RF2494 Просмотр технического описания (PDF) - RF Micro Devices

Номер в каталоге

Компоненты Описание

производитель

RF2494

HIGH FREQUENCY LNA/MIXER

RF Micro Devices 

Preliminary

RF2494

Theory of Operation

RF Micro Devices

2.4 GHz ISM Chipset

IL = 1-3 dB

2.4 to 2.483 GHz

Gain

Select

RF2494

SSOP-16 EPP

Filter

2.4 to 2.483 GHz

LNA

Dual Gain Modes

-5 dB and +10 dB

RX

15 dB Gain

SAW

IL = 10 dB max

T/R

Switch

Dual

Frequency

Synthesizer

RF

VCO

IF

VCO

RX

15 dB

IF Amp

-15 dB to 35 dB Gain

TX

15 dB

TX

+45

*2 °

-45°

RF2189

Figure 1. Entire Chipset Functional Block Diagram

10 dBm

PA Driver

IL = 1-3 dB

2.4 to 2.483 GHz

Σ

15 dB Gain

Range

TX VGC

RX VGC

RF2948

OUT Q

Base Band Amp.

Active Selectable LPF

(fC = 1 MHz to 40 MHz)

0-30 dB Gain

OUT I

Filter

I INPUT

Filter

Selectable LPF

Q INPUT

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.

The mixer power conversion gain is +19dB when R1 is

set to 1k Ω. The conversion gain can be adjusted up

8

~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.

VCC

C1

C2 L2

L1

R1

Open Collector

Mixer Output

C3

OUT

RL

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

Share Link: