# Using Units-Only NOMINAL or MOD1 Fields to Adjust Standards in a Loop

In the MET/CAL Help System, we learn that when the NOMINAL or MOD1 fields of an instrument FSC contains only a unit of measure (without a value), the value is taken from the memory register MEM.

This behavior creates the ability to use a DO-UNTIL loop to adjust the output of a standard until a condition is met. This is useful for tasks like establishing a reference level during an oscilloscope bandwidth test.

To perform this type of loop, we use a units-only field as the adjusted value, as in the examples below. The first example shows a loop that adjusts the reference level of the oscilloscope option output for a DUT amplitude reading of approximately 600 mV(pp).

``#Define the Starting Voltage. 1.001 MATH @Voltage = 600E-3 1.002 MATH MEM = @Voltage;MEM1 = MEM#Initiate the DO-UNTIL Loop. 1.003 DO#Adjust the existing set point to move closer to desired level.#In the first loop, this will remain at the starting value. 1.004 MATH MEM = MEM1 + (@Voltage - MEM) / 2#Set the output of the standard to the current value based on MEM.#The value of MEM1 will be changed to the current set point after this#row since it is a Setup test type (S in MOD4). 1.005 5520 V 50kH LS S6 S L#Check the Amplitude of the measured waveform. 1.006 SCPI MEASU:MEAS2:VAL?[I]#Define the condition for exiting the loop. 1.007 UNTIL (MEM >= 0.599) && (MEM <= 0.601)``

This next example demonstrates using the same concept on the MOD1 field instead of NOMINAL to find the actual frequency of the 3 dB Bandwidth of an oscilloscope.

``#Define the starting frequency. 1.001 MATH @Frequency = 440E6 1.002 MATH MEM = @Frequency; MEM1 = MEM#Define the step size for the loop. 1.003 MATH @FreqStep = 10E6#Initiate the DO-UNTIL loop. 1.004 DO#Adjust the setpoint upward by the size of the step. 1.005 MATH @Frequency = @Frequency + @FreqStep 1.006 MATH MEM = @Frequency#Set the output of the standard to the current value based on MEM. 1.007 5520 600mV H LS S6 S L#Check the amplitude of the measured waveform. 1.008 SCPI MEASU:MEAS2:VAL?[I]#Define the condition for exiting the loop. 1.009 UNTIL (MEM <= 0.424)``

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