosnt look that fabulous. Many web pages out there on Shottky diodes, I recommend you do your homework. Of all the schottky's, I note that Ben Tongue recommends most highly the HP5083-2835. The high resistance Rd makes them useful for DX sets with very very high Q tanks. Even so the diodes need to be paralleled with up to 4 or 5 diodes to match correctly the Rd with the tank Rp. I have found these somewhat hard to find and expensive, especially when one requires using several in parallel. I recently measured a few 1SS98 diodes and I discover them to have characteristics extermely similar to the HP and I feel they deseve more attention. Sadly, on ebay they seem to be just as difficult to find and expensive. No free ride!
Spreadsheet for calculation of Schottky diode n and Is (modified from Mike Tuggle's spreadsheet)
The above spreadsheet is based on measurements of the diodes shown above. For the determination of Is and n, I chose at random two examples from my collection of various diodes, (all schottky in this case) and measured via a modified version of the methodology outlined by Ben Tongue and Mike Tuggle. For this work I have chosen to set Id2 about 0.5uA and Id1 about 1.0uA and then read the needed voltage. This is the inverse of the normal method but the justification is that for any diode regardless of forward voltage drop the measurement is made at the same part of the LOG I vs V characteristic. This allows comparison between all diodes. I have included the actual room temperature in the calculations in order to eliminate this variable as a source of doubt or error. All the measured parameters as well as the determined results are presented in order to facilate repeatability.
Silicon diodes have good characteristics, but an unacceptably high forward voltage drop making them a very poor choice for crystal radio unless used with bias. The 1N4736A is a Zenner diode.
Spreadsheet for calculation of Silicon diode n and Is (modified from Mike Tuggle's spreadsheet)
The above spreadsheet is based on measurements of the diodes shown above. For the determination of Is and n, I chose at random two examples from my collection of various diodes, (all silicon in this case) and measured with a modified methodology to that outlined by Ben Tongue and Mike Tuggle. In this case, with the radically different forward voltage drop of these diodes from Germanium or Schottky diodes, I have kept the Id values constant (about Id2=0.5uA and Id1=1.0uA) and varied Vd. I have included the actual room temperature in the calculations in order to eliminate this variable as a source of doubt or error. All the measured parameters as well as the determined results are presented in order to facilate repeatability.
As long as I am measuring various and sundry diodes, I figure I ought to include that most ubiquitous of modern diode, the LED. Found everywhere, these diodes are rapidly becoming the low-energy light source of choice for many lighting applications. I have read occasionally of someone asking whether these ought to be useful for radio applications as well. To this question the answer is generally a resounding "No!". The turn-on voltage is waaay above any reasonable value expected to be delivered by an antenna to a crystal set. Still, this simple answer avoids the actual question, what in fact does the characteristic curve of a LED really look like? Where is the turn-on voltage with respect to the published junction voltage, (assuming you can find that).
To provide just such a look I visited my local electronics store and bought a small handful of LED's, most with the junction voltage listed and took them home to measure. Typical LED junction voltages seem to range from about 1.8v to 2.1v or more. The turn-on voltages look closer to 1.6v-1.7v. Anyone used to working with Carborundum crystals or silicon diodes will be used to biasing their rectifier to get good sensitivity. These LED's once on have a very sharp rise and with a proper bias should work quite well as detector diodes. As a bonus you will get a sweet glow as well. OK, not as cool as the glow of a vacuum diode, but certainly more sensitive!
Spreadsheet for calculation of Light Emitting Diode n and Is (modified from Mike Tuggle's spreadsheet)
The above spreadsheet is based on measurements of the diodes shown above. For the determination of Is and n, I chose at random two examples from my collection of various diodes, (all led's in this case) and measured with a modified methodology to that outlined by Ben Tongue and Mike Tuggle. In this case, with the radically different forward voltage drop of these diodes from Germanium or Schottky diodes, I have kept the Id values constant (about Id2=0.5uA and Id1=1.0uA) and varied Vd. I have included the actual room temperature in the calculations in order to eliminate this variable as a source of doubt or error. All the measured parameters as well as the determined results are presented in order to facilate repeatability.
I have been attempting to get a handle on the main diode parameters Is, n, and Rd and how they impact the operation of a crystal radio. My method of choice is to use a graphical approach, plotting charts where variations between plotted parameters become apparent and easy to visualize. I have developed an interesting chart from the measurements above that compares many diodes with respect to Is, n, and Rd, and to understand how this fits into a matching situation with a fuzzy indication of typical tank impedance limits.
Ultimately one seeks a diode with an Rd that matches with and conjugate to the impedance it sees from the tank (+antenna). In the following discussions I will start with the cross-plot of Is to Rd and then proceed to a look at how received RF power plays a part in the selection and why low Is and n are desired.
The plot above shows the relationship between diode Is and Rd. Rd is calculated via the equation Rd = VT * n / Is where VT is the thermal voltage = k * T / q = 0.0257V at room temperature and Is and n are from the measured diodes. k is Boltzmann's constant = 1.38E-23 J/K and q is the electron charge = 1.609E-19 coulombs. On such a plot I can show lines of constant n, and plot the values for individual diodes for which I have spent considerable effort to determine the parameters Is, n and Rd (where Is is the diode saturation current, n is the ideality factor, and Rd is the diode resistance). On the plot it should be evident how changes in n or Is impact a diodes' Rd.
