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參數(shù)資料
| 型號(hào)： | MRFIC0954 |
| 廠商： | Motorola, Inc. |
| 英文描述： | 800MHZ CDMA Upmixer and Exciter(800MHz 碼分多路訪問(wèn)的上混頻器和激勵(lì)器) |
| 中文描述： | 800MHz的CDMA的混頻器和勵(lì)磁（800MHz的碼分多路訪問(wèn)的上混頻器和激勵(lì)器） |
| 文件頁(yè)數(shù)： | 7/10頁(yè) |
| 文件大?。?/td> | 129K |
| 代理商： | MRFIC0954 |
MRFIC0954
7
MOTOROLA RF/IF DEVICE DATA
APPLICATIONS INFORMATION
Design Philosophy
The MRFIC0954 has three operating states, enable,
standby, and disable. These states are controlled by the truth
table shown in Table 1. The device is fully operational during
the enable state and the bias level can be selected. A high
bias current for CDMA or a lower bias current for Analog (or
CDMA at lower powers) can be selected via the FM/CDMA
pin. In the high current CDMA mode, the quiescent current is
increased to maximize the linearity of the device. In the lower
current bias, the quiescent current is optimized for efficiency
in the Analog mode. This lower bias point is also useful in
lower power CDMA operation. The standby mode can be
used to reduce current consumption during Voice Activity
Factoring. In the standby mode, the LO buffer remains on to
prevent VCO pulling and the bandgap reference bias circuit
remains on to assure rapid device turn on. Current
consumption in standby mode is 10 mA typical. The disable
mode is used to turn the MRFIC0954 completely off.
Leakage current in this mode is only a few microamps.
The mixer is a double–balanced “Gilbert–cell” design with
a balanced LO buffer amplifier. The input and output of the
mixer are differential. However, the linearity is high enough to
tie one output to VCC and use the other as a single–ended
output. Used this way it provides around 7.0 dB of gain and
typically draws 20 mA quiescent current in CDMA mode and
16 mA in Analog Mode. An external filter is required between
the mixer and RF AGC amplifier to reduce RX band noise.
Figure 1 shows the applications circuit for the
MRFIC0954. In this circuit, the IF ports of the mixer have
been matched to 50
for testing purposes. In the actual
application, the differential IF ports of the mixer would be
impedance matched to an IF SAW filter. The differential
impedance of the mixer IF ports is 1600
. The RF output of
the mixer is configured as a single ended output. DC current
to the open collector output of the mixer is provided by
inductor, L6 (6.8 nH). Inductor L6 is also part of the matching
circuit with C13 (1.6 pF), C14 (1.3 pF) and C15 (100 p).
The RF AGC amplifier is a single–ended cascode design
employing the standard “current steering” method of gain
control. It’s ground is brought out through pin number 15 so
inductance can be added to degenerate the gain for a lower
noise floor. With 2.0 to 3.0 nH of external inductance, the
maximum gain is around 13 dB. It typically draws 9.0 mA
quiescent current in CDMA mode and 3.0 mA in Analog
mode. The RF Vcntrl signal is buffered with an on–chip
OpAmp then preconditioned with temperature compensation
and dB/V linearization before being applied to the RF AGC
amplifier.
Inductor L3 (6.8 nH) and capacitor C6 (100 pF) are for the
interstage match between the RF AGC and the exciter
amplifier.
The exciter amplifier is a simple common emitter design. It
is grounded directly to the exposed pad which results in 12
dB of gain. It typically draws 24 mA bias current in CDMA
mode and 8.0 mA in Analog mode. Inductor L4 (6.8 nH),
capacitor C7 (4.3 pF), and C9 (100 pF) provide the output
matching. L4 also provides a DC current path for the open
collector output.
Noise Power Considerations
In CDMA systems, the handset is required to dynamically
adjust its output power to specific levels. This requires a
dynamic range of as much as 90 dB from the transmitter.
Another key performance specification in CDMA systems is
the output noise power, both in band and out of band. Noise
power specifications has caused the noise figure of the
transmitter to become an important system consideration.
The cascaded noise figure of the transmitter can be analyzed
with the same equation used in receiver analysis. The only
difference is the noise source is from the transmitter
(modulator) instead of the atmosphere.
Noise
Source
Gain =
Noise Figure =
G1
NF1
G2
NF2
G3
NF3
NFcascaded
NF1
NF2
1
G1
NF3
G1G2
1
This equation above shows that the cascaded noise figure
is better if the gain is higher and the noise figure is lower for
the stages close to the noise source. For this reason, it is
advantageous to implement some of the gain control of a
CDMA transmitter in the RF section. The MRFIC0954
integrates a RF AGC amplifier after the upmixer to improve
the overall noise figure of the transmitter.
If better noise figure from the mixer is required, the mixer
RF output can be operated differentially with the addition of a
balun. Operating the mixer differentially will provide some
noise cancellation and reduce the noise figure by 5.0 dB.
Shown below is a lumped element balun that is effective in
the cellular transmit band of 824 to 849 MHz.
19
18
17
VCC
RF Out
33 nH
15 nH
100 pF
2.0 pF
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