[Elecraft] dynamic range questions

[Geoffrey Mackenzie-Kennedy]

Don,

Your suggestion that both applied signals should be in the receiver's 
passband in order to obtain a 'figure of merit' agrees with the argument for 
a more useful 3rd Order dynamic range test on amateur receivers, a 
discussion which has been taking place for several years as far as I know. 
Of course the phase noise of each test signal source has to be suitably low 
in level at a very small offset to avoid corruption of the test results, or 
if you like 'Test equipment phase noise limited' results. Assuming that the 
receiver's 3rd Order dynamic range is not limited by its own LO's phase 
noise, this requirement placed on the test equipment could be one reason for 
resisting any change to the current test parameters however unsatisfactory 
they might be. Arguably one could move up from the noise floor to determine 
the 3rd Order dynamic range.

With regard to the protection provided by roofing filters, one approach used 
during the design of specialized high performance receivers whose end cost 
was not, within reason, a controlling factor was to assume that the filter 
would not provide protection, although filter generated IMD was taken into 
account. This meant that the IMD performance of the entire receiver was 
maintained at a high level, and any protection provided by the filter was 
'icing on the cake'. From that point on in the design process one could make 
adjustments aimed at reducing power consumption and cost provided that the 
receiver's  IMD performance within the filter's passband was not 
compromised.

MDS as you appreciate means 'Minimum Discernible Signal' not a signal at the 
noise floor. IIRC the term was first intended to quantify the signal which 
could be discerned by a human or device connected to a receiver's output, 
which certainly in the human case introduces a variable. Some people can 
hear a signal well below the noise floor thanks to the filter between the 
ears, others with impaired hearing may not hear the signal until it rises 
well above the noise floor. You might be interested to read SM5BSZ's 
comments on the misuse of the term MDS in QEX March / April 2006 page 36, 
others have commented before.

Thank you for your comment.

73,
Geoff
GM4ESD


Don Wilhelm <w3fpr at earthlink.net> wrote:

> Geoff,
>
> While all the situations that you pose are valid measurements, I believe 
> that the 'figure of merit' that should be applied is with both signals 
> within the receiver passband.  That alone will indicate the ability to 
> copy a weak signal in the presence of a strong one - that is basic BDR 
> (IMHO).
> Third order blocking IMD is quite another thing - there are many variables 
> to consider.  The best that we can hope for is that the 'standardized' 
> tests will provide a good indicator of the performance on the real world - 
> a receiver with narrow 'roofing filters' should excel in that test - 
> although the test reports should specify the filter widths.  The close 
> spacing tests go a long way toward simulating the 'real world', but are 
> really only a set of conditions that are defined for lab tests - the real 
> world operating conditions may present an entirely different set of 
> parameters.
>
> Current tests do consider MDS to be the minimum signal that can be 
> demodulated with no other considerations.  Right or wrong, that is the way 
> it is for now - something must be defined for lab measurements to be 
> valid - we just trust that they are adequate to be useful in actual 
> operating conditions.
>
> 73,
> Don W3FPR