Preventing SDR overload

Discussion in 'Software Defined Radio (SDR)' started by K3RW, Nov 25, 2020.

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  1. K3RW

    K3RW Premium Subscriber QRZ Page

    I'm new to SDR so be kind :)

    I've read that SDRs overload easily, though I have not yet experienced that and I am not quite sure how it might manifest (IMD, RFI, de-sense, etc.). But I wanted to know if there are any techniques or tricks to prevent that. In the future, our contest shack we will have SDRs running basically everything on transverters.

    Some things come to mind: better shielding, ferrites, etc. I don't know other than perhaps a bandpass filter if there are any other things to look at.
     
  2. WL7PM

    WL7PM Ham Member QRZ Page

    Hang wet laundry over your antenna for sufficient attenuation.
    I saw this technique used in Belize, very cost effective: this way you don't waste the atenuated power, nor do you waste valuable electricity drying clothes.
     
    2E0CIT likes this.
  3. N6YWU

    N6YWU Ham Member QRZ Page

    Direct sampling or IQ mixer SDR?

    The dynamic range of direct sampling SDR receivers is limited by the sampling bit depth of the ADC (commonly in the range of 8 to 16 bits, depending on the SDR radio design) across the entire bandwidth of the device (and also to some degree the ADC sample rate). A strong signal anywhere in the receiver bandwidth can either overload that ADC range, or cause an AGC to turn the gain down to where your signals of interest disappear well below the LSB of the ADC. So input blocking filters on the frequencies of any nearby transmitters or other strong RF sources will help, as will single-band band-pass filters for the band in use. Some SDRs have one of more of those filter systems built-in. Other SDRs require one to insert external filtering in the signal feeds.

    My 8-bit RTL-SDR in Direct Sampling mode can get overloaded by a nearby broadcast station, so I use an external broadcast band blocking filter. My 12-bit Hermes Lite 2 has a built-in high-pass filter to block broadcast AM signals when the receivers are tuned above 3 MHz, as well as a low-pass anti-alias filter to block out VHF. My 16-bit Red Pitaya doesn't seem to need an external filter, unless perhaps I want to use it next to a non-QRP transmitter.

    Taylor-style IQ mixer SDRs have some filtering inherent in the front-end mixer design, so may require less front-end filtering, internal or external.

    As with any receiver, you want to prevent local digital (etc.) noise sources from getting into the antenna, computer, power, etc. cabling.
     
    K3RW likes this.
  4. KK4NSF

    KK4NSF Ham Member QRZ Page

    A lot of that depends on the type of SDR you have. The newer SDRplay RSPdx is far less prone to overloading. Some of the cheaper radios overload when you just look at them crosswise.

    The best thing I've forund to prevent overloading is the use of band-pass filters to cut out strong off-band filters. (there was a thread about that recently) The other is to turn down the RF gain.
     
    K3RW likes this.
  5. VE7BPB

    VE7BPB Ham Member QRZ Page

    Following along on the above thoughts, arguably the best of the lower cost (<$1000) SDRs regarding dynamic range and filtering is the Airspy HF+ Discovery. Which just so happens to be on sale right now for $118 US (30% off) at the company store, Airspy.us.

    That radio would make a very good starting point for your operation, I think.

    regards, Roy
     
    K3RW and N6YWU like this.
  6. K3RW

    K3RW Premium Subscriber QRZ Page

    For the VHF multiop station we have a couple of ANANs as the SDR IF interfaces for transverters. The station will be QRO through 10GHz minus 2 and 5GHz where it won't be high power.

    My personal SDR at the moment is a Flex 1500. I'd like to make this the basis of my own portable, rover, or single-op station through 2304. I suspect where I setup for a contest site may be more of an issue than the fixed location with the ANANs. It seems every good potential VHF site I find also has a cell tower on it (!).
     
  7. G0GSR

    G0GSR Ham Member QRZ Page

    I think that is unfair to imply that overloading is a feature or weak point of the SDR architecture.
    If a superhet RX was built as cheaply and without adequate filtering, that too would overload badly.

    The significant point about an SDR overloading is that it is disastrous (unusable), whereas a conventional RX would just display some intermodulation products but be otherwise, still useable.
    That is a feature of any digital system compared with the equivalent analogue system.

    Frank
     
    WD4ELG likes this.
  8. SM0AOM

    SM0AOM Ham Member QRZ Page

    SDR:s show a very different behaviour when approaching or exceeding their dynamic range limits.

    Differing from analogue receivers, when a strong adjacent channel signal exceeds the ADC linear range there is an abrupt disruption of reception, compared to a much more gradual degradation in the analogue case.

    Also, the frequency spacing between wanted and unwanted signals have less importance in the SDR case.

    If the spacing should be 0.1 %, 1% or 10% is largely immaterial in the SDR case, but it has a very marked influence in analogue receivers.

    The whole question "boils down" to the age-old observation that "nothing can replace RF selectivity".

    73/
    Karl-Arne
    SM0AOM
     

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