Tuesday, February 21, 2012

Reflection Bridge with Type-N connectors

I always try to avoid using "between-series" adapters - there are so many out there with questionable quality. Sometimes these adapters are just the "necessary evil". In an attempt to reduce the need for adapters, I built 3 different reflection bridges for my N2PK VNA - BNC, SMA and Type-N.
From a mechanical point of view, mating a bridge directly onto 2 fixed distance ports presents a challenge - the connectors must be aligned perfectly. These are precision connectors and even the smallest misalignment will cause stress to the pin/receptacle, uneven wear and possibly damage. Using Type-N flange connectors on the reflection bridge will form too rigid junction and they are difficult to install with such great precision. On the other hand I prefer somewhat rigid connection and don't want to use flexible coax as it introduces phase instability when bent or twisted. A few tenths of the millimeter lateral "play" in the connectors will be enough to take care of small misalignment.
To build the Type-N bridge I used a pair of male Type-N connectors (solder type, Digikey p/n ACX1132-ND) on ~3 cm pieces of semi-rigid coax (RG-402 - solid copper tube shield, not the hand-conformable type). The coax is inserted in a brass tube (very slightly larger than the diameter of the coax, 2.1 cm length, K&S Engineering 3/16 x .014 Stock #129) that goes thru the wall of the aluminum enclosure and it is soldered inside to a brass plate. A second, larger diameter brass tube (2 cm length, K&S Engineering 7/32 Stcok #130) goes over the small diameter tube but does not go thru the wall. The small diameter tube provides stress-relief on pull action, as one end of the RG-402 is soldered to it. The larger diameter brass tube goes over the solder collar in the base of the male Type-N connector and it is compressed between the enclosure and connector, providing stress-relief on push action.
The whole assembly might be a little over-engineered but it is very sturdy and gives me the few tenths of millimeter lateral flexibility at the connector end without being too flexible. It, also protects the RG-402 from accidental permanent bending and damage.

The small diameter brass tube goes thru the wall and together with the protruding copper shield of the RG-402 coax is soldered to a brass plate and the PCB's ground plane. It is critical that the holes in the aluminum enclosure are just big enough for the small diameter brass tube to be inserted with no "play"as the flexibility needs to come from the exposed length of the brass tube. An SMA connector is installed for the DDS source termination (needed for improved low frequency measurements).

Heat-shrink tubing color-coded the IN and OUT of the reflection bridge. For the DUT port I installed a high-quality Amphenol 131-445 / HP-Agilent #1250-1404 female Type N connector. This instrument grade connector has a SMA(f) on the back. A corresponding SMA(m) with really short pigtail (~4-5 mm) was used for connection to the bridge. I had to make a cut-out in the PCB to accommodate the SMAs.

Because of the custom Type-N connectors, I was able to fit the bridge in a lower profile enclosure - same type I used for my RF-IV sensors - Bud Industries CN-5701 (Digikey p/n 377-1512-ND)

6 comments:

Unknown said...

I love your labels, your Vna for example looks awesome. How do you make them ?

Cheers
Michael OE1MIS

Andrey E. Stoev said...

I designed the front panel graphics, using Corel Draw software. The method for making the front panel is described in my post from Sept 9, 2009 - http://blog.kotarak.net/2009/09/n2pk-vna-power-supply-and-enclosure.html

It is fairly easy way to make graphics for front panels up to A4/Letter size and the results are excellent! The 3M laminating sheets (top protective layer) are sold in two different versions - glossy (the ones I am using) and matte finish (which is also nice but not as dirt-resistant as the glossy). Just a matter of personal preference.
I am not sure if exactly the same materials are available in your country but you might be able to find similar stuff.

Unknown said...

Thank you, I will try that.
In our global village fortunately we are no limited to local supplies any more. :)

It is interesting, we have a very similar taste in selecting the hardware we build, the inside of my N2PK looks exactly like yours, I also built two (one small and one big), but unfortunately the outside of mine looks way uglier,... Hopefully I can change that now.

My next project is a 0-3 Ghz sa based on Scotty Wsprowls labour.

Cheers
Michael OE1MIS

Andrey E. Stoev said...

"Great minds think alike"
Good luck with your project!

W9IP said...

I'm joining this thread late, sorry. Where can I find the schematic for the bridge?

Andrey E. Stoev said...

Description of the Reflection Bridge can be found here:
http://www.n2pk.com/VNA/n2pk_vna_pt_2_ver_b2.pdf

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