11-248
RF2948B
Rev A6 040930
Theory of Operation
RECEIVER
RX IF AGC/Mixer
Being essentially high impedance, RX IF IN responds
to the input voltage (rather than power), and amplifies
that voltage by the gain specified in the datasheet, then
presents the output voltage at a high impedance (after
downconversion). For characterization purposes, a
50
Ω shunt resistor is placed on the IF signal path,
before AC-coupling to the input. A 50
Ω 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
path a gain range of 4.0dB to 70.0dB. 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 negligible 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, and DC Feedback
At baseband frequency, there are fully integrated gm-C
low pass filters to further filter out-of-band signals and
spurs that get through the SAW filter, anti-alias the sig-
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
programmable with a single external resistor, and con-
tinuously 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 drive the
linear 700mVPP signal off-chip.
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.
LO INPUT BUFFERS
RF LO Buffer
The RF LO input has a limiting amplifier before the
mixer on both the RF2494 (RX) and RF2948B (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 synthe-
sizer.
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 IF LO harmonics be kept below
-30dBc.
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