[SI-LIST] Re: Dielectric constant for FR4 at above 1Ghz frequency

ron miller wrote:

>
>
> A paper by IBM a couple years back showed that skin effect dominates 
> up to about 1 Ghz
> at which point the loss(s-11) takes a big slope upward, more than 
> double the slope below
> 1 Ghz.  On a linear(freq) vs log(db) scale the slope is constant about 
> and below the break-point
> at 1 Ghz.
>
> However, losses are not well documented by the laminate manufacturers 
> and while one may
> specify loss at 1 Ghz another at 10 Ghz etc, there is no 
> concensus.about how the loss varies with
> frequency and by manufacturer and their various grades of epoxy glass.
> .
> A standards body should probably measure the S-parameters for several 
> laminates and manufacturers.
>
> Ron Miller
>
>
>
> Alex Horvath wrote:
>
>>Exactly, it seems to be fairly common misconception that skin effect losses 
>>always dominate. Dielectric losses are proportional to frequency, although 
>>they start out lower they will always exceed skin effect losses between 500 
>>and 1Ghz or so. The biggest factor affecting this seems to the resin content 
>>of the PCB.
>> 
>>I'd like to hear more about the properties of the resin, if it can be 
>>manipulated for less loss and what are the tradeoffs. I wasn't able to find 
>>out much about resin properties on the web.
>> 
>>Thanks
>>steve weir <weirsp@xxxxxxxxxx> wrote:
>>Stuart, for common dielectrics, somewhere in the high hundreds of MHz 
>>dielectric loss overtakes loss from skin effect.
>>
>>Steve.
>>At 05:16 PM 11/24/2003 -0500, Stuart Brorson wrote:
>>  
>>
>>>True enough, but aren't the dielectric losses in FR-4 more or less
>>>frequency independent (i.e. to the <10% level that the dielectric
>>>function is constant)?
>>>
>>>Anyway, Tom is right: you need to consider the frequency variation of
>>>both Dk (the dielectric function) and Df (the dielectric loss
>>>function). However, both are typically dominated by skin effect
>>>losses, particular in the GHz range.
>>>
>>>Stuart
>>>
>>>
>>>    
>>>
>>>>To make the answer complete you need also to include dielectric losses 
>>>>      
>>>>
>>>which
>>>    
>>>
>>>>also are frequency dependent.
>>>>
>>>>Tom Dagostino
>>>>Teraspeed Consulting Group LLC Teraspeed Consulting Group LLC
>>>>2926 SE Yamhill St. Device Modeling Division
>>>>Portland, OR 97214 13610 SW Harness Lane
>>>>Beaverton, OR 97008
>>>>http://www.teraspeed.com 503-430-1065
>>>>tom@xxxxxxxxxxxxx
>>>>
>>>>
>>>>-----Original Message-----
>>>>From: si-list-bounce@xxxxxxxxxxxxx
>>>>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Stuart Brorson
>>>>Sent: Monday, November 24, 2003 1:22 PM
>>>>To: andyk@xxxxxxxxxx
>>>>Cc: si-list@xxxxxxxxxxxxx
>>>>Subject: [SI-LIST] Re: Dielectric constant for FR4 at above 1Ghz
>>>>frequency
>>>>
>>>>
>>>>If I am not mistaken, your question is equivalent to this one: Is the
>>>>dielectric function constant over the 1 MHz -- 1 GHz range? The
>>>>answer is: It is not constant, but varies by several percent or more
>>>>over that range for many FR-4s. (FR-4 is not a single material, but
>>>>is a family of materials, each different depending upon your PCB
>>>>vendor, and whatever batch of materials he was shipped on any
>>>>particular day.)
>>>>
>>>>However, the bigger problem for you to think about is the loss due to
>>>>the skin effect in your copper traces. The skin effect losses vary as
>>>>the square root of the frequency, IIRC. The frequency-dependent loss
>>>>due to the skin effect dominates over the frequency variation of the
>>>>FR-4's dielectric constant.
>>>>
>>>>Here are some references:
>>>>
>>>>http://www.tdasystems.com/library/appnotes/siml0803.pdf
>>>>http://signalintegrity.com/Pubs%5Cnews%5C4_5.htm
>>>>
>>>>Stuart
>>>>
>>>>
>>>>      
>>>>
>>>>>Hi all,
>>>>>
>>>>>I am designing a high speed PCB at over 1Ghz frequency using FR4 
>>>>>        
>>>>>
>>>material=
>>>    
>>>
>>>>>.
>>>>>I am wondering how much the dielectric constant will be affected by the
>>>>>frequency over 1Ghz as compared to 1Mhz?
>>>>>
>>>>>Thank you
>>>>>
>>>>>Regards,
>>>>>Andy
>>>>>        
>>>>>
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>>
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-- 
Ronald B. Miller, Microwave/SI Engineer  RAIL = NOISE + REFLECTIONS
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