On 02/20/2020 01:54 PM, Alvaro Pendas wrote:
I get what you mean, but maybe I did not explain myself correctly.
Let's forget about GNU Radio and focus on the ADC. The ADC resolution
is 12 bits, so it has 4096 digital levels. The question here is, does
the usrp adapts those levels to the signal it is receiving at each
moment? If that adaptation does not exist, the ACD is going to use all
the 4096 only when the analog input signal is close to the input max
of the ADC. Otherwise, only some of those levels are used. For
example, half of them (2048) if the level of the ACD input is half the
max.
You're talking about AGC -- no, it does not do AGC by default. AGC
strategies are generally best left to the downstream application.
Also, mind that, in the receiving part, I think that what you
explained is not completely right. I am working with a QPSK receiver
and I demodulate the symbols correctly (with a lot of noise), but the
output of the UHD:USRP Source are actually about 0.0003. That's why
I'm afraid of the problem I've mentioned above.
Something to be aware of is that increasing gain beyond the level where
SNR no longer improves, just gives you a louder (signal+noise), but
does nothing to improve SNR. Keep in mind that on the B2xx, the
maximum gain setting in RX is about 72dB, so if you're using a setting of
30dB (you mentioned that setting before), then you still have 40dB of
head-room in the RX gain setting...
Thank you for your patient.
El jue., 20 feb. 2020 a las 19:28, Marcus D. Leech
(<[email protected] <mailto:[email protected]>>) escribió:
On 02/20/2020 11:38 AM, Alvaro Pendas wrote:
However, the way I see it, this represents a problem in the
receiving part. Let me put it this way: the max output of the ADC
is 1, and that corresponds with the max input. That max input
would represent the case when you receive a high power signal and
you set your drive amplifier next to its max.
So, If you are receiving a low power QPSK signal, with your gain
set to 30 dB, the output of your ADC would use a really small
part of the range (let's say from -0.05 to 0.05). However, if
your digital levels go from -1 to 1 and are represented with 12
bits, using such a small part of the range would make the
quantization error a problem.
Gnu Radio uses a floating-point {-1.0, 1.0} representation, which
UHD *scales* into a range that is appropriate for whatever hardware
you're using.
So, your 0.05 is scaled to about 102 by UHD prior to presentation
to the DAC, and conversely in the receive direction.
El mié., 19 feb. 2020 a las 20:04, Marcus D Leech
(<[email protected] <mailto:[email protected]>>)
escribió:
Indeed. You’d have to use an external calibration source at
several places over your parameter space (frequency gain
sample rate)
Sent from my iPhone
On Feb 19, 2020, at 1:54 PM, Alvaro Pendas
<[email protected] <mailto:[email protected]>> wrote:
Marcus thank your for your answer,
First of all, you are right, the range is -1 to 1 (instead
of 0 to 1 as I said before). So, for example, in the
receiving part, the values you get out of the UHD Source
have**a linear relationship with the voltage of the analog
signal, but I understand there is no easy way to calculate
that level with the only information of the GNU Radio
samples. Is that correct?
El mié., 19 feb. 2020 a las 19:22, Marcus D. Leech via
USRP-users (<[email protected]
<mailto:[email protected]>>) escribió:
On 02/19/2020 12:01 PM, Alvaro Pendas via USRP-users wrote:
> Hello,
> I am using GNU Radio and the USRP B200. I have noticed
that for the
> GNU block UHD: USRP Sink, the values you pass to the
block must be in
> the range 0 to 1. I guess that means if you do not
want to lose
> resolution you must ensure that you use the full
range, that is to
> say, your minimum is 0 or close to 0, and your max is
1 or close to 1.
> Am I correct?
>
> On the other hand, what are the meaning of the values
produce by the
> block UHD: USRP Source? They must be related to the
signal power, but
> I am not sure about their range. Is the minimum value
that block can
> produce the min of the ADC output, and the max, the
max of the ADC
> output? With the USRP B200 the ADC resolution is 12
bits, are the min
> and the max always set with the same value, or does it
depend on the
> USRP configuration?
>
> I am using GNU Radio right now, but probably, just
knowing how this
> works with UHD would be enough to understand the rest.
>
> Thank you for your time,
>
> Alvaro
>
Gnu radio generally likes to have baesband data streams
scaled into
{-1.0,+1.0} which are linearly related to instantaneous
voltages at
the antenna of the hardware.amazon
To a first approximation, a value near +1.0 or -1.0 will
drive the ADC
to its maximum +/- value. But that's only an
approximation, since the
signal is processed a fair amount (linearly) prior to
reaching the
ADC/DAC, and with analog hardware there's no way of
ensuring that
a max value wont' over-drive the analog hardware.
Power of a sinusoidal signal is proportional to the I*I
+ Q*Q --
remember we're dealing with *voltages* here, so ohms law
applies...
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