[time-nuts] GPS down converter question

paul swed paulswedb at gmail.com
Tue Dec 1 20:29:15 EST 2015


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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