Hi Chris,
I sent an earlier reply which seems to have gone into the bit bucket
somewhere, possibly to due to server settings in my mail client
which I've just now changed back to what was working previously.
That first reply was mostly off topic so if it does show up, please
ignore it.
I've been following the comments on your topic and just wanted to
toss something in the mix. It appears the response of your actual
filter is more rounded and the skirts less steep than the model. I
leave it to those with more knowledge to correct me, but I believe
this suggests Q of components in your filter is significantly lower
than Q assumed by the model. Using higher Q components, if possible,
may give you more attenuation of unwanted out-of-band signals. I
built one of these filters and my response appears to be in closer
agreement with the model.
Not speaking to a solution but a possible cause of the unexpected
filter response: If you used capacitors with reasonably good
tolerance, perhaps the inductors came out a bit low in value. I've
forgotten whether you used pre-made inductors or wound your own
using cores of some type. If the latter and the cores are ferrite,
the specific batch of cores might be enough to throw it off. I
believe ferrite materials typically don't have tight tolerance.
Paul
On 08/23/2018 07:08 AM, Chris Wilson wrote:
Hello LF'ers
Is anyone able and willing to advise on my band pass RX filter design
please? I have made a band pass filter for 137kHz reception and
checked it with my SDR in real time. It is shifted a little high and I
am wondering what I need to change to pull it a bit lower? Any
practical advice or component values welcome. Adjustability would be
nice! Thanks
Filter design is at http://www.chriswilson.tv/lf-circuit.jpg
Real world results at : http://www.chriswilson.tv/passband.jpg
ELSIE plot at : http://www.chriswilson.tv/lf-plot.jpg
