Now, re-reading my response and thinking a little longer, I understand your question a little better, maybe. I see where you are coming from if RF and LO are the same frequency, and it's not totally clear to me yet what happens as the phase changes. My quick back of the napkin (yes, I actually used one!) suggested outputs from a single DBM at zero phase difference of 0 * Flo, and the input noise plus the LO noise would seem to me to be additive, not multiplied. Basically a phase detector. But then I recalled that for PN measurements, the LO and test signal have to be maintained at 90 degrees shift for there to be a zero volts DC component to use for steering but the noise sidebands are then what is measured.
I guess I really need to go play in the lab a bit. Tom Holmes, N8ZM -----Original Message----- From: Tom Holmes <[email protected]> Sent: Thursday, August 26, 2021 2:27 PM To: 'Discussion of precise time and frequency measurement' <[email protected]> Subject: [time-nuts] Re: uncertainty/SNR of IQ measurements HI Jim... >From my admittedly limited understanding of IQ demodulators, the first thing >done is to split the signal power (signal, noise, and all) evenly between two paths, which then ideally feed identical double balanced mixers (I'm thinking of a hardware implementation, obviously) whose only difference is the quadrature phase of the LO. So both paths are seeing the same SNR at that point. So my first guess would be that the relative phase of the LO to the input signal would only affect the phase of the output from each path, but the noise content ( or modulation if there is any) would not be any different between the two paths. I'm not aware that a single DBM used as a downconverting mixer shows any preference to the phase angle of the input to the LO. Tom Holmes, N8ZM -----Original Message----- From: Lux, Jim <[email protected]> Sent: Thursday, August 26, 2021 1:37 PM To: Discussion of precise time and frequency measurement <[email protected]> Subject: [time-nuts] uncertainty/SNR of IQ measurements This is sort of tangential to measuring time, really more about measuring phase. I'm looking for a simplified treatment of the uncertainty of I/Q measurements. Say you've got some input signal with a given SNR and you run it into a I/Q demodulator - you get a series of I and Q measurements (which might, later, be turned into mag and phase). If the phase of the input happens to be 45 degrees relative to the LO (and at the same frequency), then you get equal I and Q values, with (presumably) equal SNRs. But if the phase is 0 degrees, is the SNR of the I term the same as the input (or perhaps, even, better), but what's the SNR of the Q term (or alternately, the sd or variance) - Does the noise power in the input divide evenly between the branches? Is the contribution of the noise from the LO equally divided? So the I is "input + noise/2" and Q is "zero + noise/2" If one looks at it as an ideal multiplier, you're multiplying some "cos (omega t) + input noise" times "cos (omega t) + LO noise" - so the noise in the output is input noise * LO + LO noise *input and a noise * noise term. I'm looking for a sort of not super quantitative and analytical treatment that I can point folks to. _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
