On 11/09/2020 05:29 PM, Lukas Haase via USRP-users wrote:
Yes, I am aware of that but my question related specifically to
https://files.ettus.com/performance_data/ubx/UBX-without-UHD-corrections.pdf
because I want to sanity-check my measurements (and I want to know how
much off I am compared to the figures). Hence my question is how the
"Gain" in the "RX Figure" in this document is defined. It's not
written anywhere.
I think a more general version of what you're asking is "what was the
exact test setup, and what software was used to
derive these values?". THAT I cannot answer, and it's likely that
whoever did that performance data is no longer
an Ettus employee. But the "gain" listed is just the gain *setting*,
which does not directly give you the total gain
between the antenna and ADC input.
Thank you, that I did not know.
For me the result is:
TX Frontend 0, RX Frontend 0: Gain range PGA0: 0.0 to 31.5 step 0.5 dB
RX Codec A, RX Codec B: Gain range digital: 0.0 to 6.0 step 0.5 dB
TX Codec A, TX Codec B: None
Is the "RX Codec" the "VGA on some motherboards ahead of the ADCs" you were
referring to above?
The word 'codec' is often used in the Ettus world to refer to ADCs and
DACs. The associated gain element is the
ADC digital gain setting.
A related question (of course, I studied the the schematic
https://files.ettus.com/schematics/ubx/UBX-160_revE.pdf as well): For both TX
and RX path the only programmable element are attenuators (HMC624LP4E, 6-bit).
That would fit nicely the "PGA0" from above: 0 to 31.5dB in 0.5dB steps are 64
steps (6 bit).
The other gain elements (TX: NBB-400, PHA-1+, RX: MGA-62563, NBB-400 are
static).
Why would you painstakingly gain up a signal, just to attenuate it again? Why a
programmable attenuator instead of a real programmable amplifier?
In the RF world, 99% of "programmable gain amplifiers" are actually just
RF amplifiers with attenuators on one side of
the signal or the other.
You *CAN* change the gain of an RF amplifier a bit by playing with bias
settings, but when you do that *other* important
properties of an RF amplifier will change, usually negatively.
So, in an RF-gain "line-up" that strives to offer decent noise figure at
the same time as having variable gain, it is typically
the case that an attenuator is sandwiched between a pair of low-noise
amplifiers. The equivalent noise figure of the
attenuator is notionally "washed out" by the presence of the LNA in
front of it. This is not a particularly weird or
radical arrangement and is generally considered sound RF
engineering. So, it's all about preserving noise figure while
still allowing variable gain at least at modest attenuation
settings. At higher settings it's likely that you're attenuating because
the input signal is "loud" enough that noise figure is less important.
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