ADC Overload

Hi All,

I notice that one or two KiWi SDR's which are using good antennas (myself included) are suffering from ADC overload due to strong Broadcast stations, especially during the night on the short wave bands.

I have built a very simple filter that I use to place a very narrow 10dB deep notch on frequencies where I have problems with strong broadcast stations.

It simply consists of a number of series tuned resonant circuits connected in parallel (shunt) with the KiWi SDR input. The frequencies and number of tuned circuits can be varied in order to suit your own RF environment.

I used Elsie http://www.tonnesoftware.com/elsie.html and RF Sim 99 http://www.electroschematics.com/835/rfsim99-download/  (both free)  to help design and fine tune the values.

22uH inductors are used in order to provide the required bandwidth and attenuation for each frequency, but you can change the values as required. 

Fixed value capacitors with trimmer capacitors connected in parallel were used to fine tune the circuits to the required frequencies. 

The filters are built into a small metal box with BNC connectors that simply fits in line with the antenna feed. 

You can tune two or more notches to the same frequency if you have a really strong signal.

The following diagram shows the circuit and frequency response curves.

image

By adding a simple filter like this, it's possible to reduce the level of very strong stations to below -20dB on the spectrum display, which is usually the point at which ADC overload starts to become problematic.

Regards,

Martin - G8JNJ
KA7U

Comments

  • A better version of the diagram can be found here


  • Accepted Answer
    Great Info!  I have a similar problem in that I live near 3 or more strong AMBC stations. I just put my Kiwi on the air yesterday and my initial solution was to add a 1800KHz HPF but I was going to start building something simllar to what you have next week, so your design is a big help!

    -Jim

    WA2ZKD

  • Hi Jim,

    I'll do a better write up and put it on the SDR page of my website over the next day or so.

    Almost any small mounded chokes will do, but I found it important to use mica or polystyrene capacitors. Small ceramic may work but the 'Q' is often very low and they drift slightly in value with changes in temperature.

    Regards,

    Martin - G8JNJ
  • Updated notes now added to my SDR webpage.


    Regards,

    Martin - G8JNJ
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