On Mon, Dec 1, 2008 at 4:16 PM, Bill W4ZV <[EMAIL PROTECTED]> wrote: > > > > Joe Subich, W4TV-3 wrote: >> >> >>> 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". >> >> The concept of processing gain is completely different than >> blocking dynamic range. The maximum large signal capability >> of any DAC has an absolute limit based on the largest integer >> the DAC can resolve and the DAC reference voltage. When more >> than one signal is present within the "window" (DC to maximum >> frequency) of the DAC the DAC needs to be able to handle the >> sum of the instantaneous peak (vector maximum) voltages not >> their average levels. >> >> No amount of decimation and processing will reduce the >> instantaneous peak voltage that the ADC must handle to >> prevent overload. The clipping (overload) level results >> in distortion (IMD) or imposes an operating condition that >> must be prevented by the application of hardware AGC (gain >> reduction) ahead of the ADC (blocking). Decimation and >> processing gain are only of value as long as he hardware >> is operating within its linear range. >> > > ...which is why the hybrid approach of a narrow roofing filter before the > ADC works so well in Orion and the K3. The narrow roofing filters limit > extremely strong signals outside the passband which would otherwise kick in > ADC protection AGC. I don't think there's a way around the fundamental > limitation of ADC resolution in the direct conversion designs. > > Phil, on the QS-1R specifications webpage, I see BDR is specified at 125 > dB...very similar to the Flex 5000's 123 dB. I haven't seen any independent > measurements of the QS-1R yet, but hope one will be forthcoming by ARRL. > Until SDR receivers are in the same >140 dB league of the K3, I'm not sure > the contest community will bite this apple. When higher resolution ADCs > eventually become available at a reasonable price, I'm sure the situation > will change. > > 73, Bill >
There is no "ADC protection AGC" in the Perseus or the QS1R. Perseus ADC begins clipping somewhere around -3 dBm without attenuation. QS1R begins clipping at +9 dBm. Perseus already outperforms the K3's receiver regardless of the ARRL's claimed 140 dB BDR of the K3. The ARRL test of the Perseus was in error (which has been pointed out) and hopefully they will be publishing a correction soon. As more ops get experience with these direct sampling receivers, there will be many more converts. Unfortunately there is a huge amount of misinformation out there regarding SDR hardware and this is a contributing problem. None of this is meant to disparage the excellent K3. I am eying one myself. -- 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
