Agilent HP-AN301-1--Low-noise-division-of 10MHz-oscillators free download

File name HP-AN301-1--Low-noise-division-of 10MHz-oscillators.pdf

Introduction As the frequency of a crystal oscillator increases it becomes increasingly difficult to produce high stability resonators. Over a period of about 20 years, the frequency of the state-of-the-art high performance crystal oscillators has increased from 100 k Hz to I MHz, then to 5 MHz. Currently 10 MHz is the most common frequency available in the highest performance crystal oscillators. Although 10 MHz has become accepted as the standard crystal oscillator frequency, there are still many applications in communications, navigation, and instrumentation which require 5 MHz, I MHz or other frequencies. Many times the system designer would like to have a local oscillator or timebase which is not a standard frequency. The choices are either to purchase a special frequency oscillator, often at a significant cost increase, or to design special interface circuitry to take advantage of the standard frequency oscillator. The purpose of this application note is to assist the system designer in designing interface circuitry that will produce required frequencies by division (+2, +4, +10 and others) of a standard 10 MHz oscillator. The system designer, interested in high stability oscillators, would like an interface to the system which minimally degrades the oscillator performance. In theory, frequency division improves the signal to noise ratio. Practically, interface circuitry cart be designed which induces only a small amount of deterioration in performance. This note describes specific circuits which have produced as little as 6 dB to 8 dB of phase noise degradation (at 10 kHz from the carrier) in divide-by-ten and divide-by-two circuits, respectively, when operating on the output of an HP 10811 oscillator. It is a very high stability, ovenized, 10 MHz crystal oscillator which, with a good interface, can provide a highly stable, non-10MHz signal to a system. The total system might look like Figure 1, with the oscillator and interface circuitry supplying the timebase signal or local oscillator to the designer's main system. The input conditioning modifies the oscillator output so that the divider receives the proper signal levels, then the output conditioning provides amplification to the signal levels needed for the time base or local oscillator requirement. Figure 1 Noise Description Oscillator noise performance is specified in terms of time domain stability and/or phase noise. Time domain stability and phase noise are the time and frequency domain measurements, respectively, of the same noise. Time domain stability is divided into two different measurements, long term and short term. Long term stability measures the amount of change over a day or more and is usually expressed as an aging rate. The aging rate is crystal dependent and, in good oscillator design, is not affected by the oscil