[SI-LIST] Re: Antenna currents and digital Ground

  • From: "davidpauljones2003" <davidpauljones2003@xxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Wed, 08 Oct 2003 11:49:02 -0000

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
> 
> 
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