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How do I delay and phase match a cable with a VVM - RF Cafe Forums
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CyberOli
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Post subject: Create a frequency shift with a phase shifter
Posted: Thu May 06, 2010 5:35 am
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Joined: Thu Jan 18, 2007
8:09 am Posts: 12 Location: Belgium
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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?
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CyberOli |
Post subject: Re: Create a frequency shift with a phase shifter
Posted: Fri May 07, 2010 10:40 am
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Joined: Thu Jan 18, 2007
8:09 am Posts: 12 Location: Belgium
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Ok.
I figured out myself. That was really
stupid in fact...
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biff44 |
Post subject: Re: Create a frequency shift with a phase shifter
Posted: Thu May 27, 2010 3:24 pm
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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.
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