N2PK VNA - Not your Dad's grid dip meter

History of my implementation of the N2PK VNA

If you have not heard about N2PK VNA before, I recommend you to read through the set of comprehensive materials available from Paul N2PK's WEB site. From there you can follow other links to get even more information.

Paul's WEB site also has information how to subscribe to his VNA Yahoo Group mailing list, so that you could keep yourself informed about changes, upgrades, troubleshooting techniques, part replacements etc.

Since the VNA was first introduced, there have been some changes done to the schematics and I decided to take the challenge and make a board layout that would reflect all changes done to date. As the result, a set of board revisions was produced, each having its advantages. For now I decided to stick with only one of them, which both Paul and I like the most. Many thanks to Paul, N2PK, for taking time to review the layouts and providing valuable comments. Note that this layout is only a rework of the original Paul's design, I only moved some components around, extended the ground plane etc.

The original VNA board was designed to have a slow ADC and a single detector. Once Paul released an update for a faster ADC, and announced an S-parameter Test Set will probably be coming, many builders have added a second board for the fast detector to their projects. The second board is in fact the same board as the 1st one, but has only the detector and power supply components mounted. Additionally, the builders had to take care of splitting the output of the Local Oscillator DDS to feed two detectors, which required having one more board for the power splitter and additional cost. I decided to place both detectors on a same board, and add a second filter for the LO DDS. Because earlier Paul pointed out placing the second filter in close proximity to the RF DDS was found to cause higher stray coupling from the RF DDS to this output port, than to the first output port, I took additional precausions, which hopefully will reduce or fully eliminate this problem. What was done is the RF DDS was rotated 90 degree counter-clockwise, as well as its output transformer T110, and all associated components. The output of the RF DDS anti-alias filter was diverted away from the LO DDS filters to the opposite side of the board.

The board size is 6.3'' x 2.6''. It was designed to fit standard enclosures - many of the Hammond aluminum die-cast products are sized 16cm (6.3'') in one dimension. Such enclosure would still have plenty of room for a built-in power supply.

Main board top DDS area

Main board top detector area

Main board bottom

When the work was coming to the end, I realized I was still missing the reflection bridges. However, after careful evaluation of different options, and how the device could be used in normal activities, I decided not to place the bridges onto the VNA board. This is mainly because the device would lose its versatility. It is much more preferable, I believe, to have the RF output and Detectors inputs brought to the front panel, and use the external bridges. In this case they can be whatever the user wants, not necessarily the Minicurcuits T1-6T ones. Having the signals on the front panel also gives me endless possibilities in producing different testing configurations.

Nevertheless, I decided to make a reflection bridge board, too. The bridge schematics was borrowed from the VNA documentation. The board has multiple vias along the edges and traces to reduce the ground plane resistance. The bottom of the board has tinned edges free of the solder mask in case I need to solder the board directly to the enclosure, or through a finger stock. The board was designed to fit the Hammond 1590A die-cast aluminum enclosure, and the idea was borrowed from Greg W8WWV's page. Refer to his nice article on the bridge construction techniques.

This board is sized 3'' x 1.3'' and can take either N-, UHF-, or SMA/SMB-type end launch receptacles. Based on Greg's experience with the 1590A enclosure, the spacing between connectors was made 2 inch. To fully equip a dual detector VNA, two of these reflection bridges would be needed.

The bridge board easily fits the Hammond 1590A die-cast enclosure. The corners of the board need to be slightly rounded to make it sink to the center of the enclosure.

Bridge in enclosure

The board traces are 50 Ohm transmission lines, and as long as the board FR4 material allows, should be usable up to VHF. Specifically because of this the traces and the copper along the trace edge are free of the solder mask.

Hammond 1590a

-Update February 19th. - My hot air station has arrived, and over the weekend I finished most of the board.

assembled bottom

-Update February 20th. - No drugs involved, the syringe is filled with SMT soldering paste.

still soldering

Overall quality of soldering is not too bad for a first timer with SMT.

-Update February 22th. - The board is almost complete, needs only the header connectors. The top side took me three to four times as longer to complete as the board bottom.

assembled top

The small pitch ICs (AD9851 and LTC2440 ones) were a bit painful to solder. Had to use a desoldering wick to remove a few solder bridges. All other ICs came out just fine.

AD9851

As far as I can tell, there was no problem fitting the parts in (I was expecting some of them may bump into each other). Like with the original layout, the transformers leads had to be trimmed in order to fit the footprint on the board.

 

Shown below is a screenshot of the Detector 2 noise floor. The VNA board with no enclosure or any grounding, sitting on the bench, surrounded by the switching power supplies and overhead daylight bulbs showed -110dB noise up to 50MHz. Nice.

second detector noise

 

Read the Noise Test Results article here.

Learn about building a reflection bridge for your measurements here.

Go back to the Noise Test Results article and get yourself updated on the latest news on that.

View the last prototype board pictures and learn about the things discovered during testing.

Important information about building your dual detector VNA board. Documentation, the BOM, hints and tips.

See some sample screen shots done at different times.

Check the boards kits availability here.

 

Thanks for dropping by and reading this page.

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Contact: miv@makarov.ca