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TA0022 डेटा पत्रक PDF( Datasheet डाउनलोड )


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भाग संख्या TA0022
समारोह Low Cost HBT Upconverters
मैन्युफैक्चरर्स RF Micro Devices 
लोगो RF Micro Devices लोगो 
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TA0022 pdf
TA0022
nal resistors. The LO input impedance is 50. The IF
amplifier portion of the mixer has a set of signal degen-
eration resistors on the transistor emitters. These
resistors are designed to increase the mixer linearity by
reducing the gain in the IF amplifier and keeping the
signal level well below LO signals. Unfortunately, the
same resistors also increase the overall noise figure.
The resistor values were carefully calculated for opti-
mal linearity and the noise figure. There are several
design techniques used to improve the overall noise
figure. First, the transistors in the IF amplifiers are dou-
bled and paralleled. The paralleled transistors reduce
rb and re the transistors and reduces the overall noise
figure. Second, a couple of capacitors are place on the
collector of the mixer output transistors. These shunt
capacitors act as low pass filters and reduce high fre-
quency mixed spurious signals. Again, the values of
these capacitors were carefully calculated to meet the
conversion gain and linearity requirements. Third, a
good bypass capacitor on the current mirror further
reduces the overall noise figure.
The LO portion of the mixer contains a two sets of
cross-coupled transistors to alternately turn the paired
transistors on and off. Typically, an LO buffer amplifier
(e.g., a limiting amplifier) is required to provide a suffi-
cient voltage swing (ideally square waves) to drive the
mixer. However, to conserve the overall current and to
make the IC small, the LO buffer amplifier is omitted.
The mixer is designed for an LO drive level of -6dBm or
greater for best mixer performance.
13
The final stage of the ICs is a low noise RF amplifier.
The RF amplifier takes the high impedance differential
output of the mixer and converts it to a 50single
ended RF output. The RF amplifier is a push-pull class
B amplifier where the current for each transistor flow
for approximately one half of the input cycle and pro-
vides a different half of the overall signal. Since the cur-
rent is only flowing one half of the time for each
transistor, the required linearity can be met with about
half of the current than classic “A” type amplifier.
One of the biggest advantages of CDMA technology
compared to FDMA or TDMA is the capacity expan-
sion. Unlike FDMA, where available spectrum is
divided into 30kHz wide channels, or TDMA, time shar-
ing of multiple users in 30kHz, in CDMA, multiple users
share the same spectrum simultaneously. The limita-
tion is not depended on the number of physical chan-
nels but on the channel condition. As the number of
users increase, interference and the probability of
higher data error rate will increase also. For this rea-
son, the linearity requirements in CDMA is very
demanding and important. The ACPR is a method of
testing the IC linearity. It is a relative measurement of
the energy in a 1.23MHz bandwidth CDMA waveform
to the energy in the adjacent channels with a given off-
set (+880kHz, and +1.98MHz for Cellular systems and
1.25MHz for PCS systems). The following graphs
show the typical ACPR performance of RF9908 and
RF9938. Figure 2 is the RF9908 ACPR with +885kHz
offset. Figure 3 is the RF9908 ACPR with +1.98MHz
offset. Figure 4 is the RF9938 ACPR with +1.25MHz
offset.
Figure 2. RF9908 ACPR at ±885 kHz Offset
13-104
Copyright 1997-2000 RF Micro Devices, Inc.

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