Monday, March 30, 2015

SNA JR



I started working on the SI5351 antenna analyzer that I was playing with.  I wanted to check different low-pass filters to clean up the square-wave out of the 5351, and needed a simple way to do this.  I had been following the Poor Hams Scalar Network Analyzer group on Yahoo.  I even purchased a set of boards, but had not built them yet.  I  wondered  how hard it would be to make a very simple scalar network analyzer out of  the AD9850 antenna analyzer I had built.   I really wanted something small to use for checking bandpass and other filters without having to connect to a PC.

Basically a SNA compares the signal going to a Device Under Test with what comes back from the DUT over a frequency range.   Most of the circuits I have seen use some sort of a Log amplifier-detector to measure the return signal.  I have a couple of  8307 log detectors, but from everything I had read they would require a lot careful layout and shielding to get it working  in an Altoids tin along with the rest of the circuitry.  And probably the use of surface mount components.

I decided to try just use two basic diode RF detectors and changed the amplifiers so I could adjust the gain.  I use one to measure the direct output of the 9850 DDS module, and the other for the output of the DUT.   I kept the same control function as in the Antenna analyzer  A short push on the encoder button starts a sweep of the selected band.   Once a scan is done you can use the encoder to scroll through the sweep.  I display the frequency and compute the return value from the DUT  in db. relative to the output of the DDS module.  The USB connector is available and different start and stop frequencies can be entered if needed when working with IF stages.

After a simple adjustment of the amp gains for equal values with the output looped directly to the input, I was measuring nearly 50 db  loss with the loop-back removed.  Just using some standard value resistors, in a pi attenuator I got a very nice looking sweep that was within a few db. of the 40 db. I had designed it for.  Since I only used standard value  5%resistors, I though this was good enough.




Then I used  the program ELSIE to design a 14mhz low-pass filter, again used standard values for L and C that I had on hand .  Really happy with the results I got.






Finally I grabbed 3 crystals out of a bag without checking frequency or other parameters, and  threw together a basic crystal filter.   Used the USB interface to  set the sweep range, I was  really pleased with the results I was able to obtain. 

 The software still needs a little tweaking and a couple of additional functions I want to add, but I think this will be a very nice tool.   I will also see about adding a buffer amplifier for the DDS.  One thing I like about this method, is that I can directly measure the gain/loss of a filter or amplifier.  Trying different buffer amplifiers I fed them through an attenuator, and could measure the actual gain of the amplifier over its frequency range. Amazing what you can stick in Altoids tins, even if I to stack two so I could include a battery pack .


This project was put up on the Solder Smoke Daily News blog, and linked by Hackaday.com.    From the number of page hits and comments there appears to be quite a  bit of interest in building the SNA JR.  The most interesting comment I saw was.


Have you ever stopped to think how many electronic projects would never have existed if Altoids had not been invented?


So for those interested in building one of their own, I put the schematic , Eagle cad board layout , and Arduino code in a public Dropbox folder.

https://www.dropbox.com/sh/clvgswb3u6jpdvj/AACCLvg37CQ-MeHXid6fMcLda?dl=0

Everything fits on a single sided circuit board, with only one jumper for 5 volts to power the op-amp.  Hardest part is stacking everything to fit in the Altoids tin.
Since this was the prototype I did not want to solder the Nano and DDS modules directly to the board.  I used a couple of cut down 40 pin low profile IC sockets for these devices.  I then shortened the pins on the Nano and DDS by about half.  This gave me clearance for the 128x128 TFT display. The only other thing I had to do was cut the heat sink tab off of the 7805 to fit in the tin.



The two gain adjusting potentiometers were mounted on their side and glued down to the board.
Slots were cut in the Altoids tin to allow adjustment, and a also a slot for access to the USB connector on the  Arduino.



I stacked this on top of another Altoids tin which holds the 2 cell battery holder for 3.7 volt Lithium batteries.  There is a SPDT slide switch that selects either battery power or external power through a jack on the side of the tin.

All controls are done through the Rotary encoder and push-button.
When displaying the SCAN screen a short push on the button starts a scan.
After the scan is completed  use the rotary encoder to scroll through the waveform.
Holding down the button for more than a second switches to the next frequency range.
If keep it held down it will continue to cycle through the ranges.

During a Scan, if you hold the button down it will display an alignment screen.
This sets the frequency to the beginning frequency of the selected range. Displays the  output and return voltages and the difference in db.  To align the SNA,  place a jumper cable from the input to output connector.  Select the frequency range you want, start the scan and hold down the button.  Then adjust the two potentiometers for equal readings around 2000.  It is fairly easy to saturate the amplifiers with a readying of 2046 so keep it down around 2000.  For most cases you can just adjust it on the 1-40 Mhz. range.



 In the latest version of software I have added settings options that allow me to remove the PC interface.  It will probably not be incorporated in the final software.



Still working on the software and some hardware refinements.  The schematic shows a buffer amplifier that I plan on trying, but have not built a board with that yet.
I will keep this updated as changes are made.




After the last local QRP club meeting I was asked about the possibility of making kits available.  Still trying to decide if I want to go this route, or just make the design for boards and software available.  If anyone would be interested in a kit of this project drop me a e-mail at duwayne@kv4qb.us   Depending on the response I will decide which way I will go.










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