On Wed, 21 Jan 2009 11:49:27 -0800, Michael Tope wrote:
>I would also use a relatively
>low impedance for the shunt element in the voltage divider. This will
>reduce errors caused by probe and stray fixture shunt capacitance at
>higher frequencies (and also the phase angle error if the DUT is
>reactive and not resistive).
EXACTLY! There's nothing "magic" about 50 ohms -- it's simply a value that
we often get for free built into our spectrum analyzer voltmeter. We could
improve accuracy at lower values of unknown Z by using lower values of load
Z. Something as simple as adding a second 50 ohm load across the voltmeter
input yields a 2:1 improvement. One reason 50 ohms is convenient is that
there it's easier to find 50 ohm loads that are likely to be good to a
fairly high frequency. The tricky part is connecting them in parallel
without adding capacitance. :)
73,
Jim K9YC
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