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SL6619TP1N データシート(PDF) 9 Page - Mitel Networks Corporation |
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SL6619TP1N データシート(HTML) 9 Page - Mitel Networks Corporation |
9 / 21 page 9 SL6619 OPERATION OF SL6619 Low Noise Amplifier To achieve optimum performance it is necessary to incor- porate a Low Noise RF Amplifier at the front end of the receiver. This is easily biased using the on-chip voltages and current source provided. All voltages and current sources used for bias of the RF amplifier, receiver and mixers should be RF decoupled using 1nF capacitors. The receiver also requires a stable Local Oscillator at the required channel frequency. Local Oscillator The Local Oscillator signal is applied to the device in phase quadrature. This can be achieved with the use of two RC networks operating at their 23dB/45 ° transfer character- istic. The RC characteristics for I and Q channels are com- bined to give a full 90 ° phase differential between the LO ports of the device. Each LO port also requires an equal level of drive from the oscillator. This is achieved by forming the two RC networks into a power divider. Gyrator Filters The on-chip filters include an adjustable gyrator filter. This may be adjusted by changing the value of the resistor con- nected between pin 13 and GND. This allows adjustment of the filters’ cutoff frequency and allows for compensation for possible process variations. Audio AGC (Fig. 3) The Audio AGC consists of a current sink which is control- led by the audio (baseband) signal. It has three parameters that may be controlled by the user. These are the attack (turn on ) time, decay (duration) time and threshold level. The attack time is simply determined by the value of the external capacitor connected to TCADJ. The external capacitor is in series with an internal 100k Ω resistor and the time constant of this circuit dictates the attack time of the AGC. i.e. tATTACK = 100kΩ3C18 The decay time is determined by the external resistor connected in parallel with the capacitor CTC. The decay time is simply tDECAY = R173C18 When a large audio (baseband) signal is incident on the input to the AGC circuit, the variable current source is turned on. This causes a voltage drop across R13. The voltage potential between VREF and the voltage on pin 31 causes a current to flow in pin 30. This charges up C18 through the 100k Ω internal resistor. As the voltage across the capacitor increases, a current source is turned on and this sinks current from pin 32. The current sink on pin 32 can be used to drive the external AGC circuit by causing a PIN diode to conduct, reducing the signal to the RF amplifier. RF AGC The RF AGC is an automatic gain control loop that protects the mixer’s RF inputs, Pins 3 and 5, from large out of band RF signals. The loop consists of an RF received signal strength indicator which detect the signal at the inputs of the mixers. This RSSI signal is then used to control the LNA current source (pin 1). Regulator The on-chip regulator should be used in conjunction with a suitable PNP transistor to achieve regulation. As the transis- tor forms part of the regulator feedback loop the transistor should exhibit the following characteristics: HFE.100 for VCE. = 0·1V If no external transistor is used, the maximum current sourcing capability of the regulator is limited to 30 µA. Automatic Frequency Control (Fig. 4) The Automatic Frequency Control consists of a detection circuit which gives a current output at AFC OP whose magni- tude and sign is a function of the difference between the local oscillator (fLO) and carrier frequencies (fC). This output current is then filtered by an off-chip integrating capacitor. The integrator’s output voltage is used to control a voltage control crystal oscillator. This closes the AFC feedback loop giving the automatic frequency control function. For an FSK modu- lated incoming RF carrier, the AFC OP current’s polarity is positive, i.e.current is sourced for fLO,fC,fLO14kHz and negative, i.e. current is sunk, for fLO.fC.fLO24kHz. The magnitude of the AFC OP current is a function of frequency offset and the transmitted data’s bit stream. If the carrier frequency, (fC), equals the local oscillator frequency, (fLO) then the magnitude of the current is zero. BIT RATE FILTER CONTROL The logic level on pin 26 controls the cutoff frequency of the 1st order bit rate for a given bit rate filter capacitor at pin 27. This allows the cutoff frequency to be changed between fC, 2fC and 0·43fC through the logic level on pin 26. This function is achieved by changing the value of the current in the 4f detector’s output stage. A logic zero (0V to 0·1V) on pin 26 gives a cutoff frequency of fC a logic one (VCC220·3V to VCC2) gives a cut off frequency of 2fC and an open circuit at pin 26 gives a cutoff frequency of 0·43fC. |
同様の部品番号 - SL6619TP1N |
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同様の説明 - SL6619TP1N |
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