Preliminary
RF2494
Theory of Operation
IL
= 1-3 dB
RF2948
RX VGC
RF Micro Devices
2.4 to 2.483 GHz
2.4 GHz ISM Chipset
RF2494
SSOP-16 EPP
Gain
Select
OUT
Q
SAW
= 10 dB max
IL
RX
RX
TX
LNA
15 dB
IF Amp
-15 dB to 35 dB Gain
15 dB Gain
Dual Gain Modes
-5 dB and +10 dB
OUT
I
Filter
2.4 to 2.483 GHz
15 dB
TX
Base Band Amp.
Active Selectable LPF
(fC = 1 MHz to 40 MHz)
0-30 dB Gain
T/R
Switch
RF
VCO
Dual
Frequency
Synthesizer
+45
°
IF
VCO
*
2
-45°
Filter
I
INPUT
RF2189
Σ
15 dB Gain
Range
Filter
Selectable LPF
10 dBm
PA Driver
Q
INPUT
TX VGC
IL
=
1-3 dB
2.4 to 2.483 GHz
Figure 1. Entire Chipset Functional Block Diagram
The RF2494 contains the LNA/Mixer for this chipset.
The LNA is made from two stages including a common
emitter amplifier stage with a power gain of 13dB and
an attenuator which has an insertion loss of 3dB in
high gain mode, and 17dB in low gain mode. The
attenuator was put after the LNA so that system noise
figure degradation would be minimized. A single gain
stage was used prior to the image filter to maximize
IP3 which minimizes the risk of large out-of-bad signals
jamming the desired signal.
The mixer power conversion gain is +19dB when R1 is
set to 1kΩ. The conversion gain can be adjusted up
~5dB or down ~7dB by changing the value of R1.
Once R1 is chosen, L2 and C3 can be used to tune the
output for the SAW filter.
8
VCC
C1
L1
R1
C2
L2
C3
OUT
RL
The mixer on the RF2494 is also two stages. The first
stage is a common emitter amp used to boost the total
power gain prior to the lossy SAW filter, to convert to a
differential signal to the input of the mixer, and to
improve the noise figure of the mixer. The second
stage is a double balanced mixer whose output is dif-
ferential open collector. It is recommended that a “cur-
rent combiner” is used (as shown in figure 2) at the
mixer output to maximize conversion gain, but other
loads can also be used. The current combiner is used
to do a differential to single ended conversion for the
SAW filter. C1, C2 and L1 are used to tune the circuit
for a specific IF frequency. L2 is a choke to supply DC
current to the mixer that is also used as a tuning ele-
ment, along with C3, to match to the SAW filter’s input
impedance. RL is the SAW filter’s input impedance.
Open Collector
Mixer Output
Figure 2. Current Combiner for Mixer Load
The cascaded power gain of the LNA/Mixer is 29dB,
which after insertion loss in the image filter (~3dB) and
IF SAW filter (~10dB), still gives 16dB of gain prior to
the IF amps. Because of this, the noise figure of the IF
amps should not significantly degrade system noise
figure.
The LNA input should be matched for a good return
loss for optimum gain and noise figure. To allow the
designer to match each of these ports, 2-port s-param-
eter data is available for the LNA, and 1-port data is
available for MIXER IN and LO IN.
Rev A0 010730
8-57
RFMD [ RF MICRO DEVICES ]
