The input impedance can be set to 1 MΩ or 50 Ω by toggling the corresponding softkey.
CAUTION: Switching the impedance to 50 Ω when the input voltage is above 12 Vrms may cause permanent damage to the input circuitry.
The input signal’s amplitude can be attenuated by 1 or 10 by toggling the softkey marked 1x/10x. Use attenuation whenever the input signal exceeds the dynamic input voltage range ±5 V or else when attenuation can reduce the influence of noise and interference. See the section dealing with these matters at the end of this chapter.
Switch between AC coupling and DC coupling by toggling the softkey AC/DC.
Use the AC coupling feature to eliminate unwanted DC signal components. Always use AC coupling when the AC signal is superimposed on a DC voltage that is higher than the trigger level setting range. However, we recommend AC coupling in many other measurement situations as well.
When you measure symmetrical signals, such as sine and square/triangle waves, AC coupling filters out all DC components. This means that a 0 V trigger level is always centered around the middle of the signal where triggering is most stable.
Signals with changing duty cycle or with a very low or high duty cycle do require DC coupling. Fig. 3-4 shows how pulses can be missed, while Fig. 3-5shows that triggering does not occur at all because the signal amplitude and the hysteresis band are not centered.
NOTE: For explanation of the hysteresis band, see page 4-3.
If you cannot obtain a stable reading, the signal-to-noise ratio (often designated S/N or SNR) might be too low, probably less than 6 to 10 dB. Then you should use a filter. Certain conditions call for special solutions like highpass, bandpass or notch filters, but usually the unwanted noise signals have higher frequency than the signal you are interested in. In that case you can utilize the built-in lowpass filters. There are both analog and digital filters, and they can also work together.
Analog Lowpass Filter
The counter has analog LP filters of RC type, one in each of the channels A and B, with a cutoff frequency of approximately 100 kHz, and a signal rejection of 20 dB at 1 MHz.
Accurate frequency measurements of noisy LF signals (up to 200 kHz) can be made when the noise components have significantly higher frequencies than the fundamental signal.
Digital Lowpass Filter
The digital LP filter utilizes the Hold-Off function described below.
With trigger Hold-Off it is possible to insert a deadtime in the input trigger circuit. This means that the input of the counter ignores all hysteresis band crossings by the input signal during a preset time after the first trigger event.
When you set the Hold-Off time to approx. 75% of the cycle time of the signal, erroneous triggering is inhibited around the point where the input signal returns through the hysteresis band. When the signal reaches the trigger point of the next cycle, the set Hold-Off time has elapsed and a new and correct trigger will be initiated. Instead of letting you calculate a suitable Hold-Off time, the counter will do the job for you by converting the filter cutoff frequency you enter via the value input menu below to an equivalent Hold-Off time.
You should be aware of a few limitations to be able to use the digital filter feature effectively and unambiguously. First you must have a rough idea of the frequency to be measured. A cutoff frequency that is too low might give a perfectly stable reading that is too low. In such a case, triggering occurs only on every 2nd, 3rd or 4th cycle. A cutoff frequency that is too
high (>2 times the input frequency) also leads to a stable reading. Here one noise pulse is counted for each half-cycle.
Use an oscilloscope for verification if you are in doubt about the frequency and waveform of your input signal. The cutoff frequency setting range is very wide: 1 Hz – 50 MHz
Toggle between manual and automatic triggering with this softkey. When Auto is active the counter automatically measures the peak-to-peak levels of the input signal and sets the trigger level to 50% of that value. The attenuation is also set automatically.
At rise/fall time measurements the trigger levels are automatically set to 10% and 90% of the peak values.
When Manual is active the trigger level is set in the value input menu designated Trig. See below. The current value can be read on the display before entering the menu.
The Auto-function measures amplitude and calculates trigger level rapidly, but if you aim at higher measurement speed without having to sacrifice the benefits of automatic triggering, then use the Auto Trig Low Freq function to set the lower frequency limit for voltage measurement.
If you know that the signal you are interested in always has a frequency higher than a certain value flow , then you can enter this value from a value input menu. The range for flow is 1 Hz to 100 kHz, and the default value is 100 Hz. The higher value, the faster measurement speed due to more rapid trigger level voltage detection.
Even faster measurement speed can be reached by setting the trigger levels manually. See Trig below.
Follow the instructions here to change the low-frequency limit:
- Press SETTINGS->Misc->Auto Trig Low Freq.
- Use the UP/DOWN arrow keys or the numeric input keys to change the low frequency limit to be used during the trigger level calculation, (default 100 Hz).
- Confirm your choice and leave the SETTINGS menu by pressing EXIT/OK three times.
Value input menu for entering the trigger level manually.
Use the UP/DOWN arrow keys or the numeric input keys to set the trigger level. A blinking underscore indicates the cursor position where the next digit will appear. The LEFT arrow key is used for correction, i.e. deleting the position preceding the current cursor position.
NOTE: It is probably easier to make small adjustments around a fixed value by using the arrow keys for incrementation or decrementation. Keep the keys depressed for faster response
NOTE: Switching over from AUTO to MAN Trigger Level is automatic if you enter a trigger level manually.
Converting “Auto” to “Fixed”
The trigger levels used by the auto trigger can be frozen and turned into fixed trigger levels simply by toggling the MAN/AUTO key. The current calculated trigger level that is visible on the display under Trig will be the new fixed manual level. Subsequent measurements will be considerably faster since the signal levels are no longer monitored by the instrument. You should not use this method if the signal levels are unstable.
NOTE: You can use auto trigger on one input and fixed trigger levels on the other.