
2000 Sep 29
11
Philips Semiconductors
Product specication
SDH/SONET STM16/OC48 main ampliers
OQ2538HP; OQ2538U
Noise gure measurements
The noise figure is the ratio of signal-to-noise ratio at the
input (Si/Ni) to signal-to-noise ratio at the output (So/No)of
the amplifier. This definition is true for both single-ended
and differential amplifiers, provided the correct values for
Si/Ni and So/No are substituted in the formula. The noise
figure is measured using the differential set-up shown in
Fig.10. The total noise on the output (No in dBm) is
measured using the spectrum analyzer at the frequency of
interest. From this value, the actual (differential) noise
figure for that frequency (spot noise figure) can be
calculated using the following formula:
The factor 2 in the denominator is present to compensate
for the fact that S21 is the single-ended power gain,
whereas the differential power gain is applicable in this
situation. Ni can be replaced with the available noise
power at the input, which is kT under matched conditions
(k is Boltzmann’s constant). The formula expressed in
dBm makes calculation easier:
,
assuming log(kT) is
173.8 dBm (T = 298 K) and
No measured in 1 Hz bandwidth and expressed in dBm.
For the OQ2538HP, in the differential configuration
(including the 100
matching resistors), this yields a
typical noise figure of 11 dB.
While the performance of this measurement set-up cannot
match that of a dedicated noise analysis system, the
results are comparable for an amplifier with a noise figure
of 11 dB.
F
S
i Ni
S
o No
-----------------
N
o
2S
21
N
i
---------------------------
N
o
2S
21
kT
----------------------------
==
=
FN
o
=
S
21
3
+
() 173.8
+
–
dB
[]
Fig.10 Noise figure measurement set-up.
handbook, full pagewidth
MGM112
100
50
SMA
termination
IN
INQ
OUT
OUTQ
100 pF
50
semi rigid
100
100
100
50
SMA
termination
50
SMA
termination
50
semi rigid
50
semi rigid
50
semi rigid
OQ2538HP
test PCB
IN
SPECTRUM
ANALYZER
Zo = 50
VEE = 4.5 V