On Sun, 02 May 2004 09:04:33 -0500, John Fields
wrote:

On Sun, 02 May 2004 14:09:44 +0100, Paul Burridge
wrote:

On Sat, 1 May 2004 15:30:18 -0400, "Ralph Mowery"
wrote:

The generators are more of a voltage source. They will deliver their rated
output when loaded to the proper impedance. Load it with 50 ohms and see if
it gives the rated output. If not try 70 ohms, or another value. One of
the reasons for using a 6 db pad is that it helps isolate the impedance of
the generator and receiver.

My main sig gen states "output EMF using 6dB pad" next to the socket.
WTF is a "6dB pad"?

---
A 6dB attenuator.

---
Also, less commonly, a device used to match the impedance of a
generator to the impedance of a transmission line or a load, or the
impedance of a transmission line to the impedance of a load.

The impedance of the generator, line, and/or load are assumed to be
purely resistive, and there is always loss associated with the
transformation. Devices which exhibit the least loss are called
"minimum loss pads", and

http://www.maxim-ic.com/appnotes.cfm/appnote_number/972

gives a good description of the process involved in designing one.

The reference given to Bruno Weinschel and the ITT handbook (as we
old-timers call it) is particularly good, and if you're interested in
RF (or just about anything else...) and you can buy a copy of it you
should.

BTW, since a pad is usually rated in terms of the power it's supposed
to lose between its input and its output, the reference to "output
EMF" on your generator may be what it supposed to be with a 3dB pad on
its output.

Easy way to find out would be to build a 3dB and a 6dB pad to find
out. For a 50 ohm tee pad, here are the resistor values you'll need
for -3dB:

0dBIN--[8.55]-+-[8.55]---3dBOUT
|
[141.93]
|
GND-----------+-----------GND


and for -6dB:

0dBIN--[16.61]-+-[16.61]---3dBOUT
|
[66.93]
|
GND------------+-----------GND

The input goes directly to your generator, (assuming its output
impedance is 50+j0 ohms) a 50 ohm resistor gets connected across the
output of the pad, and you'll measure the voltage across the 50 ohm
resistor.

Use non-inductive resistors (carbon comp if you can get them) and keep
the leads short. That is, ********KEEP THE LEADS SHORT*********.

--
John Fields