Additionally I have plotted RF signal Rs as orange circles from published coil unloaded Q (see my section on Coil Q). To convert from Qu to Ql I have made the assumption that loaded Q is about 1/5 unloaded Q for low-end sets while in performance sets it may approach 1/2.5 the unloaded Q. This is only an estimate then, but starting from actual data. To make the conversion to Rs I simply plugged the data into the standard equation R = 2pi*f*L*Q using a scaling factor to divide down Qu to Ql from high Q (1400/2.5) to low Q (100/5). This provides a nice visual display of the expected range for Rs in many crystal sets. I plot the Rs against the value of Ql used. Note that these data are for the case at about 1MHz and inductances between 200 and 300uH, as published.
Matching the diode to the tank is a matter of finding a diode with both sensitive qualities (low n) and an Rd close to the Rs presented to the diode (as per Ben Tongue's suggestions for Peak and Square Law detection). Those diodes with Is's in the 100 - 250 nA (100-280 kohm for n=1.1) or so range have the possibility to match while connected to the top of the tank coil without the use of Q-lowering taps, while untapped matching to high-end high-performance big-Litz baskets will require diodes with Is's in the 35 - 60 nA (500-800 kohm for n=1.1) range. Most "typical" germanium diodes have high Is values (>500, >60kohm) and will require a tap. Diodes such as FO-215, BAT 46, 1N60 and GAZ51 can be matched to many tanks without taps. For tanks with high quality Litz coils one will be using HP 5082-2835 or 1SS98 diodes, generally 3 to 4 in parallel to lower the Rd to the desired range. (Curiously, I have not found any diodes with Rd values in the 300 to 4500 kohm range.) Note that both resistance and impedance are frequency-specific so a match at one frequency will not remain matched across the broadcast band.
Here is an interesting plot taken from the text discussion in "Crystal Set Analysis" by Berthold Bosch. It presents received signal strength across the tuned circuit for various scenarios from threshold audibility to local blowtorch. This plot should be considered a single example specific to his location and antenna/ground system. In the text he describes his antenna an an inverted L 43m long (140') and 10m high (32'), an excellent antenna most of us do not have the real estate to erect, but offsetting this is a poor ground with Rg = 210ohms. Given the offsetting conditions I would imagine this is a good generic example of what received signal voltage and power levels presented across the LC circuit to the diode are likely to be. Bosch cited 40nV as the threshold of audible detectability (impedance matched conditions with 16kohm RF impedance / 4kohm DC phones) and for this plot I pushed it back to 20nV as it gave a superior regression fit.
This plot at left takes some actual diode I/V measurements rather than the models presented below and plots them along with the above RF voltage across the LC circuit versus RF power in mW. Here one sees that pushing for very very low Vf (via high Is / low n combinations) pushes the limit of detectable signal power from the antenna. At such minute power levels I imagine the reverse leakage current becomes significant and probably offsets the low operating point gains sought.
The above curves demonstrate the wide variation in properties and qualities that can be found in natural crystals of galena or pyrite, the two most common and best quality natural stones. In each crystal test I first poked around the crystal for some time to 1) determine a typical sensitivity for the crystal in question and 2) to locate the best hot spot with which to test. This turns out to be a non-trivial exercise on the diode test setup. In a crystal radio one need only listen for the loudest spot. With the test setup one needs test both the forward and reverse current in order to determine if the whisker is on a hot spot or not. Very tedious work! (In retrospect, if I were starting over making a test jig, I would definately add a DPDT switch to readily change between forward and reverse current measurements. I'd probably toss in a rheostat for fine adjustments as well).
For many crystals there are limited number of possible hot spots, but these may be hot indeed. For most of my "Steel Galena" samples (Tintic Utah, or Leadville Colorado) there are numerous hot spots under virtually every place I touch the probe, but in general the sensitivity is good to so so. These crystals are very kind to work with in terms of finding spots and avoiding frustration. Mirror galena on the other hand may have quality hot spots, but any hot spots at all are rare and frustratingly difficult to locate. Here my Philmore detector crystal shines with an almost ideal "Galena" response. To chase down this rabbit I purchased some lovely mirror galena from Sweetwater Missouri. I broke off a few appropriate-size chunks to pot in woods metal and test. At first I was very excited with the high currents I was seeing at moderate voltages. Figured I had struck gold. When these crystals failed miserably to rectify anything in my radios, I re-measured things in both forward and reverse directions. These crystals obey Ohm's law and act like typical resistors, not suitable for radio work at all.
For my pyrite crystals the work has been especially tedious and frustrating. With one of the crystals one "hot spot" alternated, entirely on its own, between hot and bad while I was making the measurements. I would start over and over, sometimes getting interesting readings then suddenly it would drop to low values and I'd start over, back and forth. I present this data as best as I have measured, and I don't intend to go back! You see at least one of the crystals, my "China 1", (from a lead/silver mine in Hunan) gave a sweet classic-looking curve. More to the point, "ideal" curves for natural minerals, are difficult to come by. Most crystals you use will be less than ideal. The good news is that, while listening to your set, poking about for a good spot is far easier than what I have gone through to produce these curves. Your ear will take care of you!
