[time-nuts] GPS down converter question

Herbert Poetzl herbert at 13thfloor.at
Wed Dec 2 08:10:27 EST 2015


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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