Discussion between DKNguyen and Dave Tweed

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16:13

Q: Designing a pseudo-Doppler transmitter

I am thinking about creating a CW transmitter based on the pseudo-Doppler effect (educational purposes, nothing special in mind). For this purpose, \$n\$ antennas will be placed in a straight line, separated by a distance \$d\$ between them. They will be fed with a coax (all of the \$n\$ coaxes o...

DKNguyen

The Doppler effect doesn't work that way. You need relative motion to change the distance between wavefronts.

Dave Tweed

@DKNguyen: Hence the term pseudo-Doppler. It does work, and can be used for receiving, too. Some VHF direction-finding equipment does it this way. I assume you understand the relationship between frequency modulation and phase modulation.

DKNguyen

@DaveTweed How can that possibly work though? It would need to be generating new frequencies by interferring/summing signals of just a single original frequency. Or is it doing something else?

Dave Tweed

To the OP: Everything you say is true, but you have neglected to ask an actual question. You're going to have to establish what is acceptable performance for your system and that will then tell you the acceptable value of d. Perhaps try a few examples in simulation.

@DKNguyen: The "new frequencies" are generated during the crossover from one antenna to the next, when you're effectively summing two signals that have different phases and varying amplitudes.

DKNguyen

@DaveTweed But summing a sinusoid with a phase shifted version of itself still gives a sinusoid of the same frequency, no?

Okay, so from what I'm reading it is more chasing the wavefront than any frequency shifting. So doppler in the sense of moving rather tan in the sense of frequency shifting

hobbs

16:23

You're assuming time invariance when you say "summing a sinusoid with a phase shifted version of itself gives a sinusoid of the same frequency"

when you change the phase over time, you have PM (even if it's a funny discrete version using antenna switches)

and PM and FM are roughly equivalent

DKNguyen

Is my previous comment the correct interpretation?

or tracking/locating the wavefront, if you will rather than chasing

hobbs

um, kind of, I'm not sure if I follow

Dave Tweed

The individual antennas are amplitude-modulated. This generates sidebands, and the sidebands from adjacent antennas combine in a way that looks exactly like the sidebands created by phase modulation.

hobbs

but to elaborate on what @DaveTweed was saying, there's a famous little handheld direction finder that just needs two vertical antennas a short distance apart and the ability to switch between them using a signal generated by a 555

when the signal is broadside to the two antennas, it reaches them with equal phase and switching between them has theoretically no effect

DKNguyen

What I mean is kind of like having a bunch of observers timing when someone runs by to track what direction they are coming from rather than literally measuring a frequency shift

hobbs

16:28

when it comes in end-on, then it reaches the two antennas with different phase, and switching between them creates a sideband

DKNguyen

I'll have to review the sideband thing sine I've never needed it since school

hobbs

same principle works on transmit, not that I've done it :)

anyhow it's a thing and "pseudo-Doppler" is a well-known name for it

DKNguyen

When you dewcribe the two antenna direction finder I tend to think of that as triangulation

sort of like our ears

hobbs

moving the signal in space causes a phase jump, and a continuous series of phase jumps looks like a frequency shift (plus some mess that we disregard)

DKNguyen

I see

you're talking about the act of modifying the phase requires extra frequencies and those are the sidebands

Dave Tweed

16:32

Direction finding is typically done with a circular array of antennas that are scanned at a fairly high rate. Comparing the phase of the PM signal created by this with the phase of the switching tells you the direction the signal is coming from.

hobbs

@DKNguyen yes, I think you've got it

DKNguyen

So are they kind of time stitching together the signal at different locations to make a signal signal where the phase shifts partway and then weeding out trhe sideband from that?

hobbs

and yes, circular arrays are the nice way to do the direction finding thing (it's like a discrete version of a spinning radar) but the two antenna version is very cute and actually useful for understanding

DKNguyen

I kind of need something like that for a monocopter

anyways, thanks. that was enlightening

hobbs

gladly

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