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#1
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Absorptive Shielding?
Thanks to everyone who has responded. I think the conclusion is that at the
frequencies I'm interested in, and the available space inside the radio (a little RS DX-394 table radio), it's impractical to absorb the 455 kHz crosstalk energy from 2nd IF to frontend. Better to attempt to compartmentalize the radio. That may prove to be impractical also as it would appear very difficult to make small (removable) shields over the IF section that would not have gaps. However, I'll examine the pcb layout more closely to see if there are any viable paths for the sides of the shield box. 73, Tom |
#2
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"Tom Holden" wrote in message ... Thanks to everyone who has responded. I think the conclusion is that at the frequencies I'm interested in, and the available space inside the radio (a little RS DX-394 table radio), it's impractical to absorb the 455 kHz crosstalk energy from 2nd IF to frontend. Better to attempt to compartmentalize the radio. That may prove to be impractical also as it would appear very difficult to make small (removable) shields over the IF section that would not have gaps. However, I'll examine the pcb layout more closely to see if there are any viable paths for the sides of the shield box. Small gaps won't matter as long as they are shorted out at one or both ends. A well-fitting cover would do that. You'll observe that commercial gear uses very thin tinned steel? for shield boxes. The lid edges are bent into spring fingers to hold them in place. |
#3
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On Sun, 9 Nov 2003 16:00:21 -0500, "Tom Holden"
wrote: Thanks to everyone who has responded. I think the conclusion is that at the frequencies I'm interested in, and the available space inside the radio (a little RS DX-394 table radio), it's impractical to absorb the 455 kHz crosstalk energy from 2nd IF to frontend. Better to attempt to compartmentalize the radio. That may prove to be impractical also as it would appear very difficult to make small (removable) shields over the IF section that would not have gaps. However, I'll examine the pcb layout more closely to see if there are any viable paths for the sides of the shield box. 73, Tom You'll never know before you have tried, and remember that Racal solved a similar problem with their famous RA-17 series receivers using a hacksaw to make a little mark in the chassis to stop unwanted radiation from one point to another. Believe I've seen the application of those carbonized foam used in the lids of boxes which were definitely not microwave equipment Such things are impossible to predict It is also some definite requirement for the thickness of the walls to act as screen on certain frequencies, as an example could be mentioned that pcb laminates are not thick enough for good screening on 80m in an application with two oscillators which need good screening to avoid coupling to be used for third order IP measurements 73 Jan-Martin, LA8AK Amateur radio techniques http://home.online.no/~la8ak/c.htm -- remove ,xnd to reply (Spam precaution!) |
#4
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J M Noeding wrote:
You'll never know before you have tried, and remember that Racal solved a similar problem with their famous RA-17 series receivers using a hacksaw to make a little mark in the chassis to stop unwanted radiation from one point to another. Did Racal make a complete cut through the chassis or just a deep scratch? How long was it? -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
#6
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Most novice designers get burned somewhere along the line by failure to
realize what Ian is describing -- that all amplifiers are differential amplifiers, and calling one of the inputs "ground" doesn't impart magic properties. Once that lesson has been learned (alas, usually the hard way), the probability of making circuits work as designed increases dramatically. Even so, though, a surprise sometimes comes along at the last minute. I know of one expensive, high quality test instrument that had a ground path broken at the last minute because of ground current that was induced in the signal path by the field from the display CRT deflection yoke. Forewarned, I discovered 100 mA of induced current in the shield of a multi-turn delay line from the same source, in an instrument in whose design I was participating. Simply soldering the turns of semi-rigid coax together reduced the effective transformer secondary to one turn from 20 or so, making it possible to reduce the coupling and its effect to an acceptable level by other means. But I remember that shortly after I went to work at Tektronix, I was in the main lobby admiring the example of the first oscilloscope model produced by that company, which was on display there. Its success and the reputation it gathered as a quality test instrument was instrumental in launching the company. It had two chassis, one above the other, supported by four L - cross section aluminum legs between the chassis. One of those four legs had very neatly been cut, leaving a small gap. A sturdy piece of phenolic was mounted between the gapped ends, again very neatly, with countersunk flathead screws. I remember that seeing this gave me a good feeling that ground currents could be a problem for the best of 'em. The 'scope in the lobby was a production model, but somewhere, I'm sure, there was a prototype much like the Racal device -- with a hacksaw cut across the leg. Roy Lewallen, W7EL Ian