Lux, James P wrote: ... >>> But not so trivial to provide the DC blocks in the ground side of the coax. >>> The LNA is almost certainly some MMIC with RF ground==Vss >> Not any worse than providing the DC block on the center of the coax. > > There is a significantly different EMI/EMC impact for breaking the shield > than breaking the center conductor. Might not be an issue for a GPS Active > Antenna, but it's something to think about (we have some coaxial DC blocks > at work that break both, but also apparently make mighty fine slot radiators > at 30 GHz).
Yes, but we are talking about a GPS hockey puck, not a 30GHz device. At these "low" frequencies, life is pretty easy. A little careful pc layout work, and you could do the DC block using the board almost entirely. >>> And, don't forget that these are cost sensitive devices. 4 diodes and their >>> installation and board real estate costs money, as would the extra couple >>> capacitors for the DC blocks, etc. >> There is certain to be one diode on the hot lead anyway... engineers get >> nervous >> when they leave out stuff like that.. > > No.. I doubt there's a diode in the hot lead. These sorts of devices is a > low cost device designed to be hooked up a specific piece of equipment. > They'd trust that they designed the equipment it's connected to is of the > correct polarity. Hook it up backwards and it fries. > > There's also the forward voltage drop issue.. One or two diodes in series is > a significant power consumption bump in a system where you're evaluated on > microjoules/fix. Not a watch, but a hockey puck antenna. The receivers that use hockey puck antennas are typically powered by a car's electrical system. They have to ditch 8V anyway, might as well lose some of it in a pair of diodes. > so adding a bridge would eliminate that >> diode. A quick check of Mouser gives me a cost difference (qty 1000) of one >> dime >> for the added bridge and capacitor... 12 cents if there was no diode in the >> original >> circuit. Circuit boards on devices like hockey puck antennas tend to be >> sparsely >> populated, so I don't think it would make any difference there. > > There's a non zero cost at the manufacturer for inventory costs and incoming > inspection, as well as the additional cost for the extra components in the > pick and place machine, and the reduced machine throughput. This all adds > up. For a lot of circuits, the "other costs" could be 10-20x the actual > parts cost, especially for inexpensive passives like resistors and > capacitors. Agreed. >> It would be worth the cost if the antenna was meant to be a universal device, >> but probably not if it was intended to be used on only one receiver. > > > Which is why universal devices cost more.. (or, don't exist.. The price we > pay for leveraging off consumer commodity pricing is that what WE want isn't > made...it's something that's almost what we want.) Yep! Almost always true. The hockey puck antennas, were sold as a product by themselves, and there were a lot of receivers that used them. I think motorola and others thought of them as universal devices.... but I also think they thought of them as disposable, so it isn't clear that they cared if you burned one out by misapplication. I have a spare around the shop somewhere. If it isn't too destructive, I think I'll open it up and take a look. -Chuck Harris _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.