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參數(shù)資料
| 型號： | RF2938 |
| 廠商： | RF MICRO DEVICES INC |
| 元件分類： | 無繩電話/電話 |
| 英文描述： | 2.4GHZ SPREAD-SPECTRUM TRANSCEIVER |
| 中文描述： | TELECOM, CELLULAR, RF AND BASEBAND CIRCUIT, PQFP48 |
| 封裝： | TQFP-48 |
| 文件頁數(shù)： | 10/20頁 |
| 文件大?。?/td> | 528K |
| 代理商： | RF2938 |
2-10
RF2938
Rev A8 010418
11
T
T heory of Operation
RECEIVER
RX IF AGC/Mixer
The front end of the IF AGC starts with a single-ended
input and a constant gain amp of 15dB. This first amp
stage sets the noise figure and input impedance of the
IF section, and its output is taken differentially. The rest
of the signal path is differential until the final baseband
output, which is converted back to single-ended. Fol-
lowing the front end amp are multiple stages of vari-
able gain differential amplifiers, giving the IF signal
path a gain range of 9dB to 52dB. The noise figure (in
max gain mode) of the IF amplifiers is 5dB, which
should not degrade the system noise figure.
The IF to BB mixers are double-balanced, differential
in, differential out, mixers with 5dB conversion gain.
The LO for each of these mixers is shifted 90° so that
the I and Q signals are separated in the mixers.
RX Baseband Amps, Filters, Data Slicers, and DC
Feedback
At baseband frequency, there are multiple AGC amplifi-
ers offering a gain range of 0dB to 30dB. Following
these amplifiers are fully integrated gm-C low pass fil-
ters to further filter out-of-band signals and spurs that
get through the SAW filter, anti-alias the signal prior to
the A/D converter, and to band-limit the signal and
noise to achieve optimal signal-to-noise ratio. The 3dB
cut-off frequency of these low pass filters is program-
mable with a single external resistor, and continuously
variable from 1MHz to 35MHz. A five-pole Bessel type
filter response was chosen because it is optimal for
data systems due to its flat delay response and clean
step response. Butterworth and Chebychev type filters
ring when given a step input making them less ideal for
data systems.
The filter outputs, with +6dBm gain, drive the linear
500mV
PP
signal off-chip, but also connect internally to
a data slicer which squares up the signal to CMOS lev-
els, and drives this “data” signal off-chip. This data
slicer is a high speed CMOS comparator with 30mV of
hysteresis and self-aligned input DC offset. This data
slicer can be independently disabled if only the linear
outputs are desired.
DC feedback is built into the baseband amplifier sec-
tion to correct for input offsets. Large DC offsets can
arise when a mixer LO leaks to the mixer input and
then mixes with itself. DC offsets can also result from
random transistor mismatches. A large external capac-
itor is needed for the DC feedback to set the high pass
cutoff, and this capacitor is reused to set the DC input
level for the self-aligned data slicer.
RSSI and V
GC
Operation
The receive signal path also has an RSSI output which
is the sum of both the I and Q channels. The RSSI has
about 60dBm of dynamic range and the RSSI charac-
teristic is optimized to give best linearity and dynamic
range at a VGC setting of 1.4V. It is recommended that
the system sets VGC to 1.4V to take an RSSI reading
to make channel activity and signal level decisions,
then adjusts VGC to obtain optimum dynamic range
from the I
OUT
and Q
OUT
outputs.
LO Input Buffers
RF LO Buffer
The RF LO input has a limiting amplifier before the
mixer on both the RF2444 (RX) and RF2938 (TX). This
limiting amplifier design and layout is identical on both
ICs, which will make the input impedance the same as
well. Having this amplifier between the VCO and mixer
minimizes any reverse effect the mixer has on the
VCO, expands the range of acceptable LO input levels,
and holds the LO input impedance constant when
switching between RX and TX. The LO input power
range is -18dBm to +5dBm, which should make it easy
to interface to any VCO and frequency synthesizer.
IF LO Buffer
The IF LO input has a limiting amplifier before the
phase splitting network to amplify the signal and help
isolate the VCO from the IC. Also, the LO input signal
must be twice the desired intermediate frequency. This
simplifies the quadrature network and helps reduce the
LO leakage onto the RX_IF input pin (since the LO
input is now at a different frequency than the IF). The
amplitude of this input needs to be between -15dBm
and 0dBm. Excessive IF LO harmonic content affects
phase balance of the modulator and demodulator so it
is recommended that a simple n=3 low pass filter is
included between VCO and IF LO input. The IF LO
input requires a DC bias current of +6.5
μ
A. This can
be accomplished with a 270k
resistor to V
CC
for 3.3V
operation. Failing to provide this will cause a phase
imbalance in the IF LO quadrature divider of up to 8°,
which in turn causes a similar imbalance in the I/Q out-
puts and the T
X
modulator.
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