So in short you have single tone oversampled into FPGA and you want average of I^2+Q^2.Hello support team,
My main requirement is to use I/Q sampling on the RF input (sine wave) using the direct digitization method and convert these cartesian data into polar form to generate the amplitude using the formula of sqrt(I^2+Q^2). I am using FPGA for this implementation.
RF input's highest frequency component is 60 MHz (sine wave, no modulation) . I am using oversampling to generate I, Q samples using DDC algorithm along with the decimated filter to generate the I, Q samples. right now my I, Q samples data rate after decimation is 61.44 MSPS.
If I provide this much of high sample rate results to generate the amplitude without averaging, I am getting the different samples at the output of which If i am doing averaging, I am able to get the stable amplitude output.
so, what I am understanding from the study and other application notes that there is a requirement of averaging such as moving averages to get stable result. so when we do the averaging, the output sample rate after averaging will be reduced. So below are my queries.
1. As the CIC filter is equivalent to the moving average, as CIC is the RRS filter while moving average is the non recurrsive averaging filter. so, which one is more suitable?
2. As my input RF signal is pure sine wave, what should be the suitable averaging factor should I use to get the accurate and stable amplitude measurement? My input sine wave is clean and with less harmonics.
Hello,No idea how you arrive at 61.44 MHz decimated rate. CIC decimator is required to run with integer frequency ratio, preferably power of two.
Regarding possible aliasing, you didn't specify the input signal bandwidth, only maximal frequency, that's no the same. Signal bandwidth is also important to decide about filtering. If you say there's no modulation, bandwidth is very small.
yes, your understanding is correct. sine tone oversampled into FPGA and I have shift it down to DDC down to DC. so, my question is that only If i use CIC decimation filter( which is RRS average filter equivalent to moving average) to further decimate at I, Q only. (let's say up to 3.07 MSPS from 61.44 MSPS using decimation factor of 20). then also I will get the amplitude samples from the sqrt(I^2+Q^2) from these 3 MSPS samples. In order to get the estimation of signal power in peak voltage, the averaging within some time duration is required at the amplitude output as well, for which I am asking about suitable method.So in short you have single tone oversampled into FPGA and you want average of I^2+Q^2.
just use mixer(based on DDC) to push the tone to dc. No need for decimation. Once I/Q are dc levels you can get what you want. The only issue left if there is noise. You can get rid of noise using simple running average. The level of noise will decide how many taps you need to get clean dc levels and make sure your averaging gain is unity.
There is no need to decimate. Just use running average filter of suitable number of taps.Hello,
I have used decimation filters only after DDC to get 61.44 MSPS decimation rate. and yes my input signal is pure sine wave without modulation. so bandwidth is small only. just one thing is that my frequency variation is within +/1 MHz (i.e 59-61 MHz).
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yes, your understanding is correct. sine tone oversampled into FPGA and I have shift it down to DDC down to DC. so, my question is that only If i use CIC decimation filter( which is RRS average filter equivalent to moving average) to further decimate at I, Q only. (let's say up to 3.07 MSPS from 61.44 MSPS using decimation factor of 20). then also I will get the amplitude samples from the sqrt(I^2+Q^2) from these 3 MSPS samples. In order to get the estimation of signal power in peak voltage, the averaging within some time duration is required at the amplitude output as well, for which I am asking about suitable method.
I assume averaging power itself will do instead of I/Q separately if power is your target.Hello,
Thank you for your response. Ok, I got your point. If I use only basic averaging at the I and Q samples, then also, after the sqrt root operation, I am assuming that at the amplitude output also, Averaging is required.right? otherwise how it is possible to get stable output.
Just apply DDS on sampled I/Q. No decimation needed.Hello,
Ok, I got your point. That , I can do possible way of decimation at the I, Q samples with sample rate and then all the I and Q samples ( let's say with the 6 MSPS ), then you are suggesting that I do the sum of squares for all the samples to get power and then do the averaging to get stable output. Is my understanding correct?
I´m not sure I understand this correctly.I assume averaging power itself will do instead of I/Q separately if power is your target.
It is not about square root but power as sum of squares. Anyway if target is rms you still need to get squares then average them then get root of sum of squares.I´m not sure I understand this correctly.
But just want you to keep in mind that
* first averaging, then doing the square root
gives a different result than
* first doing the square root, then doing the averaging
Klaus
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