help with navtex/medium wave receiver sensitivity and decoding

[Homer J Simpson]

no, by just summing you'd still get two nulls. (at 135 and 215 degrees)

No. Only the nulls are sharp. You can just mix the signals.



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[joseph2k]

It seems to me you could perhaps used a tuned circuit there, with
sufficient bandwidth.
In fact the active antenne you describe perhaps needs an LC to ground.
Else indeed any spike will overload the FET, easy with a 'metal plate'.

Actually accurately placed high loaded Q (over 100, can use up to 1000) in
the RF section will actually help this mode (50 baud fsk 170 Hz shift).

 

[joseph2k]

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.

 

[bigorangebus]

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

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.

 

[joseph2k]

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.

For US use, only 518 kHz is used. If you use really high q filters (200 or
more) dual filters is most appropriate. Cheap units cannot afford to do
this. The distance between the two "carrier" frequencies is way too far
for single high Q rf tuned circuits. Multiple high Q tuned circuits works
a lot like combined AM and FM IF strips.

 

[bigorangebus]

bigorangebus wrote:
On 13 Feb, 19:13, [email protected] (Mark Zenier) wrote:




For US use, only 518 kHz is used. If you use really high q filters (200 or
more) dual filters is most appropriate. Cheap units cannot afford to do
this. The distance between the two "carrier" frequencies is way too far
for single high Q rf tuned circuits. Multiple high Q tuned circuits works
a lot like combined AM and FM IF strips.

I'm in Great Britain...and theres some useful local weather forcast
information on 490kHz. I have to do some research on filters. Many
thanks for your time with this Joseph