FREQ4

Frequency List, Alternate Form 4

Description: Defines a set of frequencies used in the solution of modal frequency-response problems specifying the amount of "spread" around each natural frequency and the number of equally space excitation frequencies within the spread.

Format:

Example:

Field	Definition	Type	Default
SID	Set identification number.	Integer > 0	Required
F1	Lower bound of frequency range in cycles per unit time.	Real ≥ 0.0	0.0
F2	Upper bound of frequency range in cycles per unit time.	Real > 0.0, F2 ≥ F1	F1
FSPD	Frequency spread, +/- the factional amount specified for each mode which occurs in the frequency range F1 to F2.	0.0 < Real < 1.0	0.10
NFM	Number of evenly spaced frequencies per spread mode.	Integer > 0	3

Remarks:

1. FREQi entries must be selected with the Case Control command FREQUENCY = SID.
2. There will be NFM excitation frequencies between (1 – FSPD)*f_i for each natural frequency in the range F1 to F2.
3. In the example above, the will be 11 equally spaced frequencies across a frequency band of 0.8*f_i to 1.2*f_i for each natural frequency that occurs between 10 and 1000.
4. The frequency spread can be used also to define the half-power bandwidth. The half-power bandwidth is given by where is the damping ratio. Therefore, if FSPD is specified equal to the damping ratio for the mode, NFM specifies the number of solution frequencies within the half-power bandwidth. See Figure 1 for the definition of half-power bandwidth.

   

   Figure 1. Half-Power Point and Bandwidth

5. Since the forcing frequencies are near the structural resonance, it is important that some amount of damping be specified.
6. The DFREQ model parameter specifies the threshold for the elimination of duplicate frequencies. Duplicate frequencies will be ignored if

   

   where DFREQ is defaulted to 1.0x10^-5 and and are the maximum and minimum solution frequencies of the combined FREQi entries. (See Section 5, Parameters, for more information on DFREQ.)