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PDF AN1892 Data sheet ( Hoja de datos )
| Número de pieza | AN1892 | |
| Descripción | SA900 I/Q Transmit Modulator | |
| Fabricantes | Philips | |
| Logotipo |  |
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RF COMMUNICATIONS PRODUCTS
AN1892
SA900 I/Q transmit modulator for 1GHz
applications
Wing S. Djen
1997 Aug 20
Philips Semiconductors
1 page  
Philips Semiconductors
SA900 I/Q transmit modulator for 1GHz applications
Application note
AN1892
881
DPLX
BPF
IF = 83.16
LO = 964.16
836 Σ
90°
VCO = 128.16
IQ
Figure 8. FDD System Using SA900
881MHz
881MHz
LNA
SA601
SR00944
83.16MHz
CH 1. Rx
870.03MHz
VCO
953.19MHz
915
SWITCH
IF = 45
BPF
LO = 960
915 Σ
90°
VCO = 45
IQ
Figure 9. TDD System Using SA900
SR00945
450kHz
450kHz
MIXER
VCO
82.71MHz
IF
AMP
LIMITER
FAST RSSI
SA637
TO DSP
DUP
SERIAL INPUT + PROG LATCH
CH 1. Tx
825.030MHz
PA
836MHz
MICROCONTROLLER
INTERFACE
IQ
836MHz
BUFFER
VGA
VGA
CONTROL
953.19MHz
PHASE
SHIFT
NTWK
SA900
÷6
128.16MHz
21.36MHz
MAIN ÷3971
5/8
MAIN
SA7025 PHASE
DET.
REF ÷89
AUX
PHASE
DET.
PHA
AUX ÷2757
SERIAL INPUT
CLK MICRO DSP
SET CLK CLK
FREQUENCY PLAN (EXAMPLE ONLY)
1st LO = 953.19MHz (870.03 + 83.16MHz)
CHANNEL 1
2nd LO = 82.71MHz (83.16MHz - 450kHz)
REFERENCE = 21.36MHz
SR00946
Figure 10. IS-54 front-end chip set from Philips
DESIGNING WITH THE SA900
Baseband I/Q Inputs
The baseband modulation inputs are designed to be driven
differentially for the SA900 to operate at its best. The I and Q inputs
should have a DC offset of VCC /2, which is externally provided by
common DSP chips. If all four inputs are biased from the same
source, the device can tolerate ±0.5VDC error; however, inaccuracy
of DC bias between I1/I2 or Q1/Q2 causes reduced suppression of
the carrier. Thus, it is important to have a well regulated DC supply
for I and Q signal biasing. The bandwidth of the inputs is much
higher than the specified 2MHz. Approximately 2dB of power loss
will be experienced if the I and Q inputs are 50MHz.
The SA900 generates a minimum of 0dBm of power to a 50Ω load
when the amplitude of the I and Q signals are 400mVP-P. The
output power will decrease by 6dB for every 50% decrease in I/Q
amplitude. Single-ended I and Q sources can be used but are not
recommended due to the degradation in carrier suppression (more
than 10dB compared to differential). In addition, the entire noise
performance of the device will suffer. VCC /2 should be applied to I2
and Q2 pins if the part is driven single-endedly.
Transmit Local Oscillator
The transmit local oscillator path consists of a TXLO input buffer, LO
output buffer, VCO, image rejection mixer and phase shift network.
Together with a few external components, this section provides the I
and Q carrier for modulation.
The TXLO inputs and LO outputs are designed to be used in an
external PLL which synthesizes different frequencies for channel
selection. The RF signal being generated is fed into TXLO inputs
and then comes out of LO outputs to complete the system
synthesizer loop. The TXLO inputs are differential in nature and
1997 Aug 20
8–5
5 Page 
Philips Semiconductors
SA900 I/Q transmit modulator for 1GHz applications
Application note
AN1892
A. Those spurs could be N*TXLO, N*VCO, TXLO+VCO, N*XO, and
TXLO ±N*VCO.
Q. Can external circuitry be added or modified to reduce the
broadband noise floor below -136dBm/Hz?
A. Customers can put a bandpass SAW filter at the output of the
TXLO to improve the broadband noise floor.
Q. Can the SA900 generate BPSK signal?
A. Yes, it can. Feed the baseband signal into I1 and I2 and leave
Q1 and Q2 open or tie them to VCC /2.
Q. What is the response of the image rejection filter?
A. It is actually a SSB mixer; not an image rejection filter.
Q. What happens if the VCO is not used?
A. There will not be any signal at the DUALTX and AMPS output if
the VCO is not used.
REFERENCES:
“Implementation of a 900 MHz Transmitter System Using Highly
Integrated ASIC”, Wing S. Djen and Prasanna M. Shah,
Proceedings of the 44th IEEE Vehicular Technology Conference,
June 1994, pp. 1341-1345.
“Digital and Analog Communications Systems,” Leon W. Couch II,
Macmillan, 1990.
“Cellular System Dual-Mode Mobile Station-Base Station
Compatibility Standard”, IS-54-B, EIA/TIA, April 1992.
“Physical Layer on the Radio-Path”, GSM Standard, July 1988.
“π/4-QPSK MODEMS for Satellite Sound/Data Broadcast Systems”,
Chia-Liang Liu and Kamilo Feher, IEEE Transactions on
Broadcasting, March 1991, pp. 1-8.
“PCD5070 GSM Baseband Interface”, Preliminary specification,
Philips Semiconductors, September, 1992.
APPENDIX
Assume an imperfect I/Q modulator with gain error, K, and phase
error, φ, modulated by quadrature I/Q signals (SSB up-conversion)
wm. Then the signal, s(t), at the output of the I/Q modulator
becomes,
s(t) + K cos(wct ) f)cos(wmt) * sin(wct) cos(wmt ) 90o)
(EQ. A.1)
Using trigonometric identity and let ωc - ωm = A and ωc + ωm = B,
we obtain,
s(t)
+
K
2
cos[At
)
f]
)
K
2
cos[Bt
)
f]
)
1
2
cos[At]
*
1
2
cos[Bt]
(EQ. A.2)
Assume the information is in LSB, i.e. A, and the spur is the USB,
i.e., B, we have,
Signal
+
K
2
cos
A
cos
f
)
1
2
cos
A
*
K
2
sin
A
sin
f
(EQ. A.3)
Noise
+
K
2
cos
B
cos
f
)
1
2
cos
B
–
K
2
sin
B
sin
f
(EQ. A.4)
To find the power, we have to evaluate the envelope (amplitude) of
these two signals. Recall that for any given bandpass signal in
rectangular form,
Bandpass signal = X cos ωt - Y sin ωt,
the envelope is
Envelope = ( X2 + Y2 )0.5
Therefore, from EQ. A.3 and A.4,
ƪǒ Ǔ ǒ Ǔ ƫSignal +
0.5
22
K
2
cos
f
)
1
2
)
K
2
sin
f
(EQ. A.5)
ƪǒ Ǔ ǒ Ǔ ƫNoise +
0.5
22
K
2
cos
f
–
1
2
)
K
2
sin
f
(EQ. A.6)
Finally, the S/N ratio can be found by taking 20 log the ratio of EQ.
A.5 and A.6.
ǒ ǓS
N
+ 10 log
K2 ) 2K cos f ) 1
K2 * 2K cos f ) 1
1997 Aug 20
8–11
11 Page | ||
| Páginas | Total 14 Páginas | |
| PDF Descargar | [ Datasheet AN1892.PDF ] | |
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