Forum: GNU Radio USRP1 gain

Aaa0fc4458ead49e4f9317634a32ae92?d=identicon&s=25 Nemanja Savic (Guest)
on 2013-09-19 17:04
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Hi ll guys,

I am using for my application usrp1 and wbx board. From the
documentation I
got that maximum amplification value of 20 db, but in my flowgraph I am
able to specify without problems more gain for usrp source block. If I
put
>20 db I really see changes in noise level, which means that something
really happens. Can somebody say what happens for >20db gains, and why
there is no warning or error when i tru o do that?

Thanks and best,
558c40b97bd1af8d912424757714bda9?d=identicon&s=25 Marcus D. Leech (Guest)
on 2013-09-19 17:11
(Received via mailing list)
On 09/19/2013 11:03 AM, Nemanja Savic wrote:
> Thanks and best,
>
> --
> Nemanja Savic'
>
>
> _______________________________________________
> Discuss-gnuradio mailing list
> Discuss-gnuradio@gnu.org
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The WBX has 31.5dB of gain range.
Aaa0fc4458ead49e4f9317634a32ae92?d=identicon&s=25 Nemanja Savic (Guest)
on 2013-09-19 17:22
(Received via mailing list)
Ok, but as far as I have seen it is fixed and the only adjustable gain
should be at PGA in front of the adc. Am i right?
558c40b97bd1af8d912424757714bda9?d=identicon&s=25 Marcus D. Leech (Guest)
on 2013-09-19 17:33
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The WBX has 31.5dB gain control in the RX path.


--
Principal Investigator
Shirleys Bay Radio
Astronomy Consortium
http://www.sbrac.org
Aaa0fc4458ead49e4f9317634a32ae92?d=identicon&s=25 Nemanja Savic (Guest)
on 2013-10-09 16:53
(Received via mailing list)
Hi,

as far as I can see the only thing on wbx board that has adjustable
feature
in rx path is attenuator, everything else has fixed gain. Am I missing
something here?

Best and thanks,
Nemanja
558c40b97bd1af8d912424757714bda9?d=identicon&s=25 Marcus D. Leech (Guest)
on 2013-10-09 16:56
(Received via mailing list)
On 10/09/2013 10:51 AM, Nemanja Savic wrote:
> Hi,
>
> as far as I can see the only thing on wbx board that has adjustable
> feature in rx path is attenuator, everything else has fixed gain. Am I
> missing something here?
>
> Best and thanks,
> Nemanja
>
The way variable gain is nearly-universally implemented in RF systems is
with an attenuator in the RF path.  So, that 31.5dB variable attenuator
   *is* the variable gain.


--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org
Aaa0fc4458ead49e4f9317634a32ae92?d=identicon&s=25 Nemanja Savic (Guest)
on 2013-10-09 17:05
(Received via mailing list)
You are right Marcus, but it is negative gain, and I wonder why USRP
source
allows me to specify more than 20 db of gain for wbx, without any
warning.
In fact what happens when u specify some gain value, how is
configuration
of the path set?

Thanks
558c40b97bd1af8d912424757714bda9?d=identicon&s=25 Marcus D. Leech (Guest)
on 2013-10-09 17:12
(Received via mailing list)
On 10/09/2013 11:03 AM, Nemanja Savic wrote:
> You are right Marcus, but it is negative gain, and I wonder why USRP
> source allows me to specify more than 20 db of gain for wbx, without
> any warning. In fact what happens when u specify some gain value, how
> is configuration of the path set?
>
> Thanks
>
Gain is distributed by UHD starting at the antenna, and moving towards
the ADC.

In the case of WBX, the attenuator sits between two LNA modules.

And, I'll emphasize again--the way gain-control is implemented in RF
circuits is with a variable attenuator.  If you purchase an RF
   Variable Gain Amplifier chip with a voltage-control input, it's
overwhelmingly likely that the way it's implemented internally is with
   a voltage-variable attenuator.

The thing to "care" about is whether at some gain-setting value 'X', is
my signal-processing-chain "happy" with the results.
   If you're doing things like precision power measurement, you *HAVE*
to use an external calibration.  You can't just play
   math games with adding up nominal circuit-element gains, because each
element in the gain chain that provides a gain
   will have device-to-device variability of 1dB or more.


--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org
Aaa0fc4458ead49e4f9317634a32ae92?d=identicon&s=25 Nemanja Savic (Guest)
on 2013-10-09 17:22
(Received via mailing list)
Thank you Marcus, I think I can say that i learned something today.
Just for clarification: USRP or UHD knows the gain of all components in
the
rx chain of wbx. If i specify for example 12 db of gain, and the gain of
the chain without attenuator is for example 25 db, it will try to set
attenuation to 13 db. For values greater than 25 db, it will adjust pga
on
USRP main board.

Nemanja
558c40b97bd1af8d912424757714bda9?d=identicon&s=25 Marcus D. Leech (Guest)
on 2013-10-09 17:23
(Received via mailing list)
On 10/09/2013 11:20 AM, Nemanja Savic wrote:
> Thank you Marcus, I think I can say that i learned something today.
> Just for clarification: USRP or UHD knows the gain of all components
> in the rx chain of wbx. If i specify for example 12 db of gain, and
> the gain of the chain without attenuator is for example 25 db, it will
> try to set attenuation to 13 db. For values greater than 25 db, it
> will adjust pga on USRP main board.
>
> Nemanja
>
UHD knows the gain control range of each element.  It distributes gain
settings accordingly.

--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org
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