High-Speed and High-Performance Direct Digital Frequency Synthesizer Design by Jun-an Zhang & Ruitao Zhang & Guangjun Li

Author:Jun-an Zhang & Ruitao Zhang & Guangjun Li
Language: eng
Format: epub
ISBN: 9789811672668
Publisher: Springer Singapore

Reference [23] introduces a DEM scheme with low switching glitch energy. The main idea of this scheme is that if the current input code is larger than the previous input code (more current sources are required to be switched on), all current sources used by the previous input code will still be used, only the incremental current sources will be switched on randomly. If the current input code is smaller than the previous input code (some current sources are need to be turned off), then decrement current sources will be switched off randomly. This scheme reduces the quantity of current steering switch action as much as possible, so the glitch energy can be low.

The implementation scheme is shown in Fig. 3.9. Figure 3.9(a) is a conventional selection scheme of input digital code and current source, and the corresponding relationship between input code and current source is fixed. Figure 3.9(b) is the DEM scheme. In this scheme, two registers, RegA and RegB, are set to record the starting and ending positions of the current source selected by the input code respectively. These two registers will be updated each time when the input code is changed. A 1-bit pseudo-random number is used to determine the direction of each increase (or decrease) of current source, and the update of RegA and RegB are also controlled by this pseudo-random number. When R = 1, the value of RegB keep, and RegA = RegB ‐ input. When R = 0, the value of RegA keep, and RegB = RegA + input. For example, as shown in Fig. 4.​9(b), where input = 4 becomes input = 2. When R = 1, the selected current source changes from U1, U2, U3, U4 to U3 and U4, with RegB = 5 unchanged, RegA = RegB, and input = 5 − 2 = 3. When R = 0, the selected current source changes from U1, U2, U3, U4 to U1, U2, with RegA = 1 (unchanged), and RegB = RegA + input = 1 + 2 = 3. With the DEM scheme, the corresponding relationship between the input code and the selected current source is random, and the quantity of current source increment/decrement each time is small too.

Fig. 3.9A DEM scheme with low switching glitch energy [23]

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