Wednesday, November 12, 2014

VNA fixture PCB available from OshPark.com

Due to a number of requests to share my VNA fixture design, I have made them available at OshPark.com using the following link.  Boards can be ordered directly at that link in multiples of three very inexpensively (about $11).  Drop me a note at ko7m at arrl dot net should you encounter any difficulty in obtaining them.




The tactile switches used on the board are available at mouser.com at this link.  As always, I would be happy to help you succeed in implementing this board should you so desire.

Tuesday, November 11, 2014

Characterizing Crystals

Still learning how to use the VNA.  Grabbed a handful of Crystals and clipping them to the VNA fixture described in my previous post, I collect the following information:



I am not sure that I am doing things correctly, and certainly am not yet in a position to interpret the results seen above.  More study and experimentation will be required, but it looks like I will certainly have the necessary information to design filters.



More to come...

Monday, November 10, 2014

VNA fixture

In a previous blog post I mentioned purchasing a batch of 20 MHz crystals from China with the intent of building some crystal filters.  I am facing the need to be able to characterize my new collection of eBay crystals.  I have looked at many of the suggested techniques and out of them all, I like the idea of using a vector network analyzer for the task.

A few years ago, I purchased one of these units from SDR-Kits.



I purchased this at a time when support on Windows platforms for the necessary drivers was quite sub-optimal and as a result this device has not seen much use over the years.  However, I have dug it out, dusted it off and upgraded the firmware and USB drivers, which are now nicely supported on all versions of Windows.

Now, I need a nice fixture that can be used to characterize a batch of Crystals. Leveraging the fine work of Eldon (WA0UWH) I have built a fixture that can be used with this VNA for the task at hand.


The idea behind this is to provide a couple places where components can be connected to the fixture and swept with the VNA to be able to visualize its characteristics over a wide frequency range.

The 8 pin header is provided with three ground attach points, one signal attach point and two pins on each side that connect together.  The idea is to be able to create simple networks of discrete components and be able to sweep them or to just sweep individual components.



The buttons provide the ability to short or terminate the VNA with a 50 ohm load.  As part of the calibration process when the fixture is attached, you want to remove the effects of the fixture itself from the measurements so the VNA is calibrated with a short, through, open and load conditions before attaching components to be measured.

The two resistors are 100 ohm 0805 units that are connected in parallel across the SMA when nearest button is depressed providing a 50 ohm load.  The bottom button shorts the SMA connector when depressed.

The bare copper area at the bottom of the board allows for placing surface mount components across the gap.  These can be held in place with a non-conductive clothes pin or a conductive clip as appropriate.  When measuring surface mount crystals for example, a conductive clip will allow grounding of the crystal case as the back side of the board is an open ground plane while holding the connectors firmly against the top side of the board.



Hopefully this little fixture will help expand the usefulness of the VNA in my projects.  I will share my experiences using this characterizing crystals in a separate post.

Friday, November 7, 2014

At the tone...

Fun with Arduinos and scopes in XY display mode...  Looks good on good old analogue and high end digital scopes.  Not so much on my $400 digital model.


Dual channel 8 bit DACs and a bunch of table look-up data, and a cheap RTC if you want accurate time.



Commercial versions of hardware able to produce such a display are available.  

Thursday, November 6, 2014

New Minima-like build

My good friend Wayne NB6M has kindly loaned me his Minima-like build using my controller shield for the Arduino.  His front panel is very similar to his original Minima build, but is now sporting a 20x4 display.  He has removed the reset button from the front panel and added input for paddles in anticipation of me actually finishing the integration of my keyer code to the Minima code base.



Looking at the back of the panel, we can see the Arduino Uno and my controller shield mounted on the back of the display board.  Wayne has used #12 bare copper wire soldered to the front panel to provide attach points for the Uno and shield.  My shield will be modified to provide through-hole plating and solder pads so that it can be soldered in place.

