RF2948B データシートの表示（PDF） - RF Micro Devices

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RF2948B

2.4GHz SPREAD-SPECTRUM TRANSCEIVER

RF Micro Devices 

RF2948B

TRANSMITTER

TX LPF and Mixers

The transmit section starts with a pair of 5-pole Bessel

filters identical to the filters in the receive section and

with the same 3dB frequency. These filters pre-shape

and band-limit the digital or analog input signals prior

to the first upconversion to IF. These filters have a high

input impedance and expect an input signal of

100mVPP typical. Following these low pass filters are

the I/Q quadrature upconverter mixers. Each of these

mixers is half the size and half the current of the RF to

IF downconverter on the RF2494. Recall that this

upconverted signal may drive the same SAW filter (in

half-duplex mode) as the RF2494 and therefore share

the same load. Having the sum of the two BB to IF mix-

ers equal in size and DC current to the RF to IF mixer,

will minimize the time required to switch between RX

and TX, and will facilitate the best impedance match to

the filter.

TX VGA

Being essentially high impedance, TX 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 50Ω impedance (after

upconversion). For characterization purposes, a 50Ω

shunt resistor is placed on the IF signal path, before

AC-coupling to the input. A 50Ω signal source is

W applied directly across the shunt resistor, through a

coaxial test lead. The signal source sees the shunt

E resistor and therefore a low SWR. Voltage gain is then

N the same as power gain, simply the difference in dB

between the output power and the input power.

The AGC after the SAW filter starts with a switch and a

R constant gain amplifier of 15dB, which is identical to

the circuitry on the receive IF AGC. This was done so

O that the input impedance will remain constant for differ-

F ent gain control voltages. Following this 15dB gain

amplifier is a single stage of gain control offering 15dB

gain range. The main purpose of adding this variable

T gain is to give the system the flexibility to use different

SAW filters and image filters with different insertion

O loss values. This gain could also be adjusted real time,

N if desired.

TX Upconverter

The IF to RF upconverter is a double-balanced differ-

ential mixer with a differential to single-ended con-

verter on the output to supply 0dBm peak linear power

to the image filter. The upconverted SSB signal should

have -6dBm power at this point, and the image will

have the same power, but due to the correlated nature

of the signal and image, the output must support 0dBm

of linear power to maintain linearly.

+6dBm PA Driver

The SSB output of the upconverter is -6dBm of linear

power. The image filter should have at most 4dB of

insertion loss while removing the image, LO, 2LO and

S any other spurs. The filter output should supply the PA

driver input -10dBm of power.

N The PA driver is a one-stage class A amplifier with

G 10dB gain and capable of delivering 6dBm of linear

I power to a 50Ω load, and has a 1dB compression

point of 12dBm. For lower power applications, this PA

DES driver can be used to drive a 50Ω antenna directly.

Rev A6 040930

11-249

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