9

SL1461SA

Advance Information

Fig.8 SL1461SA VCO centre frequency uncompensated temperature stability

479.5

478.5

477.5

476.5

475.5

–20

30

80

VCO STABILITY vs TEMP and SUPPLY

TEMP/

°C

480

479

478

477

476

55 5

4.5

4.75

5

5.25

5.5

SUPPLY (V)

Fig.9 SL1461SA AGC output voltage for differing values of AGC bias resistor

AGC

2.0

1.5

1.0

0.5

–70

–60

–50

–40

–30

–20

–10

0

VOLTAGE

RF INPUT LEVEL (dBm) UNMODULATED

OUTPUT

AGC BIAS RESISTOR 5.1K

AGC BIAS CURRENT 297 A

AGC LOAD RESISTOR 3.9K

AGC BIAS RESISTOR 10.5K

AGC BIAS CURRENT 150 A

AGC LOAD RESISTOR 4.7K

AGC BIAS RESISTOR 32K

AGC BIAS CURRENT 52 A

AGC LOAD RESISTOR 10K

= 5.0 VOLTS

APPLICATION NOTES

Capture range

Under conditions when there is no RF input signal

present, the SL1461SA may react to spurious radiation from

the free running oscillator coupling into the RF inputs.

Because of the constant phase error between the VCO input

to the phase detector and the spuriously coupled signal via the

RF input, the phase comparator will drive the control voltage

to either the bottom or the top of the range.

In such a case, the capture range will be asymmetrical

about the VCO free running frequency, since any control

voltage will only be able to tune the VCO in one direction if the

tuning voltage is already at the max or min.

This effect can be avoided by driving the RF input

differentially or achieving good common mode rejection to the

VCO signal.

The lock range is independant of the above effects and

will be symmetric about the centre of the phase detector S-

curve provided the VCO is correctly aligned.

EXAMPLE

Loop out of lock

Tuning voltage =4.3V (maximum)

frequency =520MHz (maximum

It is only possible to capture signals below this frequency since

the VCO is already at its maximum frequency.

Testing of capture range should be done with the device

operating under normal conditions. An input signal of between

-35dBm to -10dBm is suitable for such a measurement.