The display board has been converted to i2c with a backpack board and the rotary encoder uses pins freed up by display being converted to i2c.  The current shield design does not incorporate the proposed pins for the encoder from the discussion list, but will be modified in the final run to be compliant.


Wayne is using a pretty conventional IF strip from the Minima, but has chosen to replace the KISS mixer and BFO mixer with ADE-1 devices.  His audio section is from a pre-existing project re-purposed for this project.  The two SMA connectors connect to the VFO and BFO Si570 outputs from my controller shield.  No low pass filter sections yet.  The current configuration makes a pretty nice general coverage receiver.


I have handed off Wayne's other Minima build to Eldon so he will have a working radio to test with during his software development efforts.

Saturday, November 1, 2014

Arduino Yun

I recently was given an Arduino Yun and have been playing around a little with this guy.


It is quite a little improvement over the basic Arduino UNO.  I have only begun to explore the possibilities, but it looks like it will be fun.

Yun is based on the ATMega32u4 microcontroller, but also is distinguished by the presence of an Atheros 9331 processor (the bit under the metal can above) that runs concurrently with the AVR processor a Linux distribution based on OpenWrt called OpenWrt-Yun.

The board has built-in Ethernet and WiFi support, as well as a USB-A port, micro-SD card slot and 20 digital I/O pins of which 7 can be used as PWM outputs and 12 as analog inputs.

The AVR processor can communicate with the Linux distribution to leverage capabilities of that platform such as networking, shell and python scripts.  The AVR side of the house has no need for a separate secondary processor as is found in the UNO to handle USB communications as it is built in.

The Yun device can appear to a connected computer as a mouse and keyboard in addition to a virtual serial / COM port.

A block diagramme can be seen below of the architecture.  The bridge component facilitates communication between the two processors allowing sharing of abilities such as running shell scripts, access to network interfaces , USB host functionality and the SD card.


Out-of-the-box the Yun is WiFi enabled and it acts like an access point.  You can connect your computer to the access point and then configure the Yun network.  I joined it to my home WiFi network, rebooted it and now I can upload scripts to it over WiFi.  Here is a shot of it sitting on top of a 10,000 ma/hr battery that is powering it as a stand alone unit that I can program from my laptop connecting via WiFi.



Here is a screen shot of an ssh session to the Linux side of the device and the normal Arduino IDE side-by-side.



 It will be fun to see what new ideas will result from this enhanced functionality.

Thursday, October 23, 2014

Minima controller shield - Using second Si570 as BFO.


I have implemented changes to the Minima controller code developed by Eldon Brown (WA0UWH) and others to use the second Si570 on my controller shield as the BFO oscillator.  This change allows easy setting of your Minima BFO oscillator frequency to the desired point on your crystal filter passband curve, be that at the -3 dB point or further down the skirt.




The IF edit mode of the controller firmware will allow the adjustment of the BFO frequency.  Separate frequencies are maintained for USB and LSB.

A separate issue is regarding the calibration of both of the Si570 devices on the shield against some frequency standard.  For most folks that have a crystal oscillator for the BFO, the IF edit mode has probably been used to calibrate the dial to some frequency standard and this is fine.  However adjusting the IF frequency offset from the displayed frequency only allows you to match the value that the controller uses to whatever frequency your crystal oscillator BFO is tuned to whereas with the second Si570, you can actually move the BFO frequency in software which is not the same thing as calibrating the dial.




The calibration of the Si570 devices as I see it is a separate matter from setting the desired BFO frequency as you certainly don't want to change where the BFO frequency sits on the IF passband just to calibrate the dial.  So, I anticipate providing a different mechanism that will allow moving the VFO frequency (and BFO frequency for those using a second Si570 as the BFO oscillator) without changing the displayed frequency in order to determine the error offset for each Si570.  This error offset can then be stored in eePROM and applied automatically from that point forward to the appropriate Si570.

More to come on this topic...