HP HP-Bench-Briefs-1977-05-08 free download

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SERVICE INFORMATION FROM HEWLETT-PACKARD MAY-AUGUST 1977 FRACTIONAL N DIVIDE-BY-N LOOP SIMPLIFIES In order to more readily understand FREQUENCY how a fractional N loop works, one must first understand the more tradi- SYNTHESIS tional phase lock loop with a divide- by-N element. The fractional N loop is by Kenneth Jessen, really a modified divide-by-N loop. HP Loveland Division INTRODUCTION Figure 1 illustrates the basic concept involved in a phase lock loop. The FR.Y..CY in., Hewlett-Packardis introducing several phase detector compares the Voltage new instruments that use a new fre- Controlled Oscillator (VCO) output to Figure 2. A Phase Lock Loop With a the reference signal and produces a Divideby-N Element to Produce a Range quency synthesis technique known as of Frequencies in Steps Equal to Integral fractional N. This new technique over- tune voltage proportional to the phase Multiples of the Reference Frequency difference of these two inputs. The tune comes many of the limitations of the voltage is "cleaned up" by passing it I I C traditional divide-by-N loop. Any given divide-by-N loop can only produce fre- quencies which are integral multiples through a low-pass filter to suppress noise and high frequency components. divide-by-N block can be stepped to produce frequencies that are integral The polarity of the tune voltage is such multiples of the reference. The VCO of the reference frequency. The 33308 can now produce a range of discrete Automatic Synthesizer, for example, that it will pull the VCO frequency in a direction to phase-track th