Image Processing Reference
In-Depth Information
short-term stability, low cost, high reliability measurement system. A description of a
classical DMTD method is given in Section 2. Some of the tests of the cross-correlation
algorithm using simulated data are discussed in Section 3.2. The design of DFSA including
hardware and software is proposed in Section 3.3-3.4. In section 4 the DFSA is applied to
measure NTSC's cesium signal and the results of noise floor of DFSA is given. Future
possible modifications to the DFSA and conclusions are discussed in Section 4.
2. Principle of DMTD method
The basic idea of the Dual Mixer Time Difference Method (DMTD) dates back to 1966 but
was introduced in “precision” frequency sources measurement some 10 years later (S.
STEIN, 1983). The DMTD method relies upon the phase measurement of two incoming
signals versus an auxiliary one, called common offset oscillator. Phase comparisons are
performed by means of double-balance mixers. It is based on the principle that phase
information is preserved in a mixing process. A block diagram is shown in figure 1.
Fig. 1. Block diagram of a dual mixer time difference measuring system
DMTD combines the best features of Beat Method and Time Interval Counter Method, using
a time interval counter to measure the relative phase of the beat signals. The measurement
resolution is increased by the heterodyne factor (the ratio of the carrier to the beat
frequency). For example, mixing a 10 MHz source against a 9.9999 MHz Hz offset reference
will produce a 100 Hz beat signal whose period variations are enhanced by a factor of 10
MHz/100 Hz = 10 5 . Thus, a period counter with 100 ns resolution (10 MHz clock) can
resolve clock phase changes of 1 ps.
Search WWH ::




Custom Search