A simple homebrew high pass filter

12744054_901043640014242_2651293633303686752_nMany thanks to Jon Hudson, of SDRplay, who posted the image above along with the following note on Facebook:

David, WA7JHZ has designed and assembled this neat 2.6 MHz high-pass filter (HPF) for use with SDRs. He says that this simple input band-pass filter (BPF) might be of interest to anyone suffering from strong AM broadcast stations that are causing overloads…..David suffers from three nearby AM radio broadcast stations that overload the front ends of several of his receivers, including the SDRplay RSP and this is an ideal, low cost solution. He built this circuit from junk parts and commented that Amidon T44-2 iron powder cores would have made a better design, but that he decided to keep costs down with this design.

This design is simple enough for almost anyone to build and could help your receiver’s front end from overloading. Many thanks to David for the design/schematic and to Jon for sharing!

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26 thoughts on “A simple homebrew high pass filter

  1. Tha Dood

    I like it!!! I could use this to block out my AM Carrier-Current station on 610KHz from getting into stuff. Hmmm??? Aren’t you one of those engineers that offered advice on an LPFM help-site about 3 years ago? Damn… Can’t even remember the site’s name now.

  2. Dave AA7EE

    Thanks for this, Thomas. I Googled “broadcast band high pass filter” and came across this post. I’m putting an HPF into a simple direct conversion receiver and am nearly done, but in the future, will give this circuit a try. Much as I enjoy winding toroids, I rather like that this uses molded inductors, and will fit into a fairly small space.



  3. Guy Atkins

    I recently purchased one of these high pass kits and an enclosure from an Australian company, and look forward to assembling it: http://www.minikits.com.au/electronic-kits/filter-kits/hf-highpass-filters/HPF9-1M8

    The price was just $14.95 USD at the current exchange rate, but I did pay extra for the matching enclosure.

    It’s a 9th order Chebyshev filter at 1.8 MHz with less than 1 dB insertion loss from 1.8 to 100 MHz, and I expect it to work very well. Since it’s a kit with all parts and a PCB, it half way between a “DIY from scratch” and a completely built commercial high pass filter.

    1. 13dka

      Well 60โ‚ฌ is a rather steep price, making DIY even more interesting. I also perceive this as a tiny little bit spammy, Mr. Walter. ๐Ÿ˜‰ While I have the honor to type to you, why are all the Bonito products still equipped with BNC connectors? You offer cables with most products but again, all BNC only. My scanner, my SDRs, my active UHF antenna and possibly everything that will be build from now on in this part of the industry… everything has SMA now and hence my cable drawer is full of SMA cables and I find BNC connectors on stuff I might want to buy rather a nuisance.

      1. Dennis Walter

        Hello Mr. or Mrs 13dka,
        it is 60,-โ‚ฌ – 19% VAT. Thats less than 50,-โ‚ฌ and you really get a good working solution, with CE, WEEE, ROHS…etc. Just plug and play. Of course, if you like DIY, go ahead and do it.
        The BNC discussion is endless. 90% is using it. Some wants N and some SMA. We also like SMA, no question. It would saves us a lot of money. We really made up our minds if we should do the next one with SMA. But in the end it is still BNC, because 90% of the customers using this.
        Keep listening

        1. TP Reitzel

          I agree. The form factor for interfacing with humans has been reduced about as much as possible. I find SMA problematic, not BNC. I use BNC for nearly every connection as connectivity is quick and reliable enough.

  4. 13dka

    Thank you German public radio for saving me from having to build one of those! ๐Ÿ™‚ Now if you could just turn off those obnoxious FM transmitters as well, all would be so hunky-dory around here. ๐Ÿ™‚

  5. Edward

    This would not be a problem in tube radios because they can take the overload with their wide dynamic range. I ran into this problem when I got my first transistor shortwave, and had a very similar solution.

    1. Michael Black

      That’s not quite right. A badly designed tube receiver could overload. Even when they didn’t need front end selectivity for image rejection, receivers generally had lots. The Racal RA-17 had a high first IF thanks to the Wadley Loop, and front end selectivity could be bypassed for a simple low pass filter, butI’m not sure how often it was used without the tuned front end.

      It’s all a balance, many mixers weren’t strong enough, and too much amplification before the mixer just made it worse.

      I remember articles where different receivers were compared, and some that looked good were bad, while others that looked like they’d be bad, weren’t. I also remember a QST article where the author sets out to build a very overload resistant receiver, starting with a 7360 balanced mixer (like in that Squires-Saunders receiver), and he pondered using a transmitting tube as the RF amplifier, before picking a lesser tube.

