Re: help with navtex/medium wave receiver sensitivity and decoding

On Feb 12, 4:57 pm, "bigorangebus" <bigorange...@xxxxxxxxx> wrote:
On 12 Feb, 21:13, "john jardine" <j...@xxxxxxxxxxxxxxxxxxxxx> wrote:





"bigorangebus" <bigorange...@xxxxxxxxx> wrote in message

news:1171304228.284296.308490@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Does anyone have any tips on increasing sensitivity for a medium wave
receiver trying to distinguish navtex 170Hz phase shifts on a low
power carrier at 518kHz?!

Ive been on and off trying to do this project for my boat for a year.
I say off and on, its become most frustrating! I can't use a loop
antenna because its just too big and too directional for use on a
boat. So I'm currently using a Nasa Marine active aerial (it is just
a small plate attached to a standard fet common source driver
circuit). I have that going down some coax through capacitive
impedance transformer and inductor creating some extra front end
selectivity, to the input of a 612 mixer, getting 6kHz IF (after being
mixed with a lo). I'm driving the 612 differentially, as its a slight
improvement over single ended. From here Ive tried using several op
amp filter circuits, and they all perform adequately.

Heres my problem, the receiver just picks up too much noise.
Everything interferes with it, even the scan on my oscilloscope. Even
without this, taking the aerial out of the room, the actual noise in
the system is too high to pick up anything but the strongest navtex
signals (which are decoded). Of course AM radio broadcasts are way
above the background noise, and navtex signals are much lower power.
And AM radio has the help of our ears to tune out the noise.

It works better when closer to the transmitter of course, but the
navtex spec says you should be able to pick up stations 400 miles
away, and i'm only just decoding the local one 100 miles away.

So my question is, does anyone know of any special techniques on the
RF side that can improve my noise handling/signal integrity? (my
expensive sony worldband receiver seems pretty good at it on SSB).
And....does anyone know how the upper market receivers get such a good
signal? My receiver just samples the input frequency (as does the
Nasa low end I think), so is very susceptible to any interference.

Grateful and very interested to hear views on this.

Many Thanks
Andy

I'm at a different location of course but a speccie analyser fed by a high
impedance buffer with a 18" telescopic aerial plugged in, just shows noise
and more noise. Biggest signal was a national radio station about 900kc at
8mV. 0-1Mc background about 0.5mV. Only starts cooling off above about 10Mc.
Using 1kc bandwidth gave a few carriers (modulation=?) around the 518kc area
but they were at the 20uV level and sitting barely above a buzzsaw of noise
filling all the frequencies and sourced from the aerial, local equipment
and PC.
Don't know 'Navtek' but I'd try very strong bandpass filtering at 518kc
after the FET. (you've probably not really enough wideband signal voltage to
overload it and may as well take benefit from the increased signal level).
The mixer is also going to give image frequency 6kc responses, these are
unfilterable. I'm guessing you're running the LO at 512kc from a 4060 type
xtal divider. The 3rd 5th 7th and 9th harmonics will easily mix with
up-spectrum buzzsaw noise and shift it down also to the 6kc slot.
Commercial stuff will filter, mix up to maybe a 60Mc IF to lose image
responses, filter and then mix back down again to whatever, (and then filter
again and once more for luck :).

--
Posted via a free Usenet account fromhttp://www.teranews.com

Interesting... yes the incoming signal is 518k and I'm mixing it with
512k generated from an 18.430M crystal divided down. This generates a
bit of noise, and there is some chance of that getting into the
system. However, that was the first thing i checked..I disabled it
and the noise level visible on a scope drop by perhaps 10 to 20%, pre-
filter, however there is very little evidence of a drop after
filtering. The signal is a freq shift of 170Hz so detecting it is
difficult even with a clean signal, and I need to do it digitally
using the lo generator as a reference, otherwise theres no hope of
tracking the 170Hz as a crystal is not accurate enough to use absolute
frequencies.
Basically you're looking at an intermittant signal (perhaps a 10min
burst every 3 hours in peak times that are close enough to see), which
is from a transmitter putting out 100W to 1kW, 100 miles away, which
is substantially less than a commercial AM station.
Also, the inverter on my low voltage kitchen lights screws the signal
audible on my multiband sony recever, as does the TV set, my
oscilloscope if it gets too close, any PC and the LCD display that
shows the Navtex information. So its a bit of a harsh environment for
it.
The FET front end already has a 60Q bandpass filter, which ive tuned
for peak (need some lee way as the next step is to dual band and pick
up 490khz which is the local navtex), i then go into a tuned
circuit(est Q80) with another can again peaked, and output this into
the differential inputs of the 612. So two filters pre 612, then band
pass filter with a Q of 30 before trying to detect the signal.
I would have run the LO higher to make filtering a little easier, but
I need a low LO for my software to decode.
I'd love to try it on an HP, but so far Ive been outbid on ebay! I
think I'm being a cheap on my bids...! I'll see if I can borrow one to
check where the noise might be coming from.
You can hear navtex by tuning near it on SSB, its sounds a lot like
RTTY.
Andy- Hide quoted text -

- Show quoted text -

There are some fundamental differences between the way an E field
antenna (like a probe) and an H field antenna (like a loop) respond to
locally generated noise.
Your may want to investigate the Ryan storm scope that can determine
the distance of a lighting strike by comparing the relative amplitude
of the e and H field. You may want to inquire over at
rec.amateur.antenna. And you may want to consider using two
perpendicular loops.

Mark



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