10

If a faster indication is required, comparable with the loop

lock up time, the capacitor will need to be 2-3 times smaller

than the time constant calculation suggests. The time to

respond to an out of lock conditions is 2-3 times less than that

required to indicate lock.

Charge pump circuit

The charge pump circuit converts the variable width up and

down pulses from the phase detector into adjustable current

pulses which can be directly connected to the loop amplifier.

The magnitude of the current and therefore the phase detector

gain can be modified when new frequency data is entered to

compensate for change in the VCO gain characteristics over

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 one. 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 V

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 V

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 V

be

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 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 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 micro amps

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 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 the temperature

and with the programmed charge pump multiplication 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 +3.4V 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.

It is possible to disable the charge pump by taking pin 39

low. In this case any leakage current will cause the oscillator

to drift off frequency. This feature may be useful where having

achieved lock an external phase detector of the user's choice

can be employed to suit a specific application.

F

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 are enabled when pin 22 is high and

disabled when pin22 is low, 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 pull down

resistor to save current and if the outputs are required an

external pull down resistor should be fitted. The value should

be kept as high as possible to reduce supply current, about

2.2k. being suitable for monitoring with a high impedance

oscilloscope probe or for driving an AC coupled 50 Ohm load.