MPSKIQ and Direct Conversion TRX using an IQ-Modulator
by Michael Keller
The MPSKIQ program can be used for a simple direct conversion TRX design. The necessary
Hilbert Transformation is already installed on the DSP and can be activated. When activated, both
the IN & OUT stereo channels act as an IN & OUT for the complex de-modulation signals. The
program delivers the necessary 90 degrees out of phase AF signals for an IQ-UP converter. When
receiving, the AF-output of an IQ-DOWN converter can be fed directly into the stereo AF-input of
the EZKIT. MPSKIQ is able to decode the complex IQ input and can easily be changed between
upper and lower sidebands. In addition to demodulating the MPSK, MPSKIQ can also demodulate
USB and LSB telephony.
The following graphic shows the principal connections to the EZKIT:
Some insights into IQ-mixers
To understand the mystic IQ-mixer, lets think about the well known real signal mixer which
everybody will probably know, and forms part of most or the simple direct conversion RX designs.
Remember, when mixing (multiplying) two sinus signals e.g. an RF signal from the antenna
f1=1001kHz with a local oscillator LO of f2=1000kHz, the result is the difference and the sum of
these frequencies. This means that at the output of the mixer, we will find two frequencies at
f3=1001kHz-1000kHz=1kHz and f4=1001kHz+1000kHz=2001kHz. The mixing product of
f4=2001kHz can easily be removed by a simple low pass filter. After filtering the result is an AF of
1kHz. If f1 is a keyed CW signal, the AF f3 at the output will also be a keyed AF at 1kHz.
Up to now all is OK, but what happens when there is another RF signal from the antenna at
999kHz?
Lets think about it. We again get the difference and sum i.e. f3=999kHz-1000kHz= minus 1kHz and
f4=999kHz+1000 kHz=1999kHz.
Yeah, the 1999kHz is no problem, but what's with the stupid minus 1kHz?
Negative frequencies, what's this?
Nothing special, you will also hear a 1kHz AF at the output of the simple real mixer, which cannot
produce the minus 1 kHz. It also delivers an AF at 1kHz, which overlays the AF resulting from the
RF at 1001kHz
Signals lying up or down the same distance from the LO will result in the same AF frequency at the
output.
Let's consider: we can not decide, if a received AF signal results from an RF lying below or
above the mixing frequency. That's exactly the reason why the simple direct conversion receiver
concept cannot decide between USB and LSB. It can demodulate DSB but when receiving USB or
LSB, the resulting AF will be overlapped by noise resulting from RF lying on the opposite
sideband.
How to overcome this nasty disadvantage. We need a mixer, which can decide whether we get
positive or negative frequencies as a result of the mixing. The key to solving this problem is to
understand the mystic negative frequencies.
Negative frequencies
A primitive direct conversion TRX consists of a local oscillator pair with two identical frequencies
with a phase difference of 90°. That results in the possibility to decide whether the result of the
mixing process is a negative or a positive frequency.
What is a negative frequency? To get an idea, lets have a look to the following graphics.
Imagine that the phasor is turning around with a specific speed or cycles per second. The projection
(shadow) of the phasor to the real-axis will result in a sinoid time signal x(t). It should be obvious
that the projection of a phasor running in clockwise or counter-clockwise direction should result in
the same projection x(t), even if the phasor is running in positive or negative direction. The
direction of the phasor gives the decision of whether it is a positive or negative frequency. Simple
projection of the signal to the real axis gives us no information, what we need is an additional
projection to the j-axis. The difference between positive and negative frequencies is the phase
relationship between both components.
If the LO has f2=1000kHz and f1=1001kHz, the result after low pass filtering is the 1 kHz sinoids.
With f2=1000kHz and f1=999kHz the result is the -1 kHz sinoids. With just one mixer, the decision
can not be made whether there is 999kHz or 1001kHz, but after mixing the input with two
frequencies at equal frequency, but 90° out of phase, we get two signal components, forming a so
called complex time signal. The two components of the complex time signal are called Inphase and
Quadratur. Inphase components result from mixing with a cosinus and Quadratur components from
mixing with a sinus time signal of the same frequency, so giving this special mixer the name IQ-
Mixer.
Now we are half way there. We have a complex time signal with a positive or negative frequency.
Unfortunately we can not hear complex signals, only real signals. But the solution is very easy.
The inphase signal is processed by an all pass filter, which doesn't change the amplitude, but causes
a phase shift of 90° degrees, independent of the input frequency. Such a filter is called a Hilbert
Transformation. The ideal Hilbert Transformation like the ideal low pass is not realizable and only
an approximation is possible. At low frequencies it's very difficult, which is easy to understand,
because a delay of 90° phase shift at DC means an infinity long delay, which is not possible to
arrange. So any physical implementation of such a filter is limited to a certain frequency range f>0.
That is no problem, when the modulating signal has no DC, which is true for PSK and telephony.
An analog solution for a Hilbert Transformation is possible, but the accuracy is limited due
tolerances in the components. Much better is an implementation using a digital signal processing.
The program MPSKIQ already contains the necessary filter functions.
The graphics shows, how to extract the positive or negative frequencies. By adding both
components of the signal after phase shifting, we cancel the positive frequencies, subtracting will
cancel all negative frequencies. This is done just by changing the sign of the adder. So we can
switch between reception of USB or LSB.
The same ideas are also valid for generating a SSB signal, just by changing the direction of the
mixers. MPSKIQ already delivers the necessary 90° phase shifted audio signals.