Crystal Radio Set Systems: Design, Measurement, and Improvement Volume II A web book by Ben Tongue First published: 10 Jul 1999; Revised: 01/06/10 i NOTES: ii 185 PREFACE Note: An easy way to use a DVM ohmmeter to check if a ferrite is made of MnZn of NiZn material is to place the leads of the ohmmeter on a bare part of the test ferrite and read the The main purpose of these Articles is to show how resistance. The resistance of NiZn will be so high that the Engineering Principles may be applied to the design of crystal ohmmeter will show an open circuit. If the ferrite is of the radios. Measurement techniques and actual measurements are MnZn type, the ohmmeter will show a reading. The reading described. They relate to selectivity, sensitivity, inductor (coil) was about 100k ohms on the ferrite rods used here. and capacitor Q (quality factor), impedance matching, the diode SPICE parameters saturation current and ideality factor, #29 Published: 10/07/2006; Revised: 01/07/08 audio transformer characteristics, earphone and antenna to ground system parameters. The design of some crystal radios that embody these principles are shown, along with performance measurements. Some original technical concepts such as the linear-to-square-law crossover point of a diode detector, contra-wound inductors and the 'benny' are presented. Please note: If any terms or concepts used here are unclear or obscure, please check out Article # 00 for possible explanations. If there still is a problem, e-mail me and I'll try to assist (Use the link below to the Front Page for my Email address). Second note: The two dates following the Article titles are, respectively, the original publication date and the date of the last revision. iii 184 in the Fair-Rite catalog as being above 350 degrees C. The annealing process reduces the permeability somewhat, but reduces the loss factor substantially. The low loss-factor property of the annealed perminvar ferrite can be easily degraded by mechanical shock, magnetic shock or just physical stress (as from a tight mounting clamp). The Fair-Rite catalog sheet for type 61 ferrite cautions "Strong magnetic fields or excessive mechanical stresses may result in irreversible changes in permeability and losses". Actually, the changes are reversible if one goes through the annealing process again. The MMG catalog, issue 1A, in writing about perminvar ferrites, adds: "Mechanical stresses such as grinding and ultrasonic cleaning increase the permeability and lower the Q, especially at the higher frequencies, although the changes in Q at the lower frequencies may be very small. I suspect that there is now much less pressure on ferrite manufacturers to deliver a low loss product than in the past. Since time is money, maybe they now skimp on the annealing process. Several years ago I took some 4" x 0.5", mix 61 rods I had purchased from CWS ByteMark and had them re- annealed at the plant of a local ferrite manufacturer. The Q of a litz-wire coil using the re-annealed core, at 2.52 MHz, was increased by 12%. This indicates that the core was not originally properly annealed, or had been subjected to some mechanical or magnetic shock after being annealing by the manufacturer. I was informed, when I asked, that coil Q at high frequencies could be expected to increase by up to100% from the pre-annealed value. I chose the best of these re- annealed rods to be my "best ferrite core" rod in this Article. Original web location for book: One source informed me that few ferrite manufacturers http://www.bentongue.com/xtalset/xtalset.html perform the annealing process anymore. Toroids made of type 61 material are still made here in the USA. iv 183 ohms overwhelming the increase in DC conductor resistance TABLE OF CONTENTS: from 0.16 to 2.62 ohms. P 001 ARTICLE 17 New ways to Increase Diode See Table 3 for measured inductance and Q values of an Detector Sensitivity to Weak Signals, and a way to determine inductor similar to inductor BB, but wound with 125/46 litz if a diode detector is operating above or below its linear-to- wire. Here the Q is even greater than in Table 7 because litz square-law crossover point construction is less sensitive to proximity and skin effect losses than is solid wire. P 009 ARTICLE 18 Get 3 dB More Output for Greater Volume on Strong Stations plus… Thanks must go to Brian Hawes for making me aware of the FEMM program and showing me how to use it. P 016 ARTICLE 19 An explanation of how the "Mystery Crystal Radio" works Part 6: Perminvar ferrite, and what the term means P 024 ARTICLE 20 How to Measure the Impedance Normal nickel/zinc