Tham khảo tài liệu 'oscilloscopes episode 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 68 Oscilloscopes repetitive waveforms right up to and beyond the chopping frquency though there is little point in so doing. The choice of trigger source is very important when working with a dual trace oscilloscope. As mentioned earlier when discussing the waveforms encountered in a decade divider stage if the frequencies being displayed on the Y1 and Y2 traces are different but related one should trigger from the lower frequency whether it be displayed on the Y1 or the Y2 trace. Dual trace scopes usually have a mixed trigger facility this means that when used in the alternate mode with internal triggering the sweep will be triggered from the Y1 channel when displaying the Y1 trace but on the next sweep will display the Y2 signal triggered from the Y2 channel. Consequently both traces will be perfectly synchronized with their respective displayed signals and the traces will appear to have a fixed stable relationship. In fact the signals displayed on the two channels could have totally unrelated frequencies as would be apparent if triggering from Yf were selected in which case the Y2 trace would not be synchronized and vice versa. In the mixed triggering mode in fact the oscilloscope is simply equivalent to two entirely separate single channel scopes each internally triggered from its own signal. Nevertheless mixed triggering can be very useful for keeping an eye on two unrelated waveforms simultaneously provided this fact is borne firmly in mind. Care is needed even when the two frequencies are harmonically related or identical. Mixed triggering will show the 0 reference output and the 90 quadrature output of a quadrature oscillator as being in phase whereas triggering from the reference input will show the correct 90 phase difference between the two sine waves. The moral is to use mixed triggering only when it is specifically required and to regard the selection of the appropriate triggering arrangements as an essential part of setting up a dual trace scope. Many
