Re: [802.3af] voltage offset proposal (was 1-point detection,monotonicity)
Hi John
thank you, i guess a graph or some visual aid (graph) would make the original spec
readable, but when it is speced as a "text" it leaves things open to mis-interpretations.
roger
At 05:07 PM 8/3/01 -0400, John Jetzt wrote:
>Out of this discussion, I believe that I now have a better way
>to spec the PD signature. It has been misleading to say that
>the conditions for the required slope were that:
> v is between 2.8 and 10.0 volts, and
> i is between 0 and 500 ua.
>This leads some to believe that one could use any current
>between 0 and 500 to probe for the signature.
>Whereas one could probe with any voltage between 2.8 and 10.0
>and this will work for any valid signature,
>not every current between 0 and 500 will work all the time.
>Rather it depends on the offsets in the signature.
>Therefore, I propose to modify the PD signature requirements
>to be as below. The new wording has a required resistance over
>a voltage range, a required max voltage offset,
>and a required max current offset. When these are plotted out,
>we get the working current range of 0 to 500 ua:
>
> Signature resistance: same min and max as before, under the
> condition that the voltage is between 2.7 and 10.1 volts.
> (This has the slight voltage change to provide some tolerance
> overlap for the PSE.)
>
> Voltage Offset for the signature resistance: 0 to 2.7 volts.
>
> Current Offset for the signature resistance: 0 to 123 ua.
>
>This gives us the same overall possible current range as before,
>i.e., 0 to 500 ua. However, I believe that "500" number was
>somewhat arbitrary, but round. Now that we are specifying the
>current offset, let's make it round instead. Call it 100 ua,
>and the new possible current range would be 0 to 477 ua.
>
>John Jetzt wrote:
>
>> Roger,
>>
>> The 500 ua was chosen to allow the PD to have some amount of
>> current offset. Here is a way to visualize the slope requirement
>> of the PD: In the V-I space we define a rectangular area
>> where the sides are
>> top: v = 10.0 volts
>> bottom: v = 2.8 volts
>> left: I = 0.0 ma
>> right: I = 0.5 ma
>> The requirement on the PD is to have a V-I characteristic that
>> enters the space through the bottom and leaves through the top,
>> and inside the rectangle the characteristic must have the required
>> constant slope.
>>
>> In order to achieve this characteristic, the PD could theoretically
>> have a pure voltage offset of up to 2.8 volts. However, once the
>> characteristic enters the space it must have the constant slope.
>>
>> Similarly, the PD could theoretically have a pure current offset
>> of up to 0.123 ma.
>>
>> The above is the requirement on the PD. It does not matter if the
>> PD has an offset, and it does not matter how the PD achieves an
>> offset. It only matters that the slope in the area has the
>> required constant value.
>>
>> The requirement on the PSE is to look within the space for the
>> characteristic. The PSE is told what the must-detect slopes are,
>> and the PSE is warned that there might be an offset.
>>
>> Therefore, the must-detect characteristics for the PSE are:
>>
>> a) 19K to 26.5K ohm resistance between pairs,
>> b) no more than 110 nF of capacitance,
>> c) a voltage offset of at most 2.8 volts DC in the signature,
>> c') a current offset of at most 0.123 ma in the signature.
>>
>> John
>>
>> R karam wrote:
>>
>> > Hi John
>> >
>> > I think the 10.1v Max conflicts with a 500ua max out of the PSE
>> > during detection for 500ua* max Rsig=26.5k (if i got this right)
>> > that puts is at 13.25v max. do you know where the 500ua spec
>> > came from?
>> >
>> > also are we happy with the 2.8v being one VBE (diode drop away)
>> > say at cold we allow 0.8v per diode that is 1.6v is this too tight?
>> > thank you for taking the time to do this - there is no shortage of
>> > confusion on the numbers...
>> >
>> > regards
>> > roger
>> >
>> > >I have a proposal for the signature voltage offset requirement
>> > >as seen by the PSE. However, before I generate an official
>> > >comment to the editor, I am sending it to the reflector
>> > >for comment. I believe the proposal below is in
>> > >the spirit of our agreements, and it also eliminates
>> > >the one-point detection "loophole" in our former discussion.
>> > >
>> > >The Proposal (in two parts):
>> > >Part 1: Change the valid detection signature maximum
>> > >voltage offset that the PSE might see to 2.8 volts,
>> > >instead of 1.5 volts (see p.8, l.11).
>> > >Part 2: Change the valid detection signature
>> > >voltage range for the PD V-I characteristic
>> > >to 2.7 - 10.1 volts, instead of 2.8 - 10 (see p.16, l.22).
>> > >
>> > >Rationale for Part 1: the 2.8 volt offset:
>> > >Several meetings ago we agreed that the detection
>> > >test voltage would be 2.8 to 10 volts.
>> > >The 2.8 number was selected to allow for
>> > >voltage offsets.
>> > >
>> > >Where did the 1.5 number come from?
>> > >I think this is what happened.
>> > >The PSE Detection Source Material gave its "must pass"
>> > >requirements in the form of circuits that could include
>> > >the "typical" two diodes. D1.2 put this into words.
>> > >
>> > >Rationale for Part 2: the 2.7 - 10.1 PD voltage range:
>> > >This leaves a 100mV band for qualification testing,
>> > >and ensures that a slightly-out-of-spec PSE
>> > >will still detect a slightly-out-of-spec PD.
>> > >(Thanks, Dave D.)
>> > >
>> > >John Jetzt
>> > >