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Re: A Noisy Question
Hi Jim,
I have a question about modeling noise in a real system.I typically use an LFSR and then average the uniform distribution as you mention above. If I'm using this noise to "simulate" an analog noise signal, and I want to "sample" that signal with a jittery clock signal, or I want to alias the signal to baseband, I will filter the noise to say 4x the bandwidth I really want, generate my clock sample times, and then interpolate (using MATLAB here, not LTSpice). For example, see the spectra in this doc it contains simulated noise power ratio, clock jitter, etc. I use PRBS generated noise so that I can reproduce the simulations in hardware in FPGA-based DSP. Cheers, Dave |
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Re: FFT Resolution
? RC and John Thanks? for the response,but I am blinded by science now as this is my first attempt to model FT behaviour I would like to be clear on at least one matter? " 'the value of the window" Is this the same as the number of cycles AKA Span ? Is it possible to display the harmonics of say a 10MHz square wave to 100Hz resolution ? My 30 year-old HP spectrum analyzer can Can anyone illustrate? with the pulse and Tran statements using a square wave at any freqency ---In LTspice@..., <ron_liff@...> wrote: In any event the resultant "spectral accuracy", even if optimized for some set of conditions, resolution, and bandwidth of spectrum, will be directly dependent on the accuracy of represented distortion (read non-linear) effects. ? - In general large scale spectra will tend to more accurate representation, whereas the smaller the spectral component the more the sensitivity, and deviation, from the actual results in a real world circuit. This effect is exploited in so-called "Harmonic balance" type solvers to great success. ? - Cordially - RC On Tuesday, December 10, 2013 2:58 PM, John Woodgate <jmw@...> wrote:
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In message <l87u7c+ebvu@...>, dated Tue, 10 Dec 2013,
"skleiser@..." <skleiser@...> writes: > Given (in this case) a square wave of fixed frequency, fast but harmonic bandwidth doesn't depend on those things. Embedded in the Help on the B source is: "In LTspice, the impulse response is found from the FFT of a discrete set points in frequency domain response. This process is prone to the usual artifacts of FFT's such as spectral leakage and picket fencing that is common to discrete FFT's. LTspice uses a proprietary algorithm that exploits that it has an exact analytical expression for the frequency domain response and chooses points and windows to cause such artifacts to diffract precisely to zero. However, LTspice must guess an appropriate frequency range and resolution. It is recommended that the LTspice first be allowed to make a guess at this. The length of the window and number of FFT data points used will be reported in the .log file. You can then adjust the algorithm's choices by explicitly setting nfft and window length. The reciprocal of the value of the window is the frequency resolution. The value of nfft times this resolution is the highest frequency considered." The significant words are: "The reciprocal of the value of the window is the frequency resolution." Frequency resolution is the same as the observed 'harmonic bandwidth'. > The harmonic energy is averaged over the bandwidth, so widening the bandwidth tends to reduce the observed amplitude. -- OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk Nondum ex silvis sumus John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK |
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Re: A Noisy Question
monettsys
--- In LTspice@..., Jim Wagner <wagnejam99@...> wrote:
Greetings folks - I have a question about modeling noise in a real system. Lets say I start with a uniform distributed sequence of random values with a span of +1 to -1. I know that I can average over N of these values and get a Gaussian sequence with a mean of zero and a standard deviation of 0.5. This much is pretty easy.I don't think that will give a very good approximation to Gaussian noise. You might want to try the Box-Mueller method. Now, these values represent discrete time samples. How do I turn them into real signal that has a defined bandwidth? Yes, I can put the sequence through a low-pass filter, but I have to define a time interval between samples. Do I need to set this time at 1/10X the filter bandwidth? 1/100X? And, does it matter whether I take the values as though the output of a first-order sample/hold, or do linear interpolation between them? Or something else? And, what sort of filtering is normally assumed? Single pole? Two or three poles?For the Box-Mueller, just take the output signal. You don't need S/H, interpolation, or any other tricks. You may need to adjust tripdV and tripdt. There are several Box-Mueller files available. Try " and " In a hardware noise generator, you normally want the noise spectrum to remain flat well past the frequencies of interest. But if you need to define the bandwidth, how about doing a FFT on the resulting noise signal? That should give answers to your questions. ThanksMike |
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Re: Cmos 4026
John Woodgate
In message <CAFL0OtQZcJSRTgEKrDT3ObSrjGatJzBfoLanqGr7Y2uN6vZOOQ@...>, dated Sun, 15 Dec 2013, Business Kid <business.kid@...> writes:
And I have stopped thinking in 4xxx logic, because they are not recommended for new designs, to put it mildly.No doubt, but the fact that they are very slow by modern standards means that they produce much less EMI. If you don't need the speed, don't buy it. -- OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk Nondum ex silvis sumus John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK |
