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 <[email protected]> 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 <[email protected]
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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