[time-nuts] GPS down converter question

paul swed paulswedb at gmail.com
Wed Dec 2 10:36:48 EST 2015 

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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