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

[Steve Bunch]

Joe,

The presence or absence of preamplifiers and attenuators in front of the ADC in a direct-digitizing receiver is just to help you manage the signal amplitude range in which you get to place your digitizing (listening) window.  On the higher end of Mercury's frequency range, the atmospheric noise is low enough that Mercury needs additional outboard preamplification to be able to digitize signals in the same range as natural noise.  At the lower end of Mercury's frequency range, it doesn't need any additional preamplification to be able to listen to the smallest signals you could hear over natural noise.  But the ADC can be overflowed, which is where the attenuator comes in.

Mercury's switchable 20dB attenuator is there to help you manage the limited dynamic range of the A/D converter.  To a first order, 16 bits of resolution translates theoretically to a best case of about (6 * 16) = 96dB of difference between the strongest signal it can handle without overflow and the weakest you can listen to.  So if you have a signal that is anywhere in the passband reaching the ADC, NOT just within the region you're looking at!, that is 96dB stronger than the one you want to listen to, then the noise figure isn't going to matter much because you'll be overflowing the ADC and its output will contain distortion that will probably seriously affect your listening pleasure.

The attenuator gives you a way to reduce the amplitude of what's reaching the ADC to keep a strong signal (or combination of signals, or lightning crashes...) from overflowing the ADC.  Obviously, when you have an attenuator in the path, the smallest signal you can digitize also goes up by that amount -- basically, you are forced to choose between two 96dB-wide "windows" of amplitude that you can digitize, one 20dB higher than the other.  If the atmospheric noise level is really high (like it is on the lower bands), and/or the guy you're talking to is strong, you can afford to toss some unneeded small-signal capability in order to reduce the likelihood that a signal booming in will overflow the ADC.

This dynamic range problem is why bandpass filters (e.g., Alex), or perhaps a high-pass filter if you live down the street from an AM broadcaster, are very helpful -- if you stop large out-of-band signals before they reach the ADC, you won't have to surrender your weak-signal sensitivity.  This can be a difficult problem for some of our friends in Europe who live near massive 40M broadcast stations inside the 40M ham band, but for most people, some front-end filtering will greatly reduce their risk of overflow problems.

Steve, K9SRB

On Apr 6, 2011, at 1:33 PM, Joe Martin K5SO wrote:

> ***** High Performance Software Defined Radio Discussion List *****
> 
> Ah, okay; point taken.  However, there is more to this issue than simply "cancelling the gain" of the LTC6400-20 in this.  Adding an attenuator in front of the LTC6400-20 absolutely trashes the noise figure of the receiver, of course.
> 
> Indeed, adding a 20 dB attenuator in front of the LTC6400-20 to "cancel" its gain results in a significantly worse noise figure than a straight wire connection to the ADC would!  When the attenuator is in place, the noise figure of the receiver is effectively 20dB, which is a horrible noise figure.
> 
> The only reason I can see to ever insert the attenuator is if you have such strong signals that you can afford to throw away your hard earned receiver sensitivity; which, given my inherent weak-signal viewpoint seems like a terrible idea, hihi!
> 
> Anyway, thanks for the clarification, Greg.
> 
> 73,  Joe K5SO
> 
> 
> On Apr 6, 2011, at 11:42 AM, Greg - ZL3IX wrote:
> 
>> ***** High Performance Software Defined Radio Discussion List *****
>> 
>> Not quite accurate, Joe.  An LTC6400-20 provides 20 dB of gain in front of the ADC.   I think one would have to call this a preamp.
>> 
>> The switchable attenuator is in front of the preamp, and cancels its gain when switched in.
>> 
>> 73, Greg ZL3IX
>> 
>> On 2011-04-07 01:05, Joe Martin K5SO wrote:
>>> ***** High Performance Software Defined Radio Discussion List *****
>>> 
>>> There is already a 20 dB switchable attenuator on the Mercury board.  The attenuator is switched in when the "Pre-amp" selection is "OFF".  Direct connection (no attenuation) occurs when "Pre-amp" is "ON" (there is no pre-amp on Mercury, as was discussed on the reflector recently).
>>> 
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