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

[Lyle Johnson]

Hello Joe!

> ...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.

Perhaps necessarily. The assumption we made was that if you are 
encountering signals strong enough to over-range the ADC and require 
significant attenuation, the LTC-6400 would saturate and cause IMD if 
the attenuator were placed after it but before the ADC.  In a test 
version of Mercury we also discovered significant degradation of the 
overall system when we attempted to allow simple bypassing of the 
LTC-6400.  That might have been due to poor PCB layout on my part.  The 
present Mercury configuration with attenuator at the input and using the 
LTC-6400 to reduce system noise figure if the Attenuator were switched 
out seemd the best compromise at the time we finalized the design.

Mercury is a system component, not a receiver per se.  External bandpass 
filtering, gain distribution, and etc should all be part of the receiver 
system design into which one incorporates Mercury.

> ...
>
> 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.

Absolutely!

73,

Lyle KK7P

 1302131851.0