[Elecraft] K3 DSP AGC Question

Wed Feb 10 01:06:09 EST 2010

Joe,

Can you elaborate your reasons for this suggestion.  I had come to 
believe that the best situation is to place attenuation before the first 
mixer to give the greatest amount of dynamic range (headroom).

Yes, I agree that the AGC Threshold could be raised in the K3 and would 
be beneficial.  I have felt the default of 005 was too low.

73,
Don W3FPR

Joe Subich, W4TV wrote:
> Perhaps one should get the ambient well down in the ADC operating 
> range but killing gain (adding loss) before the first mixer and 
> the roofing filter is not the way to do it.  If you want to 
> better align the dynamic range of the receiver (or specifically 
> the ADC) to conditions, the attenuation should come in the 
> IF - preferably divided between the first IF and 2nd IF.  
>
> However, there would still be no apparent reason that the K3 
> should not be capable of sustaining an AGC threshold at least 
> 10 db higher than currently set with AGC THR = 008 whether 
> that be reached with preamp on, preamp off or attenuator on. 
>
> 73, 
>
>    ... Joe, W4TV 
>     
>
>
>   
>> -----Original Message-----
>> From: elecraft-bounces at mailman.qth.net 
>> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Guy 
>> Olinger K2AV
>> Sent: Monday, February 08, 2010 1:49 PM
>> To: nospam.lists at subich.com
>> Cc: elecraft at mailman.qth.net
>> Subject: Re: [Elecraft] K3 DSP AGC Question
>>
>>
>> It may be a dance, but hopefully careful intelligent musing 
>> over the matter is convincing people that they should get the 
>> ambient well down in the analog-to-digital converter (ADC) 
>> operating range. If they do, a -99 threshold is a -109 
>> threshold, and further backing off RF gain can make it a -119 
>> or -129 threshold as far as the ADC is concerned, and 
>> headroom is being used for what headroom should be used for.
>>
>> One thing for sure, there has not been a good-enough 
>> presentation of what is going on, and people really don't 
>> seem to get it.  For something that is plain indisputable 
>> physics, people are still running PRE and max RFgain on 80 
>> with fast AGC, and are honestly dismayed and confused about 
>> the inevitable outcome, incorrectly blaming the rig for being 
>> natively noisy, no-work NR and NB, filter "ringing", unable 
>> to pull signals out of noise, and more.
>>
>> Maybe the variable gain by band in the MP (with a menu 
>> override) was really a better strategy for a default. This 
>> same issue dogged Orion owners, and by the chatter a lot of 
>> them never understood either.
>>
>> There IS something entirely separate with the headphone audio 
>> and HI-Z headsets, high volume levels, maybe a sharp 
>> saturation point, and mush, just haven't nailed it yet.
>>
>> 73, Guy.
>>
>> On Mon, Feb 8, 2010 at 12:01 PM, Joe Subich, W4TV 
>> <lists at subich.com> wrote:
>>     
>>>       
>>>> Likely?  I don't know.
>>>>         
>>> Likely is a better choice since raising the AGC threshold 
>>>       
>> seems to be 
>>     
>>> the primary goal of much of the "PRE/ATT/RF Gain dance" we've seen 
>>> recently.
>>>
>>> The hardware AGC (HAGC) would seem to provide an ultimate limit on 
>>> level to the ADC - unless the ADC can't handle the 
>>>       
>> transient peaks - 
>>     
>>> so a higher threshold for the DSP derived AGC would be 
>>>       
>> helpful in many 
>>     
>>> ways.
>>>
>>> 73,
>>>
>>>   ... Joe, W4TV
>>>
>>>
>>>
>>>
>>>       
>>>> -----Original Message-----
>>>> From: elecraft-bounces at mailman.qth.net 
>>>> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Lyle Johnson
>>>> Sent: Monday, February 08, 2010 11:24 AM
>>>> To: elecraft at mailman.qth.net
>>>> Subject: Re: [Elecraft] K3 DSP AGC Question
>>>>
>>>>
>>>> Hello Joe!
>>>>         
>>>>> Assuming that the threshold values have not changed in a 
>>>>>           
>> major way 
>>     
>>>>> since Jack's measurements, would it be possible to 
>>>>>           
>> extend the range 
>>     
>>>>> of threshold values - say to AGC THR=016 at about -80dBm 
>>>>>           
>> (assuming 
>>     
>>>>> 2 dB per step)?
>>>>>
>>>>>           
>>>> Possible?  Yes.
>>>>
>>>> Likely?  I don't know.  There are a lot of side effects 
>>>>         
>> that happen 
>>     
>>>> when the threshold is raised...
>>>>
>>>> 73,
>>>>
>>>> Lyle KK7P
>>>>
>>>>         
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>
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