USRP1 gain

GNU Radio
1

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,

2

On 09/19/2013 11:03 AM, Nemanja S. wrote:

Thanks and best,

–
Nemanja S.’

Discuss-gnuradio mailing list
[email protected]
Discuss-gnuradio Info Page
The WBX has 31.5dB of gain range.

3

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?

4

The WBX has 31.5dB gain control in the RX path.

–
Principal Investigator
Shirleys Bay Radio
Astronomy Consortium

5

On 10/09/2013 10:51 AM, Nemanja S. 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 L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

6

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

7

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

8

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

9

On 10/09/2013 11:03 AM, Nemanja S. 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 L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

10

On 10/09/2013 11:20 AM, Nemanja S. 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 L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium