On 2015-02-13 3:15 PM, Wayne Burdick wrote:
Another interesting issue regarding the Flex 6xxx series (and radios with a similar direct-digital-sampling RF architecture) is that "A/D Limit" descriptor in the blocking dynamic range column in Sherwood's chart. An ADC limit is a hard one; it does not gracefully soft-limit in the presence of strong signals, like an amplifier stage. Multiple signals in the passband can cause additive peaks that hit this ADC limit sooner, causing ghost signals (IMD) to appear. 3-tone (or higher) testing may be needed to quantify the effect of the ADC limit in DDC receivers. Perhaps "A/D Limit" can then be converted into a useful number.
In an e-mail exchange with Chen, W7AY some months ago he indicated the "number of tones" seems to be somewhere around 10. However, I'm sure that is fuzzy and depends a lot on the frequency relationship and if they are all of similar strength or there is a wide dynamic range. Perhaps the best way to test dynamic range in direct digital sampling receivers is noise power testing. Run a "notched noise signal" of a known level into the DDS receiver and increase the noise power until the noise floor in the notch increases 1 dB. 73, ... Joe, W4TV
Wayne N6KR
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