In the photo I indicate groupings based on an easy measure of performance. I note the current in milliamps for each set where the plate voltage is set at 0.5V. The greater the current the better are your chances to get a sensitive crystal, assuming Ohm's Law is not followed! Crystals in the "dead zone" on the left will have their woods metal re-melted for new detector crystals and the bad ones tossed, its tough love for crystals. I find that easily half the potted crystals I make are tossed this way, and only a few can be considered superlative.
I have tested seven different tube types including 6.3V rectifier diodes that take a lot of power to run and are not really suitable for battery use, a 9v dual diode tube (20D1), a pentode/diode tube (1S5) that is designed to run on a battery at 1.5V and 50mA, and a couple miscellaneous but cute tubes. In testing the 1S5, I found the filament never glowed incandescant at 1.5V and, on measuring, was barely sensitive to anything. I cannot imagine this tube would make much of a diode for crystal radio use. I tested different manufacture 1S5 tubes from two different suppliers. No dice. Finally, I tried to push the tube to operate at higher-than-specified voltages. At 4.3V the I-V curve was still very flat, but shifted slightly higher, crossing the Amp axis at about 0.4 mA, (see graph). At 5.5V one of my tubes gave up the ghost and I didnt do more. I pretty much rule the 1S5 out for my crystal radio work. Looks like i'll be needing the power supply when I get around to building/operating that Fleming Radio.
A question remains as to what in fact is different about these tubes to give such different characteristics. While the I/V characteristic of most vacuum diodes pretty much follow the Langmuir-Child Law, the steepness of the I/V characteristic is largely due to the geometry of the tube elements and the volume of electron space-charge between them. JB Calvert's Theory of Vacuum Tubes informs one that this is the property called Perveance in a vacuum diode. Measuring this requires plotting I raised to the 2/3 power against V and taking the slope of the best-fit line through the data. This slope raised to the 3/2 power is the perveance. The following chart illustrates graphically the relation bewtween the tube characteristic and perveance. For the 5726 tube I plot the characteristic in dark blue for the tube heated with the design 6.3 volts. In light blue I have plotted the measurements and raised them to the 2/3 power. One sees immediately that the new curve is linear. The slope of the curve raised to the 3/2 power yields the perveance factor for the tube.
(Note in addition that when running the tube at a more sensitive point with 4 volts only on the heater the perveance is slightly less than spec.).
With my measurements calculating the perveance is practical and quickly done, results as follows:
Tube Perveance R2 6C19P 2.54 0.997 5726 2.19 0.998 6DN3 5.40 0.997 20D1 2.02 LC? 0.985 (may not follow Langmuir-Child) 5C12P 0.32 0.999 1S5 0.063 0.999 2D1S 0.155 LC? 0.975 (may not follow Langmuir-Child) 6G2 0.050 0.999
The perveance numbers above range from 0.02 to over 2 mA/V^3/2 (with >2 being good for small-signal detection) and clearly shows my choice of the 6C19P to be an excellent one. I am surprised to see a huge 5.4 for the 6DN3 diode, a color television damping diode. It takes a Novar 9-pin socket and is a large tube so doing more with this tube may have to wait a bit.
Calvert's 2001 published data
Tube Perveance 6AL5 2.42 6H6 0.50 7Y4 0.58 2X2A 0.017 6V3-A 2.3 6AX4GT 1.42 6AV6 0.085 1A3 0.075
The high-perveance diode tube types I tested show an interesting property in that their characteristic curves do not pass through the origin of the graph. These curves are wholly acceptable as crystal radio detectors although they will require power to operate. What I have noticed with such tubes is that when I turn them off after using them with a radio, on cooling they go through a period of much increased sensitivity (very loud) before fading to nothing. It is as though running them at the full 6.3V lowers their full potential as crystal radio rectifiers. The reason is in the high perveance of the tubes, the anode is already proximate to the space charge before any plate voltage is applied. Effectively, while the plate (anode) "Zero" voltage is measured with respect to ground, the plate itself is still positive with respect to the cathodic space charge. Small currents will continue to flow even with a negative bias to the plate, (Contact potential).
In order to explore this idea, I ran a series of tests with the tubes running at lower operating voltages (effectively diminishing the space charge thus lowering the perveance) as shown:
Here you see the impact of lower operating voltages. With the tube operating between 3 and 4 volts the characteristic curves begin to pass through the origin as in regular solid-state diodes and crystals. It is precisely in this voltage range that I found the radio sensitivity, as measured by loudness to my ear, to be most pronounced. The diode characteristic needs to pass through the origin of the I-V graph for rectification at highest sensitivity. This takes me to the 20D1 tube which operates at 9v but only 200mA. This will still take a power supply to use, but the good thing is that the characteristic curve passes through the origin. Running at its design parameters the tube looks a lot like the 6C19P at 4v.