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

On 17 Feb, 11:15, joseph2k <quiettechb...@xxxxxxxxx> wrote:
bigorangebus wrote:
On 13 Feb, 19:13, mzen...@xxxxxxxxxx (Mark Zenier) wrote:
In article <1171304228.284296.308...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,

bigorangebus <bigorange...@xxxxxxxxx> wrote:
Heres my problem, the receiver just picks up too much noise.

1. Go with a ferrite loopstick antenna. Try pointing it vertically
so the null is at the zenith, and straight down into the ground.
A lot of the noise at that band is electrostatic, which your active
antenna seems to be designed to pick up.

2. Try for the narrowest bandwidth at the front end. Tune the antenna,
or have a high Q LC filter, or even build a crystal or ceramic resonator
filter at 518 (and 490) kHz. It only needs to be 500-1000 Hz wide.
That applies to your IF bandwidth, too.

2a. Since your IF is 6 kHz, watch out for images. That could double
your noise if the front end doesn't reject at the image frequency.

3. Look at "RTTY Terminal Unit"s, ie. fsk demodulators. The standard
design was two filters as narrow as possible (determined by the bit
rate) that were compared. One side could drop out and the other
half could still compare signal at that shift with the "no signal"
voltage.

Mark Zenier mzen...@xxxxxxxxxx
Googleproofaddress(account:mzenier provider:eskimo domain:com)

Regarding using a vertical ferrite loopstick antenna (I presume this
is a ferrite bar would with a coil as usual), if you put it
vertically, and the transmitter is a vertical mast..does this cause a
sensitivity issue? Normally ferrite rods are held horizontally. But
my antenna theory is basic so I'm probably missing something...

My problem with using the RTTY model is that lo drift will take me
right off the sweet spot (only have a 170Hz to play with between mark
and space).

IThanks Mark, this is very useful

Perhaps an old analog PLL would clean up the signal for further processing
in teh digital world.

--
JosephKK
Gegen dummheit kampfen die Gotter Selbst, vergebens.
--Schiller


Thats an interesting idea...I would need a slow moving error loop to
avoid pulling it all over the place. Unfortunately at an IF of 6kHz
and a 100baud Ive only for 60 cycles in which to lock and recognise a
particular frequency, but perhaps more like 20 cycles to allow for
reasonable edge detection between a logic 1 and zero. If i were using
a to part conversion it would be a neat solution and I'd have a higher
1st IF to minimise images.

I'm trying to resist putting a very tight filter on the front end as I
would like to be able to choose between 518kHz and 490kHz, although
practically this may be a reason why a certain commercial low end
receiver I tried (which is also dual channel) was so hit and miss.

.

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