Main Functions and Characters:
  -  Output 2 sine wave and 2 square wave
     AD9850  0-40MHz
     AD9851  0-70MHz
     The wave increases when the frequency more than 20-30MHz, the waveform will be unclear.
     Square Wave     0-1MHz
  -  Low-pass filter with 70MHz, so the waveform better than SN.
  -  Parallel and serial data input can be selected by a jumper.
  -  The pin of produced DA benchmark are lead out, convenient for adjustment and application of the output waveform.
  -  The comparator of benchmark input voltage generated by variable resistor, adjust the resistance can get different square wave.
  -  AD9850 module adopt ACTIVE CRYSTAL with 125MHz, AD9851 module adopt ACTIVE CRYSTAL with 30MHz.

The main difference between AD9850 and AD9851
  - AD9850 clock frequency up to 125MHz, the AD9851 clock frequency up to 180MHz. Output maximum frequency AD9851 than AD9850.
  -  AD9850 do not have 6 times frequency, but the AD9851 have.