Thursday, April 30, 2015

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

Sunday, April 19, 2015

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.




Saturday, April 18, 2015

BITX Modular The Backplane

I had been looking around for a QRP transceiver to build, and decided to do something based on the BITX .  The simple design with the use of a common bi-directional amplifiers for several stages makes it ideal for building a modular system.  Except for the final amplifier, I broke the basic transceiver  down to 7 basic assemblies.

1.  Band pass filter and Driver Amplifier
2.  Bi-directional RF Amplifier
3.  Mixer
4.  Bi-directional IF Amplifier
5.  Crystal Filter
6.  Bi-directional IF Amplifier
7.  Modulator\Product Detector  Audio\Mic Amplifier.

      With these modules in mind I looked at what common signals would be used by each module.  The BITX  amplifiers switch direction by supplying 12 volts to one of the two amplifiers, and grounding the supply line on the inactive amplifier.  These signals are labeled  as R and T in the BITX schematic, and are controlled by either a relay or switch.  Other modules also need switched 12 volts for both receive and transmit. Most  modules require an input and output which are swapped when going from receive to transmit.
   While the individual modules could just be mounted in a chassis, I  decided that I would build a back-plane similar to what you would find in a computer, and what I have seen in some instrumentation systems I worked on.  I incorporated relay switching of the 12 volts for the R and T signals.  I brought out all the signals to two  connectors , one that contains signals related to the receive path and the other the transmit path.  I located the un-switched  12 volts along with the transmit connector.  I  placed a ground between each signal and added bypass capacitors on the supply lines to minimize noise.
Schematic
Layout and Etched boards








Assembled Backplane
Backplane with Balanced Modulator- Audio board














 For the modules I am going with a 'Muppet' type construction using double sided circuit boards.  Hopefully the grounded solid copper layer on the back of the boards will provide adequate shielding.  For the connectors I am using  .1in. right angle headers on the modules and pins on the Backplane. 

I etched up a Backplane and the Balanced Modulator - Audio Baord using toner transfer.  After etching I used a new 'Liquid Tin' solution  form MG Chemicals that I wanted to try.  It is fairly expensive, but the results are great. The boards look nice and  solder better than just bare copper.

Now just need to check out the 12 volt and R/T switching and I can move on and build and test the Balanced modulator Audio Board.