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Signi\L PURITY CONS IDERI\TIONS for FREQUEN<:Y Sn1 Tiles I ZED Llel\DEND EQU I Pr1ent David L. Kel:Na General Instrument

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Signi\L PURITY CONS IDERI\TIONS for FREQUEN<:Y Sn1 Tiles I ZED Llel\DEND EQU I Pr1ent David L. Kel:Na General Instrument SIGNi\L PURITY CONS IDERI\TIONS FOR FREQUEN<:Y Sn1 TilES I ZED llEl\DEND EQU I Pr1ENT David L. Kel:na General Instrument, Jerrold Division ABSTRACT BASIC OVERVIEW OF THE PLL As cabl~ television system band­ Before we evoluate the system for wirlths increase and frequency plans pro­ its noise performance, let us first re­ liferate, more manufacturers are turning view the b~sic operation of a chase­ to synth~sized frequency agile head~nd locked loop. A simple phase-locked loop channel converters. Uith this new ap­ consists of a voltage controlled oscil­ proach using phased lo~ked loops and dual lator, or VCO, a digital divider, a chase conversion come spurious signals and detector, a reference frequency sour~e, nois~ sources not encountered before in and an integrator or loop filter. Figure crystal controlled channel converters. 1 represents such a system. Important characteristics of these These system components function ~s headend converters including phas~ noise, a s~rvo loop sue~ that when the·vco is spurious sign3ls generated by the compar­ phase-locked to the reference, the output ison frequ~ncy, and residual frequency frequency and phase of the digital divi­ a~d phase modulation, are evaluated for der is equal to the frequency and phase their subjective impact on the output of the referenc~. This makes the average signal to the cable. D3ta is presented output of the phase detector zero and, which shows the correlation between sub­ therefore, the output of the loop filter jective picture degradation and measured remains unchanged. Should a disturbance headend synthesizer noise contribution. cause the VCO oscillating at "N" times the reference to shift frequ~ncy or ~hase, the digital divider output would not be coherent with t~e reference. 7his makes the output of the phase detector INTRODUCTION nonzero causing the loop filter to change its averag~ DC output voltage, which In the past, designers of cable sys­ forces the VCO back to the proper fre­ tem headend equipment hardly concerned quency and phase (REF. 1). themselves with phase noise. This is be­ cause most systems relied on crystal NOISE SOURCES IN A PHASE-LOCKED LOOP oscillators wh0re the phase noise is not a major concern due to the inherent low Now let us take a look at the mech­ noise an~ stability of such circuitry. anisms that can oroduce noise within the The current trend is toward greater use phase-locked loop. Signals that are of frequency synthesis and phase-locked integer multiples of th~ reference fre­ loop controlled conversion processes. quency will inevitably be present at the This is due to the attractiveness to both output of the phase detector. These ref­ the manufacturer and use of frequency erence frequency components can cause programmability in the multichannel en­ spurious outputs by modulating the VCO. vironment ~nd is further driven by the Optimum phase detector characteristics decreasing cost of associated components. and the loop filter design can reduce these signals to an acceptable level. Noise is influenced by each section (REF. 2, 6, 8.) of the phase-lock8d loop system an~ ade­ quate performance is obtain~d only by Loop Filter Noise careful ctesign of all circuits. Roth product and system designer must deter­ Since the loon filter drives t~e VCO mine the phase noise performance level tuning line, any noise at the output of that is required for subjectively accept­ the loa~ filter produces phase noise on able signal quality in the intended ap­ the oscillator output. Furthermore, any plication. noise on the phase detector output is 39 C4APC OUTPUT PLL SYSTEM (SIMPLIFIED) CHANNEL 768 MHZ FREQUENCY IF SIGNAL OUTPUT (MODULATED) (MODULATED) OSCILLATOR POWER AMPLIFIER -------------------------------------~ vco PRE SCALER r-...1-"" 800-1400 MHZ +32 LOOP FIL TEA AND AMPLIFIER PROGRAMMABLEr-~-~ DIVIDER th REFERENCE FREQUENCY DIVIDER f~EFERENCE PROGRAMMING GENERATOR INPUT '-----------"""i U68 ....f---- INPUT 48 MHZ ----------------------------------------------------------------------------- FIGURE 1 modified by the filter transfer function, VCO Noise added to the loop filter output, and modulated onto the VCO. The amount of One of the more significant cont~i­ oscillator phase noise is a function of butors to output noise is the VCO itself, the tuning sensitivity, Kv, and the although all of the phase-locke~ loop amount of noise reaching the VCO's tuning components add noise to the output s~ec­ port. trum. The noise analysis of oscillators is difficult because the active device is For example, let's look at a VCO operating in its nonlinear region. How­ with a sensitivity of +35 MHZ per volt ever, if we examine the oscillator