[time-nuts] Thunderbolt - trouble locking with some types of antennas

Graham / KE9H ke9h.graham at gmail.com
Tue Aug 1 15:11:13 EDT 2017


Dider:

This is a CDMA signal. (With a 'chip' rate that far exceeds the information
rate.)

If you put a different correlator on every multipath signal, which are each
differently delayed in time, then they can be independently demodulated.
(Or time shifted and added back together with some quality indicator for
weighting.)
So, in CDMA, multipath is used as a form of (time) diversity reception and
will improve the signal to noise of the combined signal.

By definition, the signal with the least time delay either is, or is
closest to, the most direct path.

You don't have to necessarily fully demodulate this early signal by itself,
just know what its timing is. (And remember where it was, if fading in and
out.)

Since multi-path is a destructive mechanism in most narrow band radio
systems, the above may not be intuitive to people not familiar with CDMA
and "rake receiver" based systems.

Multipath helps, not hurts, these systems, as long as the multipath delays
are most of one chip apart, or more.

--- Graham

On Tue, Aug 1, 2017 at 1:11 PM, Didier Juges <shalimr9 at gmail.com> wrote:

> "The newer the receiver, the more horsepower in the silicon. In the case of
> GPS, that
> gives you more correlators to do DSP. The sensitivity improvement is a
> direct result
> of that. If you take a look at the guts of a TBolt, they date to the late
> 1990’s. That’s
> a long time in silicon years …."
>
> It seems that more correlators would speed up the time to first fix, not
> necessarily the sensitivity, particularly I do not see how it would
> directly affect the capability to stay locked when signal strength
> fluctuates?
> On the other hand, more correlators may help when there is multipath and a
> whole bunch of extraneous signals are fed into the receiver, so maybe the
> apparent lack of sensitivity is really the inability to see the signal from
> the chaff, not necessarily sensitivity in terms of noise figure.
>
>
> On Mon, Jul 31, 2017 at 3:06 PM, Bob kb8tq <kb8tq at n1k.org> wrote:
>
> > Hi
> >
> > > On Jul 31, 2017, at 3:32 PM, Didier Juges <shalimr9 at gmail.com> wrote:
> > >
> > > The Thunderbolt is well known for not having the best sensitivity among
> > GPS
> > > receivers. It seems that timing receivers in general, particularly
> those
> > of
> > > the same generation as the Thunderbolt are not as sensitive as
> navigation
> > > (possibly newer) GPS receivers. It may be because they are expected to
> > run
> > > with amplified antennas?
> >
> > The newer the receiver, the more horsepower in the silicon. In the case
> of
> > GPS, that
> > gives you more correlators to do DSP. The sensitivity improvement is a
> > direct result
> > of that. If you take a look at the guts of a TBolt, they date to the late
> > 1990’s. That’s
> > a long time in silicon years ….
> >
> > Bob
> >
> >
> > >
> > > Based on the spec you wrote, it looks like your antenna has no gain, so
> > > definitely I would expect less than good performance.
> > >
> > > My 3 Thunderbolts have been running with inside antennas (2 pucks and
> one
> > > Trimble Bullet) but my ham shack is upstairs and other than the ceiling
> > and
> > > the shingle roof, there are no other obstructions and they are doing OK
> > not
> > > great (all 3 go on holdover somewhat regularly). I am now running one
> > > downstairs (while I work on the new software for the TB Monitor) with
> 50
> > > feet of RG-58 going to a HP 58532A antenna somewhat in the clear but
> > only 8
> > > feet above ground with significant obstructions in pretty much all
> > > directions due to the low height and the Thunderbolt is happy as a clam
> > in
> > > spite of the significant losses in the cable. The HP antenna works much
> > > better than the Trimble Bullet antenna.
> > >
> > > On Mon, Jul 31, 2017 at 12:19 PM, Jerry <jsternmd at att.net> wrote:
> > >
> > >> Due to access problems, I run my Thunderbolt with a Symmetricom 58532A
> > >> antenna placed indoor near a window facing South… can’t get much worse
> > but
> > >> most of the time it will be locked onto 3 or 4 satellites.    I
> recently
> > >> bought a www.leobodnar.com <http://www.leobodnar.com>  GPSDO for my
> SDR
> > >> ham
> > >> radio setup.  I was very surprised to find that this minimalist GPSDO
> > using
> > >> a small patch antenna with internal LNA placed near my window had
> > satellite
> > >> & PPL lock within a few seconds.  It requires 3-4 satellite locks for
> > its
> > >> PPL.  However, when I attached the patch antenna to my Thunderbolt –
> > >> satellite signal strength were zero or minus for all satellites.  The
> > specs
> > >> for the patch antenna are listed below.
> > >>
> > >>
> > >>
> > >> I would appreciate any advice understanding this behavior.
> > >>
> > >>
> > >>
> > >> Jerry NY2KW
> > >>
> > >>
> > >>
> > >> Center Frequency 1575.42MHz±3 MHz
> > >>
> > >> V.S.W.R 1.5:1
> > >>
> > >> Band Width ±5 MHz
> > >>
> > >> Impendence 50 ohm
> > >>
> > >> Peak Gain >3dBic Based on 7×7cm ground plane
> > >>
> > >> Gain Coverage >-4dBic at –90°<0<+90°(over 75% Volume)
> > >>
> > >> Polarization RHCP
> > >>
> > >> LNA/Filter
> > >>
> > >> LNA Gain (Without cable) 28+/-3dB
> > >>
> > >> Noise Figure 1.5dB Typ.
> > >>
> > >> Filter Out Band Attenuation (f° =1575.42MHz)
> > >>
> > >> 7dB Min f0+/-20MHZ
> > >>
> > >> 20dB Min f0+/-50MHZ
> > >>
> > >> 30dB Min f0+/-100MHZ
> > >>
> > >> V.S.W.R <2.0
> > >>
> > >> DC Voltage 2.7V/3.0V/3.3V/5.0V/3.0V to 5.0V/other
> > >>
> > >> DC Current 5mA /11mA/15mA Max
> > >>
> > >>
> > >>
> > >>
> > >>
> > >>
> > >>
> > >> _______________________________________________
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