Image Processing Reference
In-Depth Information
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.
t 2 - t 1
t 1
t 2
Measurement interval Tau
Fig. 2. Beat signals from double-balance mixers
3. Frequency measurement using digital signal processing
To remove the effect of common offset oscillator phase noise and improve the accuracy of
measuring frequency, we proposed to make use of digital signal processing method
measuring frequency. A Multi-Channel Digital Frequency Stability Analyzer has been
developed in NTSC.
3.1 System configuration
This section will report on the Multi-Channel Digital Frequency Stability Analyzer (DFSA)
based upon the reformed DMTD scheme working at 10MHz with 100Hz beat frequency.
DFSA has eight parallel channels, and it can measure simultaneously seven oscillators. The
block diagram of the DFSA that only includes two channels is reported in Fig. 3.
Common offset reference oscillator generates frequency signal, which has a constant
frequency difference with reference oscillator. Reference oscillator and under test oscillator
at the same nominal frequency are down-converted to beat signals of low frequency by
mixing them with the common offset reference to beat frequency. A pair of analog-to-digital
converters (ADC) simultaneously digitizes the beat signals output from the double-balance
mixers. All sampling frequency of ADCs are droved by a reference oscillator to realize
simultaneously sampling. The digital beat signals are fed into personal computer (PC) to
computer the drift frequency or phase difference during measuring time interval.
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