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A Simple Homebrew L and C Tester

Discussion in 'Homebrew and Kit Projects' started by K5UOS, Feb 28, 2010.

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  1. K5UOS

    K5UOS Ham Member QRZ Page

    I have been helping a new ham get started homebrewing. I convinced him that one of the essential workbench items is a good LC meter or some means to measure L and C. Pauls posts regarding LC Meters have been interesting. It is hard to compete with a $13 dollar meter.

    One of the popular LC Meters is the AADE meter that offers a digital readout. The AADE meter originated from an article by Bill Carver in the Communications Quarterly and also shown in EMRFD by W7ZOI.

    http://www.qsl.net/wm5z/cq199301b.pdf

    My friend Glenn, KE5LPB, is building the meter from the article this weekend. I decided to build it for the 3rd time. My meter requires either hand calculations, a computer program or a spreadsheet. I have a 1000pF .25% General Radio Standard Capacitor and assume I can calibrate the homebrew meter very close to that tolerance. The meter was built from the junk box and cost zero dollars.

    This is the simple board without the inductor which is a 23.8uH inductor wound on a T130-6 toroid and fills the large open space on the board.

    [​IMG]

    This is the finished LC Tester.

    [​IMG]

    [​IMG]


    This is the simple spreadsheet to calibrate and calculate. With this simple meter the results are essentially as accurate as the standard you use to calibrate the meter. The 1001.975 pF result is the SM capacitor in the LC tester. It is a 1% 1000 pF Silver Mica. The two clips measure approximately .8pF. The 1001.975 compares to 1001.505 average measured on my last homebrew LC meter for the same SM capacitor. I calibrated both of my HB testors with the .25% Standard. The variance is .0475%.

    [​IMG]
     
  2. VK2TIL

    VK2TIL Ham Member QRZ Page

    This is real back-to-basics (sorry, Agilent :) ) stuff; it's what home-brewing is all about.

    After your mate Glenn has built this device and used it a few times he will have a real "feel" for what L & C are; not an abstract view but a "hands-on" understanding.

    You mentioned W7ZOI; you may know that Wes has recently become interested in crystal sets.

    This is a man to whose door some of the world's finest engineers come humbly knocking. Wes has worked and pioneered in some of the most esoteric fields of electronics.

    Yet here he is, wondering "Why is it so" about a "simple" tuned circuit.

    We all have a lesson to learn from this man.

    PS. I forgot to say that the equations can be plugged into a basic scientific calculator; it's often a pain to fire-up the computer for a simple job.
     
  3. K5UOS

    K5UOS Ham Member QRZ Page

    Hi Kerry,

    I was going to include the programmable calculator but I never had much experience with them. Also, our PC is never off at home.

    I have had many years experience with spreadsheets and like to recommend them for this type of application and for many other ham design applications.

    Glen, KE5LPB, finished his Junkbox Bill Carver LC Meter and it worked. He opted for a rough etched board rather than "ugly" style. I think he is excited because he built a working oscillator. Notice the wooden dowel in the center of the toroid to hold the core.

    I attached a Colpitts schematic that I used in my first LC Tester. It is 14 components w/transistors. Less, if you use the simpler FET buffer. Any buffer will likely do the job.

    [​IMG]

    Glens Meter board:

    [​IMG]
     
  4. KL7AJ

    KL7AJ Ham Member QRZ Page

    Very cool indeed. This should be required by law to get your ham ticket. :)

    Another alternative "must" for any ham shack is the conventional analog LCR bridge or "Q" meter. All the major kit manufacturers produced something like this...in fact the Heathkit component bridge with its famous green eyeball indicator still haunts my earliest nightmares. Well, not actually...the 6E5 was a very comforting green glow. Now the Gonset Goonybox with it's chrome LIDDED green eye WAS a veritable nightmare. A cyclops of the first degree. :)
     
  5. VK2TIL

    VK2TIL Ham Member QRZ Page

    Good work; Glenn should be pleased.

    I know how he feels about building a working oscillator; I still get a "buzz" when a home-built oscillator works!

    My first L/C tester was a homebrew bridge; it was based on a Practical Wireless design.

    PW had some great projects 30 or 40 years ago; I still use the GDO I based on a PW design. I also used the GDO with suitable standards to measure L & C at higher frequencies.

    I use an AADE meter these days; I've had it for years and it performs very well.
     
  6. KL7AJ

    KL7AJ Ham Member QRZ Page

    Might also mention that an LCR meter...even a bridge type "meter" like the LCR740 differs a bit from an R-X bridge in a couple of ways. Your standard LC meter uses a fixed frequency oscillator, typically in the 1KHz range which makes calculations a lot simpler...in fact, generally they have direct readout of the component values, if they're calibrated that way.

    However, many R.F. components can be quite frequency sensitive, which means you need to measure them AT the frequency of operation. In this regard an RX bridge (or even a noise bridge) is the way to go.
     
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