X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from [67.8.179.94] (account marv@lancaironline.net) by logan.com (CommuniGate Pro WebUser 5.0.9) with HTTP id 1079438 for lml@lancaironline.net; Mon, 24 Apr 2006 19:37:10 -0400 From: "Marvin Kaye" Subject: Re: [LML] Re: Avionics=Black Art To: lml X-Mailer: CommuniGate Pro WebUser v5.0.9 Date: Mon, 24 Apr 2006 19:37:10 -0400 Message-ID: In-Reply-To: <004801c667a3$09e12cc0$6401a8c0@axs> References: <004801c667a3$09e12cc0$6401a8c0@axs> X-Priority: 3 MIME-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1"; format="flowed" Content-Transfer-Encoding: 8bit Posted for "terrence o'neill" : Brent, Many thanks. Very electrically challenged Terrence N211AL From: Brent Regan """ Shields Up! ...snip... The shield will work with only one end tied to ground because, like an antenna, the circuit is partially copper and partially an oscillating magnetic field. However, energy intercepted at the far end of the shield must transverse the length of the cable, and all its DC resistance, before finding ground. If there is a ground at both ends then the energy only has to travel, at most, half as far, and there are two parallel paths to take. Now imagine you have a device that draws 3 amps at 14 volts and is connected by a 24 gauge 25 foot power cable. The DC resistance of 25' of 24 gauge wire is 0.66 ohms. Multiply this times 3 amps and the voltage drop in the wire is 2 volts. 2 volts in the power wire AND 2 volts in the ground wire. This means that if you put a volt meter between the aircraft ground and the case of the device you would see a 2 volt potential. Now imagine you need to pass a digital signal from the device and that signal is referenced to ground. When the device sets the signal to "low" it may be ground at the device but it will be at 2 volts at the other end of the cable so the receiver may not interpret the signal as "low". Remember that the signal path does not have significant current flow so there is not the voltage losses that are present in the power cable. ...snip... Many engineers do not sufficiently understand or appreciate the issues. For example, Hamid and I designed a tablet computer for Canadian Marconi Electronics (CMA-1100 Pilot View) and as part of that effort, took the system to a test facility to verify the electromagnetic radiation of the system was within limits. The closest the system got to busting the radiation limit was 3db (1/2 the allowed power) at about 150 MHz. When CMC repeated the test with a CMC built system they reported their system was over the limit by more than 3 db, 4 times the emissions we measured. Many phone calls and thousands of dollars in travel and chamber time later we discover that CMC did not follow our recommendation for terminating the power and serial communication shields directly to the connector body at both ends. Rather, they terminated using the CMC standard method of soldering the shield to a 22 gauge wire and then grounding the other end of the two inch long wire. Replacing the cable with one that had properly terminated shields fixed the problem. CMC did not believe this to be true until we did side by side tests at an independent facility with a CMC engineer present. The above is a very brief review. I have omitted and simplified much for the sake of brevity. Regards Brent Regan """