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Zakładka z wyszukiwarką danych komponentów |
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Zakładka z wyszukiwarką danych komponentów |
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LM3661AM Arkusz danych(PDF) 5 Page - National Semiconductor (TI) |
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LM3661AM Arkusz danych(HTML) 5 Page - National Semiconductor (TI) |
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5 / 6 page  Applications Information (See Internal Schematic) OSCILLATOR The Colpitts type oscillator is internally biased with a regu- lated current source which assures proper operation over a wide supply range The collector base and emitter termi- nals are at pins 4 1 and 2 respectively The crystal which is used in the parallel resonant mode may be replaced with an appropriate inductor if the application does not require the stability of a crystal oscillator In this case the resonant frequency will be determined by the inductor in parallel with the series combination of C1 and C2 TLH5586 – 4 so Cte(C1)(C2)(C1aC2) and fOe 159 0L(Ct) MIXER The mixer is double balanced to reduce spurious responses The upper pairs are switched by the oscillator while the RF input is applied to the lower pair (pin 16) R43 sets the mixer input impedance at 36 kX The mixer output impedance of 18 kX will properly match the input impedance of a ceramic filter which is used as a bandpass filter coupling the mixer output to the IF limiting amplifier IF LIMITER The IF amplifier consists of six differential gain stages with the input impedance set by R2 at 18 kX to properly termi- nate the ceramic filter driving the IF The IF alone (without mixer) has a b3 dB limiting sensitivity of approximately 50 m V The system bandwidth is limited to about 5 MHz due to high impedances in the IF which are necessary to meet low power requirements The IF output is connected to the ex- ternal quad coil at pin 8 via an internal 10 pF capacitor FM DEMOD AUDIO OUT A conventional quadrature detector is used to demodulate the FM signal The Q of the quad coil which is determined by the external resistor placed across it has multiple effects on the audio output Increasing the Q increases output level but because of nonlinearities in the tank phase characteris- tic also increases distortion (see Typical Performance Characteristics) For proper operation the voltage swing on pin 8 should be adequate to drive the upper rank of the multiplier into switching (about 100 mVrms) This voltage level is dependent on the internal 10 pF capacitor and the tank Rp voltage divider network After detection and de-em- phasis the audio output at pin 9 is buffered by an emitter follower OP AMP The op amp inverting input (pin 10) which is internally refer- enced to 07V receives dc bias from the output at pin 11 through the external feedback network Because of the low DC bias maximum swing on the op amp output with 10% distortion is 500 mVrms This can be increased when oper- ating on supplies over 23V by adding a resistor from the op amp input to ground which raises the quiescent DC at the output allowing more swing (see figure below for selection of added resistor) The op amp is normally utilized as either a bandpass filter to extract a specific frequency from the audio output such as a ring or dial tone or as a high pass filter to detect noise due to no input at the mixer The latter condition will generate a signal at the op amp output which when applied to pin 12 can mute the external audio amp For max swing VOUTe(VSbVBE)2 (from internal circuit) so (VSbVBE)2e07 1aR2R3J therefore R2 R3 e VSbVBE 14 Jb1 TLH5586 – 5 Increasing OP Amp Swing SQUELCH TRIGGER CIRCUIT The squelch trigger circuit is configured such that a low bias on the input (pin 12) will force pin 13 high (200 mV below supply) where it can support at leasta1mA load and pin 14 to be a low impedance typically 15X to ground Con- necting pin 14 to a high impedance ground reference point in the audio path between pin 9 and the audio amp will mute the audio output Pulling pin 12 above mute threshold (065V) will force pin 13 to an impedance of about 60 kX to ground and pin 14 will be an open circuit There is 100 mV of hysterisis at pin 12 which effectively prevents jitter 5 |
Podobny numer części - LM3661AM |
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Podobny opis - LM3661AM |
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