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Voltage Source, Digital Random Bit Generator

Voltage Source, Digital Random Bit Generator

Digital Random Bit Generator Voltage Source Netlist Format

The format for a pseudorandom digital bit generator voltage source is:

Vxxxx n+ n- RBG=2 VLO=val VHI=val [VTH=val] [TD=val]
TR=val TF=val BITWIDTH=val [SEED=val] [DCD=val]
[BITLIST=#bitlist] [BITFILE=file_reference][TONE=val]

n+ and n- are the positive and negative nodes.

**Digital VRBG Voltage Source Parameters**
Parameter	Description	Unit	Default
RBG	Random bit generator keyword. Must be specified as RBG=2 to select the digital RBG voltage source.	None	1 (RBG source)
BITFILE	Reference to external file with the bit values	None	None
BITLIST	Explicit list of bits to generate	None	None
BITWIDTH	Duration of bit at the level of Vth [Must be greater than 0.0]	Second	1.0e-9
DCD	Duty cycle distortion (Fraction of BITWIDTH, 0 < DCD < 1)	None	0
SEED	Integer seed for random bit pattern	None	None
TD	(Positive) delay time to start of bitstream [Must be greater than or equal to 0.0]	Second	0.0
TONE	Frequency to use for harmonic balance analysis, should be a submultiple of or equal to the driving frequency and should also be included in the HB solution setup	Hertz	0.0
TR	Risetime from VLO to VHI [Must be greater than 0.0]	Second	Calculated
TF	Falltime from VHI to VLO [Must be greater than 0.0]	Second	Calculated
VLO	Logic LOW voltage value	Volt	0.0
VHI	Logic HIGH voltage value	Volt	1.0
VTH	Threshold voltage at which BITWIDTH is measured, also initial voltage	Volt	(VHI +VLO)/2

Digital Bit Generator Voltage Source Netlist Examples

V23 23 33 RBG=2 VLO=0 VHI=3

V43 43 53 RBG=2 VLO=0 VHI=3 SEED=1023

In the first example (V23), a random bit sequence is generated starting from an internally-generated (pseudorandom) seed.

In the second example (V43), the random bit sequence starts from the given SEED, and so will be the same each time the simulation is run.

Notes

1. The RBG source is a time-domain element, suitable primarily for transient analysis simulations. The RBG source can be used in harmonic balance analysis only when a BITLIST is given. See Using the Digital RBG Source in Harmonic Balance for details.

2. The BITLIST argument is ignored when a file_reference is supplied via the BITFILE argument.

3. When no BITLIST or BITFILE argument is supplied, the RBG voltage source generates a pseudorandom bit sequence, starting from a random seed value. When no BITLIST or BITFILE is present, the optional SEED can be used to control the bit sequence. The SEED must be an integer value (the maximum absolute value is the maximum size of integers on your system). Using the same SEED guarantees that the same sequence of bits will be generated on each simulation.

4. When a BITLIST or BITFILE is supplied, the RBG source generates the specified sequence of 0 and 1 bits, and repeats the sequence until simulation terminates. When a BITLIST or BITFILE is present, any SEED value will be ignored.

5. In all cases, the sequence of generated bits starts after the time delay given by TD, and continues until the stop time (tstop) of the transient analysis is reached. Each bit changes state with rise and fall times given by TR and TF. and bit duration given by BITWIDTH, measured at the threshold voltage level, VTH.

6. The following diagram illustrates the RBG operation for a source defined as:

V23 Port1 0 RBG=2 VLO=1 VHI=3 VTH=2 TD=0.5 TR=0.5 TF=1
+ BITWIDTH=3 BITLIST=#101

7. If TD is negative, an error occurs and the source is ignored.

8. If both TR and TF are omitted, TF and TR are set to BITWIDTH/10.

9. If TR is given and TF omitted, TF is set equal to TR.

10. If TF is given and TR omitted, TR is set equal to TF.

11. If TR or TF is negative or zero, an error occurs and the source is ignored.

12. If PW is negative or zero, an error occurs and the source is ignored.

13. Using an External File

The parameter BITFILE =file_reference refers to an external file containing the bit data. See File References in the Nexxim Netlist File Format topic for details.

The format of the VRBG data file is:

#bitlist

Where bitlist is a sequence of 1’s and 0’s without any whitespace.

14. Using the Digital RBG Source in Harmonic Balance

The RBG VCVS is primarily a time-domain element, best simulated with a time-domain tool such as transient analysis. The RBG source can be used with harmonic balance analysis only when an explicit BITLIST is provided. With a random sequence, the circuit cannot reach a steady state required for harmonic balance. For harmonic balance analysis, one test tone must be a submultiple or equal to the bit-frequency of the voltage source. To ensure that the desired HB frequency is used with a RBG VCVS source, qualify the source by adding a TONE=frequency entry at the end of the instance statement. That frequency also appears as an argument in the .HB statement.

The default bit-frequency is:

1/[(BITWIDTH + [TR + TF]/2) ´ (number of bits in BITLIST)]

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