Converting dBFS to dBm with an SDR receiver based on the LTC2145-14 clocked at 125 Mhz

Hello,

I am relatively inexperienced with ADCs so my apologies if this question is a bit naive but I could not find any clear answer here.

I am using an SDR receiver build on the LTC2145-14  clocked at 125 Mhz  in order to measure power of weak sinusoidal signals. The analogue frontend of the ADC is quite simple, it is the one in the datasheet of the device, a 50 ohm resistor is connected to the high impedance differential A+ and A- analogue inputs and a wideband 1:1 balun connects this 50 ohm resistor to the input sma port of the receiver. No analogue op amp in the way, just a very loss loss anti-aliasing filter  with a 52 Mhz cut off frequency to  prevent reception of signals above the Nyquist frequency of 62.5 Mhz. With this set-up the input impedance of the receiver is a nice and fla 50 ohm from 100khz to about 50 Mhz. 

To measure power, I simply convert the signal received in dBFS into actual true world dBm.  Using an external rf power meter, I have determined empirically that the relationship between "dBFS" and "dBM" in my set-up is -10 dB . It does not change  much across the whole dynamic range of the SDR receiver and across its frequency range either. For instance a signal generator delivering  -40 dBm into my 50 ohms input impedance rf power meter will correspond to a signal reading of -50 dBFS when it is connected to my 50 ohm input impedance SDR receiver. 

The trouble is that I am unable to explain this result by looking at the datasheet of the LTC2145-14!!   The SENSE pin 63 of the LTC2125-14 chip is set to "0V" which corresponds to 1v peak to peak full scale, so +4 dBm into a 50 ohm load. So if 0 dBFS is corresponding to +4 dBM then according to the same logic,  a signal of -40dBm should correspond to a signal of -44 dBFS but as explained above, I measure -50 dBFS and not -44 dBFS !!! There is a 6dB difference that I can not explain in anyway!

I concluded it is the SDR software that is responsible for this difference but I am not too sure really.  Can someone better explain me where this 6dB difference comes from ? Is it the SDR software that I use that explains it ?  Is the reasoning I am making erroneous and if yes, can someone explain me why ?

Thanks
Regards
Peter 



correcting spelling mistake in the title
[edited by: on7yi at 2:49 PM (GMT 0) on 4 Jun 2019]
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  • 0
    •  Analog Employees 
    on Jun 11, 2019 11:47 PM

    Hi Peter,

    I'm not familiar with the LTC2125 and the SDR software at all, but I thought I'd bring up a couple of semi-random points I use when doing a similar exercise with a different family of ADCs. A lot of this will be common sense but we might as well cover these for completeness and as a sanity check.

    • dBm is a unit of power. In calculating power from voltage and resistance, we need to be sure to use the rms value of the voltage. dBFS is also a relative power measurement, but it is with respect to the full-scale voltage of the ADC. The voltage range of ADCs would be a value that is compared to the peak-to-peak value of an AC signal. So, when converting from dBm to dBFS you need to keep the conversion of rms to peak-to-peak in mind.
    • Calculation of voltage from power requires a known resistance value. Make sure in your different scenarios that you are representing the load resistances properly. If the input impedance of the ADC is high, the load resistance will be determined by what is on the board.
    • There will be an insertion loss associated with your balun. Might this account for some of the discrepancy?
    • How does your power meter work? Does it introduce any additional load that might produce unexpected results?
    • Is there a gain adjustment in the ADC that might be affecting your results?

    You are probably beyond this but I thought I'd mention some of these generic things in case they help.

    I hope your project goes well.

    Doug

Reply
  • 0
    •  Analog Employees 
    on Jun 11, 2019 11:47 PM

    Hi Peter,

    I'm not familiar with the LTC2125 and the SDR software at all, but I thought I'd bring up a couple of semi-random points I use when doing a similar exercise with a different family of ADCs. A lot of this will be common sense but we might as well cover these for completeness and as a sanity check.

    • dBm is a unit of power. In calculating power from voltage and resistance, we need to be sure to use the rms value of the voltage. dBFS is also a relative power measurement, but it is with respect to the full-scale voltage of the ADC. The voltage range of ADCs would be a value that is compared to the peak-to-peak value of an AC signal. So, when converting from dBm to dBFS you need to keep the conversion of rms to peak-to-peak in mind.
    • Calculation of voltage from power requires a known resistance value. Make sure in your different scenarios that you are representing the load resistances properly. If the input impedance of the ADC is high, the load resistance will be determined by what is on the board.
    • There will be an insertion loss associated with your balun. Might this account for some of the discrepancy?
    • How does your power meter work? Does it introduce any additional load that might produce unexpected results?
    • Is there a gain adjustment in the ADC that might be affecting your results?

    You are probably beyond this but I thought I'd mention some of these generic things in case they help.

    I hope your project goes well.

    Doug

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