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Design >> RF Design >> effect of DC-offset on Performance
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Message started by aaron_do on Oct 28^th, 2008, 8:13pm

Title: effect of DC-offset on Performance
Post by aaron_do on Oct 28^th, 2008, 8:13pm

Hi all,

I was running some simulations on an RX without any DC-offset cancellation and i found some inconsistencies. I added some mismatch to the limiting amplifier (about 5 %) and checked the output...

1) With the PXF analysis the overall RX gain dropped below 0 dB from about 90 dB.

2) With the transient analysis, the output voltage (peak to peak) remained the same, however there was a reduction in the duty cycle.

The second point seems more correct since if there is mismatch in the limiting amplifier, it just means a larger difference between the + and - is needed to switch the limiting amplifier output. Obviously because the PXF analysis is a small signal analysis, it was not able to switch the limiting amplifier at all, and the output was saturated.

My question is this. How concerned are we with DC offset in this kind of system, where before limiting, the signal has already been filtered (using analog domain filtering)? Suppose we have an FSK modulated signal, is the duty cycle important?

thanks for any input,
Aaron

EDIT:

OK seems like the signal really is desensitized. Back to the drawing board...

Title: Re: effect of DC-offset on Performance
Post by ACWWong on Nov 2^nd, 2008, 7:16am

as you have found, DC offset in a limitting strip is very important to control... in the simplest cases it does indeed impact duty cycle as you have said. Duty cycle can be very important, depending on your demodulator for your FSK system... one which does zero crossing/edge detection may well be very sensitive to it especially if it is using both positive and negative edges or on relative I and Q channels in its demodulation.
DC offset must be treated as a co-channel interference, as such you must make efforts for sufficient Signal/DC ratio for demodulation, otherwise DC offset will just saturate your receiver and it'll be deaf.

Cheers
aw