[Elecraft] Added LIN OUT measurements

[Jim Brown]

On Wed, 3 Sep 2008 14:00:27 -0700 (PDT), Paul Fletcher wrote:

Paul,

I's clear from your email that you don't understand how a ham receiver works. 
See comments interspersed. 

>Could you do me a favour please? I'm interested in what happens on the
>headphone output if you re-run the tests using a 2.7 or 2.8kHz filter with
>the audio tone set low enough in frequency so that the 3rd harmonic lies
>within the filter passband (i.e. below 900Hz). 

First, the 2.7 or 2.8 kHz filters are operating at 8.8 MHz, not at audio. They 
are 2.7 kHz or 2.8 kHz WIDE at the 8.8 MHz IF. Thus, they have NO effect on 
the measurement of audio distortion in the manner that Jack is doing it. 

>I use the Heil phones which have a 200 ohm impedance so if you could use that
>a the load that would be great. 

Another misconception. Headphones and loudspeakers do not have a constant 
impedance -- indeed, their impedance isn't even resistive!  Rather, their 
impedance varies widely over the audio spectrum. International stardards 
define the impedance of loudspeakers and headphones as the MINIMUM value of 
the impedance within their operating bandwidth. See my tutorial about this in 

http://audiosystemsgroup.com/RFI-Ham.pdf

Now, the load impedance IS relevant to distortion measurement -- in general, 
amplifiers produce the greatest distortion when they are providing the most 
current, so a lower impedance load would draw more current for the same output 
voltage. BUT -- different headphones have different voltage sensitivities. 
That is, it takes more voltage to get the same loudness in one headphone than 
another. AND, a lower impedance headset will take less voltage to get the same 
power. See how interwined all this stuff is? 

The closest that Jack could come to honoring your request is to measure the 
distortion when actually driving a set of those headphones with a specified 
output voltage over a range of frequencies (because Z, and thus I, is a 
variable). 

73,

Jim Brown K9YC