Guido and others: You are neglecting the fact that you get 100 dB of dynamic range in 60 MHz (if the A/D clock is running at full speed). BUT If you want to take that 60 MHz and look at a 4 kHz channel anywhere in it, the you should filter and decimate/downsampling to (say) 8 kHz samples. If you could make perfect filters then you can get a large increase in dynamic range or apparent number of bits in the digitized signal by downsampling and filtering.
This is how radio astronomers and many others take 1 bit samples and get large dynamic ranges. They oversample like mad for the target bandwidth and then downsample and filter. The AD9958/9 are supposed to have the best spur performance according the Analog Devices design engineers. Bob Guido wrote: > Wayne, > > Using a wideband ADC (the 16 bit, 130 MSPS/sec, LTC2208 for example) > >> seems like a poor choice for a narrow band radio compared to the >> simpler, cheaper and less power hungry Tayloe mixer approach. >> >> > > For an effective number of 16 bits, this receiver will have a dynamic range > of about 100 dB. Assuming that the receiver will have a MDS of -140 dBm, > this means that it goes in saturation at about -40 dBm or 10µW, which is 22 > mV over 50 ohm. Maybe this result in some for problem for the few local > medium and nearby shortwave stations, preselection is required to attenuate > these signals. But there is some good news too. Mixing the sampled signal > digitally, does not introduce additive spurious components. > > Which DDS has the best spur performance: AD9952 (14 bit DAC) or > >> the AD9958 (10 bit DAC)? >> > > > Even in a 14-bit DDS the narrow-band spurious is about -80 dBc (optimistic > view: not taking the wide-band spurs of -60dBc into account). Using a DDS in > a Tayloe setup will raise the noisefloor of the receiver which would worsen > the MDS. Let assume a MDS of -140dBm again, this means that weak signal can > no longer be received due to the raising noisefloor, when there is somewhere > in the band a signal that is stronger than -60dBm! The weak signal reception > of this receiver setup is more vulnerable compared to the 16 bit ADC > receiver. > > Guido > > > -- AMSAT VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats, NJQRP/AMQRP, QRP ARCI, QCWA, FRC. ARRL SDR Wrk Grp Chairman Laziness is the number one inspiration for ingenuity. Guilty as charged! ------------------------ Yahoo! Groups Sponsor --------------------~--> You can search right from your browser? It's easy and it's free. See how. http://us.click.yahoo.com/_7bhrC/NGxNAA/yQLSAA/ELTolB/TM --------------------------------------------------------------------~-> 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/
