Wednesday, September 9, 2009

N2PK VNA - Power supply PCB and front panel

I received the PCBs for the PS board from the board fab house (Sunstone circuits) and I must say that the fabrication quality is pretty good. The schematics are based on the N2PK/OM3LZ power supply board with very minimal changes on my part. Basicly, there are two voltage regulators: +12V Low-Dropout linear type and +5V switching regulator, LC noise filters for each regulator and two "crowbar" type over-voltage protection circuits combined with over-current and reverse-polarity protection. In other words - the power conditioning is very solid and provides a good protection for the VNA board from a number of potentially dangerous events.
There is an additional +9V regulator circuitry (installed on the bottom of the PS support plate) for the Accessory connector - it is used to power transverters, S-parameter test fixture or the RF-IV sensor.
The PCB layout design was done with Cadsoft Eagle. I started with the OM3LZ board as a reference but at the end I changed the layout a bit. These pictures are of my ver. 1.0 board. The final version of the PS PCB is ver. 2.0 and that one is even more compact and with a smaller footprint. Not sure if the 2.0 board will ever see daylight since I am all set with my PS needs for now.

The PS is done with mixed thru-hole/SMD technology - first the SMDs are installed and then the rest of the components. I added pads for an SMD LED on the +5V line and a current limiting resistor for it. This LED serves as a reminder that the unit is powered when operating with the covers off and it is totally optional to install.


On the bottom side I have a space for an optional LDO 12V linear regulator (Digikey p/n 576-2206-ND (Mircrel MIC29150-12WT)) . This allows for an external power supply with a wide range of voltages - typically filtered DC 13V to 24V. Because of the extremly low drop-out voltage ( 0.35V @ 1.5A) of this chip, the VNA can be powered by a standard 13.8V PS and still be in regulation. For portable use I'll probably power the unit from a 2 x 7.2V Li-Ion battery packs. This regulator can be omitted and bypassed with a jumper but then extra attention needs to be paid to the input voltage in order to prevent the over-voltage protection from triggering (at the price of a blown fuse). There is a couple of extra bypass capacitors associated with this regulator.

Unfortunately, it wasn't cost-effective to put a silk-screen for the SMD components on the bottom but I don't think it is a big issue.

The front panel of the VNA. First step is to drill all holes in the panel. The graphics for the panel are actually a "sandwich" of two layers - printed layer and protective layer. I used Corel Draw to design the graphics layout (any other vector graphics software like Adobe Illustrator will work too). The layout is then printed with a Color Ink Jet printer (printer driver: best quality, transparency) on a sheet of "Avery Clear Full-Sheet Labels" (Avery 8665 or 18665 (better) from OfficeMax/Staples). The aluminum panels must be cleaned, de-greased with alcohol and dry. During application of the printed layer, one should be careful not to smudge the printing or leave fingerprints and must try to prevent any air bubbles from forming at the same time. (I used a piece of the base paper from the label stock (the glossy, waxy side as an "applicator", rubbing the sheet while applying it to the surface). The graphics must be carefully aligned with the holes on the panel during application. Then, the print layer is protected with a second layer of a durable clear self-adhesive plastic sheet - 3M Scotch Laminating Sheets (LS854- 10M or -10G (the last number shows how many in a package, M for Matte, G - for Gloss). After applying the first (printed) layer, compressed air and soft brush were used to remove any dust particles, then applying the protective layer is done the same way - applied slowly while watching out for air pockets . (Remember - it is a "one shot" deal - if something goes wrong during the application of the laminating layer there is no going back - you have to start over with new print layer).

The resulting surface is smooth, dirt and scratch-resistant and because the printed layer is transparent, the front panel has almost the same brushed aluminum/metallic look as the enclosure. Instead of using transparent print layer, a solid-color stock can be used too, but IMHO it looks "flat" and not as attractive as the natural metallic look. Once the two layers were "sandwiched" and pressed well together, I use scalpel blade to carefully cut out the openings and the excess around the edges.

The finished panels came out very nice and professionally looking - practically "commercial product" grade. I am really happy with the results - I think this will be my method for printing front panels from now on.

While looking for a front panel layout solution, I came across an interesting product - Ink Jet printable laptop skin (sold in Office Max). It is a white, self-adhesive vinyl sheet and I think it could be used for front panel labeling as well but I really wanted to preserve the aluminum finish look so I opted for the see-through label sheets. Another possibility is to use one of the products by http://www.texascraft.com/

The front panel is installed on the enclosure along with the BNC connectors and LO jumpers. I decided to color code the connectors because of their number on the front panel. It will be easier to work with the VNA and keep track of all connections.

