[Yaesu] S Meter Calibration

[Al Lorona Jr.]

For those interested in the electronics behind the mystery, here's why
the numbers seemed weird on the recent S meter thread.

When you set your signal generator for 1 uV and connect it to a
receiver, in this case the FT-1000 in question, the receiver only sees
half of that, or 0.5 uV, developed across its 50 ohm input impedance.

Why is this? Because the generator itself is a 50 ohm source. Do you
remember what happens when you have a voltage divider of two 50 ohm
resistors and take the output across only one of those resistors? This
voltage dividing principle holds for any two equal resistances.

	1 uV ---
		|
		|
		|
		50 ohm
		resistor
		|
		|
		|<-------- measure the voltage here
		|
		50 ohm
		resistor
		|
		| 
		|
		___
		 _   Ground
		 -


The upper 50 ohm resistor represents the impedance of the generator, and
the lower represents the receiver's input impedance.

Well, many manufacturers feel it is confusing to say, "Set your signal
generator to 2 uV so that 1 uV appears at the receiver input." They'd
rather just tell you to set the generator to 1 uV and do all the
necessary calibration with the resulting 0.5 uV signal. Thus, Yaesu has
calculated the S meter settings with this actual 0.5 uV signal in mind.
They did it properly, however, because 50 uV actually appearing at the
antenna input really does register S9 on the S meter.

R,

Al W6LX

--
FAQ on WWW:               http://www.qsl.net/k7on/yaesu.html
Submissions:              yaesu@contesting.com
Administrative requests:  yaesu-REQUEST@contesting.com
Problems:                 owner-yaesu@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm