You're right, it is misleading to compare third-order intermodulation
distortion dynamic range (IMD3) specs of a direct-sampling receiver and
a superhet.
With a superhet, the distortion tends to increase linearly with signal
level, 3 dB per dB for third-order products. So you can measure IMD3
using different techniques and signal levels and get approximately the
same answer.
With direct-sampling receivers you can get almost any result you want
simply by slightly changing the test parameters. Even measuring the
same receiver using supposedly the same technique, the results are often
inconsistent.
In my opinion, the best apples-to-apples comparison of direct-sampling
and superhet receivers is blocking dynamic range (BDR). It is a simple
measurement of the ratio between the strongest signal the receiver can
handle and the weakest signal it can hear.
BDR tends to be limited by phase noise when the interfering signal is
close to the desired signal. This is not a measurement artifact but
reflects actual on-the-air performance. Direct-sampling receivers tend
to have an advantage close-in, although a superhet can also do well if
the synthesizer is carefully designed (for example the K3s :=)
Alan N1AL
On 12/24/2017 06:19 PM, WILLIE BABER wrote:
How is it possible to compare the dynamic range of a direct conversion
radio with a superhet, where a mode-specific filter can be placed at
the first mixer, with the expectation that direct conversion would
give similar dynamic range as a superhet?
I haven't heard yet a technical way to explain this, especially given > how Wes Hayward defined how to measure receiver dynamic range many
years ago.
73, Will, wj9b
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