Article Correction

[Paul Mooney]

Hi All,

Oops ! another correction to my article  "A Low Cost Introduction to 
Software Defined Radio" appearing in the Nov-Dec issue of the AMRAD 
Newsletter.  In the next to last paragraph, the text beginning with the 
words "Because of the frequency folding .....etc, is incorrect.

Consider the computer's display screen.  It displays frequency, starting 
with zero frequency (corresponding to crystal frequency Fosc) at the 
left screen edge and extends to the right edge corresponding to 22 kHz.  
An incoming 40 meter signal, at a frequency 15 kHz above Fosc appears on 
the screen as a pip displaced 15 kHz to the right of zero frequency.  
Now visualize a virtual screen displaying signal space extending below 
Fosc from zero frequency to -22 kHz.  A 40 meter image signal at a 
frequency of 15 kHz below Fosc would appear in this virtual display as a 
pip displaced 15 kHz to the left of zero frequency.  If we slide the 
virtual display to the right, under the real display such that zero 
frequency of the real display is still at its left edge, then the left 
edge of the virtual display corresponds to -22 kHz and the right edge is 
at zero frequency.  The pip due to the image signal then appears, in the 
real screen display, at an indicated frequency ( 22 - 15) kHz or 7 kHz.  
Note however, the actual image frequency, received, is Fosc - ( Fosc - 
15 ) = 15 kHz,  indistinguishable from a signal at ( Fosc + 15 ) - Fosc 
= 15 kHz, hence the "frequency folding effect " about zero frequency.

73,     Paul   K4KRE