A New Wideband Upconverter for High-Frequency Radar and Communication Systems
Analog Devices has recently released an SPI-programmable RF upconverter that can translate baseband or IF signals to extremely high transmission frequencies.
Analog Devices has recently released an SPI-programmable RF upconverter that can translate baseband or IF signals to extremely high transmission frequencies.
“High frequency” is a relative concept. If you’re working with a precision op amp, a signal frequency of 10 MHz or even 1 MHz might be considered fairly high. On the other hand, if you’re a microwave engineer who designs radar equipment or satellite communication systems, you may not be very impressed by frequencies in the 1–10 GHz range. For me, anything above 5 GHz is intimidating, and when I see a part that has a specified frequency range of 24 GHz to 44 GHz (such as the ADMV1013), I’m really not sure what to say....
Do electrical frequencies actually go that high? Do the laws of physics still apply at 44 GHz?
The Need for Upconversion
Though it is possible to directly modulate a high-frequency signal, RF systems commonly use a component called a mixer to translate an information-carrying low-frequency signal to a higher frequency that is suitable for wireless transmission. This process is called upconversion, and it can begin with a baseband signal or an intermediate-frequency (IF) signal. Mixers also perform downconversion, and in this article you can read about a basic downconversion mixer that consists of nothing more than a square-wave oscillator signal and a voltage-controlled switch.
The importance of upconversion is particularly clear in applications that require transmission frequencies way up in the gigahertz range. It’s easier to generate a complex modulated waveform when you’re working with low-frequency signals, and this is especially true if you have a software-defined radio in which the waveforms are created in software and then introduced into the analog realm by a digital-to-analog converter. You certainly won’t be using a DAC to generate a 30 GHz analog waveform, but you can use a DAC to generate a baseband or IF waveform and then achieve the final transmission frequency by means of upconversion.
The ADMV1013
This new silicon-germanium (SiGe) microwave upconverter offers a number of impressive features. As you already know, it supports extremely high transmission frequencies. The maximum RF frequency is 44 GHz, and this spec doesn’t mean that the performance deteriorates until it becomes intolerable somewhere around 44 GHz. Rather, the plots in the datasheet indicate that performance is quite stable throughout most of the operational range. For example:
Plot taken from the ADMV1013 datasheet.
The input to the ADMV1013 can be I/Q baseband waveforms or an I/Q IF signal. The device exhibits very consistent performance over a wide range of baseband frequencies, including low frequencies, as demonstrated by the following plot:
Plot taken from the ADMV1013 datasheet.
Though I typically think of baseband signals and IF signals as occupying different frequency bands, this distinction is not present in the ADMV1013 datasheet. Instead, for both of these input pathways, most of the frequency range overlaps: the baseband ports operate from DC to 6 GHz, and the IF input range is 0.8 to 6 GHz.
However, these two types of input signals are associated with different operational modes. “IF mode” is single-sideband upconversion; this mode is selected by setting a bit, via SPI, in one of the configuration registers, and the SPI interface also allows you to optimize sideband suppression.
When baseband input signals are used, IF mode is disabled and the device performs direct conversion to RF by means of quadrature modulation.
Diagram taken from the ADMV1013 datasheet.
Local Oscillator
To generate a very high-frequency RF signal, you need a very high-frequency local oscillator. However, the LO signal that you deliver to the ADMV1013 doesn’t need to be in the same frequency range as your transmitted signal, because the device incorporates a 4× frequency multiplier into the LO path.
Diagram taken from the ADMV1013 datasheet.
Evaluation Board
The ADMV1013 comes in a land grid array package (and there are 40 lands), and high-quality PCB design for a 30 GHz circuit is not exactly a simple task. If you want to gain some experience with this upconverter or determine if it’s suitable for your application, the evaluation board is definitely preferable to a custom prototype. The following diagram, taken from the ADMV1013-EVALZ User Guide, gives you an example of an effective test setup.
Have you had any interesting adventures in the land of high-frequency RF design? If so, leave a comment and let us know what you’ve learned.
