The 98 dB figure for 16-bit DAC SNR is just a theoretical maximum. The
actual SNR is lower. The Noise Spectral Density of the AD9146 is around
-163 dBc/Hz, so the SNR in the Nyquist bandwidth of 400 MHz is 77 dB
(-163 + 10 log 400). I guess the UBX has a 160 MHz low pass, so 81 dB is
probably a better estimate.
https://training.ti.com/sites/default/files/docs/TIPL%204703%20-%20Understanding%20Signal%20to%20Noise%20Ratio%20and%20Noise%20Spectral%20Density%20in%20High%20Speed%20Datat%20Converters_3.pdf
Ron
On 12/7/20 00:33, Lukas Haase via USRP-users wrote:
Hi Marino,
Which digital attenuator do you mean?
* The 46.43dB attenuation that I use in the TX-RX loopback is just
composed of static, passive attenuators
* The receive gain is set to 20dB. Hence I assume I do not make use
of the 6dB "digital gain" in the ADC and from the remaining 31.6dB
gain I use 20. Hence I the "digital attenuator" should be 11.5dB
if that answers your question?
* At a receive gain of 20dB, the noise figure is ~5dB, according to
specs and measurements. -174+5+dB10(5e6)=-102dBm which is exactly
what I measure without a signal applied.
Thanks,
Lukas
*Gesendet:* Montag, 07. Dezember 2020 um 03:23 Uhr
*Von:* "cyberphox" <[email protected]>
*An:* "Lukas Haase" <[email protected]>
*Cc:* "Marcus D. Leech" <[email protected]>,
[email protected]
*Betreff:* Re: [USRP-users] UBX160 TX "noise figure"?
Hi Lukas,
What setting do you have the digital attenuator set to?
Kind regards
Marino
On Mon, 7 Dec 2020 at 02:05, Lukas Haase via USRP-users
<[email protected] <mailto:[email protected]>> wrote:
Hi Marcus,
Thanks again!
I did now the following experiment: I connected TX to RX
back-to-back with 46.43dB attenuation in between. I set TX gain
and RX gain to 20dB and transmit a single CW at -3dBFS.
This means my output power is *Pout=11.44dBm* (cross checked with
spectrum analyzer) and on RX I sould have Pin=-34.99dBm. Indeed,
calculating the RMS of the received signal and converting to dBm,
I get *Pin=-35.0224dBm*. Spot on!
The red line is what I receive on the PSD (blue is the TX that I
send):
As you can see from the annotation, the measured "SNR" of the
received signal is only 38.7dB. I think this is mainly caused by
the phase noise skirt (and potentially the I/Q image).
In order to keep only consider thermal noise, I add random noise
to the original CW (using randn(...)+1i*randn(...) in MATLAB)
until it matches roughly the white noise floor of the received
signal. It's *SNRoutput=50dB* (yellow line).
Now, according to our discussion below, at Gtx=20, we should have
*SNRoutput=72dB* (assuming thermal noise only).
Where could the *22dB difference* in SNR come from?
Thanks!
Lukas
PS: I am aware of phase noise, DC offsets, I/Q imbalance etc. But
as you can see from my plot, I am /only /considerung thermal
noise. The thermal noise of the receiver should be orders of
magnitude lower (at least -102dBm) so the receiver noise should
not limit the results either.
*Gesendet:* Montag, 30. November 2020 um 17:08 Uhr
*Von:* "Marcus D. Leech" <[email protected]
<mailto:[email protected]>>
*An:* "Lukas Haase" <[email protected] <mailto:[email protected]>>
*Cc:* [email protected] <mailto:[email protected]>
*Betreff:* Re: [USRP-users] UBX160 TX "noise figure"?
On 11/30/2020 01:54 PM, Lukas Haase wrote:
Hi Marcus,
That makes sense, thanks.
Would you be willing to confirm if what I am doing here is
correct?
To first order, the DAC has an SNR of 98dB (16 bit). Then I
use Fries' equation to get the NF of the following stages (for
the filter and the attenuator, the noise figure is equal to
its attenuation). The NF is dominated by the 2nd and third term.
Then I subtract the NF from the SNR which gives me an output
SNR somewhere between 92dB and 67dB. Does that sound right?
For the attenuator term, just assign it a NF (in dB) of (31.5 -
TXGAIN).
The noise figure of an attenuator is just the attenuation
value--similarly for the filter. Just pretend it's a fixed
attenuator with 0 gain.
So the 'noise figure' after the DAC is just 2+(31.5 - TXGAIN)
then factor in the gains and noise figures of the amplifiers.
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