SNA Jr Software update two

I changed the display from a 1.44" 128x128  TFT ti a 1.8" 160.x128 TFT.  Unfortunately they use different driver chips so I had to make some changes to the software , mainly a different driver library and the position of text on the screen.  While I was doing that I also added a couple of
functions to the system.

New circuit board layout moved parts around to make room for the larger display, and added a ERA-3 MMIC amplifier  with attenuator to boost output and present 50 ohm to the DUT. 

After I got the hardware working with the pickup coil to use the SNA Jr. as a Dip Meter, I decided to add a DIP function to the software.  It does a sweep from 1 Mhz. to 50 Mhz. with 50 KHz steps, and looks for the deepest dip in the signal from the RLB.  After that sweep it sets the  range  to 1 MHz.  below and above the dip frequency and then  does a normalized sweep of that frequency range.  Much faster than sweeping through in several ranges.

In the existing sweep functions, they are a single sweep triggered by a short push on the encoder button.  After the sweep is started pressing and holding the button down will enter a setup mode, where you can make adjustments to the gain pots for the sweep range you are using if desired.  Because I added the normalizing  function, I had to do a complete  sweep and then display the data.


From comments on  QRP-L,  sometimes a continuous reoccurring sweep would be better. This would be useful for adjusting filters, and other alignment procedures.

I added a FAST sweep function that does sweeps at around 1 second per sweep. To get this rate I had to remove the setup functionality, and only did one reading at each point instead of using multiple readings to reduce noise. In this FAST mode I erase the old data point and draw the new datapoint as it is acquired.

Still waiting on some parts to arrive, before I can finish testing the updated circuit board layout.  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.

SNA Jr as a Dip Meter

One of my projects needs a 70Mhz. Low Pass Filter.  I plan on using air wound coils for its construction.  Only problem is that my RLC meter does not measure down to the inductance values I need.  I remember reading somewhere that they used a RLB with a pickup coil to get similar results to what you would have with a Grid Dip Meter.


I made up a small pick up coil using an old plastic 35mm film canister, 4 turns of wire and a SMA connector. I connecting it to the RLB and SNA jr.  I quickly wound about a few turns of wire around a old pen, and soldered a 330pf cap across the coil.  The film canister is large enough that I could insert the coil and capacitor inside for maximum coupling.

 Doing a couple of wide band sweeps in the normalized mode I saw a small dip in the signal around 10 MHz.  Narrowing the sweep I found a nice dip and measured  9.259MHz.  Since I knew the value of the parallel capacitor, I plugged those values into an online LC resonance calculator.  Came up with a value of 1.01uh for the coil.  Squeezing and spreading out the coil I came up with values that looked appropriate.  With this method I should be able to wind the coils I need for the LPF.  And have another tool to play with.



UPDATE:

The   pick-up coil worked well for air wound coils, but I wondered if I could modify it for doing toroid coils?  Looking at the manual for a Grid Dip Oscillator, I saw that for toroid coils they used a coupling loop around the GDO coil  connected through a couple of turns through the toroid.   I wound a few turns of wire around the end of the film canister next to the pickup coil ,and soldered the ends to a couple of pins from a good quality IC socket.  I knew these pins would be a good fit for the connectors on the solderless bread-board jumpers I have.  The connectors on these jumpers are also small enough to fit through the toroid with windings in place.

I  wound up a toroid coil and soldered a 47pf capacitor across it.   Running the bread-board jumper through the toroid a couple times I connected to the pins on the modified pickup coil.  Did a wide band sweep and saw a little dip around 20 Mhz.  Narrowing the sweep I was able to find a dip at19.819 Mhz  This dip is not as deep as with the air wound coil, but is deep enough to use.

Using the LC Resonant frequency calculator, I got a value of  1.37 uH.  Going to a toroid coil calculator for the type of core I was using I entered the value of capacitor and number of turns in the coil.  This gave me design values of 20.18 Mhz. and 1.26 uH.  Looks like this simple pickup will work for both air-wound and toroidal coils.

