[time-nuts] GPS Filter

[J. Forster]

I'm not so sure.

A filter ahead of a preamp significantly increases the system Noise
Temperature.

GPS signals are weak and link margins are small. The receiver preamps are
already very low noise.

I'd think that a narrow filter might well drive up the systen NF to the
point it'd be useless.

FWIW,

-John

=============


> I used to make some interdigital filters and amplifiers in the early 80's
> for MDS TV reception in the 2.3 GHz range. One can easily fabricate a low
> loss narrow band filter at 1.5 GHz if need be, and as mentioned before,
> antennas should be easy as well. If this really becomes an issue, I am
> sure
> there will be a lot of solutions offered and anyone with some RF
> experience
> will also be able to handle it themselves. Regards - Mike
>
> Mike B. Feher, EOZ Inc.
> 89 Arnold Blvd.
> Howell, NJ, 07731
> 732-886-5960 office
> 908-902-3831 cell
>
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Ziggy
> Sent: Friday, March 04, 2011 7:47 PM
> To: time-nuts at febo.com
> Subject: Re: [time-nuts] GPS Filter
>
> The discussion got me to thinking about how we used to filter out nearby
> interference on amateur TV - namely by using interdigital filters. This
> led to a search for GPS interdigital filters which i did indeed find.
> See Alison Microwave website at
> <http://www.amlant.co.uk/DetailsAD430.htm> for one example of an
> integrated antenna/filter/preamp. (I'm sure these aren't cheap, but I
> haven't asked.) As for retrofitting, you could add a filter after the
> antenna/amplifier assembly but I might be concerned that the amplified
> GPS antenna is pretty wide and may have trouble with a Lightsquared
> transmitter nearby. There are passive antennas though, and there are
> in-line amps - you'd need to add the filter in between. We made these
> ourselves for 439 and 1296 MHz - GPS L1 isn't much above that so with
> some care it should be doable. The tuning can be finicky though :\
>
> Ziggy
>
> On 03/04/2011 03:31 PM, John Ackermann N8UR wrote:
>> Here's a measurement we did a few years ago on the HP 58535A:
>>
> http://www.febo.com/pages/hp_gps_splitter/port_1_hp_58535a_two_port_amp.png
>>
>>
>> John
>> ----
>> On 3/4/2011 1:31 PM, Bob Camp wrote:
>>> Hi
>>>
>>> Ok, now it's pretty obvious the RF world near your GPS will be
>>> changing a *lot* in the near future. Lightsquared and a bunch of
>>> similar outfits will be camping out right next door with very high
>>> power gear. They will be running 1.5KW from somewhere in town. GPS is
>>> running 30 watts from off planet.
>>>
>>> Has anybody tossed the various HP / Symmetricom GPS splitters on a
>>> network analyzer? If so, what do the filters in them look like?
>>>
>>> I probably should corner the market on these things before asking a
>>> question like that.
>>>
>>> The new neighbors will be at 1525 to 1559 MHz.  GPS L1 is at 1575.42
>>> MHz.  That's what we are using for timing. L2 is down at 1227.5,
>>> right now it's mainly military use. Obviously these guys are a bigger
>>> deal for civilians than the military.
>>>
>>> So the question is - do the built in splitter filters have any real
>>> rejection 15 to 50 MHz off of center?
>>>
>>> Probably worth checking. It would be a pleasant surprise if they
>>> turned out to be useful.
>>>
>>> Bob
>>>
>>>
>>>
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