section - Bitsavers

More documents

Recommendations

Info

PHASE LOCKED LOOP APPLICATIONSFM IF AMPLIFIER/DEMODULATOR WITH MUTING(561B)In this application, the loop portion of the 561 B operatesin the usual manner for FM demodulation. To introducemuting (squelch) the synchronous AM detector portion ofthe PLL is used to detect the presence of an input signaland to open a muting gate. Figure 8-46 shows a typicalcircuit incorporating the muting feature.The input <strong>section</strong> of the circuit is a broad-band, amplifierlimiter.The tuned LC network at the AM input, pin 4, isadjusted to provide a 90 0 phase shift at the I F frequency.This network is adjusted for maximum output at pin 1,demodulated AM output, with a carrier applied at the IFfrequency.Three transistors at the right of the diagram (01, 02 and03) and the 1 N457 diode form the muting gate. Gating isaccomplished by applying the demodulated FM outputthrough the 1 N457 diode and by biasing the diode on andoff as follows: During periods with no input applied, 01 isshut off and 02 continues. Therefore, the diode iseffectively back biased since its anode potential developedby the two 10K resistors across the power supply isapproximately +13.5V. When an input is applied to thecircuit, 01 is turned on and 02 shuts off, reducing itscollector potential below 9V. Thus, the diode is forwardbiased and the demodulated I F output is gated through tothe circuit output.Muting threshold adjustment is accomplished using the2.5K potentiometer. Transistor 03 is used as a biasgenerator for the differential pair, 0'1 and 02' In turn, thebias of 03 is obtained from internal PLL bias points atpins 12 and 13. Thus, the muting gate will track the PLLover wide temperature variations.TYPICAL FM IF AMPLIFIER/DEMODULATOR WITH MUTING+18V22K 20 J0.01 5.6K 10K lK 6.8KX2.S-12pF1612XINPUT51K51KCo137-4SpF561BAUDIOOUTPUT0.005 lK(2)0-1LOWI NPlJoTI'"I'" I 1'"10K 4.7K 6.8K":' ":'- -":'• Tl:20 TURNS BIFLAR NO. 36 WIRE ON 1/2 -WATT RESISTOR BODY (FOR 10.7 MHz)•• PART OF NE510AFigure 8-46FM DEMODU LA TOR (560B)When used as a FM demodulator, the 560B phase lockedloop requires selection of external components and/orcircuits to create the desired response .. The areas to beconsidered are:a.) Input Signal Conditioningb.) Tuning - VCO Frequencyc.) Low Pass Filter Selection/Gain Adjustmentd.) Output Swinge.) Tracking Range Adjustmentf.) De-emphasis Network SelectionFigure 8-47 illustrates schematically a typical FM demodulatorwith IF amplifier and limiter using the 560B PLL.The amplitude of the input signal has a pronounced effecton the operation. For the tracking range to be constant,the input signal level should be greater than 2mV rms. Inaddition, AM rejection diminishes at higher signal levels anddrops to less than 20dB for signals greater than 30mV. Ifeither the tracking range or AM rejection is critical, theinput signal should be conditioned to be in the 2 to 10mVrange, using either a limiter or a combination limiteramplifier.This circuit should limit at the smallest inputvoltage that is expected.51
PHASE LOCKED LOOP APPLICATIONSTYPICAL FM DEMODULATOR WITH IF AMPLIFIER AND LIMITER USING THE 560B+18V200pFr- - - ---,J T1* I 7.5KII51 1612 1514560BColK0.0113 10 DEMODULATEDFM OUTPUTlK1.5m. ~ 2~ ~0.01 Cd* TURNS NO. 36 BIFLAR WIRE WOUND ON 1/2-WATT, lOOK RESISTOR BODY** PART OF NE51 OAFigure 8-47The PLL is tuned by adjusting the VCO to the centerfrequency of the FM signal. This is accomplished byconnecting a capacitor across pins 2 and 3. The capacitorvalue is determined using the equation Co :::::: 300/f o , pFwhere f o, the free-running VCO frequency, is in MHz. Theexact value is not important as the internal resistors areonly within ±10% of nominal value and fine tuning isnormally