Joslas platums of the osciloskops Digital lietojumprogrammas
Pieredze stāsta us ka joslas platums of an osciloskops būtu būtu vismaz piecas reizes augstāks nekā ātrākais digitālais pulkstenis ātrums of the system under test. If we select an oscilloscope that meet this criterion, then the oscilloscope will be able to capture the 5th harmonic of the signal under test with minimal signal attenuation. The 5th harmonic of the signal is important in determining the overall shape of the digital signal. Tomēr, šis vienkāršs formula nav ņem ņem kontā faktiskais augstfrekvence komponenti ietverts in ā ātri pieaug un krīt malas ja precīzi mērījumi of ātrs malas ir nepieciešami.
Formula: fBW Lielāks nekā vai vienāds līdz 5xfclk
A more accurate way of determining the bandwidth of an oscilloscope is based on the highest frequency present in the digital signal, rather than the maximum clock rate. The highest frequency of the digital signal depends on what the fastest edge speed in the design is. Tāpēc, we first need to determine the rise and fall times of the fastest signals in the design. This information can usually acquire from the published specifikācijas of the devices used in the design.
Maksimums "real" frekvence komponents of the signāls is aprēķināts izmantojot a simple formula, and Dr Howard W. Johnson has written a book on this topic, High Speed Digital Design. In this book, he refers to this frequency component as the "fknee" frekvence. The spectrum of all fast edges satur an infinite number of frequency components, but there is a point of inflection (or "knee") above who the frekvence komponenti ir nesvarīgi in noteikšana forma of the signāls. Solis 2: Aprēķināt fknee
fknee=0.5/RT(10%-90%) fknee=0.4/RT(20%-80%)
For signals with rise time characteristics defined by the 10% to 90% threshold, the inflection frequency fknee is equal to 0.5 divided by the rise time of the signal. For signals with rise time characteristics defined according to the 20% to 80% threshold (which is the usual definition in today's device specifications), fknee is equal to 0.4 divided by the rise time of the signal. But be careful not to confuse the signal rise time here with the oscilloscope's rise time specification; what we are talking about here is the actual signal edge speed. The third step is to determine the oscilloscope bandwidth required to measure the signal based on the level of accuracy required to measure the rise and fall times. Table 1 gives the oscilloscope bandwidth needed versus fknee for various accuracy requirements for oscilloscopes with Gaussian frequency response or maximum flat frequency response. It should be remembered, however, that most oscilloscopes with bandwidth specifications of 1 GHz and below are usually Gaussian, while those with bandwidths greater than 1 GHz are usually of the maximum flat frequency response type. Table 1: Coefficients for calculating the required bandwidth of an oscilloscope based on the accuracy required and the type of frequency response of the oscilloscope Step 3: Calculate the oscilloscope bandwidth
Ejam's ejam cauri a vienkāršs piemērs:
Noteikt the minimum bandwidth required for an oscilloscope that has a correct Gaussian frequency response when mērīšana 500ps rise time (10-90%); if the signal has a rise/fall time of aptuveni 500ps (defined by the 10% to 90% kritērijs), then the maximum real frequency component of the signal, fknee =} (0.5/500ps)=1}}GHz
Ja a laiks kļūda of 20% ir atļauts kad veicot mērījumus of rise time and fall time parameters, then an oscilloscope with a bandwidth of 1GHz būtu būtu piemērots šim digitālajam mērījumam lietojumprogrammai. Tomēr, ja ja laiks precizitāte ir nepieciešams būt iekšā 3%, tad an osciloskops ar a joslas platums of 2GHz būtu būtu labāks.
20% time accuracy: osciloskops joslas platums=1.0x1GHz=1.0GHz
3% hronometrāža precizitāte: osciloskops joslas platums=1.9x1GHz=1.9GHz