All of the RF interconnects are done! Initially, I was going to use microwave semi-rigid coax as it provides the best shielding and phase stability but this stuff is too exotic (read: difficult to find/install/work with/using specialized connectors) so I opted for a special mil/aerospace version of the RG-316 by Semflex called SI316. The regular RG-316 is double-shielded with two silver-plated round braids. The SI316 is the same silver/teflon coax but it is triple shielded - it has metalized kapton foil layer between the outer round braid and the inner flat braid. This results in lower attenuation and much better shielding (aprox. 35 db better or >90 dB) than the regular "plain-vanilla" RG316 - the only thing better then this cable would be to use semi-rigid coax (shielding >110 dB).

Next item on the list is the wiring harness and the power supply board mount.

Update: I made a set of bulkhead (f) BNC to (m) SMA internal RF interconnects, using semi-rigid hand-formable RG-405 coax. I left the old SI316 cables for the LO DDS and replaced only the ones connecting both detector inputs and the DDS RF OUT to the front panel. I did not observe any better detector noise figures.

11 comments:

zenographie said...

Nice power supply ! exactly what I'm looking for. Did you intend to run a small series of it ? In this case, you have a buyer :-) I just finished the detector's shielding and dds heatsink but still nothing done for the PSU.

73'
f6itu

Andrey E. Stoev said...

I could do a small run if there are enough buyers. For a really small number the price is relatively high tho - I paid $60 for just two boards (manufactured in US by Sunstone Circuits). To get the price down we need at least 10 buyers and use one of the Chinese fab houses - even then it will be probably around $10-15.
If you have a local fabrication facility I could send you the gerber/drill files along with the schematics and BOM and you can make the boards by yourself! Another option is to contact Miro, OM3LZ - his boards are very similar (less the linear regulator, LED and a couple more filtering caps). Plus it is geographically closer to you. Again, if there are at least 10 buyers I am willing to do the run myself.

zenographie said...

Thank you Andey. That could be an interesting thread to launch on the Yahoo group. (and it won't cost a dime to do it)

But anyway, you're right... I will contact Miro first. Local facilities here in Europe are far more expensive than in the US (and China, for sure).
Thanks agn for your kind answer
73+
f6itu (grinding, drilling, punching his VNA enclosure :- ) )

Anonymous said...

I would be interested in 2 boards if you decide to offer them.
Tim W4YN@arrl.net

Anonymous said...

Hi Andrej,
I am about to start the VNA build.
Could you please send me the schematics for this supply?
(I use Eagle)

email: ferdi.kinacav@bredband.net

Best 73's
Ferdi/SM0IWR

Artie said...
This comment has been removed by the author.
Artie said...

Hi Andrey,

I get many requests for power supplies, I supply kits for the VNA. Just this week alone, I received 2 requests for power supplies. I think the demand is there and having a power supply available (PCB or in kit form) would make the project much more user friendly.

If you aren't going to do a kit, would you consider releasing the Gerber files so that I could supply kits with PCB's as an option for VNA kit builders?

Also, if you or anyone else knows of the availability of any other power supply, please let me know. As stated previously, it is #2 on the list of questions I get about the VNA kits I sell. The top question on the list is the Minicircuits transformer availability from those users outside the US.

Thanks,

Art

BobMac said...

Good Day Andrey;
I'm building the VE3IUM VNA board and will need to power it. Is there a schematic of your regulator board available?
Best regards, Bob K7HBG

Andrey E. Stoev said...

The power supply is a slightly modifyed version of the OM3LZ power supply (which on the other hand is modifyed version of Paul's, N2PK power supply). My version includes a LDO linear regulator for the 12V line. This relaxes the requirements for the external DC supply and protects the VNA. You can take a look at the schematics here - http://webpages.charter.net/kotarak/VNA_PS.pdf

Anonymous said...

Hi Andrey
Any chance you can share the files for the power supply. May be they are published somewhere already. If not, please e-mail me: ra2fz(at)hotmail.com
Thank you!
73! Ed

Miguel Angel Garcia Vela - EA4DEC said...

Hi Andrey:

Yes, I have local fabrication facility and could you send me the gerber/drill files along with the schematics and BOM to make the boards by myself?

I’m starting the building of the N2PK VNA with 5.2 Makarov board and proper PSU is something I really need it.

I use Eagle too. Maybe I can try with your version 2.0 or you could send me both to compare. Up to you.

Many thanks in advance.

E-mail is: ea4dec"at"druitel.net or my Google account.

EA4DEC – Miguel Angel