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Author Topic: Need to find inductance of unknown coil *without* scope, func generator, etc  (Read 7351 times)
KD0REQ
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« Reply #15 on: June 30, 2014, 07:24:54 PM »

OK,I have in front of me the MOtorola HEP sheet on "tips on using FETs," august 1969.  one fet, one coil, 3 to 30 volts.

imagine a pencil in your hand, on a pad, as you draw this....

there is a coil with two windings (or wind a link over it if just one coil winding,) this is an IF amplifier unless you provide in/out feedback... IF transformers are a coil and winding in parallel.

ground one coil winding side, the other goes to the gate.  the drain goes to the other coil winding, thence to positive.  the source goes to ground.  if you don't have IF transformers, but have coils, put an appropriate capacitor of the same value across each coil winding.

that's all there is to a tuned-gate, tuned-drain oscillator.  run a wire end off the drain connection and that's your antenna to the GP receiver.

it will get you in the ballpark.

most JFETs allow you to change the source and drain wires, so all you really need to know is where the gate is.  the sheet shows a HEP-802, which is like a 2N5467.  but if it's a small-signal plastic JFET, and it goes to 200 or more MHz to provide enough gain, you're in business.

this is a crappy VFO unless you have a buffer and typically some stabilization caps, but you don't need stability and repeatability here, you need a quick and dirty test fixture.
« Last Edit: June 30, 2014, 07:33:35 PM by KD0REQ » Logged
KC3JV
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Posts: 20




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« Reply #16 on: July 01, 2014, 02:01:13 AM »

Go on EBAY you will find good functional L/C meters (NEW) for under US $20.  It will arrive in less than a week and works great!

Mark KC3JV
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WB6BYU
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« Reply #17 on: July 01, 2014, 03:47:27 PM »

Quote from: KD0REQ

...there is a coil with two windings (or wind a link over it if just one coil winding,)...



Certainly easier to use a circuit that doesn't require a tapped coil or winding a link.  The Franklin
oscillator described earlier will work, or I've used a Colpitts with a pair of 200 pf feedback capacitors -
that puts effectively 100pf across the coil, making it easier to calculate the inductance from the
oscillator frequency.  You can vary the capacitors as needed if your coils are particularly large or small.

These methods assume you have either a frequency counter or a general coverage receiver to
determine the oscillator frequency, of course.



Quote

...the sheet shows a HEP-802, which is like a 2N5467...



HEP-802 is a hobbyist-grade MPF-102.
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AF7EC
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« Reply #18 on: July 02, 2014, 03:28:34 PM »

Thanks again guys for the replies.

I did check out the Franklin oscillator, and it is interesting.

As far as an L/C meter, I'll just have to save my pennies and hit eBay when I can.

The VFO that I'm building uses a JFET and is fairly stable.  I've tried making VFOs with bipolar transistors, but the biasing is a whole other problem, and the JFETs work a lot more like vacuum tubes/valves.
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WA7PRC
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« Reply #19 on: July 02, 2014, 04:33:11 PM »

As far as an L/C meter, I'll just have to save my pennies and hit eBay when I can.

FWIW, the AADE L/C Meter II measures small values of L & C that you would encounter in RF work. It also compares quite well with some expensive test gear. Neil sells some other nifty gadgets as well.

vy 73,
Bryan WA7PRC
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G3RZP
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« Reply #20 on: July 03, 2014, 12:32:42 AM »

The thing to watch with FETs is the 1/f noise. A bipolar transistor with an un-bypassed emitter resistor of around 27 ohms can often be better, depending on the transistor you use and the frequency.
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WB6BYU
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« Reply #21 on: July 03, 2014, 08:48:09 AM »

Quote from: AF7EC

I've tried making VFOs with bipolar transistors, but the biasing is a whole other problem...




Actually biasing bipolar transistors isn't difficult at all, as long as you use enough resistors.
Many of the earlier designs tried to get by with just a single resistor feeding bias to the base,
and that doesn't handle the variation among devices of the same type.

