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diff --git a/host/docs/general.dox b/host/docs/general.dox index 434b3714a..3e9dfc63a 100644 --- a/host/docs/general.dox +++ b/host/docs/general.dox @@ -80,6 +80,45 @@ second). usrp->issue_stream_command(...); ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +\section general_sampleratenotes Sample rate notes + +Sample rates as delivered to the host computer for USRP devices are constrained to follow several important rules. + +It is important to understand that strictly-integer <b>decimation</b> and <b>interpolation</b> are used within USRP +hardware to meet the requested sample-rate requirements of the application at hand. That means that the desired +sample rate must meet the requirement that master-clock-rate/desired-sample-rate be an integer ratio. Further, it is +strongly desirable for that ratio to be <b>even</b>. + +There are further constraints on the desired sample rate, such that if the required decimation or interpolation exceeds 128, +then the resulting decimation <b>must</b> be evenly divisible by 2, and that if the required decimation exceeds 256, the +resulting decimation \b must be evenly divisible by 4. + +For USRP devices with fixed master clocks (notably: USRP1, USRP2, N2xx), there are fewer effective sample rates available than +on USRP hardware that provides some flexibility in selecting a master clock. Several USRP devices support flexible master +clock selection, allowing a broader range of sample rate selections by applications. See the individual devices' manual +pages for more details. + +In many cases using USRPs with flexible master-clock rates, it is possible to achieve lower sample rates without running into +the constraints of higher decimations, simply by choosing a lower master-clock rate to keep required decimation below 128. + +\subsection general_sampleratenotes_automatic Automatic master-clock selection + +In recent versions of UHD software (3.8.5 and newer), and on some devices (currently: B2xx and E3xx series), +the master clock rate is chosen automatically (unless specified by the user). +UHD will select a master clock rate that is consistent with the desired sample rate indicated by the application. + +\subsection general_sampleratenotes_nyquist Master clock rate and Nyquist + +In selecting a master clock rate on certain USRP hardware (X3xx and B1xx), it is important to select a rate that still provides +correct alias suppression by the analog hardware. For daughtercards with a 40 MHz analog bandwidth, this means the clock rate +must be <b>at least</b> 40 MHz, with better performance to be expected with a higher clock rate. For daughtercards +with 160 MHz bandwidth, it must be <b>at least</b> 160 MHz, again, better performance is to expected with a higher clock rate. + +For hardware with fixed master clock rates, of course, this isn't a consideration. + +For B2xx and E3xx hardware, the alias suppression is handled differently by the AD936x RFIC, and master clock rate +is significantly more flexible as a result. + \section general_ounotes Overflow/Underflow Notes <b>Note:</b> The following overflow/underflow notes do not apply to USRP1, |