[time-nuts] GPS down converter question

paul swed paulswedb at gmail.com
Wed Dec 2 15:31:04 UTC 2015 

Sorry mouser electroncs.

On Wed, Dec 2, 2015 at 8:10 AM, Herbert Poetzl <herbert at 13thfloor.at> wrote:

> On Tue, Dec 01, 2015 at 08:33:05PM -0500, paul swed wrote:
> > Digikey was a strike out with 1 filter for 86 cents but
> > order was 1000 units.
>
> Sorry, but could you point me to the filter in question
> please, I couldn't find anything on digikey, but probably
> I was searching for the wrong keywords.
>
> Thanks in advance,
> Herbert
>
> > Mouser however has a wide assortment very reasonable and
> > by the single units.
>
> > Hardest thing will be soldering them.
>
> > Regards
> > Paul
> > WB8TSL
> >
> >
> > On Tue, Dec 1, 2015 at 8:29 PM, paul swed <paulswedb at gmail.com> wrote:
> >
> > > Hello to the group have indeed done the 1575 down to 35.42 to 75.42 and
> > > upconverter trick.
> > > Thats what I used for 2-3 years now and thought it was time to move
> beyond
> > > that approach. Especially due to the earlier conversation on old
> receivers
> > > and that they should still work just fine if you do not care about the
> date.
> > >
> > > I actually have 2 versions of the 35 to 75 converter. One using an
> odetics
> > > down converter and another using a starlink gps receiver. Both have
> 35.42
> > > MHz IFs. Old stuff you can get a soldering iron into.
> > >
> > > No intention to put this on the tower and mini-circuits makes a good
> BPF
> > > for the 75 MHz IF. Since I will believe the actual antenna has a 1571
> > > filter in it I was thinking of skipping it down in the shack.
> > >
> > > Will see what digikey and mouser has in the way of filters and if
> > > inexpensive may buy one. I keep thinking I may actually have one also.
> > > Thanks again everyone.
> > > Paul
> > > WB8TSL
> > >
> > > On Tue, Dec 1, 2015 at 7:53 PM, Alex Pummer <alex at pcscons.com> wrote:
> > >
> > >> for 70MHz it does not hurt to match the cable to the filter at the
> > >> antenna unit [down converter]  end  and also match the filter at the
> > >> receiver upconverter end, the cable will pick up enough noise to
> overdrive
> > >> the 70 something receiver's input  [ the "outside" field will drive a
> > >> current in the cable's shield, but not in the center conductor, that
> > >> current causes noise voltage between the two end of the cable's shield
> > >> which will end up at the input of the receiver, therefore it need to
> be
> > >> filtered out before it hits the mixer......also the down converter's
> LO's
> > >> reference is sensitive to the noise which the cable will pick up [
> will
> > >> cause phase noise ], therefore it needs to be filtered .....
> > >> That down up converter system is an interesting project but it is not
> > >> that simple as it looks
> > >> 73
> > >> KJ6UHN
> > >> Alex
> > >>
> > >>
> > >>   On 12/1/2015 2:57 PM, Bob Camp wrote:
> > >>
> > >>> Hi
> > >>>
> > >>> Here’s sort of a backwards look at it:
> > >>>
> > >>> Do you *need* an IF filter in the downconverter? By that I’m asking
> > >>> about a
> > >>> filter better than a simple LC tank. Did they put the filter in the
> > >>> downconverter
> > >>> or in the main box? I would think that putting a fancy filter up by
> the
> > >>> antenna
> > >>> would have been a less likely thing to do than putting it down in the
> > >>> main box.
> > >>>
> > >>> Bob
> > >>>
> > >>>
> > >>> On Dec 1, 2015, at 9:48 AM, paul swed <paulswedb at gmail.com> wrote:
> > >>>>
> > >>>> Thanks everyone. The Meinberg is nice and maybe available from Ebay
> by
> > >>>> Alex's link. But its 35.42 much as the Odetics down converter. I am
> > >>>> looking
> > >>>> to create a 75.42 Mhz IF.
> > >>>> Mini-circuits makes just the right parts. But had several IF
> bandwidths
> > >>>> available.
> > >>>> So will go with the 2 or so MHz filter as suggested.
> > >>>>
> > >>>> I have the typical GPS better quality high gain antenna 1/2" Heliax
> > >>>> feed to
> > >>>> a low noise gain block that makes up for the loss of a 8 X splitter.
> > >>>> I may add a 1575 filter ahead of the 10 db amplifier and then hit
> the
> > >>>> mixer. I think I have a filter. I actually question that I need the
> > >>>> filter
> > >>>> or 10 db amp. May build without it to see what happens. Can easily
> add
> > >>>> it.
> > >>>> The LO will be a mini-circuits dsn-2036 followed by a 10 db amp to
> drive
> > >>>> the mixer another mini-circuit DBM. The IF drives a bpf-a76+ and
> then
> > >>>> will
> > >>>> follow that with 30 db of gain at 75 MHz.
> > >>>> At least thats my thinking.
> > >>>> Regards
> > >>>> Paul
> > >>>> WB8TSL
> > >>>>
> > >>>> On Tue, Dec 1, 2015 at 1:36 AM, Magnus Danielson <
> > >>>> magnus at rubidium.dyndns.org
> > >>>>
> > >>>>> wrote:
> > >>>>> Hi,
> > >>>>>
> > >>>>> This is a side-track to Pauls original question, but maybe a nice
> > >>>>> little
> > >>>>> point to make now that Peter touched on the subject.
> > >>>>>
> > >>>>> To elaborate a little on C/A and multipath surpression.
> > >>>>> The multipath surpression of the receiver depends on code rate,
> > >>>>> bandwidth
> > >>>>> and correlator spacing. P-code is able to surpress more, and the
> C/A
> > >>>>> code
> > >>>>> errors look about the same as the P-code, but scaled accordingly.
> > >>>>> Increasing the bandwidth helps to reduce the C/A errors, but
> taking the
> > >>>>> next step of using narrow correlators further reduces the error.
> This
> > >>>>> is
> > >>>>> shown already in the classical Spiliker book, but further readings
> from
> > >>>>> Novatel could be nice.
> > >>>>>
> > >>>>> Increasing the bandwidth and narrowing the early and late
> correlator
> > >>>>> taps
> > >>>>> both have the effect of reducing the time over which energy goes
> into
> > >>>>> the
> > >>>>> E-L difference, and hence reducing the impact of multipath into the
> > >>>>> solution.
> > >>>>>
> > >>>>> Cheers,
> > >>>>> Magnus
> > >>>>>
> > >>>>>
> > >>>>> On 12/01/2015 06:00 AM, Peter Monta wrote:
> > >>>>>
> > >>>>> What should the IF pass band bandwidth be?
> > >>>>>>>
> > >>>>>>>
> > >>>>>>> For GPS C/A with wide correlator, about 2 MHz; if you want
> Galileo
> > >>>>>> BOC and
> > >>>>>> (eventually) GPS L1C, or legacy C/A with narrow correlator, about
> 8
> > >>>>>> MHz;
> > >>>>>> for GPS P code about 20 MHz.  Books on GNSS software receivers
> will
> > >>>>>> detail
> > >>>>>> the many tradeoffs available---if you're starting out with a
> > >>>>>> proof-of-concept lab receiver, go for 8 MHz.
> > >>>>>>
> > >>>>>> Cheers,
> > >>>>>> Peter
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