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SP8852EIGHCAR Datasheet(PDF) 10 Page - Mitel Networks Corporation

Part # SP8852EIGHCAR
Description  2쨌7GHz Parallel Load Professional Synthesiser
Download  13 Pages
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Manufacturer  MITEL [Mitel Networks Corporation]
Direct Link  http://www.mitel.com
Logo MITEL - Mitel Networks Corporation

SP8852EIGHCAR Datasheet(HTML) 10 Page - Mitel Networks Corporation

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10
SP8852E
its frequency band. The charge pump pulse current is determined
by the current fed into pin 19 and is approximately equal to pin
19 current when the programmed multiplication ratio is 1. The
circuit diagram Fig. 7e shows the internal components on pin 19
which mirror the input current into the charge pump. The voltage
at pin 19 will be approximately 1·6V above ground due to two VBE
drops in the current mirror. This voltage will exhibit a negative
temperature coefficient, causing the charge pump current to
change with chip temperature by up to 10% over the full military
temperature range if the current programming resistor is
connected to VCC as shown in the application diagram, Fig. 5. In
critical applications where this change in charge pump current
would be too large the resistor to pin 19 could be increased in
value and connected to a higher supply to reduce the effect of VBE
variation on the current level. A suitable resistor connected to a
30V supply would reduce the variation in pin 19 current due to
temperature to less than 1·5%. Alternatively a stable current
source could be used to set pin 19 current.
The charge pump output on pin 20 will only produce
symmetrical up and down currents if the voltage is equal to that
on the voltage reference pin 21. In order to ensure that this
voltage relationship is maintained, an operational amplifier must
be used as shown in the typical application Fig. 5. Using this
configuration pin 20 voltage will be forced to be equal to that on
pin 21 since the operational amplifier differential input voltage will
be no more than a few millivolts (the input offset voltage of the
amplifier).
When the synthesiser is first switched on or when a frequency
outside the VCO range is programmed, the amplifier output will
limit, allowing pin 20 voltage to differ from that on pin 21. As soon
as an achievable frequency value is programmed and the
amplifier output starts to slew the correct voltage relationship
between pin 20 and 21 will be restored. Because of the importance
of voltage equality between the charge pump reference and
output pins, a resistor should never be connected in series with
the operational amplifier inverting input and pin 20, as is the case
with a phase detector giving voltage outputs. Any current drawn
from the charge pump reference pin should be limited to the few
microamps input current of a typical operational amplifier. A
resistor between the charge pump reference and the non-
inverting input could be added to provide isolation but the value
should not be so high that more than a few millivolts drop are
produced by the amplifier input current.
When selecting a suitable amplifier for the loop filter, a
number of parameters are important; input offset voltage in
most designs is only a few millivolts and an offset of 5mV will
produce a mismatch in the up and down currents of about 4%
with the charge pump multiplication factor set at 1.
The
mismatch in up and down currents caused by input offset
voltage will be reduced in proportion to the charge pump
multiplication factor in use.
If the linearity of the phase detector about the normal phase
locked operating point is critical, the input offset voltage of most
amplifiers can be adjusted to near zero by means of a
potentiometer. The charge pump reference voltage on pin 21 is
about 1·3V below the positive supply and will change with
temperatureandwiththeprogrammedchargepumpmultiplication
factor. In many cases it is convenient to operate the amplifier with
the negative power supply pin connected to 0V as this removes
the need for an additional power supply. The amplifier selected
must have a common mode range to within 3·4V (minimum
charge pump reference voltage) of the negative supply pin to
operate correctly without a negative supply. Most popular
amplifiers can be operated from a 30V positive supply to give a
wide VCO voltage drive range and have adequate common
mode range to operate with inputs at 13·4V with respect to the
negative supply.
Input bias and offset current levels to most operational
amplifiers are unlikely to be high enough to significantly affect the
accuracy of the charge pump circuit currents but the bias current
can be important in reducing reference side bands and local
oscillator drift during frequency changes.
When the loop is locked, the charge pump produces only very
narrow pulses of sufficient width to make up for any charge lost
from the loop filter components during the reference cycle. The
charge lost will be due to leakage from the charge pump output
pin and to the amplifier input bias current, the latter usually being
more significant. The result of the lost charge is a sawtooth ripple
on the VCO control line which frequency modulates the phase
locked oscillator at the reference frequency and its harmonics. A
similar effect will occur whenever the strobe input is taken high
during a programming sequence. In this case the charge pump
is disabled when the strobe input is high and any leakage current
will cause the oscillator to drift off frequency. To reduce this
effect, the duration of the strobe pulse should be minimised.
FPD and FREF Outputs
These outputs provide access to the outputs from the RF and
reference dividers and are provided for monitoring purposes
during product development or test, and for connection of an
external phase detector if required. The output circuit is of ECL
type, the circuit diagram being shown in Fig. 7g. The outputs can
be enabled or disabled under software control by the address 0
control word but are best left in the disabled state when not
required as the fast edge speeds on the output can increase the
level of reference sidebands on the synthesised oscillator.
The emitter follower outputs have no internal pulldown resis-
tor to save current and if the outputs are required an external
pulldown resistor should be fitted. The value should be kept as
high as possible to reduce supply current, about 2·2k
Ω being
suitable for monitoring with a high impedance oscilloscope probe
or for driving an AC-coupled 50
Ω load. A minimum value for the
pulldown resistor is 330
Ω.
When the FPD and FREF outputs are disabled the output level
will be at the logic low level of about 3·5V so that the additional
supply current due to the load resistors will be present even when
the outputs are disabled.
Reference Input
The reference input circuit functions as an input amplifier or
crystal oscillator. When an external reference signal is used this
is simply AC-coupled to pin 28, the base of the input emitter
follower. When a low phase noise synthesiser is required the
reference signal is critical since any noise present here will be
multiplied by the loop. To obtain the lowest possible phase noise
from the SP8852E it is best to use the highest possible reference
input frequency and to divide this down internally to obtain the
required frequency at the phase detector. The amplitude of the
reference input is also important, and a level close to the
maximum will give the lowest noise.
When the use of a low reference input frequency say 4 to
10MHz is essential some advantage may be gained by using a
limiting amplifier such as a CMOS gate to square up the
reference input. In cases where a suitable reference signal is not
available, it may be more convenient to use the input buffer as a
crystal oscillator in this case the emitter follower input transistor
is connected as a Colpitts oscillator with the crystal connected
from the base to ground and with the feedback necessary for
oscillation provided by a capacitor tap at the emitter. The
arrangement is shown inset in Fig. 5.


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