A Digital L-C “Meter”

For many years the AADE L-C meter (http://www.morsex.com/aade/lcmeter.htm) was the ‘gold standard’ bit of test equipment for radio home brewers.  At about $100 it was a bit pricey considering that you could get a Chinese clone on Ebay for about 1/4 the price.  I wish I had bought one before Neil Hecht passed away, his device was more accurate, and had a wider measurement range than any of the clones.

The principle of operation for these L-C meters is well known.  The frequency of an L-C oscillator is constantly being measured under micro processor control.  The oscillator has known fixed values of L and C, and there is provision to introduce either an unknown value of capacitance in parallel with the known fixed value, or an unknown value of inductance in series with the known value.  In either case the frequency will go down by an amount proportional to the added inductance or capacitance.

Calculating the value of the unknown part is a simple matter of applying the math.  The formula for finding the frequency of an LC oscillator when both L and C are known can be modified to solve for either L or C when the other and the frequency are known.  The accuracy  of this measurement will depend on how accurately we can measure the frequency, and how accurate are the measurements of the internal ‘standard’ inductor and capacitor are.   It is possible to calibrate the meter by measuring a precision capacitor or inductor (1.0% or better).   Usually precision capacitors are easier to find than inductors.

A few years ago Phil Rice, VK3HBR, published a design for an easy to build L-C meter based on the AADE circuit, but using his own software.  Like the AADE meter, Phil’s design used a PIC microcontroller as a frequency counter, doing the necessary calculations in software.  A open source floating point library provided by Microchip was used to perform the necessary computations.  The original version of this meter used a separate LM311 comparator for the oscillator, a newer design made use of the PIC16F628 which has that function built in.  Other than a slight cost reduction, the two designs work about the same.  Since circuit board artwork was available for the newer design, that is the one that I chose to build.

You can find all of the details on construction from Phil’s website: https://sites.google.com/site/vk3bhr/

Here are photos of the unit as I built it:

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Being able to measure the inductance of home wound coils is very useful to the amateur radio builder.  While formulas,  computer programs, and on line calculators are available to compute the number of turns required for a desired inductance when winding toroids or slug tuned coils, being able to measure the result will save hours of troubleshooting later on.  Of course, one can also measure the resonant frequency of tuned circuits with a dip meter, and thereby indirectly measure the inductance of a coil (assuming one knows the circuit capacitance).  I’ve already described a home built dip meter in a previous post.

 

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A Heathkit Dip Meter clone

I’ve built many Heathkits over the years since the 1960’s.  My first Heathkit was their MM-1 VOM, which I owned until about 15 years ago.  By then the meter glass had been broken and replaced several times (I had a Glassier cut me a piece of replacement glass), and the range switch had become balky.  I’d long replaced it with a Heath branded Fluke 77 series meter that is my current standby.

After the MM-1, I’d built their GR-54 short wave set, a 21″ color television set, GR-17 AM/FM portable radio, one of their alarm clocks, and an SB-102 amateur radio transceiver complete with the matching speaker, power supply and microphone.

There are several Heathkits that I wish I had bought.  Some of these have been obtained used on EBay, such as an IT-21 tube checker, and SG-8 signal generator, IT-1121 semiconductor curve generator, and an IB-5261 RLC bridge.  I might have gone for one of their oscilloscopes, but I picked up two old TEK’s instead.

One Heathkit that I wish I’d bought was their HD-1250.  This was the last of the Heathkit dip meters.  Some time ago I did find a used and battered GD-1 at a ham flea market, minus its set of coils.  The line cord was badly frayed, the coil socket was broken, and the selenium rectifier quite questionable.  I replaced the rectifier with a 1N4007 and increased the value of one of the resistors to compensate for the resulting higher voltage.  The filter capacitors were replaced, and a large pin crystal socket was installed in place of the broken coil socket.  A new tube was also installed, and I then cobbled together some coils using some old tube bases with some of the pins removed as plug in connectors, with short lengths of PVC pipe for the coil forms.  I was able to adjust the turns so that the dial calibrations were ‘close enough for government work’.

The GD-1 once repaired was usable, but it was obvious that a battery operated solid state unit would be more desirable.  Also I already had a smaller home built dip meter that used a 6CW4 nuvistor and a built in power supply using two 6.3 volt, 300ma filament transformers (Rat Shack) connected back to back.

