Short new video: Why RF Circuits need shielding - Page 1
Forum section: electronics / beginners

author: w2aew
author_rank: Super Contributor
timestamp: August 10, 2014, 02:59:30 am
content: Here's a short video that describes (and shows) some of the reasons why RF circuits need shielding:

author: nadona
author_rank: Regular Contributor
timestamp: August 10, 2014, 06:36:32 am
content: Another great video from Alan.Thank you very much.

author: SeanB
author_rank: Super Contributor
timestamp: August 10, 2014, 06:56:11 am
content: Very nice demo there.

author: BravoV
author_rank: Super Contributor
timestamp: August 10, 2014, 07:12:48 am
content: Does electrical conductive material but non magnetic like aluminium or copper have difference vs steel as a shield ?

author: SeanB
author_rank: Super Contributor
timestamp: August 10, 2014, 07:24:52 am
content: Mostly with the attenuation per unit of thickness. Copper is better than aluminium which is better than tin plated steel. Best though is to silver plate the screen to reduce the skin resistance to a minimum, though you do then need to coat it with a film to reduce oxidising of the silver. Magnetic shielding is more used at low frequency, as there the magnetic fields are mostly unaffected by the shielding and pass through non ferrous materials.

author: BravoV
author_rank: Super Contributor
timestamp: August 10, 2014, 07:48:21 am
content: Quote from: SeanB on August 10, 2014, 07:24:52 amSay I use 2 sides pcb to form a box shape RF shield, and with tin solder to join at the inner copper side "AND" also outer side seamlessly covered by tin solder, so practically its 2 layers copper box.Does this make it a better shield than say common aluminium box even it has thicker wall ?Something looks similar to this, but with solder along the way at all copper joins inner and outer sides.Random image grabbed from Google.Of course we're not discussing it's mechanical strength compared to thick aluminium enclosure, only it's RF filtering performance.

author: SeanB
author_rank: Super Contributor
timestamp: August 10, 2014, 08:32:08 am
content: Depends on the copper thickness, but will work well at high frequency. In any case the major considerations are the cost and the availability of the material, along with the ability you have to shape them to the desired form. If you can deep draw copper sheet with a die and mandrel then that will work best, but will be expensive. Aluminium plate and bend to shape with a bending brace and welded is cheaper, or just bend and rivet together. Tin plated steel is used a lot because it is both cheap, good enough for most uses and easy to form and solder.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 10, 2014, 09:23:55 am
content: Attenuation is proportional to thickness and sqrt(f) (skin effect stuff), or something like that.Nonferrous materials have zero attenuation at DC, but even aluminum (if it's fairly heavy) does a good job at line frequencies. Any practical material (>= 5 mil in aluminum, steel, copper..) will do a fine job past low RF (~10MHz?). Foils and films will still do a good job in the microwaves, although not forever: CDs are, I guess, somewhere around 55 nm aluminum metallization -- blue light is just barely visibly shining through (try it some time!). The color, I suspect, is due to interference (the speed of light is much slower in metal than in plastic or vacuum), combined with the shallow depth of the layer. Interference effects will likewise show up for shields of nominal thickness at proportionally lower frequencies (effectively, where the skin depth penetrates the material thickness).Copper clad isn't very good because it's quite thin; if you have the choice between 1oz double sided copper clad, and 20 mil steel, take the steel any day (although, do make sure it's tin or zinc plated so it doesn't rust away, either).If you can't seam bond both layers of copper clad (and really... how the hell are you going to do that on all six sides, anyway!?), it's at least as good as one layer, and in most directions, as good as two. You can attempt to arrange internal fields to take advantage of this, for example, placing coils near the middle of a face, so most of the field has to penetrate two layers line-of-sight; and double-seam-bonding the four walls wrapping around said coil. Bonding of the panels above and below the coil's axis isn't a big deal, because it's not inducing much if any current in that path.Tim

author: German_EE
author_rank: Super Contributor
timestamp: August 10, 2014, 11:01:27 am
content: One thing that is not mentioned is that a shield is only effective if it does not carry any current. If we build an oscillator with a battery inside a sealed copper box then there will be no radiation, but if we ground parts of the oscillator circuit to the box the circulating currents during operation will reduce effectiveness.The secret is to build your circuit on a separate PCB and then ground it at a single point near the I/O connections.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 10, 2014, 11:34:40 am
content: Great Video!!The use of the finger is well known. Here is a picture of the Bob Widlar, Analog Guru, using his:Regards,Jay_Diddy_B

author: questron
author_rank: Regular Contributor
timestamp: August 10, 2014, 05:51:12 pm
content: great video as always, keep them coming!

author: ivaylo
author_rank: Frequent Contributor
timestamp: August 14, 2014, 03:31:42 am
content: I have a silly question here. What is the audible sound the radio reproduces? If the generator puts out 18MHz unmodulated(?) what are we hearing? That's like a kiloherz or something coming out of that speaker. And why is *that* low audible frequency changing when you move your fingers around?

author: w2aew
author_rank: Super Contributor
timestamp: August 14, 2014, 03:46:16 am
content: Quote from: ivaylo on August 14, 2014, 03:31:42 amThe radio uses a product detector, thus it mixes the incoming signal with a 'beat frequency oscillator' (after being converted through a few IF stages). What you are hearing is the output of the product detector/mixer which is outputting the frequency difference between the incoming signal and the BFO. In other words, your hearing the carrier frequency which has been translated down into the audio band. Thus, any change in the incoming carrier frequency is also translated to the audio output.

author: ivaylo
author_rank: Frequent Contributor
timestamp: August 14, 2014, 04:04:03 am
content: Got it. I think. This is fascinating (and what would you hear if you were AM modulating say a 1KHz into the signal? a video on that?). Thanks for those great videos, btw!

author: w2aew
author_rank: Super Contributor
timestamp: August 14, 2014, 12:13:03 pm
content: Quote from: ivaylo on August 14, 2014, 04:04:03 amIf you zero-beated the BFO (made it match the carrier), you'd simply hear the 1kHz audio tone. If you offset the BFO like in this video, you'd hear a mix of the carrier beat tone and audio tone shifted in frequency.

author: jlmoon
author_rank: Unknown
timestamp: August 14, 2014, 02:10:51 pm
content: Awesome presentation!

author: questron
author_rank: Regular Contributor
timestamp: August 25, 2014, 02:49:12 pm
content: good video Alan, thank you!i also watched the one on A, B, C, and AB classes of amplification. would like to find out more about C class, very interesting.what are some of the good sources for complete treatments on the C class theory, how complete waveforms could be reconstructed when they are irregular as real radio signals are, what some of the design considerations and techniques are, etc?thank you for helping!