[hpsdr] PowerSDR v1.19.3.5.diversity20 (K5SO 17AUG2011)

[Joe Martin K5SO]

Hi Helmut,

Yes, I agree, the situation is an unfortunate one that has caused and  
apparently still causes confusion among HPSDR users.   On this  
reflector there has been much discussion of the pros and cons  
concerning how the control should be labeled so I hope I don't cause  
yet another series of unnecessary discussions concerning the issue by  
answering your question.

I continue to maintain that the proper label for the front panel  
control is "ATTEN", not "PREAMP", as we DO NOT turn any pre-amp on or  
off in Mercury, instead we are inserting or removing an internal pad  
in between the antenna connector and the rf amplifier ahead of the ADC  
chip in the Mercury receiver by using the control.

As you point out, yes, if the 20 dB pad is selected the noise figure  
of the receiver will be degraded by 20 dB and your noise floor will  
rise 20 dB, unless you have external signals (atmospheric noise, for  
example) at the Mercury input connector that are quite large.  Now  
whether that is important to you or not depends upon how you use your  
Mercury receiver, especially what frequencies/bands you use.

If you spend all you time on relatively low frequency bands where  
atmospheric noise is normally the dominant noise-limiting parameter  
for your receiving system, then switching the 20 dB pad "in" may help  
you substantially by preventing overload signal conditions from  
occurring in Mercury and you will not notice an increase in your noise  
floor when inserting the pad.  However, if you are operating on bands,  
say 20 meters and higher in frequency, the noise-limiting parameter  
for your receiving system will normally NOT be atmospheric noise but  
rather it will be noise generated by the receiver itself (or  
antenna).  In this case you dramatically hurt your sensitivity to weak  
signals (by 20 dB, in fact!) by inserting the attenuator and you will  
see an obvious 20 dB rise in the noise floor level when the pad is  
inserted.

What you choose to call the front-panel control is not too important  
of course as long as you, the operator, understand what the control is  
actually doing.  The fact is that the control is inserting the 20 dB  
pad or removing the 20 dB pad, so the less confusing label in my view  
is "ATTEN", and I therefore label my PSDR versions accordingly as  
"ATTEN". I think Doug W5WC uses "ATT" for the label  and sometimes I  
keep that label but it means exactly the same thing, namely, ATTENUATOR.

I hope this explanation helps you understand the issue better and  
doesn't generate much more discussion over this technically trivial  
matter.

73,  Joe K5SO

On Sep 17, 2011, at 8:45 AM, Helmut Oeller wrote:

> ***** High Performance Software Defined Radio Discussion List *****
>
> Hi Joe,
>
> the changed lettering for the PREAMP/ATT  function is a little bit  
> confusing for me:  A pad in the signal path between ant and rx will  
> reduce the signal strength and should be indicated. MDS will be  
> reduced by the same amount. This can be watched on the meter as well  
> on the panadapter. But with a signal at the ant plug actually I  
> can’t watch a difference on the meter. I assume the software  
> emulates  more  a proper preamp- function, indicating the signal  
> value at the antenna plug. BTW  Gerd’s  mercury_eu  is using a 12 dB  
> pad instead of the 20 dB one, so things get more confused.
> May be there other reasons to provide things so?
>
> 73 Helmut, DC6NY
>

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