[A-DX] Test Perseus in QST

["Nils Schiffhauer" <dk8ok@xxxxxxx>]

Moin, moin - die US-amerikanische Amateurfunkzeitschrift QST hat in ihrer
Dezember-Ausgabe den Perseus getestet. Mit einigem Recht ist dieser Test -
der eher einer auf die SDR_Kenntnisse seines Autors als einer der
Eigenschaft des Receivers war - in die Kritik gekommen.
Prof. Dr.-Ing. Wolf-Henning Rech (DF9IC) drückte das so aus:

I just had a look into the PERSEUS review in QST 12/2008 a friend of 
mine gave me to read.

What a nonsense!

"It is worth noting that the Perseus is not confined to a Windows 
evironment. (...) If Linux is your pleasure, Microtelecom encourages 
you to try the popular Linrad application..." - OK, this is simply a 
misunderstanding how Linrad supports PERSEUS. But the review author 
should not blame Nico for his own mistake.

Looking on the measurements, I doubt very seriously their measured 
gain compression values. Either the ADC is overdriven or not. This 
does happen around 124 dB @ 500Hz BW typically. No idea why and how 
ARRL lab measures something different.

I just repeated one of their measurements here in my home lab - 
Blocking gain compression 14 MHz preamp off in 20 / 5 / 2 kHz spacing 
at 500 Hz BW
ARRL 117/105/99 dB
DF9IC 124/124/124 dB
There is just no gain compression at all as long as the ADC is not 
overdriven - as expected.

The bizarre story continues with their IP measurements. Most of us 
know that IP has no meaning for direct sampling receivers like 
PERSEUS which is reflected well in the fact that the ARRL values 
change by 25 dB depending on the intermodulating signal level. The 
dynamic range is evaluated at MDS level which is useful. Well, so 
far. But if you look into the "Key Measurements Summary" the 
indicated IMDR and the IP values stem from largely different signal 
levels, and do not correspond to each other.

A dummy reader (maybe 90% of all) who is just refering to this "Key 
Measurements Summary" gets a lot of useless and partially wrong 
information. If you compare the K3 review (January 2009) to the 
PERSEUS review based on this summary you may think the K3's close-in 
blocking performance is by far superior because the numbers are 25-35 
dB higher. No, these figures just do not include reciprocal mixing 
which is the dominating mechanism in close-in blocking performance in 
any modern (post-70s) RX - if they did include it the PERSEUS would 
be superior to the K3.

Are these people in the ARRL lab and the editors such dummies or are 
they biased?

73 Henning

Tja, Dummbaxen oder parteiisch? Man weiß es nicht. 
Ich hatte die schon zweite und überarbeitete Version (!) des
QST-Manuskriptes in den Händen und habe den Autor auf so manche Dinge
hingewiesen, wofür er sich aber nur lapidar und "irgendwie
amerikanisch-freundlich" bedankte.
Am Ende muß man froh sein, dass er seine Unkenntnis über Theorie & Praxis
der SDRs nicht gänzlich am Perseus ausließ. 

73: Nils


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