ferrites (NiZn), the types with less of an AM Band Antenna-Ground System, what one can do permeability as well as lower loss factors than manganese/zinc with the results, along with some measurements (MnZn) ferrites, are often used at RF because of their low loss at the higher frequencies. They do not suffer appreciably from P 032 ARTICLE 22 Design, construction and permanent changes in permeability or loss factor from measurement of a single-tuned crystal radio set using a two- exposure to strong magnetic fields or mechanical shock such value inductor, along with a discussion of the cause of 'hash', as grinding, or dropping on the floor. short-wave ghost-signal and spurious FM reception. A way is presented to determine if the signal operating the detector is Special nickel/zinc ferrites, called perminvar ferrites can above or below its linear-to-square-law crossover point achieve a considerably lower loss factor for the same permeability than normal nickel/zinc ferrites, and at higher P 063 ARTICLE 23 How to Make a Very Efficient frequencies. This result is achieved by adding a small amount Double-Tuned, Four-Band, MW Crystal Radio Set using two of cobalt to the ferrite power before firing, but there is a catch. Version 'b' Single-Tuned, Four-Band, MW Crystal Sets In order to actually achieve the lower loss factor, the ferrite core must be annealed by raising it to a temperature above its P 068 ARTICLE 24 Sensitivity and selectivity issues Curie temperature (the temperature at which it losses all its in crystal radio sets including diode problems; measurements permeability), and then cooling it very slowly back down of the Q of variable and fixed capacitors, RF loss in slide through the Curie temperature, and then to lower temperatures. switches and loss tangent of various dielectrics This process usually takes about 24 hours. The Curie temperature of ferrite type 61 (a perminvar ferrite) is specified v 182 P 089 ARTICLE 25 A new approach to amplifying maximize Q, do not cover the whole length of the core with the the output of a crystal radio set, using energy extracted from solenoid. the RF carrier to power a micro-power IC to drive headphones Table 7: Simulation of inductor BB in FEMM at 1 MHz, with or a speaker various conductor diameters (type 61 core material) P 097 ARTICLE 26 Highly sensitive and selective single-tuned four-band crystal radio set using a new contra wound dual-value inductor, and having a 'sharp selectivity setting'; along with a way to measure the unloaded Q of an L/C resonator P 136 ARTICLE 27 Measurement of the senstivity of a crystal radio set when tuned to a weak fixed signal, as a function of the parameters of the detector diode; including output measurements on 15 diodes P 147 ARTICLE 28 How to Reduce Diode Detector * Simulates winding the 58 turn solenoid directly on the 4" Weak-Signal Insertion Power Loss to Less than that Possible long ferrite core (solenoid ID=0.5013") instead of on a former when the Input is Impedance Matched Originally released as having an ID of0.6263". Note that the the two simulations Article #15, later withdrawn. Republished as Article #28 using a conductor diameter of 0.008995" show remarkably similar parameter values. P 154 ARTICLE 29 About Maximizing the Q of Solenoid Inductors that use Ferrite Rod Cores, including charts Table 7 shows the benefits of spaced winding when using solid of Magnetic flux density and lines, with some actual Q and wire. All the inductors in Table 7 use centered solenoids of 58 inductance measurements turns and a length of 1.624". The only variable is the diameter of the conductor, which controls the spacing of the turns (the P 185 NOTES winding pitch is held constant). The lesson here is that, when using solid copper wire, there can be a great Q benefit by space winding the solenoid and using an optimum size wire; in this case a Q of 431.9 vs 130.1 at 1 MHz, with solid wire. One can see that core losses change very little with the various conductor diameters (Hysteresis losses in ohms). Notice how, with a conductor diameter change from 0.02530 to 0.00006300", the AC copper loss decreases from 11.6 to 3.02 vi 181 ARTICLE 17A Part 5 - Ferrite-rod inductor simulation experiments; all using New ways to Increase Diode Detector Sensitivity to Weak centered solenoids 1.624" long and having 58 turns Signals, and a way to determine if a diode detector is operating above or below its Linear-to-Square-Law The solenoids used in the simulations in Table 6 all use a Crossover Point conductor having a diameter of 0.0253". The only parameter varied is the core length.