SNA Jr Software Update

When I first did the SNA Jr, I used my existing Antenna Analyzer software.  After playing with the Chinese SNA, I decided to do a rewrite of the  SNA Jr software to incorporate some of its features.  The major change was to add a settings screen, where I can set the start, stop frequencies, step size, and change modes. Since I added this I removed the PC interface.
The original sweep mode gave a relative reading, showing the  difference between the output of the SNA and the input from the device under test.  This has the advantage of directly showing the gain or loss of the DUT.

  I decided to add an additional sweep mode which normalized the  data before it is displayed, this  makes it easier to measure filter bandwidth.

 

  
 One thing I liked about the  Chinese SNA software was the VFO  (signal generator) mode with its RF voltmeter.  I added a similar mode to the SNA Jr with the voltmeter shown as both a bar  and numerical values.  Voltmeter values are still relative to the output of the DDS and shown as a db value and percentage of full scale.




Navigation is the same as in earlier the earlier version.  a long press switches between display and settings screen.  When on sweep screen, a short press starts a new sweep, holding the button while in a sweep goes to a screen to adjust levels.  Using the encoder scrolls through the sweep.
In settings mode a short press moves from one setting action to another, rotating the encoder changes selected setting. In VFO mode the rotary encoder changes the frequency by the step size.  A short press on the button toggles through different step sizes.

In the process of re-doing the circuit board layout to add a MMIC amplifier and attenuator to boost output and reduce loading by the DUT.  Also shifting some parts around to give a little more room so I can go to a slightly larger 160x128 display instead of the 128x128 I am presently using.
I will post the software and board layout when finished.  If anyone is working on building one, drop me a e-mail at duwayne@kv4qb.us and I can send you the software update.

 

Update

After I got the hardware working with the pickup coil to use the SNA Jr. as a Dip Meter, I decided to add a DIP function to the software.  It does a sweep from 1 Mhz. to 50 Mhz. with 50 KHz steps, and looks for the deepest dip in the signal from the RLB.  After that sweep it sets the  range  to 1 MHz.  below and above the dip frequency and then  does a normalized sweep of that frequency range.  Much faster than sweeping through in several ranges.


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.

BITX Modular Audio Baord

After looking at documentation for several versions of the BITX and others based on the original design, I decided to use the circuit of the BITX version 3b.  First board I started with was the
Audio /Balanced Modular.  I am using 'Muppet' style layout for the circuit boards, using double sided circuit board material.  This should give a nearly solid shield between stages. 


Most of the 'Muppet' boards I have seen use Express PC software to do the layout, but since I am more familiar with Eagle cad I went with that.   Only drawback was that I had to manually layout the ground pads. 


After playing with the layout for a while I realized this will be a fairly crowded board, and even had to stretch it out a little to get everything to fit.  I did the layout as if I were going to use 1206 size surface mount components, and then placed a .1" square over each pad.  This will give me the option of using either leaded or SMT components if I desire..


After etching the board I installed the connectors and checked how well it fit on the back-plane.
The board fits nice and comes down to the top of the back-plane, hopefully this will give adequate shielding between stages.


Not to start stuffing and testing the board.

Testing the Return Loss Bridge

One of the functions available in the software for the Chinese SNA is the ability to measure SWR using a Return Loss Bridge.  When I was laying out the attenuator boards I had room to include a RLB.  

The layout uses edge mount  SMA connectors and surface mount resistors.  I allowed space for 2 parallel resistors to get the correct value if you need.  The plan for the RLB I used had 51 ohm resistors.  You can get 51.5 ohm by using a 47 ohm  and 56 ohm in parallel.   I had some 100 ohm resistors so used 2 of them in parallel to get 50 ohms.

PCB layout

 

Finished RLB

I had just put up a new multi-band fan dipole, thought that would be a good test of the RLB and software.  I used my FoxDelta antenna analyzer to align the dipole. Doing a full scan from 1-40 Mhz., I got the following SWR Plot.

