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GPS notch filter?

 
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roughleg(at)gmail.com
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PostPosted: Sun Feb 02, 2020 5:02 pm    Post subject: GPS notch filter? Reply with quote

Rooting around in boxes at the hangar the other day I came across a "thingy" (technical term) with part number TED 4-70-54. Apparently it is a notch filter to stop radio harmonics interfering with GPS reception (see SkyGeek listing). I'm using an MGL V16 radio, and GPS in the form of a Garmin G5 (internal antenna) and a SkyFYX for the ADS-B setup, plus possibly iPad on the panel and/or handhelds. The V16 will be under the passenger's seat, the SkyFYX above the passenger's head, and the G5 on the pilot's side of the panel. Airplane is a Zenith STOL CH 750.
Is there any good reason to incorporate the notch filter in my design? And if so, where best to put it?
Pat


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PostPosted: Sun Feb 02, 2020 6:23 pm    Post subject: GPS notch filter? Reply with quote

At 06:59 PM 2/2/2020, you wrote:
Quote:
Rooting around in boxes at the hangar the other day I came across a "thingy" (technical term) with part number TED 4-70-54. Apparently it is a notch filter to stop radio harmonics interfering with GPS reception (see SkyGeek listing). I'm using an MGL V16 radio, and GPS in the form of a Garmin G5 (internal antenna) and a SkyFYX for the ADS-B setup, plus possibly iPad on the panel and/or handhelds. The V16 will be under the passenger's seat, the SkyFYX above the passenger's head, and the G5 on the pilot's side of the panel. Airplane is a Zenith STOL CH 750.

Is there any good reason to incorporate the notch filter in my design? And if so, where best to put it?

It would not hurt anything to
install it in the antenna feedline
for vhf comm. The 13th harmonic
of vhf comm energy falls in close
proximity to the gps signals at 1575
or so MHz. GPS signals are very weak . . .
in fact you cannot generaly tune them
in on a normal receiver. Their signals
are literally below the atmospheric
noise floor.

The predictable, encoded nature of
received gps signals allows digital
processing to sniff out these tiny
voices and make sense of them.

Up until the dawn of gps implementation
on civil aircraft, there was no special
attention given to DO160 qualification
of vhf comm transceivers to offer special
protection to gps spectrum . . . after
all it WAS the 13th harmonic we're fussing
about.

But it turns out that significant numbers
of vhf comm systems already in place
generated 13th harmonic energy bad enough
to degrade if not snafu a gps signal.

Hence, the 'gps notch' filter band-aid
allowed these slightly-less-than-civilized
transceivers to coexist with gps equipment
on board an aircraft.

In the interim, gps receiver performance has
taken some quantum leaps . . . you got the
things in cell phones, cameras and wrist-watches.
At the same time, DO160 qualification requirements
added a band of additional protection to gps
spectrum. You can buy surface-mount gps notch
filters to be included in your vhf-comm
offering . . . and getting your DO160
blue ribbon requires that these frequencies
be protected.

The short answer is, you probably don't
need it. But do some flight testing to see
if gps signal strength values are depressed
while transmitting . . . you'd want to do
testing at 25 Khz steps from 120 MHz to
122 MHz . . .

-OR-

Check with the manufacturer of your transceiver
as to compliance with CURRENT gps protection
requirements . . .

-OR-

Stick the thing in anyhow . . . won't hurt a thing.


Bob . . .


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roughleg(at)gmail.com
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PostPosted: Sun Feb 02, 2020 8:25 pm    Post subject: GPS notch filter? Reply with quote

Thanks, Bob. As always, a very helpful and complete answer.
On Sun, Feb 2, 2020 at 7:29 PM Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:

Quote:
At 06:59 PM 2/2/2020, you wrote:
Quote:
Rooting around in boxes at the hangar the other day I came across a "thingy" (technical term) with part number TED 4-70-54. Apparently it is a notch filter to stop radio harmonics interfering with GPS reception (see SkyGeek listing). I'm using an MGL V16 radio, and GPS in the form of a Garmin G5 (internal antenna) and a SkyFYX for the ADS-B setup, plus possibly iPad on the panel and/or handhelds. The V16 will be under the passenger's seat, the SkyFYX above the passenger's head, and the G5 on the pilot's side of the panel. Airplane is a Zenith STOL CH 750.

Is there any good reason to incorporate the notch filter in my design? And if so, where best to put it?

