Alignment Advice.

[AlanBeckett]

Joe,

I assume you're using your E2. The Output Impedance is claimed to be 75R.
That's a very odd place to put your Meter. Isn't it across the Coil?

Alan

[Joe]

I knew I should have kept quiet.

The test this morning was using the HP audio oscillator. What I was trying to do was provide Al with the information to work out the inductive reactance and therefore the inductance.

There was no meter use: the signal path was oscillator, coil, CRO and back to the oscillator.

- Joe

[AlanBeckett]

Could you daw the arrangement.

Alan

[Mark Hennessy]

Judging by the numbers provided, it's something in the 0.3H region, which doesn't sound very likely to me...

A couple of things:

1. The voltages are very small for a 'scope reading. Make measurements in 10s of millivolts at least, and here there's no obvious reason to not use 100s of millivolts, or indeed volts.

2. Putting the coil in series with the 'scope means that you're relying on the 1M input impedance - which is a bit on the high side. Stray capacitance could cause all sorts of problems.

I'm lucky enough to have a TF2700 for measuring inductors, but in the absence of that, I think I'd try putting 1k or 10k to ground, setting the oscillator to a low frequency and adjust the level so that the trace fills 4 divisions peak (8 peak to peak). Then wind up the frequency until it starts to roll off. Stop when it's reached 2.8 divisions peak (5.6 peak to peak). That's the -3dB of the filter. Then you can use f=2 x pi x R x L - assuming your oscillator goes high enough in frequency to roll over.

[pwdrive]

Hi Joe, you could loose couple the sig. gen. tune the inductance to resonance (monitor with scope) with a capacitor of known value or approx known value, note the frequency then work it out from there with the standard formulas.

Lawrence.

[Joe]

Alan: There's a drawing in Post 50.

Mark: I'll have a go.

- Joe

[pwdrive]

Hi Joe, you could loose couple the sig. gen. tune the inductance to resonance (monitor with scope) with a capacitor of known value or approx known value, note the frequency then work it out from there with the standard formulas.

Edit) And a Nonograph.

Lawrence.

Sorry Joe, I did not see Marks post.

Lawrence.

[Joe]

Set to 8V p-p at 50c/s. It's down to 5.6V at 620kc/s.

   

- Joe

[Mark Hennessy]

Hi Joe,

I reckon that's 260uH, which seems a bit more plausible.

The reactance of an inductor, XL, is given by 2 pi times F times L

In the filter above, we get the -3dB point when XL is equal to the R. Note that it's -3dB, not -6dB, because of the phase angle.

So, at the -3dB point, we know that XL equals 1000, so it's just a case of rearranging to find L. Or, L = 1000 / (2 pi times F)

Hopefully this makes sense,

Mark

[Mark Hennessy]

Of course, the above is true when Zout of the HP is low - I should have checked that with you. Do you know what it is?