TAMPEREEN TEKNILLINEN YLIOPISTO
TAMPERE UNIVERSITY OF TECHNOLOGY
TLT-5500, TLT-5506
83950
Telecommunication Laboratory Course
Receiver work
E-mail: ivi@cs.tut.fi
Room: TG208/ Ismo Viitaniemi
LAB:TG205
1.
The attenuation of spurious response
1.1 Measure the attenuation of the image frequencies for the
upper and lower channels of RX-band. (According to GSM).
Measure also the attenuation of mirror image frequencies for
the middle channel.
The image frequency is f image=f RX+2f IF in the upper injection- and
f image=f RX-2f IF in the lower injection of the LO.
fIF
fRF
fIF
fLO
fIF
fimage
fimage
fIF
fLO
fRF
The measurement is performed so that we add or subtract
71MHz to the LO-frequency of the channel in question, and
this signal is injected to the RX-port of the duplex filter of
the receiver. So the LO-frequency is set to suit the lower,
middle and upper channels.
It is important to note that for the injection, the level of the
signal is kept in the linear region of the receiver. The LO
level should be according to mixer level (for example +7
dBm). The responses of image frequencies are measured
from the IF frequency.
1.2 As it is impossible to measure all the spurious responses in
a few hours time, it suffices for us to measure those few
more important ones. In examining the spurious responses,
it is enough to examine just those, which are mixed directly
to the first IF. The frequencies, from which the interfering
signal can be mixed to the desired IF signal, can be
calculated as follows:
kfx
+
nfa
+
mfLO =
fif , where
k,n,m = 1, 2, 3............
n = 0, because there is no ftx. In the lower channel we have
LO =1006 MHz, so we sill have 31 erroneous frequencies to
consider. In the upper channel LO = 1031 MHz and the
number of erroneous frequencies is 32.
The more important ones are however those, for which the
attenuation of duplex- and RX-filters and mixers are the
worst. Try for instance:
Lower channel RX = 935 MHz
819, 970.5 = fif/2-harha, 982.333, 988.25, 991.8, 1020,
1024, 1030, 1042, 1077, 1240, 1275, 1318, 1365, 1474,
1544, 1653, 1700, 1941, 1976, 2047, 2083, 2479, 2550,
2947, 3089, 3953, 4095, 4959, 5101
Upper channel
810.6, 839, 995.5, 1007, 1013, 1017, 1045, 1055, 1067,
1102, 1271, 1306, 1351, 1398, 1511, 1582, 1695, 1742,
1991, 2026, 2098, 2133, 2542, 2613, 3022, 3164, 4053,
4195, 5084, 5226
How if we use lower injection:
LO = 864...889 MHz. For example for the lower channel:
(71-2x864)/-2 = 828.5, (71-2x864)/-1 = 1657,
(71+2x864)/2 = 899.5, (71+2x864)/1 =1799
(71-3x864)/-3 = 840.33, (71-3x864)/-1 = 2521,
(71+3x864)/3 = 887.66, (71+3x864)/1 = 2663
(71-3x864)/-2 = 1260.5, (71+3x864)/2 = 1331.5
(71-4x864)/-2 = 1692.5, (71-4x864)/-1 = 3449,
(71+4x864)/2 = 1763.5, (71+4x864)/1 =3527
Etc.
Take the response of RX –filter into consideration when
choosing the test signals!
2.
The noise figure
The noise figure of the receiver is measured using the noise
figure option of spectral analyzer. You can find the instruction
manual of the equipment from the lab-folder, which you can
find from the shelf in the room B427
The noise figure is measured form the lower, middle and upper
channels of the RX-band (935 MHz- 960 MHz). The equipment
displays the amplification at the same time.
3.
Dynamics
1 dB compression point can be measured using a network
analyzer or alternatively by using a spectral analyzer and a
signal generator together. The compression point is measured
by feeding an “RX-frequency compatible” signal into the input of
the receiver and measuring the signal power at the IF as the
power level of RX injection changes.
We can basically estimate IIP3 form the compression point. Here
however we use frequencies, which are1600 kHz and 800 kHz
from the desired signal. Now to find out IIP3, we measure the
intermodulation attenuation.
The level of test signal in GSM is –43 dBm.
The required IIP3 for the receivers depends on the inter
modulation attenuation and the required demodulation S/N:
IIP3 = L(f2)+ (KMV+ S/N) / 2.
In GSM the required IIP3 typically:
IIP3 = -43 + (-43 + 99 + 9)/2 =-10.5 dBm
Represents the permissible distortion level at the input of the modulator
KMV
2f1-f2
f1
f2
fRF =2f2-f1
According to the measurements, we can define:
IIP3 =
L(test in)at RF + {the level of the RF signal compatible with the frequency of the channel=kanavataajuinen,
for which at IF:
L(kanavataajuinen) - L(2f2 – f1) = 0} / 2
And if we are at the lower channel frf =935 MHz, so
the test signals are 935.800 MHz and 936.600 MHz.
Using a –43 dBm test signal, we measure the level of
distortion at IF (we keep the LO at the lower
channel at 1006 MHz). After this we connect the
kanavataajuinen signal to the input of the receiver
and we adjust its level, until the levels of the IFsignal and the test signal are equal. The difference
between the levels is KMV, from which we can
calculate IIP3. Note that if we have the signals at
the input, we get IIP and if we have the signals at
the output, we get OIP! Don’t get confused!
4.
The leakage of LO (other emitting disturbances)
Using the spectral analyzer, we measure the LO-signal leakage
from the duplex-filter antenna port, when LO is set to match the
lower-, the middle- and the upper channel. At the same time we
observe all the other disturbance sources in the measurement
range of the spectral analyzer.