AM and FM Band Reject Filters

[SDRguy22]

Many SDRplay users live near AM and FM stations and find that these strong signals have a detrimental effect on reception with their RSP. In some cases the built-in AM and FM filters are not sufficient. There have been numerous posts recently on how to build filters to reject unwanted signals. If one does not have the skill or inclination to build something there are several off the shelf options that may be purchased at low cost. I like the NooElec products and they are available for the very reasonable price of £10.

The AM filter has some nice features:
- Rejects AM band but passes LF and HF signals
- It will pass DC so bias-T will still work
- Fully shielded
- ESD protection
- 3 dB rolloff at 350 KHz and 1900 kHz.
- Up to 40 dB of attenuation

Can be purchased at Amazon: https://tinyurl.com/amreject

The FM filter has the following features:
- Rejects FM band but passes signals above and below
- It will pass DC so bias-T will still work
- Fully shielded
- ESD protection
- 3 dB rolloff at 80 MHz and 115 MHz. (Much sharper than the RSP built in filter)
- Up to 40 dB of attenuation

Can be purchased at Amazon: https://tinyurl.com/FMreject

I own both of these products and have been very satisfied. Both come with an SMA coupler so they are very easy to connect to your RSP.

Bill

[ON5HB]

Hi Bill,

I had the same problem here.
My solution was to use an 125MHz upconverter as it looks like the very low frequencies tend to puckup AC from solarpanels and it's near to impossible to get rid of them.

I use this:



https://www.nooelec.com/store/ham-it-up-plus.html (also at Amazon)

For me it removes all unwanted signals especially between 0-4 MHz.

I think is has to do with mixing being lifted from e.g. 1 Mhz to 126MHz, as such the next mixing isn't every 1 MHz but every 126MHz.
We have strong signals at 234KHz and without the upconverter you see it returned every 234KHz, same thing happened with my RTL-SDR dongles in the past.
I did use their AM-filters but didn't do much good, upconverting worked wonders. Also it protects everything against very high ESD.

There are more cheap Chinese converters and I tried those, don't go there as they are not frequency stable and the NoElec is.

Hopefully it helps people with VLF/HF problems.

[SDRguy22]

Bas,

That is an interesting device. I see that it has a built in noise source. Have you used it to do any filter tests?

[ON5HB]

I have used filters and attenuators before, all the time mixing started on big signals.
Ending up with AM stations as high at 40m....no joke.

Checkout my websdr: http://heppen.be:8091

There are NO AM stations repeating, not a single one as far as I know.
That is due to the up-conversion.

Everything I tested before failed, as typical they match the bandwidth and multiply.
It was Pieter-Tjerk of websdr.org that pointed this out, and I acted on this, never had any problems after.

SDR is not a typical old-fashon radio and 1-to-1 reception is on HF a recepi for problems. I/Q mixes....mixes bad on strong signals...very bad.

Filters do not work, I tested a lot of them, overloads makes mixing happen en ghost-signals pop-up all over the place.

A good filter for websdr should make the second mix outside the band....ergo, you must up- or down-mix.

At least that is my experience on solving bad/ghost-signals. Direct-I/Q is horrible on low bands.

[mike0agner]

We US listeners have the same issues, and there are 2 top names for AM and FM filters; Par electronics (yes, the former manufacturers of the popular EF-SWL antenna) and Stridsberg...viz.

http://www.parelectronics.com/swl-filters.php

http://www.parelectronics.com/fm-broadcast.php

http://www.stridsbergeng.com/filters.html

They are well known and respected by DXers for years. A bit expensive but you get what you pay for.

I wouldn't trust ANY filter that didn't at least supply a response curve. While I agree that's only 1 part of the picture, it's an important part.

As a side note, Larry installs filters in his W6LVP loop amps in case you are in a high signal area. Something that Wellbrook and DX Engineering/Pixel doesn't. But the filters above will work well for non-amplified antennas such as dipoles and similar...Mike

[mike0agner]

Here's another AM (BCB) band filter from ICE...

http://www.iceradioproducts.com/filtersrf.html#bcb

Mike