[time-nuts] Oncore, Trimble Antennae

[Chuck Harris]

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