as an and assume that the VCO output frequency amplifier that has as much gain as the is 1000 MHZ with 0 volts on the control feedback network has loss, we will be line. At this sensitivity, a positive 1 able to get a rEasonable aoproximation volt de average value on the control line of the phase noise performance. Using would give an output frequency of: the basic r=lationship for thermal nois0, we note that: kTB = -174 dBm/Hz (deci­ F out F nominal + (Vdc times Kv) bels relative to 1 milliwatt per hertz) is the noise floor of any amplifier in­ F out lCJo;, MHz + (1 volt times put. For a 1 Hz bandwdth, adding the 3 5 ~m z 1 vo 1 t l amplifier noise figure, and ajding tho gain gives the corresponding amplifier F out = 1035 MHZ output noise floor. This will be the oscillator noise floor far removed from If the 1 volt signal had been l volt the carrier. The phase noise performance RMS random noise then the output would close into the carrier will depend upon have been 1000 MHz plus and minus 35 MHz whether it is a bipolar or field effect RMS of residual phas~ and frequency noise transistor. Rather than give a rigorous modulation. (REF. 8) mathematical description here, let us 40 take a look at some measureo ohase noise Divider Noise characteristics. Figure 2 sh;ws that there are 3 basic areas of phase noise: Increasing the output frequency of the VCO inherently leads to a larger fre­ 1) The close in portion, where the quency divider which, of course, ~eans oscillator phase noise is propor­ higher divider noise. Generally sp~ak­ tional to the transistor's low ing, the output phase noise of a digital frequency nois~ characteristic. divider is equal to the input phase noise divided by the circuit divisor plus the 2) A region that is a little farther inherent divider noise. This manifests removed from the carrier where the itself as a ~inimu~ attainable noise noise is related to the Q of the floor. The practical noise limits are frequency determining circuit. -l7G dBc/KHz (decibels relative to the carrier per kilohertz) for TTL types and -155 dBc/KHz for ECL dividers. CMOS OPEN LOOP VCO PHASE NOISE dividers, working up to a frequency of about 10 MHZ are similar to TTL devices, except that they have slightly higher noise between the carrier frequency and about 10 Hz away from the carrier. (REF. 3) Unfortun3tely, digital circuits have another side effect, crosstalk. This causes the input signal to appear at the output as both feedthru and stray pickup. Most synthesizer systems are limited by 0 other factors and this effect adds less 0 dB/OCTAVE than l dB to the noise level. (REF. 3) Phase Detector Noise Response -+----------~------------~--------~~F Fo In most synthesizer applications, FOR REGION: the phase detector is chosen for reasons other than noise performance, such as CD ( ;~ r( F:, ) acquisition and hold-in range. This is because the synthesizer phase detector 0 ( :~ r (~~:\~T2 )_ does not normally operate near its noise threshold, For this reason, phase ~e­ Fosc kT tectors are evaluated for their noise re­ 0 2 Ps sponse and not as a noise source them­ selves. (REF. 9, ll, 12, 13) Fo= AVERAGE OSCILLATOR FREQUENCY Q =LOADED QUALITY FACTOR OF OSCILLATOR'S RESONATOR Reference Signal Noise a= A CONSTANT WHICH IS PROPORTIONAL TO THE FLICKER NOISE AMPLITUDE Finally, we should consider the Fm = F- Fo phaselock reference signal and realize Fosc"' EFFECTIVE NOISE FIGURE OF THE AMPLIFIER USED IN THE that the output signal c~n be no more OSCILLATOR CIRCUIT stable than the r2ference frequency k = BOL TZMAN'S CONSTANT T = TEMPERATURE IN DEGREES KELVIN stability times the digital divider Ps"' RF OUTPUT POWER OF THE OSCILLATOR ratio. Generally, we can dismiss this as a problem, by recognizing that the FIGURE 2 reference frequency in a synthesizer system is fixed. This allows us to use a high stability, low phase nois2 circuit 3) The far removed noise floor of the such as a crystal controlled source. oscillator. SYSTEM H1PACT The second item is significant be­ cause a VCO that has a wide tuning range In order to discuss how we expect necessarily has a lower "Q" and there­ the perceived signal to be degraded in fore, has more phase noise. the presence of phase noise, we must consider how the receiver circuits re­ Also, ell three r~gions are relateo spond to this type of noise. First, we to frequency. This means that the higher will determine what common effects re­ the frequency of o~eration in an oscilla­ ceivers will experience. Second, we will tor, the higher the phase noise, all take a look at the video and sound demod­ othP.r parameters being the same. (RFF. ulators. It is n?.cessary to ev3luate th2 4, 5, 7, 10) video and sound signals separately, be- 41 ca~se the difference in the modulation We can derive an equation for the type for the two carriers will cause signal to phase noise ratio of a carrier their respective demodulators to respond by defining Fl and F2 as the lowest and differently to phase noise.
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