Honored ContributorMy FIR is now working so far - thanks to Altera Guru "MSchmitt" - but I still have two problems left. Please have a look at the attached pictures - there is an unwanted offset attenuation of 6dB in the filter characteristic. What can I do against that?
Honored ContributorI don't see an indication, that the said 6 dB loss is caused by the FIR filter. But there seems to be a considerable pass band ripple respectively peaking that can't be explained by a FIR filter characteristic. So I guess there's rather a problem with your measurement setup, that hasn't been described in this post (may be otherwhere?).
Did you perform a measurement without the FIR filter? Do we see simply the 6dB loss from 50 ohm impedance matching?Honored ContributorI've added a measurement without a FIR filter in between ADC and DAC - so it seems that 3dB are lost (maybe) in the coneverters - but the remaining 3dB must have to do something with the FIR filter.
Loosing exactly half of the signal could indicate a problem with the sign!? - or the converters do not work correctly. What should I try next?Honored ContributorHonored ContributorWhat I want is no additional attenuation besides the filter characteristic. I do not have modelsim, but I can provide some data I collected when trying to find out the exact behavior of the FIR (these are pics from an agilent oscilloscope and the signal tap II logic analyzer).
Especially on the pics from the logic analyzer you can see that the output signal of the filter is missing something, occupying only half of the scaling space.Honored ContributorI agree, that some waveforms look strange. Unfortunately I don't see, how they are achieved.
I would expect this: - all filter input/putput/external signals have a signed data type - signed representation is also selected for simulation or SignalTap waveforms - the filter input signal is (almost) DC free, in other words has 0 bias in signed representationHonored ContributorEverything in this measurements is "unsigned", when there is no other remark. I repeated the whole measurements with everything "signed" and had comparable results (although the permutations were different). I found no combination that worked "right".
The filter input signal comes from a Tektronix AFG 3101, and should be completely DC-free. There is nothing in between the devices and instruments exept BNC cables and a "mini circuits" DC-48MHz alias filter at the DAC output. The miniVNA (vector network analyzer) has 10dB attenuation at each in- and output to protect it - but these are calibrated out.Honored ContributorSince it is 6dB loss, try multiplying your filter output by 2 i.e. shift one bit to the MSB adding 0 to new lsb. It could be your filter gain implementation is missing one bit.
Honored ContributorI meant "DC free" related to the digital data, signal should be around signed zero with no input. I can't see from your waveforms, if this is the case.
Honored Contributor@kaz:
Multiplying the FIR filter output with 2 lead to a output signal stuck at GND. I've tried to use the multiply operation before and after the FIR filter. @FvM: The output signal seems to have no DC component - an average measurement with an Agillent DSO7012A delivered -100µV, while measuruing an aplitude of 206mV. This is strange, because in the Signal Tap II Logic Analyzer, the output signal occupies only one half of the diagrams scaling. From tomorrow on, I will be on vacation until next week - so don't wonder if my replies do not come in time.Honored ContributorWithout knowing the design and the involved hardware, the question can't be answered I fear. Most Altera ADDA Dev Kits have AC coupled analog I/O, so the output bias doesn't say anything about the digital signal.
