Thanks Ray for your interesting and informative reply.
Yes the 38mS was chosen for two reasons:
1) The timing circuits are analogue and are variable in production due to MOSFET source/gate threshold variations. 38mS allows headroom to absorb these tolerances.
2) More specifically the 38mS is 20 mS + 15mS + 2mS since we load our IDs post production and we have to get round the extra write charge time of around 15mS.(We use 20mS Charge Phase)
The technique harvests power from the inlay pickup coil and usesextremely low power to avoid compromising the inlay tag range. It does reduce it by a couple of centimetres but our access application can accept that. In assembly we use 1mm nylon washers to standardise the physical distance and therefore the mutual inductance twixt the coils. We epoxy pot the Stealth Key in a disc not much different from the W9QL package.
The ownership of the Stealth Tag are American Calan of Northwood N.H. and I will talk to them about co-operation with T.I.
I chose the LF HDX system because of my very long term experience of FSK in short wave communications starting with Army RCA transmitters in Malta in 1955/6 in teleprinter links to Libya of all places , so the merits of FM signal capture and noise survival appealed to me ! (Aso FCC lab testing was a dawdle). the theory is sound in all situations where the external system uses the 50mS or greater power phase. In the production model I have played safe by using a trimpot to set the 38kHz and a header shorting link to disable the Stealth mechanism during ID writing - as a safety measure. Our IDs provide an addition CRC and Mfg code with the 64 bit code space just to make absolutely sure that no animal can gain false entry. The product is the Calan-Broadbent feed door which has been in use in the USA since 1970. As a long retired engineer who developed this system I decided to bring it into the 21st century in a conservative manner. The T.I. LF-HDX is a very robust and cost effective system and the lamented departure of the RI-TRP-W9QL-30 just as we were manufacturing the first product in New Hamshire rather dinted faith in T.I. and makes me very nervous about the TMS3705 which has been an entirely trouble free technology for this application and easy to manufacture inmodern assembly facilities.
Further to the experience at a Nebraska USDA facility and the absence of co-existing problems, I hear that the other equipment provider uses a dual standard reader and I am suspecting that their tags are Mifar which of course would not look for the post power phase signal. I may be wrong but that seems likely.
I will get back to you after I have talked with AMCAL in Northwood.
As I said, very many thanks for your generous exposee on co-existing concepts. Our Stealth Key works, but just how high the yield will be in production remains to be tested. - it certainly gets us round co-existence with LF-HDX Dairy Parlours.
Best regards and a Happy Xmas.
Colin - Edinburgh - Scotland.