RF2938 Просмотр технического описания (PDF) - RF Micro Devices

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RF2938

2.4GHZ SPREAD-SPECTRUM TRANSCEIVER

RF Micro Devices 

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

200mVPP 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 RF2444. Recall that this

upconverted signal may drive the same SAW filter (in

half duplex mode) as the RF2444 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

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

constant gain amplifier of 15dB, which is identical to

the circuitry on the receive IF AGC. This was, done, as

on the RX signal path, so that the input impedance will

remain constant for different TX gain control voltages.

Following this 15dB gain amplifier is a single stage of

gain control offering 15dB gain range. The main pur-

pose of adding this variable gain is to give the system

the flexibility to use different SAW filters and image fil-

ters with different insertion loss values. This gain could

also be adjusted real time, if desired.

RF2938

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

any other spurs. The filter output should supply the PA

driver input -10dBm of power.

The PA driver is a two-stage class A amplifier with

10dB gain per stage and capable of delivering 6dBm

of linear power to a 50Ω load, and has a 1dB compres-

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

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

11

Rev A8 010418

2-11

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