[time-nuts] GPS down converter question

paul swed paulswedb at gmail.com
Wed Dec 2 18:39:55 EST 2015


Bob
Just adding that this is home brew so no real boards. At the IF level thats
both large enough and easy enough to add to a ground plane. So its
reasonable to build the basics and add stuff as needed.
The LNA front end is cheap so will get that just to try. Have to look at
the hot air suggested above to see what that might cost. I shudder to think
and actually may not be reasonable to even consider.
But even the front end may be cheated I suspect. Mixer and synth will be
the mini-circuits parts. I can see how far that gets me. IF gain with low
noise and solderability is really reasonable today for 75 Mhz and can be
added.
Regards
Paul
WB8TSL

On Wed, Dec 2, 2015 at 5:42 PM, Bob Camp <kb8tq at n1k.org> wrote:

> Hi
>
> Indeed (as I suspected) that 75 MHz filter runs up the total parts cost
> quite a bit.
> First thing I’d do is take a look at the board this connects to. Does it
> already have
> a narrowband filter (at 75 MHz) on it?
>
> Bob
>
> > On Dec 2, 2015, at 10:36 AM, paul swed <paulswedb at gmail.com> wrote:
> >
> > To Bobs comments your right. That 75 MHz may not be needed because of the
> > 1575 input filter. That would save $29.
> > By the way I was shocked to see for all of $3 complete 1575 filter and
> LNA
> > chips. The only nasty challenge is the chips are extremely small. The
> cost
> > is low enough I can purchase several in case I screw it up. Its clearly
> > going to be at my maximum soldering skills.
> > Regards
> > Paul
> > WB8TSL
> >
> > On Wed, Dec 2, 2015 at 10:32 AM, paul swed <paulswedb at gmail.com> wrote:
> >
> >> My bad mixing threads here the 1575 filter is in mouser and digikey has
> >> them.
> >> The 75 MHz is straight from mini-circuits.
> >> Regards
> >> Paul
> >> WB8TSL
> >>
> >> On Wed, Dec 2, 2015 at 10:31 AM, paul swed <paulswedb at gmail.com> wrote:
> >>
> >>> 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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