24 bits on Xiegu G90?

Discussion in 'Software Defined Radio (SDR)' started by DO1JML, Oct 23, 2020.

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  1. DO1JML

    DO1JML Ham Member QRZ Page

    Xiegu specifies the conversion in their G90 as "24 bits". Most transceivers appear to use 16 or even 14 bits, so that bit depth should be sufficient.
    Why would Xiegu use 8 bits more? Is there some particular design choice before the conversion which made the increased bit depth necessary?
  2. G0GSR

    G0GSR Ham Member QRZ Page

    I suspect that this does not refer to the RF sampling ADC but the audio ADC.
    The manual quotes "24-bit data size @48kHz sampling rate" which is obviously NOT the RF sampling.

  3. DO1JML

    DO1JML Ham Member QRZ Page

    Why would they need 24 bits for the audio part? HF radio is not hi-fi. I why would they need a dedicated audio ADC at all? OTOH, the spectrum display of the G90 is indeed 48 KHz.
  4. G0GSR

    G0GSR Ham Member QRZ Page

    They don't.. so who knows why? (or even if its true).

  5. KC3PBI

    KC3PBI XML Subscriber QRZ Page

    Perhaps they included a component capable of 24-bit conversion and aren't actually using it to full potential?

    ...or are using it even though the last 8 bits are a waste, but it made sense to include for other reasons such as availability, cost etc?
  6. W6RZ

    W6RZ Premium Subscriber QRZ Page

    It's a direct conversion receiver. The ADC operates at baseband. You have to use a high bit depth to get dynamic range because there's very little decimation after the ADC (48 kHz to 500 Hz is only 19.8 dB). A typical 24-bit audio ADC has a dynamic range of about 100 dB, so the total dynamic range is around 120 dB.

    On a direct sampling receiver, you have a high speed (130 Msps) 16-bit ADC with about 77 dB of dynamic range. But there's a huge amount of decimation (130 MHz to 500 Hz is 54.1 dB). So the total dynamic range is about 131 dB.
    KF4VAR, N6YWU, KL7KN and 1 other person like this.
  7. DO1JML

    DO1JML Ham Member QRZ Page

    Thank you. This was the answer I was looking for.
  8. W6UV

    W6UV Premium Subscriber QRZ Page

    When they claim 24 bits, they probably mean 12 bits of I and 12 bits of Q. I doubt they're doing direct RF sampling at 24-bits...
  9. N6YWU

    N6YWU Ham Member QRZ Page

    Xiegu is likely using a 24-bit stereo audio ADC. Not 2 12-bit ADCs. However 24-bit consumer audio ADCs don't really have 24 bits of usable range, as the noise floor of the mixer and ADC itself likely makes the bottom few bits quite random, and thus provide an ENOB of maybe only 18 to 20 bits. Still perhaps more than 16 or 12. But far less than 24. And using a stereo ADC means it has 2 channels, one for each of I and Q (instead of L and R).

    Now the RF direct sampling ADCs in the Hermes Lite 2 transceiver are only 12-bit. But the HL2 SDR system (hardware, gateware,and software) provides enough dynamic range (due to the higher sample rate plus the sample rate processing gain) to place Hermes Lite 2 setups in the Top 50 pskreporter stats, while receiving on multiple HF bands simultaneously.

    So processing gain is a real thing.
    KC3PBI likes this.
  10. W6UV

    W6UV Premium Subscriber QRZ Page

    There wouldn't necessarily be two 12-bit ADCs. A direct sampling receiver can use a single 12-bit ADC to sample the RF, and then convert the data stream into two 12-bit I and Q streams.

    Since the Xiegu website says "The SDR structure is based on 24-bit data size at 48kHz sampling rate", in this case you're right--they're probably using a 24-bit audio ADC since they're only sampling at 48 kHz. Practical ADCs for RF direct sampling, at this point in time, max out at 16 bits--there aren't any 24-bit ADCs that sample at the >=60 MHz needed for RF direct sampling of the entire HF range.

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