| 摘要 |
A self-tuning filter that is well suited for use as a output digital filter in a direct conversion delta-sigma transmitter is constructed as a high pass finite impulse response filter (18) having a cutoff frequency of twice the desired carrier frequency. The filter is clocked using the same clock as used for the commutatation within the transmitter (11, 12, 13, 14, 16). The aliasing effect of the digital filter produces a passband centered around the carrier frequency which allows the information contained in the spectrum around the passband to be transmitted, while effectively filtering out the quantization noise produced by the commutation. When the commutator clock frequency is changed in order to change the carrier frequency, the passband automatically moves to track the new carrier frequency. The output filter may be constructed using series connected flip-flops (30(1), 30(2), 30(3), 30(n)) with analog taps (32, 33, 34, 35, 36, 37, 38, 39, 30, 41) and an analog summer connected to respective Q and Q outputs, thereby producing an analog output.
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