L2572

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

8 of 14

www.unisonic.com.tw

QW-R102-032.A

FUNCTIONAL DESCRIPTION

The UTC L2572 as optimized for application in satellite receiver systems and requiring a minimum external

component count is a wideband PLL FM demodulator. It includes all the elements required to construct a phase

locked loop circuit, with the exception of an AFC detector circuit for generation of error signal to correct for any

frequency drift in the outdoor unit local oscillator, and tuning components for the local oscillator contains a block

diagram and Fig. 1 contains the typical application.

Fig. 2-1 contains the internal pin connections

In applications the second satellite IF frequency of typically 479.5 or 402MHz is fed to the RF preamplifier normally,

which depending on application and layout has a working sensitivity of typically -40 dBm. An RF level detect circuit,

which generates an AGC signal that can be used for controlling the gain of the IF amplifier stages is contained in the

preamplifier, so it can be maintaining a fixed level to the RF input of the UTC L2572, for optimum threshold

performance. The AGC circuit’s bias point can be adjusted to cater for device input power and variation in AGC line

voltage requirement. Fig. 5 shows the typical AGC curves are shown in. That the device is recommended that be

operated with an input signal between -30 and -35dBm. That can ensure optimum linearity and threshold

performance, and when over the typical sensitivity of -40dBm can give a good safety margin.

The preamplifier’s output is fed to the mixer section which is of balanced design for low radiation. In this stage the

RF signal is mixed with which is generated by an on–board oscillator, the local oscillator frequency. The oscillator

block is optimized for high linearity over the normal deviation range and uses an external varactor tuned sustaining

network. Fig. 3 contains a typical frequency versus voltage characteristic for the oscillator. Fig.4 shows the typical

stability that he loop output is designed to compensate for first order temperature variation effects.

The mixer’s output is then fed to the loop amplifier around which feedback is applied to effect loop transfer

characteristic. Feedback could be applied either in single ended or differential mode; both modes should give the

same loop response if the appropriate phase detector gains are assumed in calculating loop filters.

The loop amplifier drives a 75Ω output impedance buffer amplifier, which could be connected to a 75Ω load and

used to drive a high input impedance stage giving greater linearity and approximately 6dB higher demodulated signal

output level too.