Thankyou for your response. I agree 100% with what you are saying. However, your suggestion of having separate ground islands for Digital and RF does not look possible with the design I am working on. This is because the Digital components on the top of the PCB are placed directly over the RF components on the bottom of the board. This is why it is necessary to have a Digital and RF reference Ground plane that essentially overlap each other. What I could do to "isolate" the digital and RF return currents is to control where I stitch these two Ground planes together. I could stitch these in the less sensitive areas of the RF circuits (away from the antenna feed). What is minimum distance would you suggest? This will cause a new problem of having two GND planes that are overlapping but not stitched together in some areas, (=parallel plate capacitor?). I am not sure if this is a problem since the two ground planes will be stitched together around the perimeter of the PCB and at reasonable distance from the antenna, creating an "equipotential" faraday cage. Do you agree with this approach? --- In si-list@xxxxxxxxxxxxxxx, "Salkow, Steven" <steven.salkow@xxxx> wrote: > Your plan to stitch grounds together, while providing a low inductance to ground, does however create a grid of grounding wherein signal ground-returns will share a portion of their current according to the ratios of the reactance of various ground paths and falling off due to LC, rapidly with distance versus frequency. Portions of the digital system transitions will added noise to the RF sections, which due to their smaller noise immunity, will have a deleterious effect. A better plan is to have a island of RF separate from the digital. All signals in/out through a bridge between these two sections. Keep the sensitive antenna far from where the single point of grounding occurs between the digital and analog sections. > The board stackup is a good plan. Where the sections divide, analog and digital, a transverse stitch of ground between the analog and digital is ok but allow multiple rows so as not to increase the ground inductance in this area due to too much metal removal. > > Keep in mind the heavily perforated area is mechanically weak and may require extra support. > > If thermal cooling is not a problem, you may add a perforated shield over the digital sections to minimize their radiation. > > Steve Salkow > Lockheed Martin > 3200 Zanker Road > San Jose, CA 95134 > (408) 473-4058 (san Jose) > (408) 742-4162 (Sunnyvale) > steven.salkow@xxxx > salkow@xxxx > (925) 462-1075 Home > (925-) 487-5946 Cell > (408) 468-7271 Numeric Pager > > > -----Original Message----- > From: davidpauljones2003 [mailto:davidpauljones2003@xxxx] > Sent: Tuesday, October 07, 2003 8:39 AM > To: si-list@xxxx > Subject: [SI-LIST] Antenna currents and digital Ground > > > Hello friends, > I am working on a wireless device and will try to simplify my query > as much as I can. > I am working on a multi-layer PCB with digital on one side and RF > circuits on the other. The Digital circuits have their own ground > plane and the RF circuits also have their own ground plane. The two > Ground planes are solid and entirely overlap eachother. The PCB stack > up is therefore as follows; Digital-Digital-Digital Ground-Power > plane-RF to digital connections-RF Ground-RF-RF (1-2-3-4-5-6-7-8). > There is one shield covering all of the digital circuits on the top > of the PCB and one on the bottom covering all of the RF circuits. > I propose to stitch RF and Digital grounds around the perimeter of > the PCB (which is also the boundary of the top and bottom shields) to > form a Faraday cage. I also intend to stitch the ground planes > together wherever a signal traverse between the two Ground planes. > I may also flood internal layers and stitch them to both grounds in > order to improve isolation from one side of the PCB to the next. > I believe this strategy will reduce emissions from my PCB and help to > isolate high frequency harmonic content from the digital circuits > signals from my sensitive RF circuits. > > This leads me to the question of the antenna ground. Here is where I > need some help/advice; > I believe the whole ground system constitutes the antenna ground. > Where will the antenna currents predominantly flow? > Will this be on the outside of the shield around the antenna feed > point? > Since all the Grounds overlap and are stitched together, will the > (high harmonic) digital return currents drown the small antenna > currents or degrade the radiated sensitivity of my receiver ? > Should I avoid stitching together my digital and RF Ground planes > close to the antenna? > > Thankyou for your time, > David > > > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxx with 'unsubscribe' in the Subject field > > or to administer your membership from a web page, go to: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxx with 'help' in the Subject field > > List archives are viewable at: > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > > > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxx with 'unsubscribe' in the Subject field > > or to administer your membership from a web page, go to: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxx with 'help' in the Subject field > > List archives are viewable at: > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu