# MAX2769 I & Q For Signal Acquisition & Tracking

Having pulled data from a MAX2769B in device state 2 (2-bit I only), I have found satellite signals by multiplying the I-only data by the respective PRN and taking the FFT of the data. Using this approach, I set the imaginary part to zero, and fill in the I*PRN data for the real part, thus real only data. If I configure the device to produce the quadrature data and attempt the same procedure, it's unclear what I should do. My first pass attempt at the approach did not produce a correlation peak at any frequency. If I scrap the Q data and fill it in with zeros, I get the same answer as before, and all is well. Should the quadrature data be used in an acquisition? Does it help? Presumably, it should be used in tracking, but then I would ask a follow-up question as to whether the quadrature data should be multiplied by the PRN sequence.

If the frequency offset is greater than about $$1/(2T)$$ where $$T$$ is the total correlation time, then the estimated frequency offset would typically be stepped by that amount ($$1/(2T)$$) between correlations combined with a threshold detector for a correlation peak to indicate the frequency is close enough for fine acquisition which can then proceed using the I Q correlation peak values as explained further here, or alternatively multiple correlators can be run in parallel when resources allow and a fast acquisition is required (Essentially what I am doing here using FFT's for fast processing to simultaneously acquire in "one-shot" the carrier offset and code offset). Reducing $$T$$ is also an option, but that will also reduce overall signal to noise ratio.
Another technique for acquisition is to down-convert to baseband using a slow frequency ramp (which is similar to step in frequency and correlate with very small and very frequent frequency steps). Below shows an example result with an actual GPS signal using a frequency ramp that sweeps from -5KHz to +5KHz over a time duration of 50 ms. (Since the GPS C/A code repeats every 1 ms, the frequency change over any correlation interval is 10KHz/50 = 200 Hz which is less than the maximum $$1/(2T)=500$$ Hz interval that I recommend above. We can see here at the 200 Hz / correlation interval ramp rate the correlation peak is sampled with approximately 7 visible peaks.
Specific to the OP's question, the above correlation results shown are the absolute magnitude using I and Q ($$\sqrt{I^2 + Q^2}$$). Below shows the complex correlation result for the same last plot above plotted on the complex I and Q plane. The maximum correlation in this result ended up around 20° but could have arbitrarily been anywhere which is why we need to use both I and Q at this point in the processing.