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The variable capacitor came from a transistor AM/FM portable radio, and the coil forms were built using 1/2″ plastic water line tubing with RCA plugs (they looked like the coils that came with the Heath ‘Tunnel Dipper’ and the HD-1250).  The nuvistor dipper worked better than the GD-1, so I eventually sold the repaired GD-1 on EBay (making a very small profit).

 

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HD-1250’s do show up on Ebay quite often.  Many times the owner thinks they are ‘rare’ and wants a king’s ransom for them, or the auction ends in a bidding frenzy with the unit going for more than it’s worth.  The schematic for this kit is available on line, and after seeing the parts list I felt that I might be able to build my own version of it, IF I could find a suitable variable capacitor.

The several dip oscillators I’d built using variable capacitors from AM broadcast receivers convinced me that this was not the way to go.  BCB tuning capacitors have two sections, with the oscillator portion about half the value of the antenna side.  I’d tried wiring them up with the larger section both ways (on the ‘gate’, ‘base’ or  ‘grid side, and also on the ‘drain’, ‘collector’ or ‘plate’) end.  One way works better on HF coils, the other on VHF.  You really want a variable cap with both sections having the same value.  You could yank plates out of the antenna side so it would be the same (at the maximum setting) as the oscillator side, but the two sections would not track over 180 degrees.  It would probably work better than stock, but there would still be the problem of having less range per coil.  Instead of needing 5 or 6 coils to cover the desired frequency range, you might need nearly a dozen. (My nuvistor dip meter has a set of 9 coils).

Then one day we cleaned out my mother-in-laws house and found an old Sears transistor portable radio  It had a three section, air spaced tuning capacitor with a built in ball bearing vernier drive.  Two of the sections were identical for the antenna and RF stages, the oscillator section was the usual cut down with about half the max value as the other two.  A portable radio with an RF stage is rather rare, this must have been a high quality radio when new.  While it did work, there had been some damage from leaky batteries, and the case was ratty from being stored in a damp location.  We didn’t need yet another AM portable radio, so I happily decided to scrap it for parts.  While a bit larger physically than I’d like, the tuning capacitor with its two 250pf sections was just what the doctor ordered for building a HD-1250 clone!

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The circuit of the HD-1250 used two transistors.  The oscillator was a high frequency NPN, and there was a dual gate Mosfet a buffer stage between the oscillator pickup and the detector stage using hot carrier diodes that drive the meter.  The part numbers given were Heath house numbers, but I determined that an MPS5179 (2N5179) would work in the oscillator.  This is a low voltage (9v) NPN transistor with an FT of over 1000 mhz.  An RCA 40673 would sub for the Heath numbered Mosfet.  I had some hot carrier (Schottky) diodes in the parts bin, as well as the other two mentioned transistors.  The HD-1250 used a 150ua meter, I had two NOS 50ua meters marked ‘Lafayette’ I’d found at a hamfest.

The next problem would be to find a suitable box to build it in.  The junque box had a few nice looking mini boxes. I tried the variable capacitor on for size and selected a box that was just deep enough to fit.  While larger than the real HD-1250, it was still comfortable in the hand.

Ever since the ‘Tunnel Dipper’, Heath has used RCA sockets and plugs for dip meter coils.  This worked fine in the tunnel dipper circuit, but the split section tuning capacitor in the HD-1250 really should require the use of a ‘balanced’ coil socket.  So I went back to the crystal socket I used in rebuilding the GD-1, and made the plug in coils using ‘Banana’ plugs, bits of PC board, and plastic water line. Five coils tune the range from 1.3 Mhz to 150 Mhz, a sixth hair pin coil extends the range to 175 Mhz, but it oscillates poorly.

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Like the Heath HD-1250, the circuit was built on two circuit boards.  Both circuit boards were etched using a sharpie marker for resist and HCl-H2O2 for the etching solution.

The board closest to the coil socket is the oscillator, the board on the other side of the variable capacitor is the amplifier-detector board.  In hind sight, I should have done this the other way.  On the highest frequencies the variable capacitor itself forms part of the coil, and on the highest frequency coils connecting the oscillator at the coil socket is taping down on each end of the coil, reducing the amount of coupling.  That’s probably why the hairpin coil oscillates so poorly.

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I added an RCA jack to the bottom of the unit.  It connects to the cathode of diode D21 in the schematic to provide a sample of the oscillator signal.  I can connect a frequency counter here for calibration of the dial scale, or a more accurate frequency readout.

I think that except for the lower frequency range (the HD-1250 goes up to 250 Mhz) my unit works as well as the original.  If I ever find a used HD-1250 at a good price I might buy one, but for now I’m happy with my clone.