Digital Signal Processing Reference
In-Depth Information
First three gain stages activated
in order to suppress noise
floor of oscilloscope (- 4 dBmV).
No active gain from RF
input to output mixer.
Predicted noise level using
kT/C only: -11dBmV
F = (N
in
+N
system
)/ N
in
4.8dBmV
RMS
50Ω load
750fF
-19dBmV
23dB
LO
RF inputs of receiver
shorted to ac ground.
Equivalent noise caused by
kT/C and first stage VGA.
True RMS noise level
measured using oscilloscope.
Figure 6.6.
Sensitivity analysis of the prototype receiver. In order to measure
the noise produced by the receiver itself, the rms noise level at the
output was measured while the rf input terminals were shorted to
ac-ground. The actual noise figure of the system depends on the
background noise of the channel.
baseband amplifier will be ignored for now. For more details about the open-
loop baseband amplifier, the reader is referred to Section 6.7.
The goal of this subsection is to determine the noise produced by the analog
front-end of the receiver. It is expected that most of the noise power originates
from kT/C noise at the output of the mixer, plus a limited amount of thermal
noise being produced by the first few stages of the variable gain amplifier.
In order to exclude noise originating from the channel, the rf inputs of the
receiver were first shorted to ac-ground. Then, the rms noise signal magni-
tude at the output of the baseband section was determined using a digitizing
oscilloscope (Figure 6.6). The total measured noise power was
42
.
2dBm
(4
.
8 dBmV measured over a 50
load), while the integrated noise floor of the
oscilloscope itself was found to be
−
4
.
6 dBmV). At the moment
of the measurement, three stages of the variable gain amplifier were activated,
which corresponds to a total voltage gain of 23
.
4 dB. As a rough estimate, it
can be assumed that the equivalent noise amplitude over the terminals of the
capacitor at the output of the pulse-to-baseband conversion mixer should be
about
−
56 dBm (
−
19 dBmV. Using the theoretical kT/C noise formula, the rms noise
voltage across this capacitor (value 750 fF) can be calculated as (6.1):
−
kT
C
=−
v
noise, rms
=
23dBmV ,
(6.1)
10
−
23
J/K, T
=
·
=
=
with k
1
.
38
300K and C
750fF
