[time-nuts] GPS down converter question

Bob Camp kb8tq at n1k.org
Tue Dec 1 17:57:32 EST 2015


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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