1. Frequency/Period Average measurements
These are the most universal measurement functions for Frequency and Period. In this mode each sample is a Frequency/Period value averaged over sample interval (which acts as gate).
This is back-to-back measurement with no dead-time between the samples (see chapter 4.6 Arming for exceptions). 4 input signals can be measured in parallel. Minimal sample interval is 50 ns. Up to 32 million samples total can be measured in a single measurement session. Resolution is 12 digits per 1 s of gate time (Sample Interval).
If the signal period is greater or equal to the set Sample Interval – each signal period can be captured. When measuring Frequency/Period Average on inputs A, B, D, E and Trigger Mode is set to Auto or Relative, wide hysteresis (see details below) is used to improve noise tolerance. In this mode 2 comparators with different trigger levels are used for each input. First trigger level (e.g. Trigger Level A) defines the upper limit of wide hysteresis band and the second one (e.g. Trigger Level A2) defines the lower limit. Trigger Mode Auto sets trigger levels to 60% and 40% of signal’s voltage range and Relative allows modifying them to fine tune the hysteresis band.
Without wide hysteresis, the signal needs to cross the approx. 20 mV in case of 1x Attenuation (200 mV in case of 10x) input hysteresis band before triggering occurs. This hysteresis prevents the input from self-oscillating and reduces its sensitivity to noise. If signal noise is comparable or higher than hysteresis band – it can result in false extra triggering producing erroneous counts. These could ruin the measurement.
Figure 11 shows how spurious signals can cause the input signal to cross the trigger or hysteresis window more than once per input cycle and give erroneous counts. Figure 12 shows that a wide enough hysteresis prevents false counts.
1.1. Frequency C measurement.
With an optional RF input prescaler the Analyzer can measure up to 3, 10, 15, 20, or 24 GHz on Input C. These RF inputs are fully automatic, and no trigger setup is required. Set Sample Interval to achieve optimal compromise between resolution (long Sample Interval) and speed (short Sample Interval). The optional RF input C contains a prescaler that divides the RF signal with an integer value (Prescaler factor), to enable the normal counting circuitry to measure the frequency. The Option 10 (3 GHz) divides by 16, and the option 110/xx (10 to 24 GHz) divides by 64.
Figure 14 shows the effect of the 3 GHz prescaler. For each 16 input cycles, the prescaler gives one square wave output cycle. An input frequency of let’s say 1.6 GHz is divided down to 100 MHz and measured by the normal counting circuitry. The display shows the correct input frequency since the microcomputer compensates for the effect of the division factor.
Prescalers do not reduce resolution. The relative quantization error is the same;12-13 digits for 1s Sample Interval (Gate Time). See Table 1 to find the prescaler factors.
| Function | Prescaling Factor |
| All input A, B, D, E, EA, ER functions (up to 400 MHz) | 1 |
| Frequency C (3 GHz) | 16 |
| Frequency C (10, 15, 20, 24 GHz) | 64 |
2. Smart Frequency/Period
Smart Frequency/Period is based on the same principle as normal Frequency/Period Average. Measurement gate is divided into 1000 sub-gates, giving additional samples and statistics resolution enhancement algorithm is applied on top of it. Thanks to that it allows to get up to 1 extra digit of resolution per 1 s of gate time (depending on input signal and measurement settings).
This comes with expense of additional constraints though. Minimal possible Sample Interval for Smart Frequency/Period is 50 us with possibility to measure up to 32000 samples per measurement session. Setting Sample Interval to 40 ms or more allows to get up to 4290000 samples per measurement session.
This is back-to-back measurement with no dead-time between the samples (see chapter 4.6 Arming for exceptions). 4 signals can be measured in parallel. Wide Hysteresis is used in Trigger Modes Auto and Relative.
Smart Frequency/Period is the best choice when one needs maximal possible resolution and can bear with lower sampling rate and sample count per session.
Please, note: resolution enhancement algorithm is based on the assumption that signal frequency is static. If it is not the case – the algorithm won’t be effective and it might make sense to fall back to normal Frequency/Period Average. Please note: section Frequency C measurement. Applies to Smart Frequency C as well.
3. Period Single
This measurement function is handy if one needs to capture individual periods of continuous signals or single cycles which are less than 50 ns. Individual periods starting from 2.5 ns can be captured.
This is not a back-to-back measurement, meaning that there is a dead-time of 50 ns between the samples if High-Speed license is installed, 1 us otherwise. 2 signals can be measured in parallel. Up to 16 million samples total can be measured in a single measurement session.
Unlike Frequency/Period Average and their Smart versions, Period Single doesn’t use wide hysteresis. So only one comparator is used on A, B, D, E and Trigger Mode Auto sets trigger level to the middle of signal voltage range (Relative Trigger Level 50%).
Please, note: when measuring non-continuous signal or single cycles, Trigger Mode Auto/Relative might fail to find proper trigger level. One need to fall back to Manual Trigger Mode in this case.
4. Frequency Ratio
In Frequency Ratio mode, the instrument measures Frequency Average of up to 4 signals in parallel and then divides the resulting frequencies of any two signals. Since the instruments measures on four input channels in parallel, two frequency ratio values can be displayed in parallel.
This is back-to-back measurement with no dead-time between the samples. 4 input signals can be measured in parallel. Minimal sample interval is 50 ns if High-Speed license is installed, 1 us otherwise. Up to 16 million samples total can be measured in a single measurement session.