FM BROADCAST BAND SUPPRESSION CHEBYSHEV N=5 BAND-STOP FILTER

General discussions
sdrom33
Posts: 62
Joined: Sun Dec 09, 2018 4:38 pm

Re: FM BROADCAST BAND SUPPRESSION CHEBYSHEV N=5 BAND-STOP FILTER

Postby sdrom33 » Sun Apr 14, 2019 4:27 pm

Thanks glovisol, very informative.

Reason: No reason

glovisol
Posts: 449
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: FM BROADCAST BAND SUPPRESSION CHEBYSHEV N=5 BAND-STOP FILTER

Postby glovisol » Sun Apr 14, 2019 5:02 pm

I tested the RSPduo in the same conditions and in the same lab position. The metal case of the RSPduo shines here in its quality, the uploaded pic shows it: no FM broadcast feedtru at all.
Attachments
FM Band Feedtru RSPduo.jpg
Tuner 1 50 Ohm port terminated into 50 Ohm SMA load
FM Band Feedtru RSPduo.jpg (149.87 KiB) Viewed 147 times

Reason: No reason

glovisol
Posts: 449
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: FM BROADCAST BAND SUPPRESSION CHEBYSHEV N=5 BAND-STOP FILTER

Postby glovisol » Wed Apr 17, 2019 2:54 pm

The filter has now been assembled into the die cast box and I could upload the final measurements. The filter attenuation in the stop-band has increased to between 45 and 50 dB and it is on average 6 db better than the commercial filter. Also, looking at the comparison, white for the designed filter and yellow for the commercial filter, the FM Broadcasts feedthru seems far worse for the commercial filter, but at 90 dB down anything can happen, with FM signals stealing directly into the RSP-1A.

In any case both the commercial and the designed filter are a useful addition to my RSPduo accessories and I must use one or the other when scanning the Airband or the VHF 2 m Ham Band to get rid of overload FM Broadcast noise.
Attachments
Filter in box.JPG
Filter in box.JPG (93.83 KiB) Viewed 82 times
Band Stop Filter shielded 3.jpg
Band Stop Filter shielded 3.jpg (338.17 KiB) Viewed 82 times
Band Stop Filter shielded 2.jpg
Band Stop Filter shielded 2.jpg (144.34 KiB) Viewed 82 times

Reason: No reason

F1BJB
Posts: 116
Joined: Wed Jul 15, 2015 4:24 pm
Location: Beauvais France

Re: FM BROADCAST BAND SUPPRESSION CHEBYSHEV N=5 BAND-STOP FILTER

Postby F1BJB » Thu Apr 18, 2019 8:51 pm

Very nice post.
Another useful trick in low pass filters is to replace part of the capacitors to ground by open quarter wave stubs of coaxial cable.
You cut the cable to notch out a strong local signal .
Normally there is not enough capacity so you add a real one to compensate.
Or you add another open stub on a higher frequency.

Reason: No reason

glovisol
Posts: 449
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: FM BROADCAST BAND SUPPRESSION CHEBYSHEV N=5 BAND-STOP FILTER

Postby glovisol » Thu Apr 18, 2019 9:49 pm

Hi F1BJB,

If you look here:

viewtopic.php?f=12&t=4307

you will find the very first experiment I did a short time ago to test the new Spectrum Analyser software with a band-stop filter quickly made with coaxial stubs. Twentyfive years ago I was designing coaxial filters made with equal element series and shunt sections directly derived from tabulated low pass filter data. In a future project I plan to demonstrate the design technique of such filter types.

Cheers,

glovisol

Reason: No reason


Return to “General”

Who is online

Users browsing this forum: No registered users and 13 guests