required. Fine tuning may be accomplished byusing a trimmer capacitor in parallel with Co or by using apotentiometer connected across the power supply with therotor connected to pin 6 through a 200n current limitingresistor.The dc gain of the loop, which sets the' lock range andthreshold sensitivity, can be controlled by the placement ofa resistance between pins 14 and 15, the low pass filterterminals . .A low pass filter connected to these te'rminalscontrols the capture range or selectivity· of the loop. Inbasic terms, it may be said that the low pass filter sets thebandwidth of the demodulated information which will beobtained. For most applications, a single capacitorconnected between pins 14 and 15 will provide the requiredfiltering. The capacitance value required can be approximatedas follows:1330C~ mfdwhere f is the desired bandwidth in Hz. For example, if thedesired information bandwidth is 15kHz, the required lowpass filter capacitance will be:1330C ~ -- = .09mfd15000The output swing is a function of the frequency deviationof the incoming signal, and is approximately 0.3V p-p for.±1% deviation. For example, a standard 10.7MHz IFfrequency has a deviation of ±75kHz; therefore, the±0.75 x 100percentage deviation equals= ±0.7% and the10.7.7%output voltage will be 0.3V p-p x - = .21 V p-p, or 74mV1%rms for 100% modulation.The de-emphasis network requires an external capacitorfrom pin 10 to ground. This capacitor Cd and the 8000ninternal resistance should produce a time constant ofapproximately 75psec for standard FM broadcast demodulation.The value of the de-emphasis capacitor for thisapplication is determined by the following formula:75 x 10-68000= 0.0094 mfdFor most applicatioilS, a 0.01 mfd value would besatisfactory since the manufacturing tolerance of theresistor is on the order of 20%.52
Page 2 and 3: TABLE OF CONTENTSSECTION TITLE PAGE
Page 4: SECTIONINTRODUCTIONPHASE LOCKED LOO
Page 7 and 8: USER'S QUICK-LOOK GUIDE TO SIGNETIC
Page 10: SECTIONTHE PHASE LOCKED LOOP PRINCI
Page 13 and 14: PHASE LOCKED LOOP APPLICATIONSIt is
Page 15 and 16: PHASE LOCKED LOOP APPLICATIONSROOT
Page 17 and 18: PHASE LOCKED LOOP APPLICATIONSINTEG
Page 19 and 20: ~PHASE LOCKED LOOP APPLICATIONSFUNC
Page 21 and 22: PHASE LOCKED LOOP APPLICATIONSFREQU
Page 24 and 25: PHASE LOCKED LOOP APPLICATIONSGENER
Page 26 and 27: PHASE LOCKED LOOP APPLICATIONSPROBA
Page 28 and 29: ~PHASE LOCKED LOOP APPLICATIONSCapt
Page 30 and 31: PHASE LOCKED LOOP APPLICATIONSTRANS
Page 32 and 33: PHASE LOCKED LOOP APPLICATIONSThe V
Page 34 and 35: PHASE LOCKED LOOP APPLICATIONSINTER
Page 36 and 37: PHASE LOCKED LOOP APPLICATIONSIt is
Page 38 and 39: PHASE LOCKED LOOP APPLICATIONSWhen
Page 40 and 41: PHASE LOCKED LOOP APPLICATIONSSIMPL
Page 42 and 43: PHASE LOCKED LOOP APPLICATIONSSCHEM
Page 44 and 45: PHASE LOCKED LOOP APPLICATIONSNARRO
Page 46 and 47: PHASE LOCKED LOOP APPLICATIONSTwo e
Page 48: PHASE LOCKED LOOP APPLICATIONSd.)e.
Page 54 and 55: PHASE LOCKED LOOP APPLICATIONSPHASE
Page 56 and 57: PHASE LOCKED LOOP APPLICATIONSTRANS
Page 58 and 59: PHASE LOCKED LOOP APPLICATIONSbaud
Page 60 and 61: PHASE LOCKED LOOP APPLICATIONSFigur
Page 62 and 63: PHASE LOCKED LOOP APPLICATIONSinput
Page 64 and 65: PHASE LOCKED LOOP APPLICATIONfTRIAN
Page 66 and 67: PHASE LOCKED LOOP APPLICATIONSRF-FM
Page 68 and 69: PHASE LOCKED LOOP APPLICATIONSVHF O
Page 70 and 71: PHASE LOCKED LOOP APPLICATIONSPRECI
Page 72 and 73: PHASE LOCKED LOOP APPLICATIONSFSK D
Page 74 and 75: PHASE LOCKED LOOP APPLICATIONSThe c
Page 76 and 77: PHASE LOCKED LOOP APPLICATIONSFLUTT
Page 78 and 79: PHASE LOCKED LOOP APPLICATIONSSALES
Page 80 and 81: PHASE LOCKED LOOP APPLICATIONSINTER

section - Bitsavers

Create successful ePaper yourself

Delete template?

Save as template?