32 www.rfdesign.com February 2007
Tx/Rx Technology
Removing the interstage transmit SAW
in WCDMA
Typically, in a WCDMA radio, there is a transmit (Tx) SAW filter between the
transceiver Tx output and the power amplifier to reduce the transmit output
from increasing the receiver’s (Rx) noise figure, as well as to reduce out-of-band
spurious and noise emission from the Tx. However, if the Tx SAW filter was
eliminated, it would save area on the PCB as well as cost. This article analyzes
the trade-offs required for removing the Tx SAW from the transceiver and
duplexer sections of the handset.
By Tajinder Manku
The interstage transmitter (Tx) SAW filter between the transceiver’s Tx output and the power amplifier has effectively two functions.
1. To reduce the transmit output from increasing the receiver (Rx)
noise figure.
2. To reduce out-of-band spurious and noise emission from the Tx.
If this Tx SAW filter can be eliminated, it would save cost and area
on the PCB. This article analyzes the trade-offs involved with removing
the Tx SAW filter from the perspective of improving the performance
of either the transceiver and/or the duplexer.
Item (1) is not a concern for GSM/GPRS/EDGE radios because
these standards are half-duplex—i.e., the Rx and Tx are not on at
the same time. However, item (2) is an important concern for GSM/
GPRS/EDGE. According to the 3GPP specification, Tx noise within
the low bands of GSM/GPRS (850 MHz and 900 MHz) cannot ex-
ceed –79 dBm over a bandwidth of 100 kHz at an offset of 20 MHz.
At +33 dBm output (as per the 3GPP specification) the noise at a 20
MHz offset has to be better than –163 dBc/Hz (i.e., –79 dBm-10log
(100 kHz)-33 dBm). This number is achievable for both the transceiver
and the power amplifier. Consequently, a Tx SAW is not required for
the GSM standard.
Both functions 1 and 2, are explored with a SAW and without a
SAW for WCDMA and various trade-offs are discussed. However, the
article addresses item (1) in much more detail since item (2) is well
understood for GSM.
Transmitter to receiver desensitization
Tx to Rx desensitization occurs because the transmit noise in the
Rx band leaks into the Rx chain and effectively increases the noise floor
of the Rx. This increase in the noise floor increases the overall noise
figure of the Rx. If we assume the E
b
/N
o
=7 dB for a bit error rate (BER)
of 10-3, the required noise figure (NF) at the antenna port, with the Tx
on, must be designed to be less than 9 dB. There are three basic sources
of noise in the transmit path that can add to the Rx noise figure:
1. The digital-to-analog converter (DAC) used to generate the
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Figure 1. System analysis diagram with a Tx SAW.
702RFDF3.indd 32 2/8/2007 11:26:13 AM
34 www.rfdesign.com February 2007
analog IQ signals.
2. The modulator (i.e., the transceiver).
3. The power amplifier.
In our initial analysis, we will assume the DAC noise is much
lower (at least 10 dB lower) than the transceiver and power amplifier
noise.
Figure 1 illustrates a generalized WCDMA system with an intra-
stage Tx SAW filter. All the variables within the following analysis
are defined within Figure 1. The Rx noise due to the transceiver at
the output of the power amplifier, N
TX-RX,PA
is given by the following
equation:
,
10 log(3.84 )
TX RX PA TX RX PPA
TX RX PA
N N P
MHz G G
− −
−
= +
+ + +
(1)
Therefore, the total in-band Rx noise at the output of the power
amplifier, N
PA
is a combination of the transceiver noise and the power
amplifier noise;
, /10 /1010 log(10 10 )TX RX PA PA RXN NPAN
− −= +
(2)
Given this, the noise at the input of the Rx, N
RX
is identified as N
PA
minus the attenuation of the duplexer, plus its own noise contributions
when the Tx is off (-174+10log(3.84MHz) + NF
TX,eff
);
,
( ) /10
( 174 10log(3.84 ) ) /10
10 log(10
10 )
PA TX RX
TX eff
N D
RX
MHz NF
N −−
− + +
=
+
(3)
Therefore, the NF at the antenna input with the Tx on is given by
the following equation;
[ ]174 10log(3.84 )
_ ( )
RXNF N MHz
RX loss switch duplexer
= + −
+ +
(4)
By using equations (1) to (4), the effective noise figure of the
Rx can be derived. In a typical application, the duplexer Tx to Rx
isolation is approximately 42 dB, the noise from the PA is typically
around –73 dBm, the noise figure of the Rx with the Tx off is typi-
cally around 3 dB, and the power and noise output of the transceiver is
+4 dBm and –153 dBc/Hz, respectively. If a low-cost Tx SAW is
used, one would expect an in-band loss of 1 dB and an out-of-band
loss of 30 dB. The in-band loss from the switch and duplexer to
the antenna is typically around 2 dB and 3 dB for the Tx and Rx,
respectively. Given all this, the gain of the PA would have to be 23 dB
to achieve an output power of +24 dBm as per the 3GPP requirement.
With all these variables, the noise figure with the Tx on would be
N
TX,on
= 6.4 dB, which meets the spec and has about 2.6 dB of margin.
