A spurious removal filter is required after the DDS

In the previous article, we explained that when using a DDS, a filter circuit to remove spurious signals is required in the subsequent stage.
Direct Digital Synthesizer (DDS) capable of changing frequency in increments of a few Hz. Part 1

This time, we will explain the case where a spurious suppression filter is used. A kit for easy evaluation of the AD9834, the EVAL-CN0304-SDZ, is available, so we will use this kit to explain the case.

First, regarding the operating environment of the evaluation kit, the EVAL-CN0304-SDZ requires the EVAL-SDP-CB1Z (Eval Controller Board) for connection to a PC and the GUI tool (AD9834 Evaluation Software) to be installed on the PC. Incidentally, the EVAL-SDP-CB1Z can connect to a variety of evaluation kits, so some people may already have it.

EVAL-CN0304-SDZの機能ブロック

Now, let's take a closer look at EVAL-CN0304-SDZ. Please refer to the functional block diagram of EVAL-CN0304-SDZ.

First, the AD9834 output is a current output, so a voltage conversion resistor is connected.
Since a 6.8kΩ resistor is connected to the FSADJUST pin, a maximum current of about 3mA is output. Since a 200Ω resistor is connected to the output, a voltage of 0.6V will be obtained. As you can see in the figure, the IOUT terminal of the evaluation kit outputs the AD9834 output as is, so if you want to design your own filter, I think you should use this output.

On the other hand, the IOUTB terminal outputs the waveform that has passed through the buffer amplifier and filter circuit.
After AC coupling, the AD9834 output is input to the AD8014 with a DC bias applied to the midpoint of the input common mode voltage. A steep 7th order elliptic filter circuit is provided after the buffer amplifier, with a cutoff frequency of approximately 18MHz.

Actually compare the waveforms of filter circuit “with/without”

The measurement points are IOUT without filter and IOUTB after filtering. Connect an oscilloscope to each and observe the waveform.

Measurement result

You can see that by passing the stepped waveform through the filter circuit, it becomes a beautiful sine wave.

念のためスプリアスを確認してみます。オシロスコープのかわりにスペアナで観測、見事にスプリアスを除去できています。

Therefore, a spurious removal filter is required after the DDS.

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