FoxDelta Analyzer Plot

  

Using the SNA software in the SWR mode I got the following.  Since one display uses a log  scale  and the other linear for the vertical axis they look a little different, but after checking the values they are within a couple percent of each other.

Just for the heck of it I hooked the RLB and antenna to the SNA Jr. and came up with this as a result.

Since I do not normalize the data on the SNA Jr. I had to manually compute the normalized values and use a Return Loss- SWR table to find the SWR at selected frequencies.  The values were also within 10% of those on the FoxDelta Analyzer.  Looks like the SNA Jr. with the RLB would be useable as a portable instrument for tuning antennas.

SNA JR and Return Loss Bridge

Using the Chinese SNA

First thing I had to do after I received the Chinese SNA was its internal calibration.

To calibrate the system you will need an in-line attenuator, 6db. if calibrating linear or 40db. for log.  Using Eagle cad software I did a quick layout for a pi-attenuator.  Since the layout for the attenuator was so small I had room to put 7 attenuators on the  blank along with a Return Loss Bridge.  I have a fairly large selection of 1206 size resistors, but still need to parallel some to get the correct values for the different attenuators.  I did the layout to allow space for 4 resistors for the shunt resistance , and 3 for the series resistance.   After etching a set of boards, I computed the required values using the pi-attenuator calculator at 

http://chemandy.com/calculators/matching-pi-attenuator-calculator.htm 

Return Loss Bridge and 40db attenuator finished

I built up the required 40db. attenuator and a couple of other values. Looking at the software, there is a function to measure impedance of the device under test.  Using one of attenuator boards I fabricated the inline 50 ohm resistor required for this. I also built up the small Return Loss Bridge I had etched.


Calibration is very easy, go into the calibrate mode and follow the instructions on screen.  Place the 40db. attenuator between the input and output connector and run one sweep,  then remove the attenuator and connect the output directly to the input and run another sweep. After this is finished the calibration data is saved to a file.

I have the Eagle files and a PDF of the boards ready for toner transfer in the Drop box at
https://www.dropbox.com/sh/p4sqp2g8gu1gey4/AACrHZ8B3eqOm6-CRfIcDn4Fa?dl=0

The Chinese- Polish- German SNA (Scalar Network Analyzer)

I had finished the SNA-JR,  because of  its size and ease of use I was very happy with it for doing basic filter testing.  There are some things that I want to do that would require a more capable piece of equipment.  I still have the PHSNA boards that I want to build someday.
Looking for something else on ebay I came across a listing for  "NWT70 frequency sweep meter 0.05-85 MHZ with 0 to 50 db attenuation" .  Looking at the description and specifications, it appears to be a SNA.  

Searching on the model number and name of the software in the listing I came up with a couple of links that look to be close to the item in the listing.
First is a Polish ham's site.
http://sp9xuh.pl/index.php?p=1_6
This is a SNA based on an ad9851 DDS and an ad8307  log detector similar to the PHSNA.  Good documentation including schematic ,circuit board layouts, and firmware.

The second site is from a German ham.
http://www.dl4jal.eu

This is a software package that is used to control the SNA.  Looking at the  documentation in German (with considerable use of Google translate) it looks like it had most of the capabilities I was looking for.  Basic functions are a frequency scan, signal generator mode, power meter.  Additional functions included some calculators for LC values, R and LC impedance matching circuits.  Using an external Return Loss Bridge, the software has a function for measuring SWR, and correct  SWR for  the feed line. Adding a 50ohm resistor in series with the output it can compute the impedance of the device under test.  It also looks like you can add a circuit like the PHSNA measurement receiver.  There is a IF offset value that can be set that should directly display the actual frequency when in the spectrum-analyzer mode.
Looking around ebay I found several similar items that have different frequency ranges, one going up to 3.4Ghz.  I ordered the 85Mhz. version., it took about two weeks to arrive from China.  After running the internal calibration procedure, it appears that the functions I desired worked the way I would expect.  Still just starting to play with it, and looking on line for a more documentation.  But so far it looks like the PHSNA boards are going to sit around un-built for a little longer.  Will post updates as a I get more familiar with the unit.