News GW Instek Waveform display comparison between GDS-1000B and TEK TBS1000B-EDU

Waveform display comparison between GDS-1000B and TEK TBS1000B-EDU

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

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.

 

TEK TBS-1000B-EDU

GW GDS-1000B

Time base

Effective Sample rate (Sa/s)

Effective Sample rate (Sa/s)

50s

5

20k

25s

10

50k

10s

25

100k

5s

50

200k

2.5s

100

500k

1s

250

1M

500ms

500

2M

250ms

1k

5M

100ms

2.5k

10M

50ms

5k

20M

25ms

10k

50M

10ms

25k

100M

5ms

50k

200M

2.5ms

100k

500M

1ms

250k

1G

500us

500k

1G

250us

1M

1G

100us

2.5M

1G

50us

5M

1G

25us

10M

1G

10us

25M

1G

5us

50M

1G

2.5us

100M

1G

1us

250M

1G

500ns

500M

1G

250ns~5ns

1G

1G

1G

1G

1G

1G

1G

1G

1G

1G

1G

1G

Note: TBS-1000B-EDU is 1-2.5-5 increment; GDS-1000B is 1-2-5 increment

The biggest shortcoming of short memory depth is Aliasing due to the lack of sample rate. Oscilloscope’s sample rate should be 2x higher than DUT’s frequency in order to restore the original waveforms.

The following example is done by providing 1KHz/1V square wave to TEK1052B-EDU and GDS-1102B via a GW Instek MFG-2260MRA function generator.

 1.Under the general situations, users can see the waveform without any problem.

TEK TBS-1000B-EDU display

GW Instek GDS-1000B display

2. If users want to see the details of the waveform, stop the waveform at low speed horizontal range and perform Zoom in. Users can clearly see that TEK TBS1052B-EDU with only 2.5K memory depth cannot restore the waveform that results in severe Aliasing. TBS1052B -EDU at 250ms/div has a sampling rate of only 1kSa/s, which cannot satisfy the Nyqeist theory: Sample rate should be at least 2x higher than input frequency. As a result, TEK1052B-EDU produced Aliasing.

Aliasing due to the insufficient sample rate

After pressing pause and zooming in, signal is obviously distorted


3. For GDS-1102B under 200ms/div, its 5MSa/s can satisfy the requirement of sample rate which is 2x higher than input frequency. As a result, GDS-1102B revealed genuine waveforms without Aliasing.

Waveforms entered roll mode under 200ms/div

After pressing pause and zooming in, signal is restore without
distortion


4.When testing higher frequency waveforms or fast-changing modulation or arbitrary waveforms, we could easily find the difference between the two oscilloscope displays.

(a) Comparison between tests with 20kHz PWM signals (the left display is TEK TBS1052B-EDU and the right display is GDS-1000B

Under stop, only pulse is shown

For fast-changing modulated waveforms, the change of the
modulated signal can be clearly observed under stop

 

After zooming in, the square waveform’s point display cannot be restored to the original modulated waveform.

Via the Zoom function, users can observe the details of the
fast-changing waveform.


(b)Comparison of Video signal tests

 The display of the fast-changing waveform under
TEK TBS1000B-EDU becomes very blurred.

GDS-1000B can clearly observe the NTSC video waveform
under the same conditions.

 

TEK TBS1000B-EDU has only the point display under oom in and the display is very blurred.

GDS-1000B can observe the details of color bust waveform
under Zoom

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.

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