Hi all,

I have two RF DC blocks (capacitor with RF connectors pre-attached and
assembled into a rugged, constant characteristic impedance unit). One is
tiny and uses SMA connectors. The other is larger and uses N connectors.

The SMA one is an Inmet 8037. The N one is a Pasternack PE8213. Both are
rated for the frequency range of 10MHz to 18GHz, with up to 200V operating
voltage (the N-type one specifies this as RMS and the SMA one just says
"200V").

The strange thing is that the smaller one has a capacitance of about 200nF
and the larger one is only 1.7nF. This is not listed in their official
specs but I measured it with an LCR meter. Sure enough, the PE8213 (1.7nF)
begins to attenuate at around 1MHz, just as you would predict by
1/(2*pi*R*C) with R=3D100 ohms (50 ohm source and load). The Inmet 8037 goe=
s
down to much lower frequency without significant effect.

Does anyone know why two similarly spec'd components would have such vastly
different internal construction and performance? One thought that crossed
my mind is that maybe the Inmet one is really a higher performance model
which failed some test and was marked as a lower-rated part, but I have my
doubts since I would expect that capacitor failure would likely prevent
reliable operation even at the restricted frequency range.

I use these to protect my spectrum analyzer from accidental DC input
voltage. I would rather use the PE8213 because it uses N connectors which
match my analyzer. To use the SMA one, I need to introduce additional
adaptors in-line. However, I do considerable work in the range below 10MHz
so I am stuck with the SMA one for now.

Sean
--=20
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