Re: DC voltage on a DC coupled spectrum analyzer


Rajesh VS
 

Thanks Chuck and Jim,
These are very valuable points, specially around DC block and co-ax
cables.
Any comments /suggestion about using fast acting lowest current pico fuses
(sub miniature ) as an added protection along the probe? about their
impact on input loading/parasitic ?

//** I almost burned the front end of my hp 8568B by accidentally plugging
it in to DC rail instead of RF out in a tracking generator board.
I was saved by the built in pico fuse **//

On Tue, May 28, 2019 at 11:07 AM Jim Ford <james.ford@cox.net> wrote:

Good points, Chuck.It's a good habit to discharge coax cables that have
been lying or hanging around for a while before connecting them to ESD
sensitive equipment. Obviously if you have just been using a cable there's
no point in discharging it, but cables are capacitors and will charge up.
Easy enough to forget that.Jim Ford Sent from my Verizon, Samsung Galaxy
smartphone
-------- Original message --------From: Chuck Harris <cfharris@erols.com>
Date: 5/28/19 9:54 AM (GMT-08:00) To: TekScopes@groups.io Subject: Re:
[TekScopes] DC voltage on a DC coupled spectrum analyzer Often, a DC block
isn't insurance at all.Every DC block I have met is simply a series
capacitor, usuallyabout 0.1uf... those for lower frequency SA's, can be
several uf,so as to not degrade the low frequency performance too much.If
you suddenly apply a high DC voltage, from a low impedancesource to the DC
block, it will apply all of the charging currentimpulse directly to the
SA's front end. This impulse can quiteeasily destroy the mixer, if it is
directly exposed without anattenuator.Best is to be cognizant of what you
are doing, and discharge longcoax runs, and antennas, before connecting,
and simply don't connectany DC charged source to the SA without using using
a highimpedance probe... preferably with a series capacitor.... 10M
scopeprobes are often a very good choice for probing in dodgy
situations.-Chuck Harrisradioconnection@cox.net wrote:> Unless you have a
need to go into the LF region, a DC blocking accessory is cheap insurance.>

--
/Rajesh

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