Spring Update…..

12 May

You may have noticed my silence for the last couple months. Probably like you, I’ve been absorbed in the many things that happen when Winter ends and Spring begins. Yard work, graduations,  weddings, social obligations, domestic projects, vacations, etc. I know I left in the middle of a couple unfinished projects, so I think it’s time to to stop and discuss where we are, how we got here, and where we’re going.

This site came about as a result of my quest for a portable QRP radio that I could use for camping outings and also as a platform for my never-ending need to experiment and “improve” things. I thought the Minima project might be it, but after considerable time spent analyzing that design, it became clear that the basic architecture is flawed and the project has been stalled for some time. I decided to take some concepts I’d been noodling for a long time and go off and develop my own radio called the “Bus Switch Special“. I got pretty far into that development and somehow decided the world needed a better way to characterize crystals for IF filters, so I started writing about that. This led me to resurrect a scalar network analyzer I had developed in 2003 and implement it with modern component modules. This became the SNAP! project on this site. I got SNAP! to the point where it needed just a few more days of my time to be able to release all the schematics, firmware and software to the web site. Then Spring hit, as mentioned above. During that time, I became aware of the very nice X1M QRP transceiver available fully assembled for under $300. I purchased one and I’m now fully involved at experimenting with how to ‘improve’ that.

That’s the way it is with me…I can’t seem to control myself. I go from project to project, always moving on when I get to about the 80% point. Don’t get me wrong – I fully intend to finish up the SNAP! project and eventually get back to the BSS-1, but right now, and probably until Winter makes me housebound, I’ll probably continue my affair with the X1M.

 

Stay tuned…

Joe
W3JDR

 

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SNA Output Level – Update

10 Apr

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

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

SNA – A Disappointing Discovery!

4 Apr

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

Trace Calibrate & Print Preview Features

30 Mar

I got these functions going again today. They’ve always been in there but I had them turned off while converting the more important code features. Basically, “Calibrate” is a menu item that let’s you specify a trace slope and trace offset so you can calibrate the screen display to your particular log detector.

Calibrate

The “Print Preview” feature copies the trace display into a print window with all the instrument setup parameters listed below, as well as an area where the user can type comments. When you press the “Print” button, the display goes out to the default system printer. There’s also a “Save Picture” button that saves the trace display window as a color .bmp file.

Print

I’m almost ready to release alpha test versions of the software and firmware to the download site. Only thing holding this up is a hardware issue where RF is either radiating or conducting from the DDS module’s clock into the log detector, preventing it from going down below about -40 dBm.

SNAP! Lives!

29 Mar

Today I got around to marrying the SNAP! firmware & software systems to my breadboard hardware system – it lives!

There still some work to be done on the hardware getting the zero-signal baseline voltage down where it belongs and on the SNAP! software system to get the log display scaled properly, but we do have functionality.

Here’s a screen capture of a 6-pole crystal filter I made about 25 years ago. Display-line markers are turned off for now. You probably can’t tell by looking at it, but the log range is a bit exaggerated and the filter is actually much flatter than it appears. I’ll be working on this.

7372 khz 6P xtal filter

SNAP! Coming Soon

24 Mar

I now have the SNA firmware and software to a point where I’m almost ready to do an Alpha test release after I confirm that it still works with the hardware. This new software system will be called “SNAP!”, for Scalar Network Analysis Program”. SNAP! software is feature-rich and includes two on-screen X-Y display lines with digital readouts as well as an on-screen spot marker with readout. All screen colors are user definable and there’s a screen-print function that lets you print out your sweep with user-comments added. You can also save the on-screen display to a file and load it back in later.  Coming in future releases will be a  ‘normalize” feature that corrects for non-flatness in the hardware’s amplitude response as well as a calibrate function that allows you to exactly match the display to the log detector’s slope & offset. All of this functionality is already in there from 2003, but it will take some time to adapt it to the major changes I made to get it working with the new hardware. The Arduino firmware will also be released as a .hex file and the protocol is already on the web site for developers who wish to adapt their own hardware to the SNAP! software system.

Once I get the initial releases of the software & firmware out, anyone who wants to breadboard the very simple hardware can have a go at it. Then I plan to begin development of a PCB that fits inside an Altoids tin.

As usual, stay tuned…