But you have to adjust the VFO offset because if you don't take that into account, the frequency it's reading is going to be off several Khz .... when used with the Flex Radio it computes this for you, with a seperate VFO (I'm using AA0ZZ's I-Q DDS VFO) I need to have the receive frequency match the display frequency on the I-Q VFO ... the Flex also offsets the frequency slightly because there's a lot of noise in the bottom couple of hundred Hz so it's not truly direct conversion but is a low IF superhet with a 11.025Khz default IF (Look in the setup of PowerSDR under the General tab in the DDS box) It also only displays 20Khz of bandwidth which avoids the noise at 1Khz on down to 0Hz (That information can be found in the QEX article) and the aliases above 21Khz as you get close to the Nyquist frequency (Having to recompute the IF offset is a big reason there is no 96Khz sample rate yet and the IF is fixed at 11.025Khz)
I don't want to have to compute frequencies myself, That's ok for a simplified experimental device like the SoftRock, I want my I-Q VFO to do that, the only exception is when I need to offset to avoid DDS spurs in the passband ... PowerSDR knows where the spurs are in the case of the Flex Radio and also automatically computes the offset so it displays the proper receiver frequency on the display when in the Spur Reduction mode, unfortunely I'll have to do it manually whne the spur problem arises JR --- In [email protected], KD5NWA <[EMAIL PROTECTED]> wrote: > > On which radio? > > On a SoftRock the QSD mixer with a 48KHz sampling rate, it converts > a 48KHz window of RF centered on the clock frequency down to audio in > a +-24KHz range, the "IF" of the radio moves around as you listen to > different frequencies, it's not a fixed place. For example; > > If your detector frequency (clock) is at 7.040KHz, > > and your listening to 7.050MHz, your IF is 10 KHz > if you are listening to 7.060MHz your IF is 20 KHz > if you are listening to 7.030MH your IF is 10 KHz but you are > listening to the negative image. > > If you want a bigger window, you would increase the sampling rate of > the audio card to 96KHz, then your window would then become +-48KHz > from the QSD oscillator frequency. > > > On a radio like the SDR-1000 it tries to keep the IF at approximately > 11KHz by moving the clock frequency, but if the requested frequency > would generate a lot of spurs, it changes the DDS frequency a little > bit to get rid of the spurs and adjust the IF frequency slightly to > end up on the requested frequency. > > > > At 08:57 AM 3/8/2006, you wrote: > >In order to double the bandwidth of an SDR, say from 20Khz ro 40Khz > >wouldn't you also have to double the IF from 12Khz up to 24Khz (or at > >least 22Khz)? > > > >JR > > > > > > > > > > > > > >Yahoo! Groups Links > > > > > > > > > > > Cecil Bayona > KD5NWA > www.qrpradio.com > > I fail to see why doing the same thing over and over and getting the > same results every time is insanity: I've almost proved it isn't; > only a few more tests now and I'm sure results will differ this time ... > > [Non-text portions of this message have been removed] > Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/soft_radio/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
