depends on actual requirement.Hello All,
My input frequency is pure sine wave RF signal ( No modulation) . My requirement is to use direct sampling reception of the input signal, generate I,Q using DDC algorithm. and my main aim is to calculate amplitude and phase from the received I and Q samples.
Now, My input frequency is slightly varying in between 1 MHz. let's say, right now my input frequency is 40 MHz, then its variation is from 39-41 MHz. So, I am planning to set the NCO frequency as 39 MHz. Then my DDC output ( complex output spectrum) will be from DC-2 MHz based on the variation of the input frequency. also, DDC output decimation is done up to 10 MSPS. so DC-2 MHz will be within the range of the Decimation filter.
and then from the generated I,Q , We will measure the amplitude and phase. I am thinking of the above, so that I don't have to vary NCO frequency every time there is a slight change in input frequency. As my main signal is sine wave and my main aim is to measure amplitude from the received I,Q samples.
Is my above approach correct to have the DDC output variable within the DC-2 MHz range and not constant DC ?
It would be very helpful if you could suggest and provide support on the above case as soon as possible.
so is your requirement to lock to tone(carrier tracking)?Thank you for your quick response. to further elaborate,
Yes, My actual target is to get amplitude/phase from the received I,Q from the input pure sine wave RF signal and my main requirement is to do this in the shortest time possible.
So, if I try to vary NCO automatically as per input frequency variation, I have to update the NCO frequency using SPI registers every time based on the difference from DC received in the DDC output. this process will take some micro sec time. As My input frequency variation would be unknown to me. so, to lock the input tone, I need to follow the above automated process.
but as my requirement is to get the exact amplitude and phase from the received I,Q samples only. so, I thought to have variable DDC output.
So, please guide , is this approch correct or you can suggest a better way to fast the process of automation NCO frequency update based on the FFT difference. my NCO freq. is updated using SPI and will take almost 3 usec for 1 KHz frequency variation.
Hi,so is your requirement to lock to tone(carrier tracking)?
If so you need some more work. fft can do but has latency. search for Costas loop.
Yes, it is fast for my further processing. I need to reduce the sampling rate to 10 MSPS. that's why i decided to use direct sampling using DDC algorithm.so you don't need tracking. Fair enough.
Just wondered why then down convert. Can't you just deal with 40Mhz direct or is it too Fast?
With such simple requirement you may also consider under sampling then using alias of tone without the need for DDC or decimation.Yes, it is fast for my further processing. I need to reduce the sampling rate to 10 MSPS. that's why i decided to use direct sampling using DDC algorithm.
Hello,With such simple requirement you may also consider under sampling then using alias of tone without the need for DDC or decimation.
yes, to calculate alias frequency (fa):Hello,
Oh I got your point. yes, thank you for your guidance. I will start thinking to use under sampling and aliasing of tone. But then , without the use of DDC , direct digital IQ detection process I need to identify , right?
Ok, so what I understand is that using sample rate in such a way , my input single tone is aliased back to lower frequency. and then I need to convert the digital samples to I and Q to further calculate amplitude? then which method i need to follow for I,Q detection?yes, to calculate alias frequency (fa):
fa = 2FN - F (double Nyquist minus signal tone)
for example if you sample at 30Msps you get tone at:
2*15 - 40 = -10 which if real will be +/-10Mhz
if you just want amplitude then you don't need IQ. You can just compute directly with some avaraging.Ok, so what I understand is that using sample rate in such a way , my input single tone is aliased back to lower frequency. and then I need to convert the digital samples to I and Q to further calculate amplitude? then which method i need to follow for I,Q detection?
Phase of a single tone changes around full cycle. Its starting point varies with ADC capture start. when comparing two tones at same frequency then one can compare the two all along or at some agreed point of circle. when tone is complex its phase is atan2(Q,I) or "angle(x)" in matlab.Hello,
Yes I have to calculate phase as well. For the 2 diff. input signal , relative phase I need to measure.
Yes, As IQ obtaining from cos/sin mixing that what I had thought before using NCO only . based on that only, I have raise the query.
yes, my both input signal is having same frequency. just the amplitude is different.Phase of a single tone changes around full cycle. Its starting point varies with ADC capture start. when comparing two tones at same frequency then one can compare the two all along or at some agreed point of circle. when tone is complex its phase is atan2(Q,I) or "angle(x)" in matlab.
Comparing phase of two tones at different frequencies is not helpful.
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?