Three major factors, including bandwidth, sample rate and memory depth, contribute the selection of a digital oscilloscope. This article is to elaborate the impact on waveform retrieval under same sample rate and different memory depth. The number of samples an oscilloscope can store is defined as memory depth. Memory depth can be calculated by Record duration divided by Sample period as shown in the formula below. As indicated, memory depth has a positive relationship with the sampling rate. In other words, waveforms can be recorded over a long period of time when stored in a larger memory depth
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Total Waveform Points Sampled = Record Duration /Sample Period=Record Duration X Sampling Rate If Total Waveform Points Sampled > DSO Memory Depth, all excessive points sampled need to be abandoned and the effective sampling rate is forced to slow down Memory Depth= Record Duration X Effective Sampling Rate Effective Sampling Rate = Memory Depth/ Record Duration When Record Duration is long, Longer DSO Memory Depth means Faster Effective Sampling Rate |
*Sample period is 1/sample rate
**Record duration = Time Base X 10 div
For relatively slow and repetitive signals, memory depth should be the primary consideration rather than sampling rate; for example, GW Instek GDS-1000B series has a long memory depth of 10M points per channel. The memory depth of 10,000,000 (per channel) points enable you to pick up and hold a complete signal with better accuracy. On the other hand, when observing a single shot event, the sample rate would be the most essential element to observe. What are waveform differences under same bandwidth and sample rate but different memory depth? We made a comparison between TEK TBS-1000B-EDU and GW Instek GDS-1000B to reveal the differences. TEKTBS-1000B-EDU 50MHz and 70MHz models have a sample rate of 1GSa/s but the memory depth is only 2.5k. GW Instek GDS-1000B also has a sample rate of 1GSa/s and its memory depth is as high as 10M per channel, hence, the sample rate of each horizontal level is at least 4000x higher than that of TEK TBS-1000B-EDU. For the maximum sample rate of 1GSa/s, TEK TBS-1000B-EDU offers sample rate per division of 250ns~5ns/div. On the contrary, GDS-1000B extensively provides 1ms-5ns/div.
(Comparison chart for the sample rate of each horizontal level is as follows )
To avoid distortion, oscilloscope’s sample rate should be at least 2x higher than the maximum frequency of DUT. The sample rate of each horizontal level for TEK TBS-1000B-EDU is 4000x lower than that of GDS-1000B, therefore, the product measurement range by TEK’s scope is substantially limited. While using slow levels or roll mode to reveal waveforms, only GDS-1000B can truthfully restore the original waveforms.
While switching horizontal levels to slower levels, every three slower levels will reduce sample rate in ten folds and the sample rate will be reduced to only 1% after switching 6 slower levels. Once users use a short memory depth oscilloscope to observe signals, the result will be misjudged due to Aliasing. It is especially worst while observing Pulse signals and modulation signals with a short memory depth oscilloscope.From the above comparison, we can conclude that the 10M long memory depth of GDS-1000B can yield higher sample rate for each horizontal level to accurately restore and display waveforms. Additionally, the new platform of GDS-1000B allows waveform update rate to reach 50,000 wfm/s so as to faithfully reveal Pulse signals and modulation signals as well as easily identify abnormal burst signals. By comparing with the TDS-1000 series, TEK TBS-1000B-EDU is rather a new product but its main upgrade is display screen from 5.7”to 7”and its internal design is still an old design, which is slow in operation and waveform update rate. GDS-1000B also provides 1M pts FFT display. We can conclude that GDS-1000B is superior to TEK TBS-1000B-EDU in terms of functionalities and applications.
