SP8853A/B
DESCRIPTION
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 4·7kΩ 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.
A basic application using a single phase detector is shown
in Fig. 6a. The SP8853 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,
withtheotherpindecoupledtoground.Thereferenceoscillator
is of conventional Colpitts type, with two capacitors required to
provide a low impedance tap for the feedback signal to the
transistoremitter.TypicalvaluesareshowninFig.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,thusspeedingupsystemoperation.Theoutputcurrent
available from pin 27 is limited to 1·5mA; if this is exceeded,
the logic low level will be uncertain.
Choice of Loop Amplifier
Theloopamplifierconvertsthechargepumpcurrentpulses
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
mostcases, anoperationalamplifierwillbeusedtoprovidethe
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.
Thisarrangementcanbeparticularlyusefulwheretheminimum
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.
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
Table4). Thisapplicationcouldalsousechargepump2output
(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
chargepump2currenttobesettogivelowreferencefrequency
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.
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
SP8853 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.
9
MITEL [ MITEL NETWORKS CORPORATION ]
