Hi Marcus, when I turn down the gain, the shape of the spectrum noisefloor remains non-flat. Also at a gain of 0, the shape is wavy. I'm not feeding any RF source to the channels: they are 50 Ohm terminated. The plots of the spectrum were produced with a third party application. However, I've recorded the samples by using the "rx_samples_to_file" tool and then plotted them with Matlab but the shape remains the same (non-flat). I am aware that the final spectrum is the convolution of all the filtering stages, but I would like to know if this is the expected noisefloor spectrum for the TwinRx (for example for UBX the spectrum noisefloor is much more flat). For this reason, I am asking if someone can provide a plot of the spectrum of both channels at a sampling rate of 100 Msps.
Thank you. Regards, Carlo Venier Il gio 22 giu 2023, 15:30 Marcus D. Leech <patchvonbr...@gmail.com> ha scritto: > On 22/06/2023 05:48, Carlo Venier wrote: > > Good morning, > > I have some questions regarding the TwinRX daughterboard. > > 1) With the following settings: > Master Clock Rate = 200 MHz > Tuning Frequency = 2450 MHz > Sample rate = 100 Msps > Gain = 93 > Every RF connector is 50 Ohm terminated > LO_OFFSET disabled > > I get a non-flat spectrum. In particular, the center portion of it is up > to 5 dB higher than at +/- 40 MHz from the tuning frequency. You can see > this behaviour in the attached figures. > > 2) The two channels have a different behaviour below 1.8 GHz (for example > at 450 MHz). The spectrum of channel :1 is a lot more wavy than the > spectrum of the channel :0. You can see this behaviour in the attached > figures. > In any case, also below 1.8 GHz the delta between min and max level is > about 5 dBs. > > 3) Sometimes, the DC component pops up. As the TwinRX channels are a > superheterodyne receivers, I don't expect that the DC component appears, > but it happens. > > Is it possible that the TwinRX I'm using is not working properly? Or am I > missing some settings? > > 4) Can somebody provide a screenshot of the spectrum of the TwinRx > channels with the same settings at 1) for the tuning frequencies of 450 MHz > and 2450 MHz? > > Thank you. > > > Best regards, > > Carlo Venier > > _______________________________________________ > USRP-users mailing list -- usrp-users@lists.ettus.com > To unsubscribe send an email to usrp-users-le...@lists.ettus.com > > What happens if you turn the gain down about 5dB? What are you using to > provide a perfectly-flat noise input? > > What are you using to collect the data that produced these plots? > > The analog bandwidth of the TwinRX is limited to 80MHz, and uses SAW > filters at the 1st IF (TA0824A and TA0851A, depending > on whether it's operating in "low band" or "high band"). The final IF > filter is a discrete multi-stage design, but I don't > know what it's filter shape is. > > IN ADDITION, there are RF input filters that are selected based on the > tuned RF frequency. The end result of all of these > filters is that the final filter "shape" is the convolution of all of > those individual filter shapes. > > I'm not sure what you mean by "sometimes the DC component shows up". I > would expect there to be spurs at some points-- > for example those that are exact multiples of internal clocks, etc. > > > _______________________________________________ > USRP-users mailing list -- usrp-users@lists.ettus.com > To unsubscribe send an email to usrp-users-le...@lists.ettus.com >
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