For this particular case, the transmit noise in the Rx band is mainly
influenced by the PA noise rather than the transceiver noise. For
example, if the transceiver noise increased by 8 dB, N
TX,on
= 6.5 dB,
which is only a 0.1 dB increase. Consequently, with a Tx SAW filter,
the affects of the transceiver noise in the Rx band can be effectively
ignored.
Now, let’s assume the SAW is removed from the Tx chain and the
transceiver noise is –153 dBc/Hz; see Figure 2. Under this scenario, the
NF at the antenna port is 10.9 dB, which fails the NF spec of 9 dB.
From the analysis, it becomes apparent that there are only two
variables that dramatically influence the amount of Tx to Rx desen-
sitization; (a) the duplex isolation between the Tx and Rx (D
TX-RX
)
and (b) the transceiver noise in the Rx band (N
TX-RX
). Better duplexer
isolation usually implies increased cost and size. Furthermore, duplexer
technology generally moves toward producing smaller scales, thus one
would expect the Tx to Rx isolation to become worse. In any case, let’s
make the assumption the duplex isolation is now 48 dB (as opposed
to 42 dB). With 48 dB of isolation, N
TX,on
= 7.8 dB, which meets the
spec of 9 dB, but only has 1.2 dB of margin.
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Figure 2. System analysis diagram without a Tx SAW.
Noise Figure (NF) Specification Requirement = 9 dB
Scenario Total NF Margin Pass / Fail
With SAW 6.4 dB 2.6 dB Pass
Conventional duplexer
without SAW 10.9 dB N/A Fail
Better duplexer
without SAW 7.8 dB 1.2 dB Pass*
Better Tx noise
without SAW 7.8 dB 1.2 dB Pass*
Table 1. Trade offs without the SAW filter.
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36 www.rfdesign.com February 2007
The other variable is the transceiver noise in the Rx band. To im-
prove this usually implies increasing the current consumption within
the transceiver, which would in turn reduce battery life. In any case,
let’s assume N
TX-RX
= -160 dBc/Hz (opposed to –153 dBc/Hz) and the
duplex isolation is still only 42 dB. Under this scenario, N
TX,on
= 7.8 dB
that meets spec, but only has 1.2 dB of margin. So far in our analysis
we have assumed that the noise due to the DAC generating the base-
band IQ signals can be ignored. However, this assumption may not be
valid if the noise required from the transceiver needs to be better than
–160 dBc/Hz. For the DAC noise to be neglected it would need to
Bands Uplink (MHz) Downlink (MHz) Duplexer Close in Spurs Measured Min Power
Lower Upper Lower Upper Spacing (MHz) Upper Lower BW (kHz) dBm
IMT, I 1920 1980 2110 2170 190 1805 1880 100 -71
PCS, II 1850 1910 1930 1990 80 1930 1990 3840 -60
DCS, III 1710 1785 1805 1880 95 1805 1880 3840 -60
850, V 824 849 869 894 45 869 894 3840 -60
be better than –170 dBc/Hz. If we assume a 1 V swing at the DAC
output, the SNR required by the DAC output would be 170 dBc/Hz-
10log(3.84 MHz), or 104 dB. Assuming 45 dB of quantization noise
filtering, the DAC itself requires about 59 dB of SNR. This would
result in a 9-bit DAC at a clocking rate of 26 MHz.
In summary, Table 1 depicts the trade offs without the SAW filter.
Transmit out-of-band spurious requirements
Table 2 illustrates the closest spurious requirements for four popular
WCDMA bands. These numbers can be translated into a noise require-
ment at the output of the transceiver if no Tx
SAW is used;
_ ( / ) min_
10*log( ) 24
=
− −
noise TX dBc Hz power
BW dBm
(5)
The hardest requirements in terms of noise
are bands I and V. These requirements are
–145 dBc/Hz at 40 MHz offset and –150 dBc/
Hz at 45 MHz offset, respectively.
However, not only does the noise have to
be below these settings, care has to be taken
to make sure the spurious components are
below these values. However, for band I, five
exceptions are allowed within the band limits
of 1805 MHz and 1880 MHz.
Conclusion
When an interstage Tx SAW is used, the Rx
sensitivity is optimized for best performance.
By removing the Tx SAW there becomes a
design trade off between duplexer technology,
the transmit noise in the Rx band, and the Rx
noise figure. If the Tx SAW is removed, the
specification requirements of either/both the
transceiver and duplexer have to be better. The
Tx must be designed to have less noise in the
Rx band and less spurious power out-of-band.
The duplexer, on the other hand, has to have
better isolation characteristics between the
Tx and Rx. However, increasing the duplexer
requirements may not be the right choice given
better duplexer performance may come at a
higher cost and more PCB area. RFD
Table 2. Spurious requirement for all the key WCDMA bands.
ABOUT THE AUTHOR
Tajinder Manku is chief technology officer
and founder of Sirific Wireless Corporation,
Richardson, Texas. Prior to Sirific, Manku
served as an associate professor at the
University of Waterloo in Canada, where
he earned his B.S in solid-state physics
and a PhD.
l o c k o n w i t h R a k o n
find the best product to suit your needs at: www.rakon.com
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702RFDF3.indd 36 2/8/2007 11:26:16 AM
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