LC generator

Hello Andy,

To be honest I don't know why you want to do that with opamps. All you need is a good RF transistor, an amplifier after that which doesn't have to be particularly linear, a Schmitt trigger that's fast enough and then off into a frequency counter.

Look at how "grid dip meters" are designed. These are strikingly simple circuits. Their oscillating stage is able to work from a few hundred kHz to several hundred MHz. Examples can be found in the ham radio literature and probably on the web as well. The name grid dip meter is a left-over from the tube days. I still have one with tubes ;-)

Regards, Joerg

Hello Andy,

Not the ones I know. They have a variable capacitor on the inside and a set of inductors that you can plug in. Or you can connect an unknown inductor to the socket. There are only these two connections.

That is a very wide range. It will only oscillate if the DUT has enough Q. Lots of Q. Basically, if a good grid dip meter cannot make it oscillate chances are that nothing else will.

Can't you measure the unknown capacitance or inductance in another way? Even a regular DVM in the lab does a pretty good job there. For inductors we use an analyser but mostly just because it is there.

Regards, Joerg

Hello Winfield,

If you'd donate your new 4192A to Andy he might not need to build all this ;-)

Regards, Joerg

Hello:

I've designed an LC generator supposed to accept a wide range of tank capacitance values with the tank inductance fixed, and a wide range of tank inductance values with the tank capacitance fixed. The objective is to be able to measure the unknown tank capacitance/inductance as precisely as possible (better than 1%).

Before I go forward and buy the missing parts and solder the whole thing, I thought I might ask the s.e.d. community to take a look at the schematics, to spot any obvious problems / easy improvements.

I've posted the schematics in a.b.s.e. under the same Subject.

Thank you in advance for your feedback.

-- Andy I.

I have looked up some "grid dip meter" schematics on the Net, what I've found suggests that those are three-point oscillators, using a set of interchangeable LC tanks to cover an extended frequency range.

I think I need to explicitate some requirements to the device: the operational mode is to connect either an unknown capacitance in parallel with the tank, or an unknown inductance in series with the tank's inductance - the same as for the well-known aade meter. This entails that the L/C ratio varies very very much: from 1.2e8 to 4e2 [H/F] (C between 680pF and 0.2uF for the L fixed at 82uH, and for L between 82uH and 82mH for the C fixed at 680pF), and that only 2 points of connection are available for the tank, the output frequency is between about 15kHz and 670kHz. With these requirements, do you see a simpler circuit? Sure, the comparator is missing from the schematics, but that's an easy part.

Regards,

-- Andy

Absolutely.

The DUT inductor goes in series with L1. The DUT capacitor goes in parallel with C8 (sorry for the numbering!). The output is the output of eitehr U8 or U1 (goes to a frequency counter).

An etalon capacitor is temporarily connected in parallel with C8, and based on the frequency shift, the reference capacitance of the tank is calculated.

Then when the DUT is connected, based on the frequency shift, its value is calculated.

So the operational mode is the same as for the well-known "aade" meter.

The objective of going pure sine wave (and the harmonics are pretty low here, the signal is 80mV pp at U1 output), is to attain as much absolute precision as possible. Better than 0.5%, and for L down to 10nH.

Because the ratio L/C varies widely from 1.2e8 to 4e2 [H/F] (C between

The negative feedback coefficient is set with the two FET-based attenuators in series. Two attenuators were set up in series because of the required 80dB attenuation range.

The output signal is rectified (germanium diode here) and filtered before closing the oscillator AGC loop. The filter values were chosen to provide as oscillation-free as possible AGC regulation on one hand, and the maximum precision of the amplitude on the other.

