A limited attenuation high-pass filter for the KiwiSDR

Figure 1: Inside the "limited attenuation" high pass filter, housed inside a small, die- cast aluminum enclosure two which BNC connectors were mounted. Click on the image for a larger version.

One of the issues common with using a broad-band, direct-sampling SDR (software-defined radio) like the KiwiSDR is that of overload by strong, low-frequency signals, such as those on the AM (mediumwave) broadcast band - but there's another problem that should be considered as well:  The high generally-high signal levels at lower HF frequencies.  If one looks at an spectrum analyzer connected to a broad-band receive during the evening, one will immediately note that the lower the frequency, the higher the signals seem - particularly the background noise.

This becomes problematic if one is using an antenna with a relatively flat gain across the entire HF band - and one wishes to make the receiver usable at the top end of the HF band.  As an example, I have a KiwiSDR connected to an antenna that is rated to cover from 3 to 30 MHz with roughly constant gainbut noted that at the top end of the frequency range, around the 10 meter amateur band, the overall system gain was not quite sufficient to "hear" the background iononspheric noise.

The obvious solution to this gain deficit is to install an RF amplifier - which I did - but this had the effect of increasing the already-strong signals below about 10 MHz even more, resulting in occasional "OV" indications on the KiwiSDR's S-meter signalling to me that the RF levels were a bit high.  While this wasn't too much of a problem during normal conditions, if the lower HF band were particularly noisy - as often occurs in the summer with thunderstorms on the same continent - reception across the entire HF spectrum was compromised when the loud static crashes would occasionally saturate the A/D converter.

It occurred to me that while I had about the right amount of system gain on 10 meters, I had far more than I needed at lower frequencies, so I set about designing a filter that would reduce signals at the low end of the HF spectrum, but have minimal effect at the upper end.

A "limited" high-pass filter:

The obvious solution to this would be the addition of a high-pass filter - but there's a problem:  Even a minimal high-pass filter would have increasingly-higher attenuation at lower and lower frequencies - potentially in the many 10s of dB - but we don't want to get rid of the lowest frequencies.  What we need is a filter that will "knock down" signals by a significant amount - but not so much that they become inaudible.

In analyzing the signal levels, I determined that the goal of the design would be to leave signal levels above about 10 MHz unaffected, but reduce the signals below 8 MHz or so by 10-15dB.  This amount of attenuation (about 2 "S" units) would significantly reduce the amount of RF energy entering the A/D converter at these frequency (about 2 "bits" worth) but analysis of the noise floor and signal levels at these lower frequencies indicated that I would still be able to hear the noise floor.

The diagram of this filter is shown below:

Figure 2:  Diagram of the "limited attenuation" 10 MHz high-pass filter. This filter attenuates by about 12dB below 10-12 MHz, reduce the overall signal power reaching the A/D converter of the KiwiSDR.

The diagram above, as rendered by "LT Spice", depicts the load (the receiver) as R2, a 50 ohm resistor.  No real attempt was made to make this filter's input and output impedances "flat" across the entire HF spectrum - and to be sure, below about 14 MHz it's a bit high, but this will have little practical effect on its operation.

As tested on a spectrum analyzer with a tracking generator, the insertion loss is 12-13dB from DC to about 4 MHz at which point it gradually drops to about 2dB at 11 MHz and then dropping to about 1dB by 30 MHz.  When doing an "A/B" comparison with and without the filter on the KiwiSDR, the waterfall above 10 MHz looked unchanged by the signal below about 7 MHz were much less "bright" - and most importantly, the occasional "OV" indications on the S-meter pretty much stopped appearing altogether.

The components for construction of this filter aren't critical:  The capacitors are NP0 (a.k.a. C0G) ceramic while L1-L3 are wound using 30 AWG enameled wire with L1 and L2 having 15 turns and L3 having 12 turns on T37-2 toroidal cores.  L4 is a an inexpensive molded inductor, and its value can be anything from 2.2-3.3 uH, or one could wind 25 turns on the same T37-2 toroidal cores as used for L1-L3.

Conclusion:

This filter seems to be very effective in reducing the total signal power from lower HF frequencies while having minimal effect at higher frequencies.  Because the signal+noise levels from a broadband antenna are much higher at the lower end of the spectrum, it may be possible to reduce these signals by 2 "S" units or so without dropping the background noise - or the signals themselves - below the noise floor of the receiver.

For information on a filter system that is specifically designed to attenuate AM (MW) broadcast band signals, see the article "Managing HF signal dynamics on the RTL-SDR (and KiwiSDR) receivers", also on this blog.

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This page stolen from ka7oei.blogspot.com