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RF9938: Low-Cost HBT Upconverters for CDMA Transmitter ICs
With -10dBm input at the IC, typical Adjacent Channel
Power Rejection (ACPR) for the RF9908 is -37dB for
the 880KHz offset and -58dB for the 1.98MHz offset.
The typical ACPR for the RF9938 is -43dB. The nominal power supply current for the RF9908 is 18mA and
25mA for the RF9938. The overall performance is
affected by less than +/-1dB over temperature, frequency, and the LO drive level. Both ICs are pin compatible and are packaged in a standard 8-lead plastic
SOIC package.
To support the increasing demand for Code Division
Multiple Access (CDMA), the latest digital communication technology in cellular industry, RF Micro Devices
(RFMD) developed CDMA chip sets for both receiver
and transmit chain.
The CDMA chip sets were
designed and manufactured on an advanced GaAs
Heterojuction Bipolar Transistor (HBT) process from
TRW. The HBT process offers several advantages
over other processes such as high Ft, low noise transistors, and process consistency. The CDMA program
is two-fold: The first chip set, which was designed for
cellular band, consist of a Receive and a transmit IF
AGC Amps(RF9907 and RF9909), a Receive LNA/
Mixer(RF9906),
and
a
Transmit
Upconverter(RF9908); the second chip set, designed for PCS
band, consist of a Receive LNA/Mixer (RF9936) and a
Transmit Upconverter(RF9938). This paper describes
the two Transmit Upconverters for this CDMA program.
Figure 1 is a simplified circuit topology of the mixer. As
shown in the figure, the mixer is a Gilbert Cell based
multiplier with an IF amplifier at the bottom and the LO
switching transistors at the top. Double balanced mixers such as this offers the best port to port isolation
and low LO feed-through at the output. Minimum port
to port isolation is 30dB for both upconverters and the
minimum spurious signal in a 60MHz bandwidth is
50dBc.
Design architecture is the same for both cellular and
PCS designs. The PCS upconverter, RF9938, is a
modified RF9908 with performance optimized for the
PCS band. Both ICs operate at 3.6VDC with 0dB conversion gain.
Input ports for the IF and LO are configured for differential operation. However, the LO can be driven single
ended if one of the input port is AC grounded. Input
impedance for the IF amplifier is 265Ω, set by the inter-
VCC
MIXER OUT
MIXER B OUT
13
TECHNICAL NOTES
AND ARTICLES
LOA
LOB
IFA
IFB
RF MICRO DEVICES, INC.
GREENSBORO, NC
Title
PCS UPCONVERTER - MIXER
Size
Document Number
REV
A
Date:
April 1, 1996
Sheet
of
Figure 1. Simplified Circuit Topology
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nal resistors. The LO input impedance is 50Ω. The IF
amplifier portion of the mixer has a set of signal degeneration 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 optimal 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 doubled 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 frequency 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.
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 offset (+/-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.
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 sufficient 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.
TECHNICAL NOTES
AND ARTICLES
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 50Ω single
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 provides a different half of the overall signal. Since the
current 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 expansion. Unlike FDMA, where available spectrum is
divided into 30kHz wide channels, or TDMA, time
sharing of multiple users in 30kHz, in CDMA, multiple
users share the same spectrum simultaneously. The
limitation is not depended on the number of physical
channels 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 reason, the linearity requirements in CDMA is very
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Figure 2. RF9908 ACPR at ±885 kHz Offset
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Figure 3. RF9908 ACPR at ±1.98 MHz Offset
TECHNICAL NOTES
AND ARTICLES
13
Figure 4. RF9938 ACPR at ±1.25 MHz Offset
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TECHNICAL NOTES
AND ARTICLES
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