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Re: Cmos 4026
Business Kid
I thought I was the only one with any 4000 chips left. I never saw or met the 4026. There are a few on ebay if you want to prototype :-) I am guessing that you are simulating an idea which you are going to program. Use a separate 74xx counter and 74xx bcd decoder chip if you must, but I also fail to see the point. And I have stopped thinking in 4xxx logic, because they are not recommended for new designs, to put it mildly.On 14 December 2013 10:55, <xcheta1@...> wrote:
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A Noisy Question
Greetings folks -
I have a question about modeling noise in a real system. Lets say I start with a uniform distributed sequence of random values with a span of +1 to -1. I know that I can average over N of these values and get a Gaussian sequence with a mean of zero and a standard deviation of 0.5. This much is pretty easy. Now, these values represent discrete time samples. How do I turn them into real signal that has a defined bandwidth? Yes, I can put the sequence through a low-pass filter, but I have to define a time interval between samples. Do I need to set this time at 1/10X the filter bandwidth? 1/100X? And, does it matter whether I take the values as though the output of a first-order sample/hold, or do linear interpolation between them? Or something else? And, what sort of filtering is normally assumed? Single pole? Two or three poles? Yes, this is for an LTspice project. And, the reason I am being a bit coy about specific values is that I would like to apply this in a variety of communication systems with varying operating frequencies and bandwidths. In some cases, the system bandwidth will be very broad (maybe 100 MHz) encompassing HF frequencies with lots of ambient noise as well as VHF that is much quieter. Thanks Jim Wagner Oregon Research Electronics |
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Convert ADS and/or AWR (MWO) RF FET Models to SPICE
Does anybody know how to convert RF FET models that were made for ADS or AWR (MWO) to SPICE models.? Seems some RF device manufacturers (NXP, Freescale, ...) do not make SPICE models for their parts.? Not good if you can't afford one of these expensive simulators.
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Re: i need help
Then ask in the Hspice list?
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Show quoted text
Jim Wagner Oregon Research Electronics On Dec 14, 2013, at 5:01 PM, Á¢ Çf wrote:
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i need help
?I really need ?the hspice netlist of Dflipflop with reset please help me><~~ i already have dflipflop without reset .SUBCKT DFF gnd vcc d clk q q_bar X1 gnd vcc 1 2 3 nand2 X2 gnd vcc 3 clk 2 nand2 X3 gnd vcc 2 clk 1 5 nand3 X4 gnd vcc 5 d 1 nand2 X5 gnd vcc 2 q_bar q nand2 X6 gnd vcc q 5 q_bar nand2 .ends .SUBCKT nand2 gnd vcc a b vout ?? Mx1 vout b vcc vcc ?p w=1.5u l=0.18u Mx2 vout a vcc vcc ?p w=1.5u l=0.18u Mx3 vout a 2 gnd ?n w=1u l=0.18u Mx4 2 b gnd gnd ?n w=1u l=0.18u .ENDS |
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hspice netlst
I really want the hspice netlist about d flip flop with reset!!
someone know it?? please told me~~ ex:d flip flop without reset .SUBCKT DFF gnd vcc d clk q q_bar X1 gnd vcc 1 2 3 nand2 X2 gnd vcc 3 clk 2 nand2 X3 gnd vcc 2 clk 1 5 nand3 X4 gnd vcc 5 d 1 nand2 X5 gnd vcc 2 q_bar q nand2 X6 gnd vcc q 5 q_bar nand2 .ends .SUBCKT nand2 gnd vcc a b vout ?? Mx1 vout b vcc vcc ?p w=1.5u l=0.18u Mx2 vout a vcc vcc ?p w=1.5u l=0.18u Mx3 vout a 2 gnd ?n w=1u l=0.18u Mx4 2 b gnd gnd ?n w=1u l=0.18u .ENDS |
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Re: LTSpice 4026
Xcheta1 wrote, "...? This is one of those situations that makes me ask, Why do you want to simulate that?
I don't mean that there isn't a good reason. ?I just don't see one.
The CD4026 is a digital counter, with digital inputs and outputs. ?SPICE is used when you need to know whether some voltage is 8.47 volts or 9.75 volts; or if you need to know the risetime, or to look for overshoot ... things of an analog nature. ?Those things MIGHT be concerns, but often they are not. ?When the wires or PCB traces are short, often you can just hook up the ICs, and it works, without having to know the voltages with millivolt precision.
Just wondering.
Regards,
Andy |
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LTSpice 4026
HI there,
I Would like to build a speeometer. My plan was that i use the CMOS 4026 because i need the Carry Out for my 7-Single-Digit-Nummeric Display but in LTSpice i can?t find the 4026 to Simulate my circiut. Please help my i search a alternate component for the CMOS 4026 with the same application and where i can use at LTSpice. Thank you for Help Xcheta1 |
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Re: LTspice models
John Woodgate
In message <1386959812.25712.YahooMailNeo@...>, dated Fri, 13 Dec 2013, Milan Djordjevic <inductpower1984@...> writes:
I became a member of your group, because to him that the user of the simulator and need some models.Your message cannot be understood. Please get a friend to help you to write in better English. You may be able to get an on-line translation of your message from your language to English. -- OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk Nondum ex silvis sumus John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK |
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