RF2948B Просмотр технического описания (PDF) - RF Micro Devices
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RF2948B Datasheet PDF : 18 Pages
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RF2948B
Theory of Operation
RECEIVER
DC feedback is built into the baseband amplifier sec-
RX IF AGC/Mixer
tion to correct for input offsets. Large DC offsets can
Being essentially high impedance, RX IF IN responds
arise when a mixer LO leaks to the mixer input and
to the input voltage (rather than power), and amplifies
then mixes with itself. DC offsets can also result from
that voltage by the gain specified in the datasheet, then
random transistor mismatches. A large external capac-
presents the output voltage at a high impedance (after
itor is needed for the DC feedback to set the high pass
downconversion). For characterization purposes, a
cutoff.
50Ω shunt resistor is placed on the IF signal path,
before AC-coupling to the input. A 50Ω signal source is
applied directly across the shunt resistor, through a
coaxial test lead. The signal source sees the shunt
resistor and therefore a low SWR. Voltage gain is then
simply the ratio of the output voltage to the input volt-
age.
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
LO INPUT BUFFERS
RF LO Buffer
The RF LO input has a limiting amplifier before the
S mixer on both the RF2494 (RX) and RF2948B (TX).
This limiting amplifier design and layout is identical on
N both ICs, which will make the input impedance the
same as well. Having this amplifier between the VCO
G and mixer minimizes any reverse effect the mixer has
I on the VCO, expands the range of acceptable LO input
levels, and holds the LO input impedance constant
S when switching between RX and TX. The LO input
power range is -18dBm to +5dBm, which should make
E it easy to interface to any VCO and frequency synthe-
Dsizer.
path a gain range of 4.0dB to 70.0dB. The noise figure
(in max gain mode) of the IF amplifiers is 5dB, which
W should not degrade the system noise figure.
E The IF to BB mixers are double-balanced, differential
in, differential out, mixers with negligible conversion
N gain. The LO for each of these mixers is shifted 90° so
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
that the I and Q signals are separated in the mixers.
RX Baseband Amps, Filters, and DC Feedback
R At baseband frequency, there are fully integrated gm-C
low pass filters to further filter out-of-band signals and
O spurs that get through the SAW filter, anti-alias the sig-
F nal prior to the A/D converter, and to band-limit the sig-
nal and noise to achieve optimal signal-to-noise ratio.
The 3dB cut-off frequency of these low pass filters is
T programmable with a single external resistor, and con-
tinuously variable from 1MHz to 35MHz. A five-pole
O Bessel type filter response was chosen because it is
optimal for data systems due to its flat delay response
N and clean step response. Butterworth and Chebychev
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 IF LO harmonics be kept below
-30 dBc.
type filters ring when given a step input making them
less ideal for data systems. The filter outputs drive the
linear 700mVPP signal off-chip.
11-248
Rev A6 040930
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