[TenTec] Anomalous signal on Omni VI

[Dr. Gerald N. Johnson]

On Wed, 2009-05-20 at 20:43 -1000, Ken Brown wrote:
> I am listening to a signal with my Omni VI. I have the rig tuned to 
> 7.005 CW. When I tune upward the audio note gets lower, as it would if I 
> were listening in upper sideband mode. As I continue to tune upward past 
> where zero beat would be, the signal does not reappear, as it would if 
> it was a really strong signal actually on 7.006 that was blowing through 
> the 9 MHz IF filters, and being heard when tuned to 7.005. So I have 
> concluded the signal must be an image. By my calculations when tuned to 
> 7.005 LSB (or CW with an Omni VI, since Omni VI receives CW in LSB) the 
> LO is at 16.008 MHz and the image would be at about 25.011 MHz. When I 
> listen on 25.011 I cannot find a signal. I have not been able to figure 
> this out. Any suggestions? I have not heard any other signals that tune 
> as if the receiver is listening to USB when it is in CW mode.
> 
> Thanks,
> 
> DE N6KB
> 
Its more complicated. The LO actually is a 5.005 MHz and then there's a
crystal at 11 MHz moving it up to 16.005 MHz. But there's also a weak LO
signal at 11- 5 MHz = 5.995 MHz. So there are LO components at 5.005,
5.995, 16.005 and harmonics of those. The VFO mixer board works at
filtering out the unwanted, but its not absolutely perfect. If the
incoming signal is strong enough any one of these can mix it to the IF
and tune in the wrong direction.

73, Jerry, K0CQ