also it's easy to make a negative capacitance circuit tunable or frequency-dependent by varying the gain of the negative feedback loop portion so you can adjust the oscillator operating frequency that way without a varicap, too
Let it ring in the 10-100Mhz region, square it up to logic levels and then use a divide by a suitable factor of 2^N to bring it into the region where a uP can measure the frequency reliably.
-- Regards, Martin Brown
What's a "quadrature synchronous detector"?
It sounds a bit like my idea to use a phase detector
Right but two synchronous detectors, one in phase and the other 90 degrees off of that.
GH
If you model the finite Q as a shunt resistor, and connect it to a negative impedance to form an oscillator, the shunt resistance is precisely canceled at the point of steady-state oscillation.
Anyhow, an oscillator is a simple way to measure inductance. At moderate Q, the equation works.
To the mathematically challenged an easy way to state this is the frequency of a resonant circuit varies with the value of the loss resistance, But not very damn much. My physicist friend taught me that.
Would it affect the frequency differently if the R in the C or L or external?
points. I assume they are related.
Mikek
After careful measurement, I have never been able to find any evidence of negative resistance in a cc Colpitts or how to control it. Instead, I use the Bode criteria. That is
For examples, see Oscillator.zip at
I never said it didn't. If you had read my post, or looked at the link, you would find that Q has a minor effect on the frequency for Q of 4 or greater.
"Q must, for example, be less than four to make it differ by 1%"
Radiotron Designer's Handbook Page 449
A bigger problem is iron core inductors. As you pointed out earlier, different measurement methods can give different results.
It's a one-component Fourier meter, doing a multiply/accumulate on a periodic signal.
Only by 2:1. And it's easy to measure frequency to a part per million.
Bode should be Barkhausen
I definitely not going to set-up my monitor for every odd picture. I once set it up for color balance and background illumination levels.
Get yourself a decent flatbed scanner and scan it to proper B/W GIF (not JPEG). Flatbed scanners are available in multifunction printers
Yes of course but in one of his post's the OP was complaining their hardware had problems measuring small differences in frequency.
piglet
There are some tricks you can use. These make it trivial to measure to 1e-12 in 1 second. 1e-14 is a bit tougher. 1e-16 is for the pros. 1e-18 is theoretically possible but difficult.
He could buy a cheap counter and get to 1 PPM and be done.
A clearly hand-drawn schematic can be photographed with a cheap camera, or even a cell phone camera, and be perfectly legible.
Jpegs work fine.
I think this circuit works:
Ink on grid paper, cheap camera.
All my sketches have a title and a date, so if anyone sees it years later they know what it is.
A whiteboard can be nicely photographed too.
Simple. Make a 2N3904 emitter follower running at 25 mA or so from split supplies. Ground the base via a couple of inches of wire, bypass the collector, and watch it oscillate. No tapped tank required.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
R.F. Design magazine featured a circuit some years ago that drives an inductor with a 100kHz sinusoidal current, and measures the resulting in-phase and quadrature voltages.
That method is robust, yielding independent measurements of inductance and effective series resistance.
I have a scan of the original article somewhere(*), but this seems to be a mostly-faithful reproduction:
You'll notice some interesting compound op-amp arrangements, which the original article explains provide a 2nd-order frequency compensation, which drastically improves the phase response. (That's important, otherwise phase errors eat into the I-Q scheme's accuracy.)
(*) Ah yes, here it is: "Simple Digital Inductance Meter with 0.1nH Resolution" Roger A. Williams, LTX Corp., R.F. Design, October 1987, p50-55
Cheers, James Arthur
A more modern version could use a 50 ohm sine source, digitize at 4F, and do some math.
50 ohms? The scheme needs a current source. That way the resistive and reactive components are easily measured.
I started to cobble together a 'modernized' version a ways back with variable frequency excitation to cover larger inductances, and a discrete current source that was faster than the op-amp version. But I diverted and never finished it, can't recall why.
The Kelvin drive scheme is elegant--no error from long leads.
Cheers, James Arthur
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