Building a prototype Dual Detector VNA

Getting closer to completion of the VNA board

What a nice feeling when after many late night testing and tweaking, desoldering the components and soldering them back onto the board, making thousands of Google queries, time comes to move on. All removed during multiple tests components are now back where they belong to, the shack desk is clean and ready to receive a fresh pile of jumper wires, bobbins of solder and flux bottles.

What I noticed a while ago, was that working on a project is much easier, and the project has more chances to be finished if you build it the way that gives fast and easy access to its functional parts. Normally this translates to installation of peripheral connectors to get the signals in and out of the boards instead of soldering the wires permanently. I now try to follow this rule even when I build circuits on the pegboards. Takes a few seconds to get the board out for parts replacement, to clean the flux, drill holes, or show to your colleague.

Same approach was used with the N2PK VNA dual detector board. In addition to the SIP row connectors existed in the original design, I wanted to interface the RF signals by terminating them on the board with the SMA or SMB type RF connectors. The benefits received were hard to overestimate, especially when working around the enclosure. The board now is pretty much finished, and I posted a few pictures of it below. You may find a few extra components installed, and a few of them missing, but that were the ones needed only for the experimentation, or which were not critical to my use, such as the master oscillator buffer IC and its output connector, which I did not install at all.

 

Download a bigger picture of the board done with a higher resolution. Sorry, that was the most my old Sony camera was capable of.

Observations and changes to the design

The first tests I did with the board showed noise level at a -100dB mark. Mounting the board in the enclosure improved this figure by 10dB dropping it down to -110dB.

It appears the board and ambient temperature changes contribute the most to the detectors offset drift. The offset drift affects calibration. This mainly affects low signal level measurements and may not be that critical for most of measurements done in transmission mode, but you have to be aware of that. My experiments showed that heat removal, such as usage of a big heatsink or a fan, may not be the best method to fight thermal drift. In this case the drift still exists, slowly rising with time (for hours). Instead, use of a small to moderate size heatsinks for the AD9851 ICs and the master oscillator gives much better drift curve, which rises fast during first 20 minutes, and then the change stabilizes within a microvolt or two.

In order to help the builder to secure the DDS and master oscillator heatsinks, I will update the layout with two mounting holes. They will be located to the left of the DDS ICs in the ground plane area. The particular methods of mounting of the heatsinks is left to the builders engineering skills.

Credits

I would like to thank the people who helped me in numerous ways to get this prototype board running. Here is the names in alphabetical order:

• Edgar Brown, N6OU
• Milosh Rankovic, VA3VEF
• Paul Kiciak, N2PK
• Tim O'Rourke, W4YN

Contact: miv@makarov.ca