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)
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.
output
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.
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.
Supply Voltage
The SL6659 will operate reliably trom 2·5V to 7·5V. The supply
line must be decoupled with 470nF using short leads.
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
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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