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" <[EMAIL PROTECTED]> To: <elecraft@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. _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
