[time-nuts] GPS Filter

Mike Feher mfeher at eozinc.com
Sat Mar 5 01:59:23 UTC 2011


Well, as you said John, for FWIW. In this case not much. As said low loss,
so increase in noise temp would be minimal, and, if it makes a difference
between an overloaded front end or a 0.5 dB loss in NF, it will be welcome.
Heck, we use filters in front of most of our Satcom LNBs at 21 GHz with
minimal effect. Been there and done that, as the saying goes. If necessary
the small increase in noise temp can easily be overcome by a slightly larger
aperture, especially since a view of the full sky is not really necessary
and birds below certain elevation angles are typically ignored by software
settings of one's own choosing. And again, I was talking about people who
have done this and can do it again. It would not be an issue for me. 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 J. Forster
Sent: Friday, March 04, 2011 8:47 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] GPS Filter

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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