White, G3SEK wrote: J M Noeding wrote: You'll never know before you have tried, and remember that Racal solved a similar problem with their famous RA-17 series receivers using a hacksaw to make a little mark in the chassis to stop unwanted radiation from one point to another. [...] More than a "little mark" - a big cut, deliberately intended to interrupt an unwanted current path through the chassis. Such things are impossible to predict If you stop to think about it, such things often *can* be predicted... or at least explained with the benefit of hindsight. When we connect a signal from point A to point B, we often forget about the return path through 'ground'. But every signal path must also have a return path - that's why we call these things "circuits". Even if we shield the signal conductor to provide a preferred return path, there are still other return paths available in parallel with that, through the box. Return currents will divide between these paths according to the RF impedance that each path presents (Ohm's law), so you will always get some RF currents in the body of the box. The box is just another RF conductor, and when currents flow through it, voltages will appear. The key thing is to remember that there's no magic that makes the box a perfect zero potential. For all us Sixties children: "'Ground' is a delusion, Grasshopper." Therefore in a complex project you will always get some interaction between different stages. What's hard to predict is how much effect this will have on overall performance. The problem that plagued Racal was spurious receiver responses. Racal was a startup company with a revolutionary all-band receiver concept... but it involved several oscillators in different stages, that were getting into the wrong parts of the receiver and causing "birdies". They couldn't sell a single unit until that problem was fixed. They had already gone to the huge expense of a diecast chassis divided into many shielding compartments... and it wasn't working. Very soon, the money would run out and Racal could cease to exist. Hence the desperate solution with the hacksaw - and it saved the company. Legend has it that the prototype receiver (complete with saw cut) was on display in Reception for many years... maybe they shoulda kept the hacksaw too? I once had a problem of this type with spurious responses from a receiver that was constructed in three (apparently) shielded diecast boxes with coax signal interconnections. The boxes were stacked side by side inside the case of the receiver. Remembering the Racal experience, something made me shove a piece of folded paper under one of the boxes, to insulate it from the receiver case. The spurs disappeared! Looking further into this, the real problem was with the power interconnections between the boxes, which I'd been ignoring until then. RF was getting into the power rails, and contact between the boxes formed a ground loop. Unwanted signals were being transferred between the two boxes via this 'back channel'. With better bypassing of the power rails, it was possible to re-connect the boxes to the case of the receiver. |
#7
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Roy Lewallen wrote:
Most novice designers get burned somewhere along the line by failure to realize what Ian is describing -- that all amplifiers are differential amplifiers, and calling one of the inputs "ground" doesn't impart magic properties. Once that lesson has been learned (alas, usually the hard way), the probability of making circuits work as designed increases dramatically. Even so, though, a surprise sometimes comes along at the last minute. I know of one expensive, high quality test instrument that had a ground path broken at the last minute because of ground current that was induced in the signal path by the field from the display CRT deflection yoke. Forewarned, I discovered 100 mA of induced current in the shield of a multi-turn delay line from the same source, in an instrument in whose design I was participating. Simply soldering the turns of semi-rigid coax together reduced the effective transformer secondary to one turn from 20 or so, making it possible to reduce the coupling and its effect to an acceptable level by other means. But I remember that shortly after I went to work at Tektronix, I was in the main lobby admiring the example of the first oscilloscope model produced by that company, which was on display there. Its success and the reputation it gathered as a quality test instrument was instrumental in launching the company. It had two chassis, one above the other, supported by four L - cross section aluminum legs between the chassis. One of those four legs had very neatly been cut, leaving a small gap. A sturdy piece of phenolic was mounted between the gapped ends, again very neatly, with countersunk flathead screws. I remember that seeing this gave me a good feeling that ground currents could be a problem for the best of 'em. The 'scope in the lobby was a production model, but somewhere, I'm sure, there was a prototype much like the Racal device -- with a hacksaw cut across the leg. Roy Lewallen, W7EL This thread has got me wondering if I could reduce some of the residual 'birdies' in my R8B (there aren't that many) which I suspect are caused by one or more common mode ground loops involving the synthesizer, by implementing the chassis cutting technique. I'm not about to take a hacksaw (Dremel tool) to it using the trial and error method, so how does one analyze the situation to get a feel for what might be going on? -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
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