[Elecraft] "TX IMD problem"

[Kok Chen]

On Apr 4, 2010, at 4/4    6:50 AM, chen dave wrote:

> I use K3 internal 2 tones test function + SDR-IQ+ SpectraVue3.

Are you ignoring the crest factor of the two tone test signal?

To get 100 watts average RF power from a "two tone" signal, the  
instantaneous peak power has to be nearly 200 watts.  Assuming that  
you are not clipping the A/D converter of the receiver, I suspect that  
your K3's PA may either be clipping on the voltage rail or the PA is  
being ALC'ed.

This is why when I tested the PSK31 transmit IMD on my K3 (here: http://homepage.mac.com/chen/Technical/K3/Digital/digital.html 
  ), I had limited the output (average) RF power to 50 watts.

Note that when BPSK31 (or QPSK31) transmits the idle Varicode, it is  
precisely a "two-tone" signal (i.e., double sideband suppressed  
carrier signal that is modulated by a sinusoidal AF tone).  In the  
case of PSK31, the AF tone is a sine wave at 15.625 Hz (the I and Q  
signals are cosinusoidally varying between each PSK31 symbol at a  
period of 31.25 symbols/sec).

73
Chen, W7AY