RF Considerations

Direct I/Q Modulation or IF Upconversion ?

There are several ways to get a digital modulated RF carrier on the desired output frequency. Basically we can distinguish two different RF strategies. One of these strategies starts with a digital modulated carrier which is generated on a low IF frequency. In most cases this is done at a frequency range close to 36 MHz. This is nowadays easily possible with the highly integrated chips which combine interpolating filters, numerical controlled oscillator, digital IQ mixers and on board high resolution DAC's into a single device. These devices run typically at 100-200 MHz clock frequency. Examples of these devices can be found at Analog Devices or Intersil. Because the digital modulated signal is just at IF, it has to be upconverted by a very linear upconverter which includes a mixer, a local oscillator signal (synthesized), additional bandfiltering for image rejection etc.

Advantages of upconversion of the modulated lower IF signal are:

• No problems of feedback of the output signal after upconversion which is a cause of problems with the second method.
• Some of the complex functionality which is needed for a digital modulator does not have to be implemented in FPGA, for example interpolation stages are already present in the modulator ASIC.
  

Disadvantages of the upconversion system with the modulated lower IF are:

• Higher complexity of the RF path and therefore higher costs.
• Need for low group delay band filter which is needed for image rejection after upconversion.
• Requires high IP3 mixer for distortionless upconversion.

The second strategy modulates the RF carrier directly on the desired RF output frequency by means of an analog I/Q mixer which is basically the same as a Single Side Band mixer as also previously shown at the communication theory section.


Figure 1. Basic architecture of I/Q modulator

The local oscillator signal which is represented in above diagram as the term cos(wt) is practically implemented as an external synthesized signal. This LO signal is generated by a VCO which is controlled by a phase locked loop. A lot of I/Q mixers (or IQ modulator) are commercially available from different vendors. A big advantage of the I/Q mixer when it is used for D-ATV is the fact that no further upconversion is needed because the modulation is done at the final RF frequency. The idea is similar as the widely used FM ATV systems which also contain a VCO which is modulated directly on the desired output frequency although the modulation principles and methods itself differ. Unfortunately, the direct conversion principle introduces also some disadvantages. One of these problems occurs when the modulated output signal is unintentionally fed back to the LO input. In this case a lot of problems arise and the signal gets heavily distorted. Therefore in practice carefull PCB design, shielding and good decoupling is needed. Another type of impairment occurs when the I and Q amplitude drive levels are not equal. This type of impairment causes IQ mismatch errors which has some effect on the resulting EVM of the signal. Depending on the type of modulation scheme, this might have more or less impact. Fortunately, QPSK which is used for the D-ATV DVB-S systems is quite robust.

If we look at the different types of D-ATV projects which exist at this moment we see that the DL8DW group uses the first topology, which is IF upconversion, while the Adacom group and we have chosen for the direct conversion principle due to above mentioned technical advantages/disadvantages. So.. although some people might give some opinion which system might be better; in principle both stategies are good and final performance is determined by the actual implementation. However, if you choose for the IF upconversion system then you'll have to design both the upconverter and also the power amplifiers. With the direct conversion system you can just keep your effort limited with the PA design only.

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