| 摘要 |
<p>An apparatus for generating a control signal including pulses occurring at a first frequency f1 and having an overall mean frequency f2 which is in an exact ratio to the difference between the frequencies of a reference signal N and an offset signal O has a common oscillator 18 operating at the frequency of the reference signal N. The offset signal O is produced from the oscillator 18 by a first channel 24a of a 2-channel direct digital synthesizer (DDS), and the frequency f1 is generated by its second channel 24b. The offset signal O controls the rate at which data is entered into a first-in first-out store (FIFO) 54. The signal from the second channel 24b of the DDS triggers sub-sequences of pulses to produce the required control signal, which modifies the extraction of data from the FIFO 54 occurring at the rate of the reference signal N. Each subsequence may comprise one pulse, no pulse or two pulses. The modifications introduced by the control signal exactly balance the accumulation of a surplus or deficiency of data which arises from the frequency offset between the signals N and O. The apparatus may be used to synthesize signals for testing communications equipment such as for SONET and SDH networks. <IMAGE></p> |
