Hi Sean,

I know it's unlikely but perhaps the Inmet has a resonance somewhere
below 10 MHz.

Friendly regards, Bob

On Thu, Jun 26, 2014, at 06:46 PM, Sean Breheny wrote:
> Hi all,
>=20
> 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.
>=20
> 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").
>=20
> 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
> goes
> down to much lower frequency without significant effect.
>=20
> 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.
>=20
> 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.
>=20
> Sean
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