STB5701

Functional description

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4.3.2

Image Rejection Mixer

Figure 5.

Mixer block diagram

An excellent feature of the STB5701 is the integrated image rejection mixer. This device

was designed to eliminate the need for a costly front-end SAW filter for many applications.

The advantage of not using a SAW filter is increased sensitivity, simplified antenna

matching, less board space, and lower cost.The mixer cell is a pair of double-balanced

mixers that perform an IQ down conversion of the 350-400MHz RF input to the IF

these signals to achieve ~35dB of Image Rejection over the full temperature range.

Low-side injection is required due to the on-chip Image Rejection architecture.

4.3.3

ASK/FSK demodulator and data filter

The received signal strength indicator (RSSI) voltage is proportional to the log of the down-

converted RF signal at the IF limiting amplifier input. It also used as demodulator for

amplitude-shift \ keying (ASK) modulation. The signal coming from the RSSI amplifier is

converted into the raw data signal by the ASK/FSK demodulator. The ASK or FSK

modulation selection is set by I2C interface.

The dynamic range of the RSSI amplifier is exceeded if the RF input signal is about 80

dB higher compared to the RF input signal at full sensitivity.

In FSK mode, the S/N ratio is not affected by the dynamic range of the RSSI amplifier but at

FSK receive mode the RSSI output provides a field strength indication.

Coming from the RSSI the FSK signal is fed to the input of the FSK demodulator. After

buffering the signal is fed to a phase discriminator. The phase shift is generated by

an external 10.7MHz Ceramic discriminator connected to FSK_Discr. (PIN 21).

The FSK demodulator is intended to be used for an FSK deviation of 37.5 kHz. Lower values

may be used but the sensitivity of the receiver is reduced in that condition. After

demodulation a 2nd order Sallen and Key filter is provided in order to suppress unwanted

frequency components.

s

I

s

Q

RFAmp

Image Rejection Mixer

IF Filter

I2C

(SC_FILTER)

(31)

Vcc_MIX

LNA

VCO