[time-nuts] GPS down converter question

Bob Camp kb8tq at n1k.org
Wed Dec 2 17:42:34 EST 2015


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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