Hi-- I'm a newbie to this forum, so please forgive
unfamiliarity with posting a new topic question
on: Spectrum analyzers and dc blocks.
often need to look at some RF signal coming down
a coax feedline from an antenna with a built-in
LNA. This means there is low-voltage on the coax
powering the antenna mounted LNA. Since spectrum
analyzers (a Tek 495P in my case) cannot have DC
on their RF input, a DC block must be used.
I have been cautioned that many DC blocks have
large enough values of series capacitance to be
a risk to the mixer diode in spectrum analyzers.
Many DC blocks, in order to achieve a low-frequency
response below a few hundred KHz, use series capacitors
as large as 1.0 uF or even 2.2 uF.
told that a discharged 1 uF capacitor will take
more than enough power to charge-up to kill the
mixer diodes (12 microjoules at 5V = 120 ergs. The
burnout spec of typical Schottky mixer diodes is
hard to find, but seems to be around 1 to 10 ergs
for at least some parts.
I have been warned
that a DC block with a 1uF or 2uF capacitor is very
risky to use. It has been suggested that, unless
I specifically need very low frequency response,
that 0.1 uF is about the most capacitance to risk
using in a DC block and even then, I should avoid
exposing it to more than around 5 VDC.
anyone on this forum confirm the risk to the mixer
diode from inserting a series 1 uF or 2 uF? Since
I so often have a need to look at RF coming from
amplified antennas, I had considered installing
a DC block INSIDE the spectrum analyser (with semi-rigid
line and SMA connectors). Any thougths or feedback
on this will be much appreciated!
My 911 has a "1" in front