  It would not hurt anything to
  install it in the antenna feedline
  for vhf comm.  The 13th harmonic
  of vhf comm energy falls in close
  proximity to the gps signals at 1575
  or so MHz.  GPS signals are very weak . . .
  in fact you cannot generaly tune them
  in on a normal receiver. Their signals
  are literally below the atmospheric
  noise floor.

  The predictable, encoded nature of
  received gps signals allows digital
  processing to sniff out these tiny
  voices and make sense of them.

  Up until the dawn of gps implementation
  on civil aircraft, there was no special
  attention given to DO160 qualification
  of vhf comm transceivers to offer special
  protection to gps spectrum . . . after
  all it WAS the 13th harmonic we're fussing
  about.

  But it turns out that significant numbers
  of vhf comm systems already in place
  generated 13th harmonic energy bad enough
  to degrade if not snafu a gps signal.

  Hence, the 'gps notch' filter band-aid
  allowed these slightly-less-than-civilized
  transceivers to coexist with gps equipment
  on board an aircraft.

  In the interim, gps receiver performance has
  taken some quantum leaps . . . you got the
  things in cell phones, cameras and wrist-watches.
  At the same time, DO160 qualification requirements
  added a band of additional protection to gps
  spectrum. You can buy surface-mount gps notch
  filters to be included in your vhf-comm
  offering . . . and getting your DO160
  blue ribbon requires that these frequencies
  be protected.

  The short answer is, you probably don't
  need it. But do some flight testing to see
  if gps signal strength values are depressed
  while transmitting . . . you'd want to do
  testing at 25 Khz steps from 120 MHz to
  122 MHz . . .

  -OR-

  Check with the manufacturer of your transceiver
  as to compliance with CURRENT gps protection
  requirements . . .

  -OR-

  Stick the thing in anyhow . . . won't hurt a thing.


  Bob . . .


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PostPosted: Sun Feb 02, 2020 9:15 pm    Post subject: GPS notch filter? Reply with quote

At 06:59 PM 2/2/2020, you wrote:
Quote:
Rooting around in boxes at the hangar the other day I came across a "thingy" (technical term) with part number TED 4-70-54. Apparently it is a notch filter to stop radio harmonics interfering with GPS reception (see SkyGeek listing). I'm using an MGL V16 radio, and GPS in the form of a Garmin G5 (internal antenna) and a SkyFYX for the ADS-B setup, plus possibly iPad on the panel and/or handhelds. The V16 will be under the passenger's seat, the SkyFYX above the passenger's head, and the G5 on the pilot's side of the panel. Airplane is a Zenith STOL CH 750.

Is there any good reason to incorporate the notch filter in my design? And if so, where best to put it?

It would not hurt anything to
install it in the antenna feedline
for vhf comm. The 13th harmonic
of vhf comm energy falls in close
proximity to the gps signals at 1575
or so MHz. GPS signals are very weak . . .
in fact you cannot generaly tune them
in on a normal receiver. Their signals
are literally below the atmospheric
noise floor.

The predictable, encoded nature of
received gps signals allows digital
processing to sniff out these tiny
voices and make sense of them.

Up until the dawn of gps implementation
on civil aircraft, there was no special
attention given to DO160 qualification
of vhf comm transceivers to offer special
protection to gps spectrum . . . after
all it WAS the 13th harmonic we're fussing
about.

But it turns out that significant numbers
of vhf comm systems already in place
generated 13th harmonic energy bad enough
to degrade if not snafu a gps signal.

Hence, the 'gps notch' filter band-aid
allowed these slightly-less-than-civilized
transceivers to coexist with gps equipment
on board an aircraft.

In the interim, gps receiver performance has
taken some quantum leaps . . . you got the
things in cell phones, cameras and wrist-watches.
At the same time, DO160 qualification requirements
added a band of additional protection to gps
spectrum. You can buy surface-mount gps notch
filters to be included in your vhf-comm
offering . . . and getting your DO160
blue ribbon requires that these frequencies
be protected.

The short answer is, you probably don't
need it. But do some flight testing to see
if gps signal strength values are depressed
while transmitting . . . you'd want to do
testing at 25 Khz steps from 120 MHz to
122 MHz . . .

-OR-

Check with the manufacturer of your transceiver
as to compliance with CURRENT gps protection
requirements . . .

-OR-

Stick the thing in anyhow . . . won't hurt a thing.


Bob . . .


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