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Fabricante

SL6659

LOW POWER IF/AF CIRCUIT (WITH RSSI) FOR FM RADIO

Zarlink Semiconductor Inc 

SL6659

Fig. 3 Internal schematic

GENERAL DESCRIPTION

The SL6659 is a very low power, high performance integrated

circuit intended for IF amplification and demodulation in FM radio

receivers. It comprises:

q A mixer stage for use up to 200MHz

q A limiting amplifier operating up to 1·5MHz

q A quadrature detector with AF output

q An RSSI (Received Signal Strength Indicator) output

Mixer

The Mixer is single balanced with an active load. Gain is set

externally by the load resistor, although the value is normally

determined by that required for matching into the ceramic filter. It is

possible to use a tuned circuit but an increase in mixer gain will result

in a corresponding reduction of the mixer input intercept point.

The RF input is a diode-biased transistor with a bias current of

typically 300µA. The LO input is differential but would normally be

driven single-ended. Special care should be taken to avoid acciden-

tal overload of the local oscillator input.

IF Amplifier

The limiting amplifier is capable of operation to at least 1MHz and

the input impedance is set by an external resistor to match the

ceramic filter. Because of the high gain, pins 11 and 12 must be

adequately bypassed.

Detector

A conventional quadrature detector providing audio output is fed

internally from the IF amplifier; the quadrature input is fed externally

using an appropriate capacitor and phase shift network.

RSSI Output

The RSSI output is a current source with value proportional to the

logarithm of the IF signal amplitude. There is a small residual current

due to noise within the amplifier (and mixer) but beyond this point

there is a measured and guaranteed 70dB dynamic range. The

typical range extends to 92dB, independent of frequency, and with

exceptionally good temperature and supply voltage stability.

Supply Voltage

The SL6659 will operate reliably trom 2·5V to 7·5V. The supply

line must be decoupled with 470nF using short leads.

Internal Bias Voltage

The internal band-gap reference must be externally decoupled.

It can be used as an external reference but must not be loaded

heavily; the output impedance is typically 14Ω..

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