DS335 Synthesized Function Generator
DS335 Synthesized Function Generator
DS335 Synthesized Function Generator
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
2-2 Introduction<br />
DDS gives the <strong>DS335</strong> greater flexibility and power than conventional<br />
synthesizers without the drawbacks inherent in PLL designs.<br />
<strong>DS335</strong> Description<br />
Figure 2: <strong>DS335</strong> Block Diagram<br />
A block diagram of the <strong>DS335</strong> is shown in Figure 2. The heart of the <strong>DS335</strong><br />
is a 10 MHz crystal clock. The 10 MHz clock controls the DDS ASIC,<br />
waveform ROM, and high-speed 12bit DAC. Sampling theory limits the<br />
frequency of the waveform output from the DAC to about 40% of 10 MHz, or<br />
3 MHz. The 48 bit length of the ASIC's PIR's sets the frequency resolution to<br />
about 36 nHz. These parameters and the DAC's 12 bit resolution define the<br />
performance limits of the <strong>DS335</strong>.<br />
The reconstruction filter is key to accurately reproducing a waveform in a<br />
sampled data system. The <strong>DS335</strong> contains two separate filters. For sine<br />
wave generation the output of the DAC goes through a 7 th order Cauer filter,<br />
while ramps, and triangles pass instead through a 3.5 MHz 5 th order Bessel<br />
filter. The Cauer filter has a cutoff frequency of 3.4 MHz and a stopband<br />
attenuation of 86 dB, and includes a peaking circuit to correct for the<br />
sin(x)/x amplitude response characteristic of a sampled system. This filter<br />
eliminates any alias frequencies from the waveform output and allows<br />
generation of extremely pure sine waves. However, the Cauer filter has very<br />
poor time response and is only useful for CW waveforms. Therefore, the<br />
Bessel filter was chosen for its ideal time response, eliminating rings and<br />
overshoots from stepped waveform outputs.<br />
The output from the filter passes through pre-amplifier attenuators with a 0 to<br />
14 dB range. The attenuators are followed with a wide bandwidth power<br />
amplifier that outputs a 10 V peak-to-peak into a 50 ohm load with a rise time<br />
of less than 15 ns. The output of the power amplifier passes through a series<br />
of four step attenuators (2, 4, 8, and 16 dB) that set the <strong>DS335</strong>'s final output<br />
<strong>DS335</strong> <strong>Synthesized</strong> <strong>Function</strong> <strong>Generator</strong>