[hpsdr] HPSDR Signal Overload ( In the digital processing???)

[Joe Martin K5SO]

All,

Thank you for the interesting input.  Whether you would like to call  
the LTC6400-20 a "pre-amp" or, as seems more logical, a conventional   
"1st RF amplifier stage", is only a matter of semantics in any case.

The main point is that if less overall system gain is desired from  
your radio it is better from a noise-figure/performance standpoint to  
introduce attenuation AFTER the first amplifier stage not before it,  
as the system noise figure is dominated by the noise figure of the  
first amplifier stage whether you call that first stage a pre-amp or  
not.  The noise figure of Mercury would be maintained relatively  
constant if the attenuator were introduced after the LTC6400-20  
instead before it.  By introducing the attenuation ahead of the  
amplifier you significantly degrade the noise figure of Mercury,  
unnecessarily.

The statements about atmospherics at lower frequencies being the  
dominant performance limiting feature are true, of course, but many  
including myself use this receiver in the high HF bands and microwave  
bands where such a situation is certainly NOT the case; there noise  
figure of the first amplifier stage is the dominant feature.   
Therefore, my valid point remains that noise figure is not something  
that you should squander lightly in the design, particularly if you  
are interested in detecting weak signals at higher frequencies.

Also, the comments about operating in a high-RF-level environment are  
valid but there are more suitable methods of reducing system gain than  
putting an attenuator in front of the first rf amplifier stage as we  
are doing in Mercury.  An attenuator after the first amplifier stage  
would preserve the system noise figure better even though doing so  
would not make a noticeable difference in that particular, special case.

The users of Mercury should understand what is actually being done in  
the radio when they select "pre-amp ON" and "pre-amp OFF".  The  
current configuration in Mercury is NOT a conventional arrangement in  
which a preamp is actually turned on and off and therefore Mercury  
performs differently than you would expect if you were expecting  
behavior of a conventional radio system that does use a pre-amp and  
turns it on and off.  Weak signal operators using this system at high  
frequencies should be aware of this fact.

73,  Joe K5SO



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