User`s guide

138 53220A/53230A User’s Guide
4 53220A/53230A Input Signal Conditioning
Threshold sensitivity (Figure 4-4) to the input signal is a function of the
amount of noise rejection or hysteresis. Noise rejection (the hysteresis band)
at the counter input is increased or decreased with the command:
INPut[{1|2}]:NREJection {OFF|ON}
INPut[{1|2}]:NREJection? (query form)
ON enables noise rejection, thus increasing hysteresis and decreasing sensi-
tivity by 50%. This setting should be used when noise is present in the signal
environment. However, if the threshold level is close to a peak amplitude (pos-
itive or negative), a count will not occur if the signal does not cross both hys-
teresis levels due to the increased hysteresis band. OFF disables noise
rejection which decreases hysteresis and increases sensitivity.
CONFigure and MEASure do not change the noise rejection setting. Following
a reset (*RST) or instrument preset (Preset), noise rejection is disabled (off).
Noise Rejection from Bandwidth Limiting For input signals up to 100 kHz,
reducing the counter’s bandwidth from 350 MHz to 100 kHz (see “Bandwidth
Limiting Filter”) also provides noise rejection. Additional noise rejection for
frequencies in this range is then obtained through INPut:NREJection ON.
Time Interval Errors Using noise rejection with time interval measurements
results in time interval errors due to errors in setting the threshold level and
the effects of hysteresis on the trigger and reset points (Figure 4-4). These
errors are dependent on the input signal slew rate at each trigger point.
Noise Rejection Example
//period measurement of expected 10 MHz signal,
//use channel 1
CONF:PER 0.1E-6,.001,(@1)
INP:IMP 1.0E6 //set impedance to 1 Mohm
INP:RANG 50 //set range to 50
INP:COUP AC //set AC coupling
INP:LEV 3 //set 3V threshold level (absolute)
INP:NREJ ON //enable noise rejection