(21-08-2012, 05:16 PM)AlanBeckett Wrote: While I was at it I thought I'd better make sure that the Calibration is at least reasonable. I shorted out the duff switch position, fed it with 1kHz from my J-2 and set the Output to give a reading of 100mW on the Marconi. An AVO 8 across the Input showed 0.9Vrms. Assuming that the Marconi Input Impedance really is 8R then that shows an error of 1.25% - well within what I can measure or trust. So far so good, but what is the Input Impedance of the Marconi? I put an R-Box - a very good one - between the J-2 and the Marconi and set it to show half power on the Marconi, while keeping the Output of the J-2 constant - it does vary with Load. The answer was 5R!
Anyone care to explain that?
Alan
Presumably your R-Box consisted of a single resistance inserted in series with the J2 source and the TF 893A load. In which case, my analysis proceeds as follows.
Let the source voltage within the J2 be Vs; let its internal resistance be Rs; let the value of R-Box be R, (which at the moment we will say we don't know the value of). Let the input resistance of the TF893A be Rin and equal to a constant 8 ohms.
The first measurement.
For 100 mW indicated on the meter, the voltage across Rin is 0.9 v (by measurement).
Therefore we have:
Vs / 0.9 = (Rs + 8) / 8 (ratio of potentials = ratio of resistances)
This reduces to:
Vs = 0.11Rs + 0.88 . . . . . eqn. 1
The second measurement.
For 50 mW indicated on the meter, the voltage across Rin is now 0.632 v., since
V = √(PR) = √(0.05 x 8) = 0.632 v.
We have Rs and R in series and those two in series with Rin.
Vs is developed across all of them; the 0.632 v only appears across Rin (= 8 ohms).
Therefore, as before, we have:
Vs / 0.632 = (Rs + R + 8) / 8
This reduces to:
Vs = 0.079Rs + 0.079R + 0.632 . . . . . eqn. 2
Eliminating Vs from eqn. 1 and eqn. 2, we obtain:
0.11 Rs + 0.88 = 0.079Rs + 0.079R + 0.632.
This reduces to:
0.39Rs + 3.14 = R . . . . . eqn. 3
Now we have assumed that we do not know the value of R and the value of Rs, but we have a good idea of their typical and expected values.
If R = 5, then eqn. 3 returns a value of Rs = 4.8 ohms.
However, if Rs = 5, eqn. 3 returns a value of R = 5.09 ohms.
Both of those values are well within the tolerance ranges for each of the two unknowns Rs and R. Hence your measurements are valid & consistent and, moreover, establish that the value of Rin is also 8 ohms, within an acceptable & typical tolerance band.
HTH.
Al.
