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Infiniisim and Filters

Question asked by conchiupct on May 9, 2011
Latest reply on May 11, 2011 by algoss

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Hello everybody!!

I'm using Infiniisim to compensate the effects of some cables and a passband filter of my signal. The problem is that if I compensate also the rejected band (from 0 up to 3 GHz), I obtain a increment of more than 50 dB in such rejected band. Sould I compensate only my "band of interest", for example from 3 up to 4 GHz and not from 0 to 4 GHz?

Thank you very much

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algoss May 10, 2011 6:14 AM
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Are you using S-Parameter files or directly manipulating the .tf2 files?

I have created passband filters and comb filters by directly manipulating the .tf2 files with no problems. Please post pictures or other info that might help me figure out what's going wrong...

AG
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conchiupct May 11, 2011 3:53 AM
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Dear AG,

Thank you for your answer. I'm measuring the signal received by an atenna and I'm using a physical passband filter before the input of the oscilloscope. Therefore, I'd like to compensate the effects (module and phase) of such filter. In the next figure, one can see the measured transfer function of the filter in the ENA:

I save the s2p file of such filter and with this file I use infiniisim to remove the insertion losses of such filter. In the next image the transfer function of the transformation applied by Infiniisim is shown:

Therefore, the received signal (in the band of interest), wil bee masked by "something" at low frequencies. Therefore, I'm not sure how to compensate the effect of my passband filter.

I would thank any idea.

Thank you in advance.
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- filter.bmp900.1 KB
- InfiniisimFilter.bmp2.3 MB
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algoss May 11, 2011 5:28 AM
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OK. InfiniiSim is doing exactly what you told it to do. It is trying to reverse the effects of your filter. For instance, your filter has about 60dB of loss at about 1.7GHz. InfiniiSim is try to add about 60 dB of gain at that same frequency. Your signal has essentially gone away, at that frequency.

The scope, however, has some noise at that frequency, so InfiniiSim is multiplying that noise by 60dB, and drowning out your signal. That's what is happening everywhere except in the passband.

Oscilloscopes, in theory, only have about 50dB of dynamic range (6.02*nbits + 1.76). In reality, the spurious-free dynamic range is only about 40dB. Given that, you should never try to use InfiniiSim to remove more than about 20dB of loss, and even there, you're on the edge. At any given frequency, InfiniiSim sees your signal, plus the inherent noise added by the scope. The transfer function generally adds gain, multiplying both the signal, plus the noise

InfiniiSim is not designed to do what you are trying to do. It is intended to remove the effects of low-loss devices, such as cables and PC board traces. It does this at the expense of some additional noise and other effects. It can't recover signals that have been removed by high-loss devices, such as filters.

AG
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Infiniisim and Filters

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