SNA Output Level – Update

In my last posts, I reported that there was a severe roll-off in RF level that I hadn’t seen with the AD9850 in the past. I attributed it to the cheap Chinese Ebay DDS module and went about designing a workaround. I spent all week procuring, mounting and programming a MCP4018 I2C DigiPot to control the DAC current (and thus the output level) under firmware control. My plan was to build a lookup table that automatically programs the DigiPot ‘on the fly’ to correct level variations.

Well I got almost all the way through that process and was looking at how well the DigiPot controls the RF level when I discovered there was no significant uncorrected variation anymore! Well, it turns out that my initial testing was done with a random scope probe that I pulled out of a pile of old probes I had nearby. This probe had a significant high-frequency rolloff that led me to the wrong conclusion. When I measured again with a good 100 Mhz probe and confirmed with a spectrum analyzer, the rolloff was only a couple dB, easily within the correction range of the SNAP! software’s ‘normalize’ feature. I now have a couple of brand new probes on order and all the old ones are going in the trash.

I think I’ll leave the DigiPot in the hardware design as a build option and maybe use it as a software-controlled RF output level adjustment. Meanwhile, I’ll finish building the whole rig into the Altoids tin and call it a day on the hardware design. Then I can move on to getting everything documented and released to the web site.

Joe

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SNA Level Variation Problem – A Solution Concept

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

SNA – A Disappointing Discovery!

I spent a good deal of time yesterday getting the SNA hardware rebuilt on a board that fits inside an Altoids tin. I hoped that on Monday, I could release an alpha version of the hardware, firmware and software designs so that others could reproduce it.

Today, while cleaning up what I thought were final details, I decided to check the flatness of the DDS module from 3-30 Mhz. Oh no!!! The output level of these cheap Chinese modules seems to vary by almost 10 db, dropping off severely as frequency increases! I’m not sure where the rolloff is occurring, but it seems to be in the lowpass filter. Now, I know that the 9850 chip is much better than this, as I’ve used it before. The last time was in the first incarnation of the SNA, using a DDS-60 board from AMQRP.

So now it’s back to square-one for a while to make some investigations of a good Analog Devices chip vs what came in the cheap Chinese DDS module from Ebay – maybe it’s a knockoff chip? This could take a while. Meanwhile, I’m going to compensate for it using the SNAP! software’s “normalize” feature. This feature takes out generator level variations so the displayed output is flat. I’m sure it can correct the display flatness, but it will result in lowered dynamic range as the RF level drops off.

As Gilda Radner used to say on Saturday Night Live, “It’s always something!”

Joe