      That said, early transistor receivers did tend to overload. In retrospect, I think too much attention was paid to low current, rather than best operation. It took time to get good. Balanced mixers existed in tube radios, but it took time for them to generally arrive in solid state receivers. The FET was touted as the solution when it arrived about 1964, but then about 1971 bipolar transistors were back, but “hey, we need to run more current through them so they won’t overload”.

      High IFs have probably helped in recent years. A low end 455KHz IF receiver didn’t have much image rejection, adding more selectivity to the front end caused losses so more amplification was placed there. Badly done, it jus overloaded more. A high IF means easier front end selectivity, and less amplification before the mixer.

      Building a good preselector for these SDR receivers might be a good thing.


      1. Edward

        As you say that all depends, My HQ140X has a decent tuned RF amplifier in front of the mixer that does a good job of attenuating out of band signals It’s image rejection is good but not terrific but there is no IMD products (that thing survived all these years and I use it when my solid state radios don’t make the grade) . However my digital Radioshack DX380 with a 50 mhz IF has a LPF filter that rolls off at 40mhz and is pathetic on any kind of external antenna, picks up signals that are not there and from IMD, generate their own QRM internally, I am surprised the high end SDR’s do not have band pass filters that are automatically switched in/out depending on what is tuned in. I don’t think accepting an AM signal is necessary when tuned in on shortwave and vice versa. But they are built to a budget not a specification.

    2. DL4NO

      Tube receivers had selective frontends because of impedance transformation issues: At least on HF a tube input circuit is mostly capacitive. So you needed a high-impedance, slightly inductive matching network – the well-known preselector of the time. With today’s bandfilters those tube receivers would have obverloaded all the time.

      Just consider how these receivers tried to prevent overload of the first mixer: They regulated the gain of the first amplifier stage – by deliberately using a non-linear tube.

      Apart from that, large-signal charcteristics of tube receivers were mostly inferior to today’s semiconductor frontends that cost only a small fraction of a good tube receivers at their time.

      A preselector did not help against nearby BC stations. Especially notorious was the 40m band. Similar sutuations existed during the more important contests.

  6. DL4NO

    The world is small these days! ๐Ÿ™‚

    My announced artickle about DX Patrol vs. SDRplay has just been published: cqDL (our “German QST”) 3/2016, p. 28-31.

    I that article I wrote that the 12-MHz low-pass filter of the SDRplay is no really good solution. It is a pity that they did not use the existing resources of their chip set to implement cross-over frequencies like 2/8/15 MHz.

    If I had a say I would provide separate inputs for up to 60 MHz and above and make the LNA switchable.

    1. 13dka

      Did you miss the latest update or did I miss a bit about the LNA? The LNA can be turned off completely now, the automatic switching depending on GR setting is gone (except in the interim ExtIO.dll for SDR# 1.4+).

      1. DL4NO

        Considering the schematics you can only “switch off” the LNA, i.e. turn off its power supply without changing the signal path.

        This appears to be a quite crude way. The MGA-68563 data sheet does not describe this mode. Your measure at least throws off the impedance matching of the antenna input and the fiters. I have no idea how good this works.

        OK: I still use SDR#.

        BTW: These days I got an USB voltmeter/amperemeter: The DX Patrol draws more than 200 mA and heats up accordingly. The SDRplay consumes about 20 mA (SW) to 70 mA (VHF).

        1. 13dka

          Ah, that sounds crude indeed.

          Re: How well that works – I can’t give you more than “empirical data” I coincidentally acquired yesterday and today, comparing some things with and w/o LNA on the RSP and the RTL stick but I did notice that using Inmarsat signals on 1.5 GHz as a benchmark (and an antenna with its own LNA), the RSP still performed considerably better (with the LNA turned off of course) than the RTL stick (which doesn’t have an LNA). So if not bypassing the LNA has detrimental effects, they’re probably rather subtle up there. Of course I can’t tell how bad they really are without actually bypassing the LNA.

          The same comparison on the FM band on a passive “antenna” (1m thick wire stuck out of the attic window) surprisingly turned out that the RTL stick was pretty much on par with the RSP (LNA off), although the RTL stick creates considerable images on 4m while the RSP does not.

          BTW the RSP is performing very, very good on FMBC with the LNA on, I’m hearing a 5kW (!) station in a distance of 426km since I got the RSP 10-15dB above the noise, while the tropo DX prediction sites don’t show/predict any elevated conditions. ๐Ÿ™‚


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