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參數(shù)資料
| 型號： | MRFIC1854A |
| 廠商： | Motorola, Inc. |
| 英文描述： | 1.9GHz CDMA Upmixer/Exciter Amplifier(1.9GHz CDMA上混頻器/放大器) |
| 中文描述： | 1.9GHz的CDMA的混頻器/激勵器放大器（1.9GHz的CDMA的上混頻器/放大器） |
| 文件頁數(shù)： | 7/10頁 |
| 文件大?。?/td> | 125K |
| 代理商： | MRFIC1854A |
MRFIC1854A
APPLICATION INFORMATION
7
MOTOROLA WIRELESS SEMICONDUCTOR
SOLUTIONS – RF AND IF DEVICE DATA
Design Philosophy
The MRFIC1854A 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 maximum power CDMA or a lower bias
current for CDMA at lower powers can be selected via the
Bias Select pin. In the high current CDMA mode, the
quiescent current is increased to maximize the linearity of the
device. In the lower current bias state, the quiescent current
is reduced to save current during 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
MRFIC1854A 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. The IF AGC is a differential amplifier
that uses the “current steering” method for gain control. The
IF AGC/mixer combination has 16 dB of gain and typically
draws 20 mA quiescent current in the CDMA 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
MRFIC1854A. In this circuit, the IF ports of the IF AGC 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 IF ports is 1600 ohms. The RF output of the
mixer is configured as a differential output. A stripline balun is
used to convert the RF output to single ended. DC current to
the open collector output of the mixer is provided by inductor,
L3 (18 nH) and transmission line, T4. Transmission lines T3
and T4, and capacitors C15 (30 pF) and C14 (3.6 pF) form
the balun/output match for the mixer.
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. The maximum gain is around 13 dB. It typically
draws 9.0 mA quiescent current in CDMA 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.
Transmission line T2 and capacitor C7 (47 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.
Inductor L2 (10 nH), capacitor C8 (30 pF), and C10 (30 pF)
provide the output matching. L2 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 MRFIC1854A
integrates a RF AGC amplifier after the upmixer to improve
the overall noise figure of the transmitter.
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