Method for locking a synthesised output signal of a synthesised waveform synthesiser in a phase relationship

A method for locking a synthesised output signal of a digital waveform synthesiser in a phase relationship with an externally generated input signal, the digital waveform synthesiser comprising a direct digital frequency synthesiser responsive to a frequency control digital word applied thereto for producing the synthesised output signal, the method comprising:

• applying an initial value of a frequency control digital word to the direct digital synthesiser, and
• sequentially applying progressively altered values of the frequency control digital word to the direct digital synthesiser until the synthesised output signal is substantially in the phase relationship with the input signal,
• wherein the amount by which the initial value of the frequency control digital word is altered to produce the second value of the frequency control digital word is a function of the absolute value of the initial value of the frequency control digital word,
• wherein the amount by which the initial value of the frequency control digital word is altered to produce the second value of the frequency control digital word does not exceed half the absolute value of the initial value of the frequency control digital word.

A digital waveform synthesiser for producing a synthesised output signal in a phase relationship with an externally generated input signal, the digital waveform synthesiser comprising:

• a direct digital frequency synthesiser responsive to a frequency control digital word applied thereto for producing the synthesised output signal,
• an input terminal for receiving the externally generated input signal,
• a comparing circuit for comparing a signal indicative of the phase of the synthesised output signal with a signal indicative of the phase of the input signal, and
• a control circuit for applying an initial value of the frequency control digital word to the direct digital synthesiser, and being responsive to the comparing circuit for subsequently and sequentially applying progressively altered values of the frequency control digital word to the direct digital synthesiser until the synthesised output signal produced by the direct digital synthesiser is substantially in the phase relationship with the input signal,
• wherein the control circuit is adapted for producing each subsequent value of the frequency control digital word as a function of the current value of the frequency control digital word,
• wherein the control circuit is responsive to the comparing circuit determining that the synthesised output signal is in a first state in which the period thereof is greater than 1/H times the period of the input signal for increasing the value of the frequency control digital word from the current value thereof, and the control circuit is responsive to the comparing circuit determining that the synthesised output signal is in a second state in which the period thereof is less than 1/H times the period of the input signal for decreasing the value of the frequency control digital word from the current value thereof, where H is a whole number of value from one upwards.

A digital waveform synthesiser for producing a synthesised output signal waveform of a frequency for applying to an external circuit to produce a selectable frequency dependent response from the external circuit, the digital waveform synthesiser comprising:

• a direct digital frequency synthesiser responsive to a frequency control digital word applied thereto for producing the synthesised output signal to be applied to the external circuit,
• a feedback terminal for receiving a signal indicative of the frequency dependent response of the external circuit to the synthesised output signal applied thereto,
• a comparing circuit for comparing a characteristic of the signal indicative of the frequency dependent response of the external circuit with a signal indicative of a corresponding characteristic of the synthesised output signal, and
• a control circuit for applying an initial value of the frequency control digital word to the direct digital synthesiser, and being responsive to the comparing circuit for subsequently and sequentially applying progressively altered values of the frequency control digital word to the direct digital synthesiser until the synthesised output signal produced by the direct digital synthesiser is of frequency to produce the selected frequency dependent response in the external circuit.