[amsat-bb] Phase shift keying question

Grant Hodgson grant at ghengineering.co.uk
Mon Jun 22 06:05:43 PDT 2009


I/Q modulators don't work that way, no free lunch and all that.  In newbie 
terms, an I/Q modulator can be thought of as a special mixer that can 
modulate a carrier signal with an arbitrary modulation format - i.e. any type 
of modulation you want - literally.  

An I/Q modulator has a Local Oscillator input (carrier), an I input and a Q 
input, and an RF output.  The modulation signal (FM, CW, AM, SSB, FSK, PSK or 
whatever) is applied to both the I and Q inputs, which in turn modulate the 
LO signal to produce the output signal.

The type of modulation is determined by the phase relationship between the I 
and Q inputs.  Couple of simple examples - lets say the modulation signal is 
a 1kHz sine wave, and we have an LO signal of 144.100MHz.  If both I and Q 
inputs are fed with this signal - in phase - then the output of the I/Q 
modulator will be AM, at 144.100MHz, modulated at 1kHz - you will hear a 1kHz 
tone on an AM receiver.

If the I and Q inputs are fed 90 degrees out of phase with the same 1kHz 
tone, then the output of the modulator will be either LSB or USB depending on 
whether the I signal leads or lags the Q signal.  If the receiver is set to 
144.100 kHz you will hear a 1kHz tone when set to the appropriate sideband.

With slightly more complex phase relationships between the I and Q signals it 
is possible to generate any type of modulation you want, and from a 
receiver's point of view you will not know how the modulation was generated.

But you can't feed different modulation channels into the I and Q inputs - 
the output would be a complex mixture of the two which would be un-

You could try Googling 'IQ modulation' for more information, although some of 
the explanations are aimed at professional engineers with good maths skills.


Grant  G8UBN

Could anyone please explain, in newbie terms, if it is possible to split a 
telemetry data stream from a satellite OBC into two data streams, sent 
simultaneously on two nbfm channels, each PSK with the phase difference 

I have in mind a conversation which I don't fully understand, about these 
the I and Q components, and this offering a doubling of the data rate 
with using one nbfm channel.

Grateful for any pointers on or off the -bb

many thanks

andy G0SFJ

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