On 28/04/2011 1:55 PM, Matt E. wrote:
There is a full WBX characterization with USRP1 here:
Noise figure with N200/N210 is 1-2 dB better.
[Just because I’m in an expansive and tutorial-esque frame of mind this
Equivalent noise figures of ADCs are (relative to RF amplifiers) just
awful. Equivalent noise figures of ADCs can be anywhere between
20 to 30dB (or more). For ADCs of equivalent “quality”, the
equivalent noise figure decreases with increasing number of bits, so all
being equal, you’d naturally expect that the equivalent noise figure
of the 12-bit USRP1 ADC to be noticeably worse than the 14-bit
N2xx ADC, and in fact, that is the case. This is a natural
consequence of ADC “physics”.
Now the total noise figure of a system tends to be dominated by the
noise-figure of the first gain stage in a sytem, that is because the
figure at any given stage N, is always conceptually divided by the
gain of the previous stage (N-1), so:
Ntot(N) = Ntot(N-1) + (Noise(N) / GAIN(N-1))
From this, you can see that the first-stage noise-figure and gain is
quite important in determining the overall noise-figure of the
move further away from the antenna, a given stage noise figure
contributes less and less to the overall noise figure, provided that it
preceded by a low-noise gain stage.
With boards like the BASIC_RX and LF_RX, there are no gain elements, so
observed noise-figure is entirely dominated by the less-than-wonderful
equivalent noise figure of the ADC. Which means that for those
boards, you need some low-noise gain ahead of them to achieve decent
weak-signal performance. I’ve used 40dB of low-noise gain (ERA-3
GaAs MMIC), with 25-45MHz bandpass filters to make repeatable
observations of galactic background noise at 38MHz with a BASIC_RX
For other cards, the RX chain typically has an LNA at the head of the
chain, followed by some amount of variable-gain amplification.
keep in mind about variable-gain amplifiers in the RF domain is that
their equivalent noise figure gets worse as you reduce their gain.
That’s typically because the gain-control is implemented as an
electrically-controlled step attenuator, and the noise-figure of an
is identical to its attenuation–so a 10dB attenuator “contributes”
10dB of noise figure at whatever stage in the gain chain it is placed.
This made obvious in the plots for the WBX–the noise-figure curve
approaches a limit of 5dB as the gain is increased, with a peak noise
figure of roughly 22dB at the lowest (0dB) gain setting.