[time-nuts] GPS down converter question
paul swed
paulswedb at gmail.com
Tue Dec 1 20:33:05 EST 2015
Digikey was a strike out with 1 filter for 86 cents but order was 1000
units.
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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