RF2494 RF Micro Devices, RF2494 Datasheet - Page 5

• Manufacturer Part Number: RF2494
• Description: HIGH FREQUENCY LNA/MIXER
• Manufacturer: RF Micro Devices

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 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.
Rev A0 010730
Preliminary
2.4 to 2.483 GHz
2.4 GHz ISM Chipset
RF Micro Devices
Filter
Switch
T/R
Select
Gain
Synthesizer
Frequency
Dual Gain Modes
-5 dB and +10 dB
Dual
LNA
RF2189
2.4 to 2.483 GHz
IL = 1-3 dB
VCO
VCO
RF
IF
15 dB Gain
Theory of Operation
SSOP-16 EPP
RF2494
RX
IL = 10 dB max
SAW
PA Driver
10 dBm
2.4 to 2.483 GHz
IL = 1-3 dB
The mixer power conversion gain is +19dB when R1 is
set to 1k . 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.
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.
RX
TX
* 2
15 dB
15 dB
TX
-45°
+45
°
TX VGC
15 dB Gain
Range
-15 dB to 35 dB Gain
IF Amp
Open Collector
Mixer Output
C1
R1
L1
RX VGC
VCC
(f
Active Selectable LPF
C
Base Band Amp.
= 1 MHz to 40 MHz)
0-30 dB Gain
C2 L2
Selectable LPF
RF2948
Filter
Filter
RF2494
C3
OUT Q
OUT I
RL
Q INPUT
I INPUT
OUT
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