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SP8861NAHP Datasheet(PDF) 9 Page - Mitel Networks Corporation |
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SP8861NAHP Datasheet(HTML) 9 Page - Mitel Networks Corporation |
9 / 13 page 9 SP8861 DESCRIPTION A basic application using a single phase detector is shown in Fig. 6a. The SP8861 is a 1·3GHz part so good RF design techniques should be employed, including the use of a ground plane and suitable high frequency capacitors at the RF input and for power supply decoupling. The RF input should be coupled to either pin 10 or pin 11, with the other pin decoupled to ground. The reference oscillator is of conventional Colpitts type, with two capacitors required to provide a low impedance tap for the feedback signal to the transistor emitter. Typical values are shown in Fig. 6a, although these may be varied to suit the loading requirements of particular crystals. Where a suitable reference signal already exists or where a very stable source is required, it is possible to apply an external reference as shown in Fig. 6b. The amplitude should be kept below 0·5Vrms to avoid forward biasing the transistor’s collector-base junction. Lock Detect and Charge Pump Operation In some systems, it is useful to have an indication of phase lock. This function is provided on pin 27 (LOCK DETECT), which goes low when the output of charge pump 2 (PD2) is between 2·25V and 2·75V and can be used to drive an LED to give visual indication of phase lock. Alternatively, a pullup resistor may be connected from pin 27 to VCC and the output used to signal to the control microprocessor that the loop is locked, thus speeding up system operation. The output current available from pin 27 is limited to 1·5mA; if this is exceeded, the logic low level will be uncertain. The circuit diagram of Fig. 6a is a basic application with minimum component count but which is neverthless perfectly adequate for many applications. Charge pump 1 output (pin3) is used to drive the loop amplifier which provides the control voltage for the VCO. When charge pump 1 is used in this mode, the PD1 and PD2 bits in the reference programming word must be set to enable charge pump 1 continuously (see Table 4). This application could also use charge pump 2 output (pin 25) or, if a higher phase detectot gain is required, pins 3 and 25 could be connected in parallel to use the combined output current from both charge pumps. The lock detect circuit can be programmed to automatically disable charge pump 1 as shown in Table 4. This feature can be used to reduce the system lock up time by connecting the charge pump outputs in parallel to the loop amplifier with resistor Rb connected in series with charge pump 2 output. This connection allows a relatively high current to be used from charge pump 1 to give a short lock up time, and a low charge pump 2 current to be set to give low reference frequency sidebands. The degree of lock up time improvement depends on the ratio of charge pump 1 and charge pump 2 currents. When the loop is out of lock, both charge pumps will be enabled and will feed current to the loop amplifier to bring the VCO to phase lock. The current from charge 2 will produce a voltage drop across Rb, allowing operation of the lock detect circuit and enabling charge pump 1. The value of Rb must be chosen to give a voltage drop greater than 0·25V at the current level programmed for charge pump 2. When phase lock is achieved, there will be no charge pump current and therefore the voltage at pin 25 will be equal to that on the virtual earth point of the loop amplifier (2·5V), disabling charge pump 1. Charge pump 1 should not be left open circuit when enabled as this would prevent correct operation of the phase detector. The output on pin 3 should be biased to half supply with a pair of 4·7k Ω resistors connected across supplies. When charge pump 2 is used to drive the loop amplifier, the lock detect circuit will only give an out of lock indication when large frequency changes are made or when a frequency outside the range of the VCO is programmed. at other times the loop amplifier is maintained at 2·5V by the action of the loop filter components. Again, a resistor connected between pin 25 and the loop amplifier, producing a voltage drop greater than 0·25V at the charge current programmed will allow sensitive out of lock detection. When phase lock detection is required using charge pump 1 only, charge pump 2 output should be biased to 2·5V, using two equal value resistors, Ra, across the supply as shown in Fig. 6c. A small capacitor, Cd, connected frompin 28 to ground may be used to reduce chatter at the lock detect output. A detailed block diagram of the lock detect circuit is shown in Fig. 3. Choice of Loop Amplifier The loop amplifier converts the charge pump current pulses into a voltage of a magnitude suitable for driving the chosen VCO. The choice of amplifier is determined by the voltage swing required at the VCO to achieve the necessary range. In most cases, an operational amplifier will be used to provide the essential characteristcs of high input impedance, high gain and low output impedance required in this application. A simple discrete design could also be used as shown in Fig. 6d. This arrangement can be particularly useful where the minimum VCO control voltage must be close to ground and where negative supplies are inconvenient. This form of amplifier is not suitable for use with charge pump 2 when the lock detect circuit is required. When an operational amplifier is used in the inverting configuration shown in Fig. 6a, the charge pump output is connected directly to the virtual earth point and will therefore operate a a voltage close to that set on the non-inverting input. Normally, this operating point should be set at half supply using a potential divider of two equal value resistors, Rx, but if necessary the voltage can be set up to 1V higher or lower without detrimental effect. When the lock detect function is required on charge pump 2 however, the non-inverting input must be at half supply. The digital phase detector and charge pump in the SP8861 produces bi-directional current pulses in order to correct errors between the reference and the VCO divider outputs. Once synchronisation is achieved, in theory no further output from the charge pump should be required. In practice, due to leakage currents and particularly the input current of the amplifier, the capacitors in the loop filter will gradually discharge, modifying the VCO control voltage and requiring further outputs from the charge pump to restore the charge. The effect of this continuous correction is to frequency modulate the VCO frequency and thus produce sidebands at the reference frequency. In order to reduce this effect to a minimum, an amplifier with low input bias is essential. |
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