| Electronics > Beginners |
| Copperless perfboard + wire wrap for RF prototyping? |
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| aneevuser:
I've been trying to play around with some simple RLC circuits, measuring resonant frequencies, Q factors and the like. I'm putting them together on a Wisher breadboard, and have been getting some very strange behaviour, which I think I've finally tracked down to inter-pin capacitances (which seem to be about 10 pF). This has been frustrating and it's taken me a long time to figure out. I'm now wondering what is a more suitable low cost method of putting together an RF circuit that is sensitive to stray L and C and I'm thinking of putting them together on copperless perfboard and wire wrapping the components instead, my reasoning being that lack of copper track will minimise stray C, and I can wire wrap up to the edge of the components to minimise stray L. However, I don't want to buy the kit just to find out that this doesn't help much, or that there's some other problem that I'll run into. Does anyone know if this is a good way to go, or if I should take another approach? |
| ledtester:
A technique called "Manhattan construction" is often used for RF circuits. Have a look at this slide deck. There are examples of the technique starting at slide 13. http://s-iihr64.iihr.uiowa.edu/MyWeb/Teaching/ece_55195_2015/Lectures/Construction.pdf |
| Howardlong:
I use these extensively for prototyping RF & mixed signal. https://www.busboard.com/surfacemountpcbs In particular: https://www.busboard.com/SP3T-50x50-G and https://www.busboard.com/SP3-100x100-G They have a solid copper groundplane and non-plated through holes. They also do PTH versions but I find that is a bit restrictive on layout at times, and it doesn't take more than a few seconds to run a through wire. Edit: I wouldn't ever use wire wrap for RF, wire wrap is an antenna. Edit2: Apart from some baseband and LF applications where S/N is unimpotant, for most RF applications, solderless breadboard is a problem waiting to happen. Embrace and hone your soldering skills, and preferably switch to surface mount! |
| Ian.M:
Soldering components to brass pins stuck in shellac or paraffin wax impregnated balsawood (or other low density kiln dried softwood you can set pins in with a pin punch or just a pair of pliers) was the origin of the electronics term 'breadboarding', from literally building a circuit on the significant other's wooden breadboard! Its still viable for point to point wired circuits using leaded components at frequencies below roughly the middle of the HF band. Panels for controls etc. can simply be screwed to one edge of the board. It can also look pretty good (1920's retro-tech) if you take care to make an aesthetic layout, use hardwood, rout and fine sand a molded profile on the edges, highly polish the shellac before assembly, and pre-drill the pin holes under-size so you don't mar the harder wood setting the pins. If you need to work at a higher frequency go Manhatten Island which with careful layout is good up to the low UHF band. Its also *MUCH* better suited to SMD prototyping, (dead bug or on small breakout boards) and even if you don't want to use SMD for the bulk of the circuit, its still worth using SMD ceramic caps for sub-uF decoupling - just tombstone them directly on the ground plane and use the top end as a circuit node. 0805 or larger 100Meg 10% SMD resistors make quick, good and cheap low capacitance standoffs for 'air wiring' parts of the circuit where you need to minimise capacitance to the ground plane. If you need low leakage standoffs, then its expensive pins in Teflon bushes in drilled holes. Above the low UHF band, the layout *is* the circuit reactive components, so you need to cut carefully designed and dimensioned striplines etc. on double sided specialist low loss PCB material, and 'fence' round the edges of ground islands with copper rivet vias to stitch them down to the ground plane, then edge solder the assembled boards into screening cans. Its not too bad if you've got a good CNC mill with a precision high speed spindle and a supply of small conical carbide cutters, and small carbide drill bits, and can go straight from CAD to PCB ready to copper rivet the via fences (and don't mind the effort containing the abrasive PCB dust so it doesn't get into the mill's ways), but is a royal PITA if you are using hand tools. Put the breadboard (of whatever construction technique) in a biscuit tin (or sweets tin - Altoids is the classic) and spot solder the lid on a couple of points on each side (or use PC case panel grounding clips) for screening. You can also build screening boxes by tin-smithing from bent and soldered tinplate. Another method is using copper clad PCB, with the seams soldered. Double sided PCB is stronger with a slightly oversize baseplate and end walls, with all seams soldered both sides. |
| bd139:
Important things are a solid, low inductance ground plane, keep all the leads short and keep everything rigid. Short leads kills stray L as much as is possible. Stray C is inevitable and unavoidable so you tend to design around this. The biggest problem I find is stray variable C. I did an example here of why you need a solid ground plane by building a really naff prototype HF LC oscillator on a "gouged PCB" method popular in the 60s and 70s and tuning it on my HF radio. The stray C here is between the traces on the board and the board underneath it. As you can see it detunes things terribly. Thus I don't use anything with isolated pads on it. Inevitably this leads to "dead bug" construction, at least for me. Typical example. Looks like s**t but from a performance perspective it's pretty excellent. Never correlate neatness with performance - they are 100% independent. So what I do: 1. Take a piece of solid single sided FR4 2. Solder all the parts to the top (and BNC connectors) keeping all the leads short. 3. Make sure it is rigid. The circuits built are high performance, reliable and cheap to build. This is good to at least 250MHz or so (I haven't gone higher) Edit: I still use stripboard for low frequency/audio stuff though. Does the job and Roth sell some nice stuff with FR4 substrate. And I still use (Wisher) breadboards but mainly for bias testing stuff where a bit of experimentation is required. As you're in the UK, Rapid electronics sell half decent FR4 single sided stuff under the RVFM brand. It's pretty good for the money. I cut it with some aviation shears I got from Machine Mart. Also a final note, worth a read, particularly the construction section: https://www.analog.com/media/en/technical-documentation/application-notes/an47fa.pdf |
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