Regards,

-- Andy

The arrangement should work fine but seems sensitive to low (=normal?) values of inductor Q. Using C8 and L1 with a Q of say 10 (series loss of 30ohms) the U1 circuit won't oscillate. Not enough positive feedback due to the lossy divider action of R7 and the working tuned circuit new Rd of about 3kohms. (R7 needs a 0.47u in series). It'd be difficult to increase the loop gain to compensate as the fets are maxed out in the ohms area and another auto attenuator circuit on the +ve feedback side is beyond the pale. My own experience (ie. years of mistakes and stupidity :) suggests it's better in these 'wide ranging Rd-Q' cases, to just feed the tuned circuit from a controlled, good quality, current source. The current source can then act as the oscillator ALC element. (I say this as I've been running a similar project). regards john

Well, almost.

I looked through a few drawings of grid dip meters and the ones I have generally do use a 2 terminal connection for the inductance, but use a dual variable capacitor in the Colpitts configuration. No problem for measuring inductance, as you suggest, but measuring capacitance would be somewhat more complicated. I couldn't find a drawing of a grid dip meter with a 2 terminal LC connection but that doesn't mean they don't exist.

Of course, if tunnel diodes were still available, you could do the whole thing with about 5 parts...

For a good review of the principles of precise RCLD measurement, go to the US Patent and Trademarks Office and look at the theory described by the assignee Genrad. You can also search under "Impedance Measurement" in the title.

Jack

There is a similar device presented here:

It uses a comparator so that it works over a wide range of LC ratios.

kevin

An inductance may give a slightly different value depending on the oscillation frequency. Inductances are measured at particular frequency so the 1% precision is more or less... utopian.

S.M.

Ok, I think I will do some more search.

I could have. But it looks like one has to make quite an investment to get better than 1% precision measurement for capacitance. And I wondered how far one can go with such a relatively simple circuit (it is not good for capacitances larger than 1uF, but that's ok).

Regards,

Andy

What? Neil's relationship is textbook perfect, with the L = 1 /(w^2 C) formula, etc. The other formulas may look a bit complicated, but that's because he adds a clever method to use measurements on a 0.5% "standard" capacitor, included in the instrument, to provide precision calibration for the rest of the measurements.

Yes, I know that circuit. I wanted a more linear circuit, in which the relationship between oscillation frequency and the tank resonant frequency would be more straightforward.

-- Andy

I had a similar reaction the first time I saw Neil's circuit, but after seeing how well it works, I realized the oscillating frequency isn't much disturbed by the things you mention (they affect the oscillator "gain"), but rather is determined by the high-Q L-C resonance, which overcomes those issues. His automatic calibration technique hides the rest of the true disturbing issues, such as stray capacitance.

I recommend you get one of his little beauties. Quite inexpensive, too.

Yes, they are too perfect, I think; these are not complicated formulas. What I am saying is that there are several other factors that should impact the real oscillating frequency in a complicated, non-reflected in the above formulas way, especially at the hi-frequency end: the comparator's threshold levels, the slew rate/propagation delay, the power supply voltage, ... Instead of trying to account for these factors, why not work with a regular oscillation.

Thanks,

-- Andy

Hello BFoelsch,

When I have to measure a capacitance in a tight spot where I can't get to with an analyzer I just connect a well known inductor to it. Then I use the dip meter to figure the resonant frequency of the resulting LC circuit and, bingo. But it happens so infrequently that I have put the dip meter on the list of units for regular battery swaps. Else it might just start to leak from old age.

Tunnel diodes were all the rage when I was young. The ultimate boutique part. Every professor marveled at them but you couldn't get them anywhere or they cost an arm and a leg. So I guess they went the same way as the unijunction transistor (I did have one of those). I prefer the bread and butter stuff such as ye olde BFS17A. Available for pennies at every street corner and blazingly fast.

Regards, Joerg

from

Part Number: 1N3712

General Purpose Tunnel Diode Price Special: $ 68.79

ouch!

martin

...

There seem to exist an offer of legacy Soviet parts from various distributors in Russia, including the Arsenid-Gallium and Germanium tunnel and reversed diodes (not expensive). If only there was a good application today. Except the micro-bugs :-)

-- Andy