[Elecraft] Poor Man's Spectrum Analyzer

[Bob - W5BIG]

Hi Matt,

You can use two resistors to make the signal source think it is terminated
in 50 ohms and make the SA think it is being driven from a 75 ohm source.

I posted a diagram of the circuit here:
http://w5big.home.comcast.net/50ohm_to_75ohm.gif

The first resistor is 43.3 ohms in series with the input to the SA.
The second resistor is 86.6 ohms to ground at the signal input.
(87 ohms isn't a standard 5% value, but you could put two 43 ohms, 5%
resistors
in series).  1% metal film resistors are even better if you are getting into
the UHF region.
The leads should be as short as possible. With this arrangement,
you have an easy-to-build wideband matching circuit without transformers.
The loss is 4db which can be accounted for when you calibrate the SA.

73/ Bob - W5BIG


----- Original Message -----
From: "Matt Osborn" <kc0ukk at msosborn.com>
To: <elecraft at mailman.qth.net>
Sent: Sunday, October 09, 2005 9:42 PM
Subject: [Elecraft] Poor Man's Spectrum Analyzer


The Poor Man's Spectrum Analyzer I'm building uses a varacter driven
cable tuner as its front end.  The cable tuner has an input impedance
of 75 ohms. Most instruments have 50 ohm input impedance and my usage
will be primarily with 50 ohm devices.

Maybe it isn't worth worrying about, but I'm looking for a convenient
way to convert from 75 ohm to 50 ohm input impedance.  I've seen some
writeups on building a 50 ohm to 75 ohm broadband unun, but the
articles were incomplete (or my knowledge level is too low to
recognize a complete article).  The analyzer covers 5MHzto 500MHz.