The noise is always there. It is just a matter of whether you can see it or not. Picture the panadapter as a moving 20kHz window over the whole frequency spectrum. When you tune VFO A, 2 things actually happen. First, the hardware is tuned (if necessary). Then the software does the fine tuning (within 3.051kHz).
So lets just say that there is some noise down at DC (which is almost surely what you are seeing). If Spur Reduction is turned off then the hardware is tuned directly every Hz and no software fine tuning is necessary. In this case, the noise would be at -11.025kHz (because of our IF offset). This is 1.025kHz off the left end of the screen. However, if Spur Reduction is turned on, at one point the noise will be centered at -11.025k -(3.051k / 2). At another point it will be -11.025k +(3.051k/2). At this point, the noise is at -9.4995kHz. This is clearly visible within the panadapter window. So why do we have Spur Reduction? It is used to avoid the worst DDS spurs with a procedure documented in the application notes of our DDS (AD9854). Basically, it reduces the allowable frequencies that the hardware can be tuned. This does not eliminate all spurs, but rather avoids the worst predictable ones due to the DDS itself. One handy way to use the Spur Reduction feature is that if you have an offending spur, you can sometimes move it out of the passband by cycling the Spur Reduction on or off. Eric Wachsmann FlexRadio Systems > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] > radio.biz] On Behalf Of Jiri Sanda > Sent: Monday, December 05, 2005 11:04 AM > Cc: flexRadio@flex-radio.biz > Subject: Re: [Flexradio] Interesting behavior when connected to a dummy > load > > > I do not understand ? > If the transmitted noise get so much worse when "spur. reduction" is on > why is it there at all ? What positive it does ? > > 73 ! > > Jiri > OK1RI > > On Mon, 5 Dec 2005, Eric Wachsmann - FlexRadio wrote: > > > Jeff nailed this one on the head. The jumping around is because when > > spur reduction is turned on, the radio hardware is only tuned every > > ~3.051kHz. We do the fine tuning using a software oscillator. Also > > worthy of note is that we use an 11kHz IF. So what you are seeing is > > the junk around DC on the left side of the spectrum. > > > > > > Eric Wachsmann > > FlexRadio Systems > > > > > > -----Original Message----- > > From: [EMAIL PROTECTED] > > [mailto:[EMAIL PROTECTED] On Behalf Of Tim Ellison > > Sent: Friday, December 02, 2005 11:23 PM > > To: flexRadio@flex-radio.biz > > Subject: [Flexradio] Interesting behavior when connected to a dummy load > > > > I was testing tonight into a dummy load and noticed some very peculiar > > behavior. > > When connected to the dummy load on 14.179 MHz (USB) I see the noise > > floor (signal) in the panadapter as a flat line from -10K to +10K Hz at > > about -145 dBm. That looks good to me > > Now I change frequency to 14.178 MHz and I see the signal rise to -130 > > dBm between -10K and -9.5K Hz. It is flat at -145dBm across the rest of > > the spectrum. Looks like more noise on the low end. Not so good. > > If I change the frequency the to 14.177 MHz I see the signal rise to > > -113 dBm between -10K and -9.5K Hz. It is flat at -145dBm across the > > rest of the spectrum This shows a *lot* more noise. > > Here come the strange part. > > I change the frequency to 14.176 MHz and the signal looks just like the > > signal on 14.179. Flat across the entire spectrum at -145 dBm. > > By changing the frequency down one kHz to 14.175, I see the signal that > > looks like 14.178 MHz. A small rise to -130 dBm at the low end again. > > If I decrement the frequency down an additional kHz to 14.174, I see the > > signal that looks like 14.177 MHz. A large rise to -113 dBm at the low > > end again. > > This pattern repeats itself through out the entire 20 meter band. The > > rise in the noise floor is a little less in magnitude at the very bottom > > of 20 meters (14.050), but by the time I get to 14.150 the rise in the > > noise floor is back to approximately -110 dBm. > > I checked other bands and it exhibits this behavior on ALL bands. > > Always a three kHz stepping to repeat the pattern. > > Anyone have a clue what might cause this behavior? I'm using the Delta > > 44 with the breakout box eliminator. > > -Tim > > --- > > <mailto:[EMAIL PROTECTED]> Tim Ellison > > <http://www.itsco.com/> Integrated Technical Services > > Apex, NC USA > > 919.674.0044 Ext. 25 / 919.674.0045 (FAX) > > 919.215.6375 - cell > > Skype: kg4rzy > > > > _______________________________________________ > FlexRadio mailing list > FlexRadio@flex-radio.biz > http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz
