Spectrum analyzers and DC blocks - RF Cafe Forums
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Post subject: Spectrum analyzers and DC blocks
Posted: Sat Apr 01, 2006 11:45 pm
Joined: Sat Apr 01, 2006
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
Posted: Sun Apr 02, 2006 12:25 pm
Joined: Wed Aug 20, 2003
Location: Santa Barbara,
Most Spectrum Analyzers (at least the ones I have
used) do not run straight into the first Mixer but
have a 10 dB pad in front specifically to protect
against this sort of problem. Should be ok. If your
analyzer does not have a pad, you can add your own.
Alternatively, why not make your own DC block
putting whatever capacitance value you want. I can't
imagine a DC block with 2.2uF having very good high
frequency response anyway.
Post subject: DC Block for spectrum analyzer
Posted: Fri Apr 14, 2006 5:54 am
Joined: Sun Mar 05, 2006
DC block for spectrum analyzer may affect
the reading on a spectrum analyzer. It is always
preferrable to buy a DC block from any reputed vendors
like Agilent (HP), RF labs, or others. For professional
quality results, a calibrated DC block is recommended.
Don't rely upon ann input attenuator (that is in-built)
of a spectrum analyzer for taking care of incoming
DC signals. It often happens that both attenuator
and input mixer go bad due to wrong input. When
buying a DC block, take care of selecting appropriate
end-connectors. If un-calibrated readings are sufficient,
a simple home-made dc block (using very low cap,
like 0.1uf) may be used. But, this is not recommended
solution, and difficult to get repeatable readings.
Hope this helps.
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