[USRP-users] [Discuss-gnuradio] AGC and Dynamic Range of ADC

Marcus D. Leech mleech at ripnet.com
Tue Sep 24 10:35:07 EDT 2013

On 09/24/2013 10:30 AM, bob wole wrote:
> On 09/23/2013 11:07 PM, bob wole wrote:
>> Can somebody please guide me on this ?
>> Bob
>> On Fri, Sep 20, 2013 at 4:44 PM, bob wole <address at hidden 
>> <mailto:address at hidden>> wrote:
>>     I have USRPN210 with WBX and RFX2400. Is there any AGC chip on
>>     N210 motherboard or WBX, RFX2400 before ADC to utilize the
>>     dynamic range of ADC ? if yes, which one? If not, then won't the
>>     varying input signal (for example signal from moving object)  to
>>     ADC affect the performance of ADC ?
>>     Bob
> >>The ADC on an N210 has over 80dB of dynamic range.  If that >>isn't 
> enough, then your application can adjust the RF gain to taste.
> >>Only a subset of applications actually benefit from the usual 
> >>hardware AGC schemes, and such schemes are invariably 
> >>application-specific, so there's
> >>no *automatic* gain control.  But your application can >>dynamically 
> make gain adjustments as it goes.
> Hi Marcus thanks for response, If there is no automatic gain control 
> then how does it is ensured that ADC doesn't saturate ? I mean what if 
> there is a signal whose amplitude is greater that ADC's highest input 
> level ?  Won't it saturate the ADC ? Also what if the signal is too 
> weak to span much of the ADC input voltage, then again we are not 
> using all bits effectively. Guide please.
> Thanks,
> Bob
> >>-- 
> >>Marcus Leech
> >>Principal Investigator
> >>Shirleys Bay Radio Astronomy Consortium
> >>http://www.sbrac.org  <http://www.sbrac.org/>
There is nothing to ensure that ADC values are neither "too little" nor 
"too big".  That's up to your application.  There's an RF gain control 
on the
   WBX, for example, that gives 31.5dB of gain control.    If you have 
very weak signals, then you have to use an external LNA.  If your dynamic
   range requirements exceed the 80dB dynamic range of the ADC, then 
your application has to account for this, and adjust RF gain accordingly.

Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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