[Hallicrafters] urm-25d rf probe

[David C. Hallam]

One thing I forgot to add.  The 50 ohm cables should be RG-223 or RG-55
double shielded coax to reduce or prevent signal leakage.


David
KC2JD

-----Original Message-----
From: hallicrafters-bounces at mailman.qth.net
[mailto:hallicrafters-bounces at mailman.qth.net]On Behalf Of David C.
Hallam
Sent: Tuesday, December 13, 2005 11:49 AM
To: Hallicrafters List; Jack Schrader
Subject: RE: [Hallicrafters] urm-25d rf probe


Jack,

You don't use a probe as such as you would on say an oscilloscope.  You need
some 50 ohm cables with BNC connectors and 2 useful accessories that you
will see in the tech manuals.  Both of these accessories can be purchased
maybe, but you can make yourself from the information in the manuals.

One is a couple of capacitors in series with some test leads.  The
capacitors are to block DC from the 25D attenuator.  A few volts of DC can
burn it out.  This is for when you are injecting signals into a high
impedance circuit and are not concerned with exact reading.  An RF tight
enclosure is not necessary and a small minibox works well.

The other is a 50 ohm load for the 25D to be inserted in the line to insure
the output attenuator sees a 50 ohm load and the 25D voltmeter reads
correctly when you are calibrating something.  This should be in an RF tight
enclosure.  RF tight die cast boxes are available for less than $10 and you
have to mount a BNC connector on each end unless you can find one all ready
made up that way.

All of this is in the tech manuals with diagrams along with a couple of
others such as a 5:1 voltage divider that is useful when you to want to work
in the low microvolt ranges.

It's also a good idea to connect a digital frequency counter to the 25D when
you are using it.  It makes setting the output frequency a whole lot easier.

David
KC2JD






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