Dear Jim, Alan, Gerhard, LF,
Agn tnx for your suggestions! OK, so my way to realise the filter was
not too worse. Tomorrow i will play with a double LC filter, just as
applied before the BF981, just for 12 kHz. This is what you see in the
photo (1x LC in parallel) but not in the schematic. I have some smaller
fixed ferrite pot coils of 33mH. I used them i the VLF grabber antenna
and they perform good. I will try to resonate one at 12,7 kHz (so for
the QRSS3 sector) but since it should be possible to cover the whole
band i will resonate the other filter at 11,6 kHz to make CW reception
possible. By varying the coupling C it should be possible to find a
filter curve with a reasonably flat passband attenuation and good level
rejection of DCF39. I will post the new spectrogram here then. And i
will check again the attenuatioin of 153 kHz.
And, it will be the first version of the antenna so it has not to be
"perfect" in the first attempt. No problem to change the filter design
when it seems not to work fine...
Since i am in holidays next week it will take some days longer until the
grabber is active. Then, i hope to see more amateur signals there than
on my VLF grabber ;-)
I plan to create 2 websites: One is just for QRSS3, a fast updating
website (each 40 seconds) with only a small window, just to show
137,65...137,75 kHz. The other website could show the region arround
136,32 kHz and 137,78 kHz, each in QRSS-60 and QRSS-120 and additionaly
a QRSS-10 window at about 137,7 kHz. I plan to use an extra PC for that
which runs 24/7...
kind regards, Stefan/DK7FC
Am 06.08.2010 00:22, schrieb James Moritz:
Dear Stefan, LF Group,
The AADE utility will do the design calculations for you... the
limitation of filters like this is that, in order to achieve a
particular shape of frequency response, bandwidth, and frequency,
there is a certain minimum value of component Q required, whatever
type of circuit you use. Generally, things that increase "selectivity"
require more Q. A narrower passband (as a fraction of the centre
frequency) requires a higher Q. A more rapid the transition from
passband to stopband requires higher Q. A higher order filter (i.e. a
larger number of LC tuned circuits in the bandpass case, to give
higher attenuation in the stop-band) requires higher Q. At 136k, Q of
100 or so is easy to achieve; with special pot-cores, or very big
coils, a Q of 1000 or more can be obtained. This practically means
filters with moderately sharp cut-off can be fairly easily made with a
bandwidth of several kHz, or with much more difficulty a bandwidth of
a few hundred Hz. So a high rejection of DLF will easily be obtained
by the input filter if it is designed to give adequate image
rejection, but that would be difficult to achieve with DCF39, only
about 1kHz above the top of the band. This is why crystal filters
became popular ;-)
As Michel says, the coupled-resonator type of filter is best for
narrow-band designs (like the "top-coupled" circuit in your drawing).
It is nice because you usually end up with inductors that are the same
value. The 3kHz ladder filter used in my 9kHz preamp circuit is a type
more suitable for a large bandwidth / centre frequency ratio - if you
try to design such a filter for a small ratio, say 5kHz BW at 137kHz,
you will end up with an impractical design with very small shunt
inductances and very large series inductances. Cauer/elliptic bandpass
filters tend to get a bit complicated... and are still limited by
inductor Q anyway.
For the 12kHz IF filter, if you wish the whole 2.1kHz of the 136kHz
band to pass through the filter, a coupled resonator filter is again
probably the most practical - but you will end up with a somewhat
asymmetrical response due to the nature of this type of design. Due to
the lower centre frequency, Q requirements are reduced, and you will
probably be able to get better rejection of DCF39. But you will need
bigger inductors to do it. You could instead design a filter with a
narrower bandwidth - say a few hundred Hz just to pass the QRSS
segment at the top of the band. In this case, you could get quite high
rejection of DCF39 with a fairly simple filter.
A better approach may be to have a rejection notch filter to attenuate
the DCF39 carrier. This could be done at the input frequency or the
IF. With bridge-type circuits, rather high rejection can be achieved
at a spot frequency with a single tuned circuit, at the expense of
some attenuation of nearby frequencies.
But as Alan suggests, you may not need much filtering - sound cards do
vary, but quite often they have suprisingly good linearity. Provided
you use the minimum possible gain to raise the band noise above the
sound card noise level, and the level of DCF39 is not high enough to
actually saturate the sound card input, it may work OK.
Intermodulation may not be serious, since there is essentially only
one strong signal reaching the sound card input, and so not much for
it to intermodulate with. Having aother signal 60dB above the wanted
signal may not be an issue. I guess the sidebands from DCF39 that
actually fall within the 136kHz band may be more of a limiting factor.
They do noticeably raise the noise level at my QTH - You are obviously
much closer!
BTW- are you really getting 60dB attenuation of 153kHz? The level of
DLF on your spectrogram seems a bit hard to believe.
Cheers, Jim Moritz
73 de M0BMU
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