STA5620

Functional description

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3

Functional description

3.1

RFA and MIXER section

The 1575.42 MHz RF signal at the output of the external SAW filter is amplified by a RF

amplifier (RFA) and then down converted by an image rejection mixer.

The good performances of the cascade configuration and the technology choice guarantee

a noise figure better than 4.5dB in typical conditions. In fact, the RFA gain is high enough to

minimize the effects on the noise figure of the following integrated stages.

The linearity of the RFA and Mixer section ensures immunity to RF blockers close to the

GPS signal. Then it allows the use of low quality external pre-selection filters.

Two ninety degrees out of phase signals are derived from the VCO and send to the input of

the image rejection mixer. A minimum image rejection ratio of 20dB is guaranteed.

3.2

IF section

The output of the mixer combiner is processed through an integrated filter able to select the

GPS L1 bands. The IF filter cuts any out-of-band signal including the mixer products. In

addition it acts as an anti-aliasing filter for the A/D converter. An attenuation of 20dB is

The IF filter characteristic is calibrated by an internal loop which compensates process,

temperature and voltage variations.

In order to let the baseband reconstruct the received information, the IF filter must not

introduce an excessive phase shift within the signal bandwidth.

3.3

Variable gain amplifiers

A cascade of variable gain amplifiers and the relevant control circuit balance the system

gain in relationship to the RF input signal strength. In that way the signal level at the input of

the A/D converter is suitably compensated.

The device is able to self-adjust the AGC gain by integrating the MAG output by a dedicated

circuit in order to obtain 33% of MAG bit duty cycle. The loop is compensated by an external

capacitor connected to the AGC_CTRL pin. The relevant voltage is used to control the

variable gain amplifiers.

The internal loop can be by-passed by setting a voltage to the AGC_CTRL input pin. A

dynamic range of around 55dB is typically achieved.

3.4

A/D converter

The task of the A/D converter is to determine the sign and the magnitude of the received

Those baseband chips with just one bit input will use only the sign bit. In that case the

AGC_CTRL pin must be connected to ground.