SL1466 データシートの表示（PDF） - Zarlink Semiconductor Inc

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SL1466

Wideband PLL FM Demodulator

Zarlink Semiconductor Inc 

SL1466

APPLICATION NOTES

Tuning procedure

The component values shown in the applications circuit

Fig. 6 are optimised for operation at an IF of 479.5MHz. The

AFC circuit can be used to fine tune the external tank as

follows:

With the SL1466 connected as in the test set up Fig. 11 or

its equivalent using 75Ω cables. Set the video generator for 1V

p-p output. Set the satellite test transmitter for a carrier

frequency of 479.5MHz, frequency deviation 13.5MHz, power

level -30dBm. Turn on the pre-emphasis filter.

Monitor the voltage levels on Pin 2 (DIGFLO) and Pin 3

(DIGFHI).

Adjust the tank coil by squeezing it slightly until the signal

on both Pins goes high (i.e. > Vcc –0.4 Volts).

These Pins remain high provided the LO frequency is tuned

to within the AFC WINDOW aperture,( ± 0.22MHz).

Optimising the loop components

The network connected from Pin 26 (VIDEO+) to Pin 24

(VIDEOFB–) and from Pin 25 (VIDEO–) to Pin 27 (VIDEO

FB+) forms the loop filter.

The components shown are based on a natural frequency

ƒn of 2.46MHz (ω n =2x π x2.46 Mrads/s ) and damping factor

ξ =2.6, and assuming Ko = 54MHz/V.

The closed loop gain of the receiver (i.e. the ratio of the

output amplitude to the input carrier frequency variation

versus frequency) has a low pass filter characteristic. Its roll off

is determined by the natural frequency whilst its in band

flatness is determined by the damping factor. Both factors will

affect the 3dB bandwidth as discussed earlier. A narrow

bandwidth will cause loss of high frequency resolution whilst

a large bandwidth will degrade the overall signal/noise in the

output waveform. Thus a selection procedure might be as

follows:

I Calculate R7 (R9) and C9 (C14) based on ƒ n = 2.46MHz,

ξ =2.6 and connect as in Fig. 5.

I Set the video generator for 1Vp–p composite video and the

test generator for a carrier frequency of 479.5MHz, frequency

deviation of 13.5MHz and power level –30dBm.

I Turn on the pre–emphasis filter. Use the 15kHz test pattern

to give black/white screen.

I Monitor the video analyser or TV set.

I Adjust the de–emphasis filter until the bar amplitude is

1Vp–p or 0% error. Reduce transmitter power level until

sparklies or streaking appear.

I Adjust component values for minimum power level when

streaking and sparklies occur together.

AGC settings

The signal level at the input to the limiter preceding the

phase detector is maintained at an level of around –20dBm or

more by an internal (device) AGC and an external (tuner) AGC

circuit.

Current pulses at the carrier frequency with mark/space

proportional to this input power are sourced out of pin 18 (AGC

TIME CONSTANT). These are smoothed and turned into a

voltage by the external components R4, C4. The time constant

R4 C4 should be adjusted so that the expected signal fading

rate can be tracked but its value is not critical, 5mSec typically.

Fig. 4 shows a typical external AGC control curve. Also

shown is the AGCTIMECONST voltage which is an indication

of the level of internal AGC gain being applied, (the control

range is the flat part of the curve).

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