Mobile Technology
More efficient design T
Thomas Rottach looks at how high performance HF generators can offer maximum design flexibility
he development of high frequency systems means that the designer requires ever more detailed specifications and as a result systems are being divided into several parts during the design phase and are being executed by specialists in those respective areas. Beside the fact that everyone can focus on their speciality, this concept provides another advantage: parallel development. However, defining explicit interfaces is necessary as a result in order to enable each group to design individually and independently. Also, each group has to be able to test its designs and verify the most important interfaces and this is where the benefit of a flexible source comes into play. A perfect HF source can provide all the signals of the complimentary but not yet available parts, and facilitate the complete integration of those parts at a later state. RIGOL has developed a new signal
source for high frequency measurement, spanning a frequency area from 9kHz to 6 GHz. The signal generator can be used as a HF sinus source or as an analogue modulation source. The functionality of this complete system can be explained using the design example of a mobile phone.
measurements can be done on mixers? The HF signal generator can be used either to replace the local oscillator (LO) or the IF (intermediate frequency) or RF (radio frequency) source. To be able to measure the mixer performance it is necessary to have an additional spectrum analyser or at least a power meter with an attached band pass filter. Which are the most important mixer measurements and which specifications and functions of the source are essential?
A view into the data sheet of a mixer shows two other parameters beside the frequency ranges which are important in selecting the right mixer. First it is the conversion loss and secondly the port-to- port-isolation (RF-IF resp. LO-IF). Other basic parameters are VSWR (IF, LO- resp. RF- VSWR) and IP3 (3rd order intercept point). Frequency stability, low phase noise and high output power accuracy are important requirements for the source. The DSG3000 developed by Rigol offers the option OXCO (oven controlled high stable oscillator with 5ppb) and the option Power Meter Controller (PMC) to be able to eliminate output power error. Mixers are grouped by the needed local oscillator input power; for example you can find labels Level 7 or Level 17 or Level 23. To be able to measure this specified mixer the used HF generator has to be able to supply LO-power that is high enough. The DSG3000 with its maximum output power of 25dBm has been designed with this in mind.
The next element under consideration is the amplifier. Here, you also have to measure and specify several parameters to ensure a distortion-free and reliable data transmission. Besides the frequency range and
A simplified block diagram provides an overview of the included function blocks. The system can be divided into two main blocks: the high frequency part and the base band part (see above).
In this article we focus on the HF part,
the reason being is that you can find many possibilities for the use of a HF generator. The mobile phone RF front-end consists of filters, amplifiers, mixers and I/Q-modules. Each of these components has to be tested and measured separately.
The best way to specify or analyse filters is to use a network analyser (NWA) or a spectrum analyser with a built in tracking generator. The main advantage of a VNA (vector network analyser) is that besides the pass band and stop band attenuation and the frequency parameters of the phase response respectively the group delay can be measured as well.
What possible applications with 32 February 2014
the gain, the 1-dB-compression point, the inter-modulation behaviour and the input and output matching 50 Ohm over the specified frequency range are important parameters.
In the example shown here (see above) the possibilities of how a RF signal generator could be used for power amplifier measurements are shown. The 1-dB-compression point defines where the power amplifier leaves its linear behaviour and starts to become non-linear. This point is specified at the position where the real, measured output power differs 1dB from the theoretical, linear extrapolation. Both, being able to perform an output power sweep over a defined range and the accuracy of the output power are very important parameters. With the help of the PMC (Power Meter Controller) option it is possible to run an output power calibration in front of the
Components in Electronics
actual test. This option enables the control of a USB Power Meter which is connected directly to the generator. The measured, real power values are stored into the DSG3000 and will be used during the test to correct the output power level in that way that the wanted power level can be guaranteed directly at your device under test.
Leaving the RF-Frontend-part and moving over to the intersection of the baseband part, beside some additional filters we can find the I/Q modulator inside the sender path and an I/Q demodulator inside the receiver path. This is more or less the border between the analogue and the digital part of the system.
The output of the I/Q modulator in the sender path is the sum of two phase modulated (sometimes also amplitude modulated) signals. It often lies on an intermediate frequency (IF).The inputs are two digital signals (I and Q). The receiver path splits the IF signal into two signals which will then be de-modulated into digital data streams.
The testing and verifying of these
The diagram at the end of the article shows schematically the possibilities of the I/Q option of the Rigol Generator series DSG3000. You can see that all described applications can be realised. To be able to simply build different I/Q base signals Rigol provides a PC based software called Ultra IQ Station for free with the DSG3000 I/Q option. The IQ modulation bandwidth of
the DSG3000 (IQ) is 30MHz for I and Q signals when using internal modulation and 60MHz for I and Q signals when using the external modulation. This means I or Q baseband signal (ext. mod) can have 60MHz; RF-I/Q signals can have 120MHz. In summary, it can be said that the DSG3000 series is a flexible RF source due to its standard functions and with the additional options available it can be used in a huge number of various applications. In addition to the functions described above there is also an of-the-shelf analogue output built in. With this it is possible to output standard signals as sine wave/square wave/ramp(3Vpp max, 1MHz). Furthermore, pulses can be defined and outputted.
modules require much more functionality than just generating CW signals or output analogue modulated signals. It is necessary to be able to generate and output digital I and Q baseband signals. Additionally, there should be the possibility to output I/Q-IF signals. If we go further into the digital part it will be an advantage to be able to include baseband I and Q signals into the generator, which come from the digital signal processing part of the mobile and generate and output I/Q-IF signals.
The system can also be enhanced with the PUG option. Based on this, it is possible to output predefined pulse trains. The system’s flexibility does not end at the interfaces, however, as all standard connections (GPIB/LAN/USB) are provided off-the-shelf.
Rigol Technologies |
www.rigol.com
Thomas Rottach is an applications engineer with Rigol Technologies
www.cieonline.co.uk
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