Use a voltage divider to give a specific voltage on the base (using resistors around 10K to 100K),
then the emitter voltage will be 0.6V less than that for a silicon transistor.  Then, given the voltage,
choose an emitter resistor to set the desired current.

For example, to bias a transistor to 2mA from a 6V supply, you could use a voltage divider of
50K and 10k to set the base voltage to 1.0V.  The emitter voltage would then be 0.4V, requiring
an emitter resistor of 0.4V / .002A = 200 ohms for 2mA of standing current.
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AF7EC
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« Reply #22 on: July 06, 2014, 07:03:24 PM »

WB6BYU and G3RZP, thank you so much for your posts. Cheesy

To those who may grumble "How in the world did this knucklehead get to be an Amateur Extra?", I will let you, and all the world know something...I have a developmental disability.  I clawed and scratched through school.  Math(s) and I have a very hard time getting along.  I see the formulas on web-sites for determining frequency, inductance, etc and my eyes glaze over.  It's not that I'm being lazy or ignorant, it's because I have never been able to get math(s) to work well in my brain.  However, I have had a soldering iron and an intense interest to build radios since I was five or six years of age!

Needless to say, I just barely passed my Tech, General and Extra exams, but here I am.  I want to build and I have a decent collection of electronic parts I've harvested from untold number of broadcast radios, TVs and other gadgets.  I've got a lot of experience soldering, breadboarding and trial-and-error'ing, and I don't imagine my desire to homebrew amateur radio stuff is going to wane anytime soon.  It's just frustrating not knowing how to arrive at the correct values for tuned-circuit parts without some very clear language on how to calculate it.

Anyway, to all my amateur radio compadres, thank you!  Smiley
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KD0REQ
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« Reply #23 on: July 07, 2014, 01:43:06 PM »

I am constantly reminded in real life that I am never the smartest person in the room.  fortunately, I am often not the dumbest, but that is disputed by some.  in any event, workarounds and perseverance counts for quite a bit in this world.

hang in, good luck, and have fun.
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G3RZP
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« Reply #24 on: July 07, 2014, 02:30:05 PM »

KF7EC

>Math(s) and I have a very hard time getting along.<

It's fair to say that I have trouble with math, although probably not as much as you. Way back, in 1969, I (just about!) learnt about Laplace transforms and Fourier analysis. (I did totally fail the exam) I actually needed to use Laplace for the first and last time in 1985  and had to get the XYL to help - and she was away at a Girl Scout camp - and she helped over the repeater! However, I've managed  over 50 years of employment in the radio industry in design and semiconductor applications with only basic algebra: I've achieved Senior Membership of the IEEE, 16 patents and some 40 plus odd published articles in various places, including papers at international conferences. Plus chairing a number of international standards committees on radio matters, including representing amateur radio at the ITU -R Study Group 1  for the IARU as a Technical Consultant..

So DON'T sell yourself short because you have trouble with math! It doesn't mean that you can't still learn about (and do)the technical side of radio.
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KT4WO
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« Reply #25 on: July 20, 2014, 07:54:23 AM »

http://www.ebay.com/itm/Digital-Transistor-Tester-Capacitor-ESR-Inductance-Resistor-LCR-Meter-NPN-PNP-/291041497713?pt=LH_DefaultDomain_0&hash=item43c36cd271



This would be the best $18 bucks you have ever spent!
I have one I built from a kit a few years back.
Took a few hours going thru junk box and had all my
inducters sorted.

This is something every homebrewer needs!

KT4WO

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KT4WO
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« Reply #26 on: July 20, 2014, 08:08:51 AM »

http://www.ebay.com/itm/Inductance-meter-capacitance-meter-frequency-meter-LC-meter-with-back-light-/121228917349?pt=LH_DefaultDomain_0&hash=item1c39ce7a65




Or better yet....$12 bucks...shipped.

KT4WO

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