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#1
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Picket-fenced ground gias next to microstrip traces
I've noticed that various data sheet application circuits for items such as RF
switches, amplifiers, etc. (e.g., http://www.hittite.com/product_info/.../hmc349lp4c.pd f ) use what I would call "microstrip traces surrounded by a ground pour 'guard' to reduce coupling to adjacent traces." Someone else, however, has suggested that the application circuits were really designed as co-planar waveguides (with grounds). Anyone else want to venture an opinion? For a 62.5 mil board, a 50 ohm microstrip's width is around 115 mils using FR-4 (k=4.7). Using TxLine 2003, for a CPW w/ground I have to reduce the gap to ~50 mils before the signal trace width reduces ~10% to ~104 mils. I take this to imply that there's not a lot of coupling between the signal trace and the copper pours (instead of the ground plane) until the gap width is comparable to the board thickness. In general, my impression has been that the use of copper flooding is more to provide isolation between adjacent traces than to change the form of the transmission line, and the use of the picket fenced vias was to insure that a large chunk of copper pour didn't suddently turn into a resonator. Does that sound correct? ---Joel Kolstad |
#2
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Picket-fenced ground gias next to microstrip traces
Joel Kolstad wrote:
I've noticed that various data sheet application circuits for items such as RF switches, amplifiers, etc. (e.g., http://www.hittite.com/product_info/.../hmc349lp4c.pd f ) use what I would call "microstrip traces surrounded by a ground pour 'guard' to reduce coupling to adjacent traces." Someone else, however, has suggested that the application circuits were really designed as co-planar waveguides (with grounds). Anyone else want to venture an opinion? Their Application Note 17 ("Design techniques enhance isolation in switch assemblies") talks explicitly about grounded-coplanar-waveguide techniques. The evaluation boards for the "349" switches certainly appear to use the CPW techniques discussed in AppNote 17, including closely-spaced plated thru-holes to tie top and bottom ground planes together. For a 62.5 mil board, a 50 ohm microstrip's width is around 115 mils using FR-4 (k=4.7). Using TxLine 2003, for a CPW w/ground I have to reduce the gap to ~50 mils before the signal trace width reduces ~10% to ~104 mils. I take this to imply that there's not a lot of coupling between the signal trace and the copper pours (instead of the ground plane) until the gap width is comparable to the board thickness. In general, my impression has been that the use of copper flooding is more to provide isolation between adjacent traces than to change the form of the transmission line, and the use of the picket fenced vias was to insure that a large chunk of copper pour didn't suddently turn into a resonator. Does that sound correct? If by "picket fenced vias" you mean the projections outward from the signal vias on the evaluation boards, I take those to be the plated thru-holesdiscussed in AppNote 17. Are you seeing something I'm not? _Interesting_ gadgets. Awfully small, but interesting; may have to ask for samples and eval boards. -- Mike Andrews, W5EGO Tired old sysadmin |
#3
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Picket-fenced ground gias next to microstrip traces
Hi Mike,
Thanks for the response -- I'll go and read that app note. If by "picket fenced vias" you mean the projections outward from the signal vias on the evaluation boards I mean all the vias going from the top side copper to the ground plane. Viewed from the side -- and with a little imagination -- those vias kinda look like a picket fense. OK, maybe not so much, but I *have* heard this term before; I'm not making it up myself! :-) |
#4
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Picket-fenced ground gias next to microstrip traces
the most important reason for the "pouring" is to reduce the inductance
of the upper side gorund plane. most of the relevant field in the microstrip is confined in the volume between the microstrip itself and the gorun plane beneath. there is a phemomenon called fring effect, whereby fields generated on the upper side of the microstrip come into play, but that occurs only at frequencies of about 5GHZ and above, not at the usual frequencies we talk about. it belong more to the microwaves' people realm. leave it alone under 1GHz. you don't have to worry about that effect at lower frequencies. Saandy 4Z5KS |
#5
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Picket-fenced ground gias next to microstrip traces
"Saandy , 4Z5KS" wrote in message
