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Old 14th November 2003, 01:54 PM   #31
Electrons are yellow and more is better!
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I know that the source impedance has strong influence of the bandwidth.

Now I'm trying to get FFT which wasn't easy to remember how it was done, but thanks to andy_c I can read what he says about it.
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Old 14th November 2003, 02:23 PM   #32
sonnya is offline sonnya  Denmark
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Hi Per.

Check the Hfe curve for the Spice model before the FFT test. You will be surpriced how bad some of them are.

Just a hint.
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Old 14th November 2003, 02:43 PM   #33
Pedja is offline Pedja  Serbia
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Quote:
Originally posted by peranders
I know that the source impedance has strong influence of the bandwidth.
Source impedance doesn’t have an influence on the bandwidth. It only forms the low pass filter with the input capacitance.

Pedja
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Old 14th November 2003, 02:46 PM   #34
Electrons are yellow and more is better!
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Simulation experts, how do I get narrow peaks in the FFT diagram?
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Old 14th November 2003, 02:48 PM   #35
Electrons are yellow and more is better!
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Quote:
Originally posted by Pedja

Source impedance doesn’t have an influence on the bandwidth. It only forms the low pass filter with the input capacitance.

Pedja
Yeah? Put in 100 ohms (or whatever) in series with the signal source and you will get reduced bandwidth.
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Old 14th November 2003, 02:48 PM   #36
Pedja is offline Pedja  Serbia
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Quote:
Simulation experts, how do I get narrow peaks in the FFT diagram?
Run a higher resolution transient analysis.

Pedja
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Old 14th November 2003, 02:52 PM   #37
Pedja is offline Pedja  Serbia
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Quote:
Originally posted by peranders

Yeah? Put in 100 ohms (or whatever) in series with the signal source and you will get reduced bandwidth.
You will get a low passed circuit. The circuit's bandwidth remains the same.

Pedja
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Old 14th November 2003, 03:15 PM   #38
Electrons are yellow and more is better!
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Quote:
Originally posted by Pedja

Run a higher resolution transient analysis.

Pedja
I have run at 16384 steps. Oki, doki. I have increased to 65536, better.....

What about the dynamics. The FFT shows only around 100 dB S/N. Has this something to do with LTSpice?
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Old 14th November 2003, 03:19 PM   #39
tiroth is offline tiroth  United States
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You may also need a longer sample train and/or shorter step times to get really nice looking FFT. I usually play with all three settings when I am trying to find low-level behavior.
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Old 14th November 2003, 03:22 PM   #40
sonnya is offline sonnya  Denmark
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In tools>control_panel

Remove all compression.

Run transient with the smallest timestep like 2msec window, 20nsec step.

Run it for at least 20 periodes.

1KHz => 20msec, 20nsec
20KHz => 1msec, 10n
10KHz => 2msec, 20n
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