Double FR4 board with ground plane, pic please ?

[Argus25]

Al,

I have developed a technique that works well to about 10 MHz. (I have not tried it higher) The photos show part of an AM pantry transmitter board and another unit which is a baby 3W PEP output AM radio station which has a VFO from 500kHz to 7.5MHz.

The idea is to isolate functional circuit sections using about 1/4 inch tall and about 0.5mm to 0.7mm thick brass strips which are perpendicular to the pcb surface, rather than parallel to it like a conventional ground plane.

The perimeter strips are ground (negative) and the central one the supply rail, which also acts as an RF isolation barrier, bypassed to the ground with 0.1uF caps and tantalums to the ground strips. This provides isolation between functional circuit systems and a very low Z earth return & power supply system at RF frequencies, still while using plated through hole spot board which can otherwise have problems at high frequencies.

The brass strips are looped over & soldered with 0.7mm copper wire to pairs of pads, and another wire bus sits between those on the pcb bottom. It also provides a very convenient way to get access to a good ground and a well bypassed supply rail when wiring the circuit. This leads to a regular looking layout as many of the R's and C's etc can connect to the brass strips on the under surface conductor and the + rail is available everywhere down the pcb center.

sorry about the blurry photo,

Hugo.

[Al (astral highway)]

Hello Hugo,

Thank you for the very illustrative photos and design ideas. Another beautiful piece of work there.

I am struck by the relevance of your solution for isolating sub-assemblies where necessary or helpful and also by your use of the brass busses perpendicular to the board.

Thank you ,

[G6Tanuki]

I like the use of those strips to provide some smidgin of RF isolation.

In the past you could get purpose-made PCB 'power distribution' strips which looked rather like what you've used but were actually a sort of flat, linear ceramic capacitor; they were a sandwich with a central bus-bar intended for the +ve supply, and on each side there was a ground-plane.

I forget who made them but they were brilliant for decoupling the +5V-at-several-amps rails when building large TTL-based circuits.

[You could also get similar 'flat' decoupling-capacitors that were designed to sit *under* a high-speed TTL device, with the capacitor's legs sharing the TTL device's PCB holes]

[Argus25]

Originally Posted by G6Tanuki

I like the use of those strips to provide some smidgin of RF isolation.

In the past you could get purpose-made PCB 'power distribution' strips which looked rather like what you've used but were actually a sort of flat, linear ceramic capacitor

Yes, most of the benefits of the brass strip come from lowering of the ground and power rail impedances, rather than electrostatic shielding. However, if the strips are made 10mm tall on the component side and about 4 or 5mm tall on the wired side too, then tops and bottoms can be put over these so fully enclose and shield the sub- circuit inside. I do this with VFO's etc. Below 10MHz the strips alone work fine.

There are still machine pin DIL sockets with axial 0.1uF monolithic ceramic capacitor built into them between the supply pins, I think RS or Farnell have them.

[Al (astral highway)]

Hey folks,

I've been working happily with some of those small green double-sided FR4 boards which have circular pads on both sides. No problems there at all. They are still not my favourite material for circuits carrying higher currents, hence my focus on this (pictured) which for me is a 'new' material' with potential, if only I can suss it out.

It's this other material, pictured below (pads and ground plane sides), which is the subject of my OP and still causing me a headache.

It seems as if it would be necessary to have some kind of rotary cutter (mini dremel, which I don't have) to carve out 'islands' on the ground plane side?

Although many examples have been posted, I'm not sure that any of them is the material I'm demonstrating here.

If they are, then please excuse my lack of insight!

As you can see, if you drill out a hole on the ground plane side, it has to be very shallow to remove just the copper from that side. Maybe this is what some of you have been describing?

Thanks,

[ms660]

Use a spot face cutter or just a drill bit (sharp) Both are hand powered:

https://www.youtube.com/watch?v=LvR4B5q5Pfc

Lawrence.

[Herald1360]

^^^WHS^^^ (And WIS https://www.vintage-radio.net/forum/...3&postcount=11 )

[Al (astral highway)]

Thanks, I think I have the measure of this now. I’m still dealing with some considerable physical limitations , which can make even a simple-looking technique elaborate until I’ve practised it, then there’s there’s a gap between the standard I want and the standard I can achieve without enough practice. I want perfect, or as near as possible to it!

There will be some ‘wasted’ board, invariably, as my higher ‘executive director of standards’ gets involved. (Please don’t worry, it’s a joke rather than an actual belief that I have such a person enshrined in me)

[julie_m]

After many years spent using a 4mm. drill bit on copper strip breadboard, some of which were earning a living, one day I finally treated myself, in the name of making up an order value to qualify for free delivery, to the "proper" tool -- called a spot face cutter. This looks like an untwisted drill bit (there won't be enough swarf for it to need spiral grooves to clear it out of the way) with a plastic handle, about 10cm. long.

What a revelation!

OK, so it wasn't the worst Thing I Have Been Doing Wrongly All My Life but even over the few twists it took to prepare a half-postcard-sized circuit board, I soon noticed how much nicer the plastic handle is to hold. It does not conduct away your body heat, nor dig into your skin, and you are holding it in a much more comfortable position.

[Herald1360]

Shove the drill bit into a file handle?

Older SFCs had some straight flutes about half an inch long. The present version looks like the bean counters have been at it. It does the job but it doesn't "look" the part.

[Al (astral highway)]

Julie, thank you, that appeals to my attachment to the highest standard of work I can achieve and sounds efficient and ergonomic!

Chris, ingenious idea, like it.

[ms660]

You could always go cave man:

https://www.youtube.com/watch?v=c6wRLJWfjFw

Or a bit more refined:

https://www.youtube.com/watch?v=pSIvqLARbdM

Lawrence.