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

  • From: "Salkow, Steven" <steven.salkow@xxxxxxxx>
  • To: davidpauljones2003@xxxxxxxxx, si-list@xxxxxxxxxxxxx
  • Date: Tue, 07 Oct 2003 12:36:19 -0700

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@xxxxxxxx
salkow@xxxxxxxxxxxx
(925) 462-1075 Home
(925-) 487-5946 Cell
(408) 468-7271 Numeric Pager


-----Original Message-----
From: davidpauljones2003 [mailto:davidpauljones2003@xxxxxxxxx]
Sent: Tuesday, October 07, 2003 8:39 AM
To: si-list@xxxxxxxxxxxxx
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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