Thanks Bill, it was the first 2 that were of interest. David G3UNA > > > > > > David Cutter wrote: > > > > Bill > > > > I can't recall seeing that figure published before; where would I look for > > that figure? > > > > David > > G3UNA > >> snip > > > > > >> The purpose of the roofing filter is mainly to reduce strong > >> (approximately > >> S9+30) signals from over-driving the ADC in the DSP. If they do that, > >> then > >> the DSP determines the final selectivity over its ~100 dB dynamic range. > >> > >> 73, Bill > > > > > > David I'm not sure which figure you meant, so I'll try to cover all bases: > > 1. S9+30 limit for over-driving ADC. This can be deduced by the setpoint > of the Hardware AGC Mod quoted from Elecraft's mod page below: > > "This modification raises the hardware AGC threshold from about S9+5 to > about S9+25 to +30. The new threshold still protects the DSP's > analog-to-digital converter, while providing better receive performance when > wider crystal filters are used." > > 2. 100 dB dynamic range of DSP. I'm not going to search for the exact > source but this is often quoted by manufacturers as the practical limit of > current DSPs using 24-bit ADCs. Theoretically it should be higher based > only on 24-bit ADC resolution but practically it's limited by other factors. > All hybird heterodyne/DSP rigs such as Orion, K3, FT-2000/9000 and > IC-7700/7800 have about the same limit. I suppose if we get 32-bit ADCs > this limit could increase. > > 3. DSP Shape Factor formula from KK7P: > > http://www.mail-archive.com/elecraft@mailman.qth.net/msg53271.html > > Hopefully I covered the figure you were questioning. > > 73, Bill > > >
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