Author Topic: Open Source HW RF Signal Generator  (Read 27278 times)

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Online Yansi

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Re: Open Source HW RF Signal Generator
« Reply #75 on: February 02, 2020, 06:33:48 pm »
Was any part of this tested and verified independently? I am not a close follower of this thread, but I still have a bit of bad taste of it being just a quick slaptogether. Nor do I see a single point to use any duino crap within designs that even uses FPGAs. (simply this is not a piece of a children toy, so why even bother making it child friendly?)

How will you manage to make any RF shielding lids? A custom milled aluminium blocks?

Is there really none ESD protection on the USB?

Also for example, from another quick look at the PCB: Why not flipping the 1.5Ghz LPF vertically? Would allow for much shorter interconnects.

I think the layout is far from what I'd call done. (hence my bad taste of a quick slaptogether). But I stay being corrected.
« Last Edit: February 02, 2020, 06:38:14 pm by Yansi »
 

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #76 on: February 02, 2020, 07:53:15 pm »
This board is a combination of all the individual boards I previously made and which I verified to be working. The only thing I didn't test so far is the FPGA-Board and its control of the AD9910 I have here. Control of the chip via SPI-Bus with the main CPU is working reliably.
The changes I've made compared to the version I have here with the individual boards I've detailed in this post: https://www.eevblog.com/forum/oshw/open-source-hw-rf-signal-generator/msg2819470/#msg2819470

Regarding your other questions:
I don't have a CNC-Machine, but I've got a 3D-Printer and plan to use conductive filament (and/or copper-foil or conductive paint) to print RF-Shielding enclosures. The holes in the PCB have a diameter of 3.4mm, which allows using M3 screws to mount the shields.

USB ESD-Protection... Oops. :)
I'll add the USBLC6 to the USB-Lines. Used that chip in other projects before and have some in my parts-bin.

The location of the filters could be optimized, yes.
If they were a bit cheaper, I'd be using tiny SMD-Filters by Minicircuits. That would allow more more filters with better attenuation of the harmonics.
« Last Edit: February 02, 2020, 07:59:09 pm by SaabFAN »
 

Online Yansi

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Re: Open Source HW RF Signal Generator
« Reply #77 on: February 02, 2020, 08:30:46 pm »
I wouldn't trust the 3D printed bullshit (yes, I'm a skepticist, especially with these types of filaments). Get a proper alu case milled (it is not that much expensive, but sure not cheap), or I'd opt to optimize the layout to use off-the shelf tinned steel sheet shielding cases.

Yup, I also go through a lot of USBLC6, both -2SC6 and -4SC6 (2 or 4 way).

There are other manufacturers of monolithic filters. Have you tried looking thoroughly enough?  Johanson technology, Taiyo Yuden, Murata, Epcos/TDK, etc etc etc. Even Mouser stocks heaps of them, for a fraction of MiniCircuits cost.

Performance of these filters suck (stopband attenuation 30-40dB), but can be cascaded with acceptable insertion loss. Very cheap. Also look at aliexpress and other far east sources. Lots of these available.



 

Offline thm_w

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Re: Open Source HW RF Signal Generator
« Reply #78 on: February 03, 2020, 06:54:16 am »
3D printing makes sense regardless of what your end result is, as it lets you test fit the dimensions. CNC setup time is not cheap compared to filament.

Not sure if anyone has tested 30% copper PLA, would be interesting to see. Its not electrically conductive across the material.
The actual surface conductive stuff, afaik, is all carbon based, which is not great for RF absorption compared to say, Cu or Al.

These might be better options over carbon, just a question of if its good enough and won't melt PLA:

https://www.mgchemicals.com/downloads/tds/tds-842wb-l.pdf
"Surface Resistance @ 50 μm 0.0020 Ω/sq"

https://www.caswellplating.com/copper-conductive-paint-4oz.html
"Good surface resistivity of <1 Ohm/sq at 1 mil dry film thickness. Attenuation: more than 75 dB from 30 MHz to 1.5 GHz."

https://www.mgchemicals.com/downloads/tds/tds-841ar-a.pdf
"Surface Resistance @ 50 μm 0.60Ω/sq"
 

Offline OwO

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Re: Open Source HW RF Signal Generator
« Reply #79 on: February 03, 2020, 08:54:46 am »
Why not use shield cans and clips?
Something like this: https://www.aliexpress.com/item/33003768263.html
I'm sure you can find other dimensions or fold these from copper foil for custom dimensions. On taobao some sellers can do custom shield cans.

There are clips that solder to the PCB:
921084-0
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Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #80 on: February 03, 2020, 06:05:50 pm »
Soldering these clips to the PCB should be possible. The surfaces free of soldermask are 5mm wide.

I was inspired to use 3D-Printed pieces by the video of TheSignalPath in which he 3D-Printed some horn antennas and Waveguide-Filters with pretty good results.

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #81 on: February 19, 2020, 07:10:35 pm »
I've done some research and the method to go (even used by at least one company I found that has invested some research in this area) seems to be coating the 3D-Printed objects with conductive paint or metallic foil.

Metallic foil is probably the best choice here price wise. Or cans out of copper sheet metal. :)


Regarding the filters: There's a ton of them available on Digikey and Mouser, but they all seem to be targeting the 2.4 GHz (WLAN)-Area. So they're outside the area of interest and when I have to order parts directly from the manufacturer, Minicircuits seems to be the better supplier.

Offline eb4fbz

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Re: Open Source HW RF Signal Generator
« Reply #82 on: February 20, 2020, 09:41:47 am »
Those microstrip lines are pretty thin! What is the dielectric thickness?

You will have big mismatches due to discontinuities created by the components pads. Those lines look very thing even compared with 0402 passives!

I would never ever try again to make distributed elements filters on FR4. Not even for <1GHz. It's not about loss, but also uncontrollable variations between boards and ultimate rejection. I would use SMD low pass filters from Murata or Johanson. They are cheap, and performance will be better and painless compared to those filters on FR4.
 

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #83 on: February 20, 2020, 08:10:42 pm »
The thickness is 0.1mm (Controlled Impedance 4-Layer Board made by JLCPCB). Dielectric constant is 4.05.
Yes, I noticed the thickness discontinuities of the pads. Unfortunately, that's one of the compromises I had to make to keep cost low.

Regarding the filters: Okay, you guys convinced me to switch to some ready made SMD-Filters :)
It also seems like Mouser has recently added Minicircuits Filters to their inventory. They do not ship to Germany though. Hopefully that'll change in the near future. So I'll redesign the filter-section with the filters by Minicircuits and update the layout of the filter-section. Should save a lot of space as well.

Online Yansi

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Re: Open Source HW RF Signal Generator
« Reply #84 on: February 20, 2020, 11:39:23 pm »
These thin transmission lines are nightmare to work with. Use different stackup or forget 50ohms. On a generic FR4 board, even controlled impedance, it will be from floor to bottom in its value.

What is also commonly done, is to use layer 1 for signal (as you have), then omit layer 2  (no copper under RF parts), layer 3 becomes RF GND, layer 4 other signals (supply, control signals).

That way you can get much thicker traces for 50R, which is welcome, as the etching tolerances won't bother that much. Also will enable you to obtain much higher impedances for distributed filter elements.
 

Offline OwO

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Re: Open Source HW RF Signal Generator
« Reply #85 on: February 21, 2020, 04:07:33 am »
There is also the 0.2mm stackup which JLC offers and is de-facto standard for boards with RF.
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