Re: Varactor ?


"Bob Masta" <N0Spam@xxxxxxxxxxx> wrote in message
news:4915956e.2914154@xxxxxxxxxxxxxxxxxxxxx
On Sat, 8 Nov 2008 06:37:18 -0600, "Dave"
<db5151@xxxxxxxxxxx> wrote:


"Andrew Holme" <ah@xxxxxxxxxxxx> wrote in message
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"Dave" <db5151@xxxxxxxxxxx> wrote in message
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Looking at the data*** for the NTE618, and wondering why it lists a
frequency (1MHz) after each entry for the interterminal capacitance.
Anyone have any ideas on this? I figured the applied voltage would be
DC. No?


They give the mean DC voltage, VR, as well. In operation, the diodes
are
subjected to a DC reverse bias with a (hopefully) smaller AC signal
component superimposed. You have to apply an AC signal to measure
capacitance! 1 MHz is a good test frequency for an AM radio tuning
diode.


Hmmm. Well, thanks for the reply... Now I just have to try and
understand
why an AC signal is necessary for measuring capacitance. Not as smart as
I
thought I was. I don't remember covering varactors in school (30 years
ago!) but I thought I understood the principle. If you would be so kind,
why *is* an AC signal necessary for the measurement of capacitance?

Thanks again,

Dave


In principle it isn't, but in practice it is. If
you had an ideal large-valued capacitor you could
apply a known DC voltage through a known
resistance and measure the time the capacitor took
to charge to a given fraction of the source
voltage. That gets pretty tricky when the
capcitance is as small as we're talking about in a
varactor... it can't handle a huge voltage (even
if you had one handy) so you'd need to measure a
ridiculously small time interval (and final
voltage). And that measurement would assume the
capacitor was ideal... which it isn't.

Also worthy of consideration, with a varying voltage on a varactor there
will result also varying capacitance.


.

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