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 Post subject: Create a frequency shift with a phase shifter
Posted: Thu May 06, 2010 5:35 am 

Joined: Thu Jan 18, 2007 8:09 am
Posts: 12
Location: Belgium
Hello guys.

So a little background to explain my problem. I work for a company that provides Internet on board high speed train. For that we use a satellite link. As you can imagine we face a Doppler shift when the train is moving.

I would like to be able to reproduce the Doppler shift in lab to be able to test satellite technology without having to go on a train to face the Doppler shift. At around 12.5 GHz (Ku Band) the Doppler shift is about 3.5 KHz (with a train running at 83.3 m/s). As i can only integrate a device after the LNB, i have to works in L Band. However the Doppler shift as to be the same.

I want to reproduce the worst case frequency shift, so the shift has to remain constant (which simplify my problem).

Then only way i see to create that is by creating a constant phase derivation with a device like a Digital Phase Shifter on the IF signal (L Band after transposition by the LNB) since a phase derivation results in a frequency shift.

Where i got stuck is when i try to put numbers on the derivation i have to apply on my signal. Using the formula of the Instantaneous Frequency and the Instantaneous Phase and after integration i came to:

Phi(t)= 2*PI*Fo*t + 2*PI*Fshift*t

Phi(t) being the instantaneous phase
Fo being the original frequency
Fshift being the Doppler shift

From that i conclude that:

Phi(t) = Phio(t) + PhiShift(t)

Phio(t) being the instantaneous phase of Fo
PhiShift(t) beinf the instantaneous phase of Fshift

Then i conclude that:

PhiShift(t) = 2*PI*Fshift*t

It appaers then that to create a frequency shift, i need to apply a constant phase derivation over which is TOTALLY independant from the frequency i am working at. For example, whatever i am working at 12.5 Ghz or at 1.2 Gz (after conversion by the LNB with a LO@11.3Ghz) which sound to me a bit... wierd.

I must miss something somewhere but i can't figure out what and where. Can someone help me with this understanding?

 Post subject: Re: Create a frequency shift with a phase shifter
Posted: Fri May 07, 2010 10:40 am 

Joined: Thu Jan 18, 2007 8:09 am
Posts: 12
Location: Belgium

I figured out myself. That was really stupid in fact...

 Post subject: Re: Create a frequency shift with a phase shifter
Posted: Thu May 27, 2010 3:24 pm 

Joined: Wed Apr 08, 2009 11:07 am
Posts: 37
yes, in fact you can simulate a doppler shift with a stepped ramp of phase. In other words between your antenna and the device under test, if you add a phase shifter and digitally program it to be 0, 22.5, 45...337.5, 0, 22.5...degrees, the frequency will appear to shift. You can get the frequency to shift upwards or downwards by the direction you program the phase sequency. This is useful in things like jammer systems on a military airplane, where the pilot wants the missile coming at him to think he is travelling at a different velocity than he really is.

Unfortunately, since most forms of mobile communications use phase modulation in their detection, you are not going to be able to use this way to simulate the doppler frequency shift. Your communications device will try to track out the shifts, and make all sorts of bit errors while trying to do so.

I would take the signal (2.45 GHz for example) use a stable frequency synthesizer and upconvert it in a mixer to 8.45 GHz with a 6.0 Ghz local oscillator signal). I would bandpass filter it, and then downconvert it back to the band of interst with either a 6.0000035, or a 5.9999965 GHz LO. If you want to change the apparent doppler shift, you just redial the frequency.

Depending on what the communications device is you are testing, you might have to lock the two LO synthesizers together with a common clock frequency.

Maguffin Microwave Consulting

Posted  11/12/2012
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