SNA Level Variation Problem – A Solution Concept

4 Apr

As I reported in my previous post, I was blindsided by the amount of level variation the Chinese DDS modules have when tuning from 3-30 Mhz…over 10 dB. There are lots of ways to address this, ranging from a pure software approach to complex hardware re-design and/or AGC leveling loops. I think I’m going to go for a hybrid approach. Here’s how:

The AD9850’s output power is set by a single resistor that controls how much current the D-A converter can supply to the load. By varying the values of this resistor, you can vary the output power. I’ve done this in the past and you can get about 30 dB of adjustment range. Using 4 or 5 of the Arduino’s unused outputs, I’m going to select from a set of resistance values that can be switched in and out in a binary addition fashion. I think that, using this approach, I can level the RF output of the DDS ‘on the fly’ to within a dB using a look-up table programmed in the Nano module. If finer control of the displayed signal flatness is required, the remainder can be done on the SNAP! PC software side using the trace ‘normalize’ feature. This approach, if it works, should address the problem with an elegantly simple hardware mod and will minimize the loss of dynamic range a pure software solution would produce. On Monday, I’ll do some Excel simulations to see what can be done with about 4 properly calculated resistors switched by the Nano. Wish me luck…

Joe

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