Block diagram on Figure 1 demonstrates input signal transformation to series of time and frequency measurement data points.
First, input signal gets into input amplifier. The generic role of input amplifier circuits is matching the input to signal source, and optionally: attenuate or amplify it, remove DC offset and/or filter out high-frequency noise.
Please note: There are several kinds of input amplifier circuits (configurable input amplifiers, prescalers, fixed input amplifiers), please refer to 4.5 Input signal conditioning/Input Amplifiers chapter for more details about them.
After the input amplifiers the signal is digitized using one (inputs C, EA, ER) or 2 comparators (inputs A, B, D, E). It works the following way: when the signal crosses the set trigger level, comparator generates a positive (in case of transition from below to above the trigger level) or negative (in case of transition from above to below the trigger level) slope. Please refer to Figure 2 for an example.
Please note: For most measurement modes on inputs with 2 comparators, output of only one comparator is used for measuring the signal from this input. However, there are measurement modes using both comparators implicitly:
- Rise/Fall Time and Slew Rate measurements use 2 comparators for getting Time Interval between lowest (10%) and highest (90%) levels of the signal (see details in 5.4.3 Rise Time, Fall Time, Rise-Fall Time);
- Pulse width and Duty Cycle measurements use 2 comparators to produce pulses on positive and negative slope of a signal and measure Time Interval between these;
- Frequency and Period Average use 2 comparators to implement implicit wide hysteresis targeted on improving noisy signals measurements (see details in 5.1.1 Frequency/Period Average measurements chapter).
Second comparator can also be explicitly selected for measurement. E.g., Time Interval A, A2 will measure time interval between signal level set by trigger level A and signal level set by trigger level A2 (can be useful for measuring characteristics of multi-level signals, e.g. TDR measurements – see 5.2.4 Measuring Time Interval between different trigger points of the same signal). It can also be explicitly used as a source of start or stop arming.
Digitized signals from physical inputs are just pulse trains which can be multiplexed to 4 internal measurement channels. Each measurement channel starts with Wide Hysteresis block. Wide Hysteresis block just passes through the signal for most measurement functions except Frequency, Period Average, their Smart versions, TIE, and Frequency Ratio for which implicit wide hysteresis is used for better noise tolerance (see 5.1.1 Frequency/Period Average measurements).
The resulting pulse train is then counted independently in each measurement channel. Measurement logic specific for each measurement mode makes snapshots of channel’s pulse counter, timestamps it and adds channel number, forming series of so-called raw results.
Raw results are further post-processed in calculation block to form series of final value-timestamps pairs.
See Figure 2 for a visualized example of signal processing happening inside CNT-104S.
