**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 = 105. Thus, a period counter with 100 ns resolution (10 MHz clock) can resolve clock phase changes of 1 ps.

116 Applications of Digital Signal Processing

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

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

resolve clock phase changes of 1 ps.

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 = 105. Thus, a period counter with 100 ns resolution (10 MHz clock) can

possible modifications to the DFSA and conclusions are discussed in Section 4.

**2. Principle of DMTD method** 

The DMTD setup is arguably the most precise way of measuring an ensemble of clocks all having the same nominal frequency. The usual idea thought that the noise of the common offset oscillator could be cancelled out in the overall measurement process. However, if the oscillator 1 and oscillator 2 are independent, then the beat signals of being fed into counter are not coherent. Figure 2 shows the beat signals that are fed into the time interval counter, thus, the beat signals of two test oscillators against the common offset oscillator are zero crossing at different sets of points on the time axis, such as t1 and t2. When time interval counter is used to measure the time difference of two beat signals, the time difference will be contaminated by short-term offset oscillator noise, here called common-source phase error (C. A. Greenhall, 2001, 2006). This DMTD method is inevitable common-source phase error when use counter to measure time difference. To remove the effect of common-source phase error need to propose other processing method.

Fig. 2. Beat signals from double-balance mixers
