On Sun, 14 Oct 2012 13:09:06 -0700, Joerg <invalid@invalid.invalid>
wrote:
>John Larkin wrote:
>> On Sun, 14 Oct 2012 07:52:40 -0700, Joerg <invalid@invalid.invalid>
>> wrote:
>>
>>> John Larkin wrote:
>>>> On Sat, 13 Oct 2012 16:06:24 -0700 (PDT), "langwadt@fonz.dk"
>>>> <langwadt@fonz.dk> wrote:
>>>>
>>>>> On 14 Okt., 00:41, John Fields <jfie...@austininstruments.com> wrote:
>>>>>> On Sat, 13 Oct 2012 08:50:12 -0700, John Larkin
>>>>>>
>>>>>> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
>>>>>>
>>>>>>> We are considering sending data over CAT6 twisted-pairs, from one FPGA
>>>>>>> to another at some 10s of meters distance. It might be prudent to
>>>>>>> transformer-couple the data, to avoid ground-loop common-mode hazards,
>>>>>>> and the obvious choice would be to use RJ45 connectors with built-in
>>>>>>> Ethernet magnetics. These seem to have inductance in the 400 uH range,
>>>>>>> which gives a low-end frequency response in the 40 KHz sort of range.
>>>>>>> The data would have to be DC-balanced, and we'd have to use NRZI
>>>>>>> coding, bit filling, whatever to avoid long runs of 1s or 0s from
>>>>>>> adding any low-frequency components. We have 80 or so streams arriving
>>>>>>> at the main FPGA from the field, so we may not have enough FPGA
>>>>>>> resources to do full 8b10b or some such encoding; we may have to
>>>>>>> invent something dumber. Our bit rates will be in the 25-125 mbps sort
>>>>>>> of range.
>>>>>> ---
>>>>>> Invent something dumber?
>>>>>>
>>>>>> It sounds like you've bit off more than you can chew, the contract
>>>>>> doesn't allow you to split, and you're blaming the FPGA for your
>>>>>> nervousness.
>>>>>> ---
>>>>> if inventing something dumber achieves the goal of making what you
>>>>> need
>>>> >from what you have, I'll call that success
>>>>>>> Anyhow, I started playing with pushing uncoded pseudo-random data
>>>>>>> through the magnetics. I used the standard LT Spice "digital" parts
>>>>>>> and found that they would NOT make a working shift register. I had to
>>>>>>> edit all the flops to add the "TD=1n" Spice directive to give them
>>>>>>> some prop delay.
>>>>>> ---
>>>>>> Since LTspice can't read your mind, and you want to use the behavioral
>>>>>> models, allowing you to define the prop delay is a good thing.
>>>>>> ---
>>>>> sure being able to define prob delay is good, but it is still a bit
>>>>> suprising that LTspice doesn't have some internal timestep delay or
>>>>> order of operations that would make a simple shift register work out
>>>>> of the box
>>>> What I found is that, if the D input of the first flop is high, then
>>>> the first clock loads 1's into ALL the flops of a shift register.
>>>> Somehow I didn't expect that.
>>>>
>>>>>>> The RC lowpass filter below gives a quick visual indication of the
>>>>>>> sequence periodicity. The sequence taps are from AoE page 657.
>>>>>> ---
>>>>>> Have you finally lost your mind?
>>>>>>
>>>>>> Your circuit shows only a basic 7 bit PRSG with no data input port and
>>>>>> no sync.
>>>>>>
>>>>>> --
>>>>> it shows what happens to a "random" 20MHz signal through a model of
>>>>> an
>>>>> ethernet transformer that was the point I assume
>>>> Right. It was to evaluate how a baseband data pattern makes it through
>>>> the magnetics. Of course, the 2^7 PRBS is a rough model of whatever
>>>> data we eventually have to ship, and we'll probably need to run-limit
>>>> or scramble our data somehow.
>>>>
>>>> NRZI would be better, but can still be fooled by data patterns.
>>>
>>> If you used mid-biased Schmitt inputs, how?
>>>
>>> Except for the very first state after power-up but that could be dumped.
>>
>> The low-frequency rolloff makes the receive comparator baseline shift
>> when it sees a long run of 1's or 0's. You can see that in my sim by
>> lowering the clock frequency. The problem is, if I want to push the
>> data rate as high as possible (el customer wants to ship a lot of data
>> ASAP) the eye diagrams at the end of the cable are by definition not
>> perfect. So the baseline shift will cause errors.
>>
>
>If you really max out the link or the RX/TX chips then you can't do
>cheap NRZ, of course. But as they say, one has got to pay for what one
>gets. So if the customer wants to multicast lots of HDTV channels across
>it then it'll take some serious hardware. I found that if the
>transitions are fast enough the Schmitt method is ok.
We'd like to go simple NRZ (and skip the 10/8 rate loss, and the
complexity of 8b10b) but we do need to limit the run lengths somehow.
My sim is a first shot at examining the effect of run length. If I had
a good cable model, I could add that, and a comparator, and do eye
diagrams. I'll probably just do it experimentally.
>
>
>> We are doing some simple cable equalization, but heroic equalization,
>> like Gb Ethernet does, isn't practical in this system.
>>
>> 8b10b goes to extremes to make sure there is zero DC component in the
>> data, and very little low-frequency stuff. It keeps a running 1s/0s
>> longterm count, and substitutes alternate 10b symbols to servo that to
>> zero.
>>
>>
>> Hey, this same customer is now insisting on worldwide EMI compliance
>> (for a box with 80 connectors, and roughly 140 high-speed i/o
>> signals). ...
>
>
>If this demand is accompnaied by a commensurate check that's ok :-)
>
>You can go to Elliott or whoever is your favorite EMC place and have it
>tested for 100-something countries. But that does get expensive. And
>keep in mind that some countries now go to 6GHz for EMC.
>
>
>I was thinking I could get one of those little USB spectrum
>> analyzers and one of those surfboard-looking antennas. I could run the
>> analyzer on a laptop. I'd haul the DUT, the antenna, and the laptop/SA
>> up to Truckee and put it all down on tree stumps for open-field
>> pre-lab testing. Whose SA did you like?
>>
>> Ever used these?
>>
>> http://www.aaronia.com/products/antennas/HyperLOG-7040-LogPer-antenna/
>>
>
>Don't know the antenna and I am not so partial to their analyzers, I
>favor another kind:
>
>http://signalhound.com/
Looks good. Thanks. $919 for 4 GHz is impressive.
>
>I got the 4.4GHz version, the 12.4GHz version came out a month later. It
>has already paid for itself by avoiding a chunk of rental costs at a
>client, plus the hassle of packaging and shipping.
>
>But keep in mind that those things are SDR so they can't deal with fast
>pulse stuff, might miss it or mis-interpret the level. There is a trick
>though, I unhook image-reject for a second once in a while, to see if
>any missed nasties pop up. So it's not quite like the old HP boxes but
>one can't beat the portability.
What do you use for antennas?
--
John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators