[time-nuts] Oncore, Trimble Antennae

Lux, James P james.p.lux at jpl.nasa.gov
Wed Oct 22 14:34:06 UTC 2008




On 10/22/08 7:20 AM, "Chuck Harris" <cfharris at erols.com> wrote:

> Lux, James P wrote:
>>
>>
>> On 10/21/08 9:31 PM, "Chuck Harris" <cfharris at erols.com> wrote:
>>
>>> Tom Van Baak wrote:
>>>>> Both the Trimble and Motorola modules use active antennae with 5V power
>>>>> - what I don't know is whether they are the same polarity.
>>>>> all my trimbles and oncores have +5 on center, ground on shield...
>>>> I'd be interested to know if any antennas are in fact the other
>>>> way around. Never even considered that.
>>> It would have been fairly trivial for them to put a bridge rectifier
>>> before the amplifier's power stuff, allowing for either polarity.
>>>
>>  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).

>
>> 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.

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.


>
> 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.)

Jim






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