RF105 Просмотр технического описания (PDF) - Conexant Systems

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RF105

900 MHz Digital Spread Spectrum Transceiver

Conexant Systems 

RF105

900 MHz Digital Spread Spectrum Transceiver

Technical Description

Baseband Filter Bandwidth _______________________

The receive baseband filters have a bandpass

characteristic. The low-pass cutoff is determined by the

GmC filters and is set by the Rgmc resistor connected to pin

19.

The GmC filter has a 3-pole Butterworth response and is

preceded by a 4-pole Butterworth Sallen & Key low-pass

filter with a fixed cutoff frequency of 1.2 MHz. The fixed

filters are designed to attenuate out-of-band blocking

signals propagating through the receive path.

The baseband high-pass cutoff is set by the bandwidth of

the DC servo loop, which in turn is set by the value of the

Cservo capacitors connected between pins 32 and 33, and

pins 34 and 35. The DC servo loop nulls out the DC offset

in the receive baseband path. It is designed to be tolerant

of the Cservo Equivalent Series Resistance (ESR), so that

common surface mount capacitors are suitable.

The baseband high-pass cutoff frequency should be set

much lower than the low-pass cutoff frequency, or else the

servo loop will become unstable.

The optimum receive bandwidth values are:

fLPF = 820 kHz, Rgmc = 875 Ω

fHPF = 20 kHz, Cservo = 0.082 µF

RF Output _____________________________________

The transmit RF outputs from the RF105 are differential

and matched to 100 Ω differential. If a single-ended

connection is required, the unused output must be suitably

terminated by a 50 Ω resistor (see Figure 4). The transmit

output power is determined by the output power control

inputs, PS1 (pin 20) and PS2 (pin 21), and by the value of

Rmod (connected to pin 14). Rmod sets the bias current into

the modulator, which is then multiplied by a factor set by

the state of PS1 and PS2. PS1 and PS2 input

programming is described in the Transmitter section of

Table 3.

The characteristic of the output Peak Envelope Power

(PEP) versus Rmod is shown in Figure 3 for a sinusoidal

120 mV peak-to-peak input signal and for high power

mode (PS1 = 0, PS2 = 0). For Rmod equal to 1.2 kΩ, the

typical RF105 output power is approximately –8 dBm in

high-power mode. This provides approximately 20 dBm of

transmit power when used with Conexant’s RF106 (29 dB

gain PA).

2.3

2.2

2.1

2

1.9

1.8

1.7

1.6

1.5

1.4

1.3

1.2

1.1

1

0.9

0.8

0.7

0.6

0.5

-12

-11

-10

-9

-8

-7

-6

-5

-4

Tx PEP (dBm)

Figure 3. Tx PEP vs Rmod

(Txd signal = 120 mVpp, 300 kHz sinusoid, Vcc = 3.0V)

2

Conexant

W117, Rev. B

Conexant Proprietary Information

September 28, 1999

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