Sunday, July 08, 2012

When You Need To Get Rid Of That Noise

Receivers are not immune to very strong signals. And they exist - it could be your local AM station, the general signal level in your part of the world, or that your radio simply cannot handle the signal level.

Conventional receivers often had a tracking preselector which helped attenuating signals outside of its passband. Older receivers more often used a manual preselector. With the advent of software defined radios, capable of recording from 192 kHz up to 4 MHz, a preselector loses its relevance. Some SDRs are "wide open", while others use a set of bandpass filters. The RF Space NetSDR for instance uses 10 sub-octave filters. The Winradio G33DDC has14 configurable bandpass filters. The Afedri SDR-net on the other side has none, while the G31DDC has a selectable high pass filter to reduce MW interference on SW.

My 100 kHz notch filter was designed by Swedish DX-er Stefan Wikander, is easy to build and was later refined by Dallas Lankford who added a 500 kHz high pass filter to the design. Read more here.

When it comes to high pass and low pass filters, there is a limited amount of off-the-shelf products. Tracey Gardner kindly directed me to the Minicircuits range of filters. Unfortunately, almost all their products are aimed at higher frequency ranges, but they have some interesting low pass filters. For the MW DX-er, the 1.9 MHz filter could be very suitable. In addition, the filters are very small with a female BNC on one end and a male BNC on the other. At around USD 35, they are quite inexpensive, too.

Other than that, it seems that you have to rely on small, dedicated enterprises. They will charge you more, but the products can be tailor-made for you, and quality will be excellent. Here are a couple of interesting manufacturers:

Clifton Laboratories offer active antennas, amplifiers, filters and other items of interest for the DX-er and radio amateur. The proprietor Jack Smith makes bandpass filters, band rejection filters, high pass filters and low pass filters with superb characteristics. They can be built to meet customers requirements. The price range is USD 40-70, metal cased with connectors. More details on their web page. My low pass filter was bought (un-boxed) from Clifton Laboriatories.

Kiwa Electronics has been around for decades. Kiwa offers a wide range of low pass filters, including "Broadcast band rejection filter", i.e. a high pass filter. Prices are in the USD 70-150 range - the more expensive ones allow DC to pass through the filter. The web site does not indicate whether customers requirements are met. I've bought a few preamps from Kiwa in the past, and I've also had a receiver modified there. The proprietor Craig Siegenthaler does an excellent job.

Apart from these two, I was advised about Rescue Electronics, which offers low pass filters for the DX-er. However, after the initial positive contact, I did not hear from the company again, not even after a follow-up email.

Are there designs out there that the hobbyist can use to make his own filter? Please use the comment field to share your ideas.


Anonymous said...

HPSDR - Alex RF Bandpass Filters
can be found here

Perhaps a little bit overkill?

/Fredrik Dourén

Bjarne Mjelde said...

Still cool though ;-)

Anonymous said...

Hi Bjarne. Regarding other filter designs or DIY's:
I suppose your'e aware of Arne Nilsson's experiments, in this case a HP filter @500kHz? Described in SWB 1749. I tested the mentioned Elsie program and "reversed engineered" Cliftons MW HP filter. All figures matches :-). Probably I will try out something myself...

Bill McFadden said...

SVC Filter designer from is handy for designing 3rd-to-7th order highpass and lowpass LC filters or various types. Plots ideal filter response as well as the response using standard-value parts. I used it to design a couple of elliptical filters that perform very well. Best of all, it's free.

JackOfVA said...


I use AADE filter design software, and there other programs such as the mentioned Elsie software.

However, obtaining performance that matches the computer design requires significant attention to detail.

For example, all inductors are adjusted for +/- 0.5% of theoretical value using an HP 4192A LF Impedance bridge, with the measurement being made at the inductor's design frequency. Capacitors are measured and selected with a similar objective of 0.5% using a General Radio 1658 DigiBridge. Fortunately capacitors show little shift in value from 1000 Hz measurements to frequencies in the low MHz range.

It's also necessary to pay close attention to the filter's physical layout to reduce coupling between inductors and also unintended coupling due to common impedance in the ground returns.

It's simple to build a filter with 60 dB ultimate rejection; it's far more difficult to build the same filter and obtain 80 or 90 dB ultimate rejection.

Jack K8ZOA

Robert said...

For use with my Afedri SDR-Net, I am looking at for their compact passive high-pass and low-pass filters with SMA connecters on each end, for only US $26 each.

For shortwave listeners with an Afedri SDR-Net, it would be simple to screw together a high-pass and a low-pass filter to get a pass band focused on a specific broadcast band.

The above webstore lists high-pass and low-pass filters with values


Kevin said...

Bjarne - what specifically did you buy for use with the AFEDRI? I just oprdered one today, and need to order filters also, to control the signals getting through. Also do you have a recomendation for an inexpensive antenna aerial? I like plug and play solutions as much as possible. Kevin N1KCG

Bjarne Mjelde said...

Kevin: I bought a low pass filter from CliftonLabs to reduce SW interference to MW. I already have a 100 kHz notch (or bandstop) filter, which is combined with a 500 kHz high pass filter to reduce LW interference to MW. Requirements will vary with location. I live in a very low-level signal area, with some important exceptions: A very strong station on 5930 kHz, a Loran C station on 100 kHz and a strong broadcasting station on 153 kHz.