[Elecraft] MCU 4.51 NB

[David Cutter]

Joe said "The only problem is that my poor antennas don't given me 
enough strong signals
to really evaluate strong signal pile-up with off air signals at the 
higher thresholds. "

Turning on the pre-amp will increase the noise level, increase the 
signal strength of those signals already there and perhaps bring in a 
few more signals to help with the testing.  It's a simple thing to do.

David
G3UNA

On 12/05/2012 22:19, Don Wilhelm wrote:
> I fail to see any advantage to turning on the preamp when it is not 
> needed.  If you can hear the atmospheric noise without the preamp, 
> leave it off (if the noise level increases when the antenna is 
> attached, you have enough gain).  By extension, if you can still hear 
> the atmospheric noise with the attenuator in, leave it in - using more 
> front end gain than is necessary for the band and antenna conditions 
> will only reduce the dynamic range of the receiver.  If you do not 
> know what that means in terms of operating, let me put it simply - you 
> may not hear that weak one that you would have heard if you had set 
> the preamp and attenuator properly.
>
> Technical discussion - the band noise is S-3 with the preamp off.  The 
> K3 will start to overload on a signal that is S9+70, and will hear 
> signals that are greater than the S-3 band noise level.  Turn the 
> preamp on, and the band noise raises to about S-5, but the K3 
> overloads on that signal that is now S9+70, so you can now only hear 
> between S5 and S9+70 where before you could hear between S3 and 
> S9+70.  You have reduced the dynamic range by turning on the preamp 
> when it was not needed.
> BTW - The S9+70 overload point I used was just an example off the top 
> of my head - I do not have the overload point for the K3 at handy 
> reference, so use that as a "for instance" rather than as an absolute 
> data point.  (also BTW - my "overload" term is technically referred to 
> as the "compression" point).
>
> 73,
> Don W3FPR
>
> On 5/12/2012 6:56 PM, David Cutter wrote:
>> Would it help the tests to engage the pre-amp? ie make things more
>> challenging.
>>
>> David
>> G3UNA
>>
>> On 12/05/2012 17:22, Joe Subich, W4TV wrote:
>>> On 5/12/2012 12:17 PM, Ted Bryant wrote:
>>> >   Was v4.51 sent to the 4.50 testers?
>>>
>>> Apparently not all (I had to request it).
>>>
>>> I've been doing some crude testing ... with 4.51 AGC THR = 15 is
>>> approximately -73 dBm/S9 (to the best of my ability to measure with
>>> the XG3 and step attenuator).
>>>
>>> With high values for AGC THR it is important to use higher values of
>>> AGC SLP otherwise a sudden very strong signal will drive the audio
>>> amp and/or headphone amp into severe distortion - particularly if the
>>> AF Gain is high.
>>>
>>> I'm finding AGC THR in the 12/13 range and SLP in the 8/10 range is
>>> very comfortable and seems to "open up" the K3 receiver.  The only
>>> problem is that my poor antennas don't given me enough strong signals
>>> to really evaluate strong signal pile-up with off air signals at the
>>> higher thresholds.
>>>
>>> 73,
>>>
>>>       ... Joe, W4TV
>>>
>>>
>