oups.com... ,,,,,,this is PRECISELY what a CPW is NOT! It's a "coplanar waveguide with ground plane." This seems more popular than strict CPWs, as far as I can tell. |
#6
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Picket-fenced ground gias next to microstrip traces
This appears to be a blend of co-planar and microstrip. Co-planar has no
ground plane, that is, everything is in one plane thus the name "co-planar". A "microstrip-like" line is flanked by two ground surfaces and there is no ground plane under it. Like this ---- - ---- Microstrip is a flat line over a ground plane. Like this - ----------- The vias along both sides of this "CPW" T-line appear to be used to constrain the fields and The Vias most certainly must enter into the characteristic impedance equation since the spacing to the line is so close. Therefore the line must be narrower than pure coplanar. I was part of a program to design a family of his same type of switch on GaAs, back in the early 90's and the co-planar line was used. 73, Steve, k9DCi "Mike Andrews" wrote in message ... Joel Kolstad wrote: I've noticed that various data sheet application circuits for items such as RF switches, amplifiers, etc. (e.g., http://www.hittite.com/product_info/.../hmc349lp4c.pd f ) use what I would call "microstrip traces surrounded by a ground pour 'guard' to reduce coupling to adjacent traces." Someone else, however, has suggested that the application circuits were really designed as co-planar waveguides (with grounds). Anyone else want to venture an opinion? Their Application Note 17 ("Design techniques enhance isolation in switch assemblies") talks explicitly about grounded-coplanar-waveguide techniques. The evaluation boards for the "349" switches certainly appear to use the CPW techniques discussed in AppNote 17, including closely-spaced plated thru-holes to tie top and bottom ground planes together. For a 62.5 mil board, a 50 ohm microstrip's width is around 115 mils using FR-4 (k=4.7). Using TxLine 2003, for a CPW w/ground I have to reduce the gap to ~50 mils before the signal trace width reduces ~10% to ~104 mils. I take this to imply that there's not a lot of coupling between the signal trace and the copper pours (instead of the ground plane) until the gap width is comparable to the board thickness. In general, my impression has been that the use of copper flooding is more to provide isolation between adjacent traces than to change the form of the transmission line, and the use of the picket fenced vias was to insure that a large chunk of copper pour didn't suddently turn into a resonator. Does that sound correct? If by "picket fenced vias" you mean the projections outward from the signal vias on the evaluation boards, I take those to be the plated thru-holesdiscussed in AppNote 17. Are you seeing something I'm not? _Interesting_ gadgets. Awfully small, but interesting; may have to ask for samples and eval boards. -- Mike Andrews, W5EGO Tired old sysadmin |
#7
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Picket-fenced ground gias next to microstrip traces
Hi Steve,
"Steve Nosko" wrote in message ... This appears to be a blend of co-planar and microstrip. The freebie program TxLine 2003 "knows" about (can compute dimensions from impedances, etc.) CPWs, CPWs w/ground planes, microstrips, etc. and you can get some feel for how wide you can make a CPW w/GP gap before it's effectively a microstrip -- when the signal trace widths became nearly the same. I dug up some more Hittite app notes, and it does seem as though they're clearly playing in the CPW w/GP arena. They do point out that a major downside is that you eat up a fair number of routing channels with all those vias. I was part of a program to design a family of his same type of switch on GaAs, back in the early 90's and the co-planar line was used. I'm surprised just how much "chaos" there seems to be in the RF IC (primarily MMIC) arena... there are plenty of old standbys like Hittite, MA/COM (now Tyco), and Watkins-Johnson, but they seem to have a pretty well stocked "stable" of components and only introduce a handful of new ones every year. On the other hand, the little guys and mergers like Freecell, California Eastern, and RF Micro Devices seems to have lots of good parts that suddenly get discontinued, product introductions that turn out to be vaporware (a friend claims that RFMD is great for this -- their data sheets are really part of their marketing department, with specs drawn strictly from simulation -- they initially quote everyone a lead time of, say, 3 months, and if they don't get orders for large quantities, they just never fab the chip in the first place), and otherwise seem a little difficult to rely on at times! ---Joel |
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