RX difference between SDR and a hardware XCVR

Discussion in 'Software Defined Radio (SDR)' started by N4RNR, Mar 27, 2019.

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  1. SM0AOM

    SM0AOM Ham Member QRZ Page

    There are two parameters that are of interest when finding out the ability to hear a weak signal,
    the first is the "sensitivity" or "minimum detectable signal", and the second is the close-in dynamic range.

    The first means very little by itself, as it is quite simple to make a receiver that can detect very weak signals.
    Also, the ambient noise floor of today is orders of magnitude higher than the weakest signal that can be received by a modern receiver.

    On the other side, the dynamic range, or the difference in signal amplitudes that can be handled at the same time, is what separates the good receivers from the rest.
    Making a receiver that permits weak signals to be received with very much stronger signals close in frequency is a challenge, and a good dynamic range is reflected directly in the price of a receiver or transceiver.

    There are a lot of arguments around how good a receiver needs to be with respect to dynamic range.
    Some extreme cases, when you operate close to one or more high-powered stations may require the very best, but more casual operations will not stress even medium-range receivers.

    The reader should be advised that conventional analogue receivers and SDR:s show very different behaviours when approaching their dynamic range limits.

    73/
    Karl-Arne
    SM0AOM
     
  2. W1VT

    W1VT Ham Member QRZ Page

    Your ability to hear weak signals is determined by the noise at your location. Very similar to being able to see dim objects in the sky with a telescope.
    You can't see much with an expensive telescope in a light polluted city like Philadelphia!
    I remember being able to see the Milky Way from Mt Equinox Vermont in the early 90s. By the late 90s there was too much development and light pollution to do that.

    Sometimes, just getting your antenna away from inhabited buildings will make a big difference.
     
    K7TRF likes this.
  3. K7TRF

    K7TRF Premium Subscriber QRZ Page

    As posted above for HF receivers the limiter is almost always local noise or overall band noise and not receivers themselves.

    But in lab or low noise environment with an antenna up high and in the clear receiver noise floor or Minimum Detectable Signal (which is determined largely by receiver noise floor) are the key specs in terms of how low a signal a receiver can 'hear'. But at least for HF receivers it's very rare our bands or local noise conditions are anywhere near quiet enough to make full use of low noise receivers. A good simple test that applies to SDRs as well as conventional analog receivers is to disconnect and then reconnect the antenna when the receiver is operating. If the audible noise increases with the antenna connected (which it almost always does) then noise sources whether distant band noise or local noise sources are stronger, and in most cases much stronger, than the receiver's own internal noise. Basically in that condition more receiver sensitivity won't help and you'd probably never know it if you compared two receivers side by side with different specified sensitivity unless one was really a dog.

    In crowded band conditions, especially if there are strong signals not far away from your operating frequency Close In Dynamic Range is a pretty important specification as it tells you how well your receiver can deal with strong signals that are close in frequency as you try to work weaker signals. That's very important to contesters and other folks that regularly deal with strong signals that are fairly close in frequency. The Sherwood receiver rankings are based on this which may or may not apply to how, when and where you operate. No harm in buying the receiver with the highest close in dynamic range specs but unless you struggle with strong adjacent frequency signals it may not be that important to the way you operate.

    Good filtering whether crystal filtering or DSP filtering, especially if the receiver applies it as early as possible in the receive chain can help a lot for digging signals out of noise. That doesn't technically change the receiver's specified noise floor but in practical terms good filtering can make all the difference when it comes to working weaker stations in noisy band conditions. Even good audio filtering can go a long ways to pulling out weak signals but it's best if filtering is inside the receiver's AGC loop so the receiver's gain structure isn't dominated by the noise or interference. IOW, audio filtering can narrow down the frequencies you hear but artifacts like AGC pumping can happen when your filtering is on the output side of the receiver and doesn't cut down that interference early in the signal processing chain.
     
  4. KF5LJW

    KF5LJW Ham Member QRZ Page

    First thing to understand is there are SDR Receivers, and then there are SDR Receivers just like their analog cousins. I have a few SDR receivers like SDP Play and Airspy HF+ and I classify them as toys that they are. Roughly as good as a Kenwood TS-590. They make for good cheap Panadapter used to find signals. However they are inexpensive and where they save the bucks is in the Analog to Digital Converters. They only use 12 to 16 bits and that translates directly into Dynamic Range. In fact a 6th grader can tell you the Dynamic Range just by knowing how many bits the A/D use, 6dB-bit. So a 12 bit A/D 6 x 12 = 72 dB, and 16 bits = 6 x 16 = 96dB.

    Not with that out of the way there are SDR's with 22 to 32 bits which is a fantastic Dynamic Range of 132 to 192 dB which blows any analog cousin out of the water. Couple that with the filter capabilities, no mixer noise, and yeah an SDR outperforms the best analog RX. On the commercial side like Wireless all use SDR based radio, and soon all Amatuer radios will be SDR and that switch is happeneing now trying to catch up.

    So do SDR and Analog RX's sound and perform the same. No when you compare apples to apples price points, SDR will perform equally or better.
     
  5. W6RZ

    W6RZ Premium Subscriber QRZ Page

    It's more complex than that. First, real life ADC's never perform to the theoretical 6 dB/bit equation. The specification that defines their real life performance is signal to noise plus distortion ratio. For the 14-bit LTC2208-14 used in the Icom IC-7300, the value is 77 dBFS at 5 MHz. This translates to 12.5 bits, which is called the effective number of bits or ENOB.

    Even 16-bit ADC's still have an ENOB around 12.5 bits. The LTC2208 used in the IC-7610 is just slightly better at 12.6 bits and the AD9467 used in the Flex Radio products is slightly worse at 12.4 bits.

    But ENOB is only part of the overall dynamic range equation. Decimation of the digital signal makes up for the remaining 50 dB or so of dynamic range. That's because we're decimating from the ADC bandwidth of 60 MHz to the final signal filter bandwidth of 500 Hz. log10 60,000,000 / 500 = 51 dB.

    So the minimum discernible signal (MDS) to overload dynamic range is about 128 dB (77 + 51). The IC-7300 and IC-7610 are about 123 dB because they use a 20 dB gain ADC driver in front of the ADC (which lowers the overload point by 20 dB, but doesn't reduce the noise floor by 20 dB).

    There are no commercially available high speed (direct sampling) RF ADC's with more than 16 bits.
     
    Last edited: Apr 16, 2019
    IZ4HDM, G0GSR and WD4IGX like this.
  6. AA7QQ

    AA7QQ Ham Member QRZ Page

    My LimeSDR-USB is a pretty nice transceiver. Of course, it's no ANAN or Flex, but a good 60 MHz bandwidth & 2 softwares getting finalized for it are exciting.
    There's even this: https://www.crowdsupply.com/lime-micro/limerfe All of the LPFs, BPFS, predrivers, drivers, changeover parts & the such to have a plug & play 35Db to 22Db 100 KHZ to 3.8 GHz transceiver.

    Ed
     

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