On Mon, Dec 1, 2008 at 1:29 PM, Bill W4ZV <[EMAIL PROTECTED]> wrote: > > > > Alan Bloom wrote: >> >> On Mon, 2008-12-01 at 06:35, ab2tc wrote: >>> Hi, >>> >>> I, too, would be very interested in seeing how "entire HF spectrum >>> digitized >>> at once" receivers would fare in the real world. I am *very* skeptical. >>> Although the Perseus (with that architecture) receiver fares relatively >>> well >>> in the recent ARRL test, >> >> It compares well for third-order intermodulation distortion, but not for >> blocking dynamic range. The A/D converter is very linear within its >> range, so you can get some fantastic TOI (third-order intercept) numbers >> if you measure with signals near the top of the ADC range. However the >> ADC full-scale signal level is less than a conventional mixer can >> handle. >> > > Correct. I believe the Perseus only has a 14-bit ADC so it's more limited > in dynamic range than the 24-bit ADCs commonly used by other SDR rigs. Its > dynamic range may be OK for IMD but today's SDRs cannot approach the BDR of > rigs like the K3 until even higher resolution ADCs become available. > > I believe the best BDR I've seen for any SDR is the Flex 5000, which has > just over 120 dB...the K3 is better by about 20 dB. > > 73, Bill W4ZV > > P.S. How many hits for "1000 " or "5000 " in the contest soapbox so far? > ZERO. (Which says something about SDRs in use by real contesters).
This is not correct. You can't just compare the number of bits without taking into consideration the ADC sampling rate and signal bandwidth. Please see the concept of "signal processing gain" or "process gain". A SDR such as the Perseus with a 14 bit ADC gains dynamic range by the process of decimation/filtering. If a 14 bit ADC is sampling at 80 MSPS and you are decimating to common filter bandwidths used for CW signals, ex. 500 Hz, there is an increase of dynamic range by 10 log(80e6/500) = 52 dB. The 14 bit ADC used in Perseus is rated at ~77 dB SNR at 15 MHz. With decimation the dynamic range is increased to approx. 77 dB + 52 dB = 129 dB. Another example is the QS1R's 16 bit ADC rated at ~77 dB SNR at 30 MHz. The ADC samples at 125 MSPS. 10 log(125e6/500) = 54 dB, so dynamic range after decimation is approx. 77 dB + 54 dB = 131 dB. Many of the better 24 bit audio ADCs used in SDRs are capable of 120 - 124 dB of dynamic range at *best*. I believe the K3's PCM1804 24 bit ADC is rated at 112 dB SNR. The AK5394A 24 bit ADC used in the Flex-5000 series radios has a rated SNR of 123 dB. For ADCs the SNR is pretty much the same as dynamic range. 73 Phil N8VB -- Phil Covington Software Radio Laboratory LLC Columbus, Ohio http://www.srl-llc.com _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
