Digital Signal Processing Reference
In-Depth Information
the receiving antenna of a 950 MHz gsm front-end. Second-order nonlineari-
ties in the input stage of the receiver will generate undesired in-band spurious
frequencies. Fortunately, because these blocker signals are far beyond the pass-
band of the receiver, they can be effectively suppressed by the band-select filter.
For this purpose, the rf input stage of a gsm receiver is commonly equipped
with a passive 35 MHz-wide saw-filter, which can provide over 40 dB out-of-
band attenuation at large offsets from the passband frequency.
It turns out that third-order nonlinearities are a far more severe issue due to their
intermodulation beat products between in-band blocker signals. Suppose two
large interferers at closely separated frequencies
ω
b
1
and
ω
b
2
are present in the
input signal. Third-order intermodulation (im
3
=
3hd
3
) generates unexpected
distortion components at 2
ω
b
1
−
ω
b
2
and 2
ω
b
2
−
ω
b
1
, which are located closely
to the frequency of the blocker signals [San99]. Unfortunately, those extra in-
termodulation components can corrupt a more distant and weaker signal that
could be of interest to the user. Since all signals are within the passband of
the band-selection filter, an irreversible loss of the weak signal may arise from
third-order harmonic distortion in the analog receive chain.
Harmonic distortion specifications of a gsm receiver
As a practical example, consider the gsm 05.05 radio specification. In
this standard, it is defined which levels of blocking signals a handheld
gsm device must be able to withstand without dropping a call. The
in-band blocking signal level that the standard imposes for a mobile
station (ms) defines the linearity requirements of the front-end.
At the same moment when two interferers with a power level of
−
49 dBm are present at the antenna terminal of the receiver, the sen-
sitivity level of the receiver must be equal or better than
−
99 dBm.
Since the baseband processor of a gsm requires a minimal carrier
/
noise-ratio of 8 dB for a successful detection of the gmsk signal, it
is straightforward to find that the maximum tolerable intermodulation
component is 8 dB below the sensitivity level, which is
107 dBm.
The third-order intermodulation requirement of the front-end is de-
fined by the ratio between the power of the beat product and the power
of the interferers (A.25):
−
im
3
[dB]
=
beat power [dBV]
−
blocker level [dBV]
=
beat power [dBW]
−
blocker level [dBW]
=−
107dBm
+
49dBm
=−
58dB
(A.26)
The linearity of the analog front-end of a receiver is commonly char-
acterized by its third-order intermodulation intercept point (ip
3
), being
