[Test-Equipment] HP-410B VTVM probe cables question

[Richard Knoppow]

----- Original Message ----- 
From: "Robert Myers" <rsmyers at rogers.com>
To: "'Discussion of Electronic Test Equipment'" 
<test-equipment at mailman.qth.net>
Sent: Monday, August 18, 2014 11:39 AM
Subject: [Test-Equipment] HP-410B VTVM probe cables question


>I have an HP-410B VTVM.
>
> 1.  Can someone tell me why the DC voltage probe 
> connection to the HP-410B
> consists of a shielded cable while the Ohms probe 
> connection is a just an
> insulated wire - or at least on mine the Ohms probe cable 
> is that way (is
> that correct for the Ohms probe?).
>
> So, is the shield really needed on the DC Volts probe?
>
> 2.  Also, can someone tell me the original specification 
> for the triax cable
> that connects the RF probe (i.e. AC Volts) to the HP-410B? 
> It shows an HP
> part number, but I think it was probably a Belden triax 
> cable.
>
> I am wondering if any newer more flexible triax cable 
> capable of handling
> the voltages work?  If so, what should its characteristic 
> impedance be (or
> does it matter, as it should be carrying only DC volts 
> created from the
> rectified AC/RF)?
>
> Thanks,
> --- Rob Myers, VE3JQL
>
     I can't be sure but I think the need for the shield on 
the DC lead is due to the presence of a 22 megohm resistor 
in the probe body.  The purpose is to provide isolation and 
reduce capacitive loading on the circuit being measured. 
The result is a very high impedance line between the probe 
and the meter that I suspect is very vulnerable to induced 
voltages.  Perhaps someone else has a better explanation.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com