An A-Z Guide to The Basics of RF Downcoverters

An RF downconverter is an important part of signal processing and mixing, in a variety of different applications, including military radar operations, cellular communications, and beyond! A radio frequency or RF mixer is a three-port device that modulates or demodulates (meaning it changes the frequency) of a signal, while at the same time preserving the essential characteristics and qualities of the original signal.

Perhaps the greatest benefit of RF technology is to allow a received signal to be amplified at a different frequency. In a radio frequency mixer, there are two input ports where radio signals come into the device. The downconverter device then calculates both the sum and the difference of these two frequencies, and creates a new output wave that matches both the sum and difference wave. When the device is used with a signal receiver, it is known as a downconverter. When paired with a transmitter, it is called an upconverter.

Mixers generally fall into two categories, passive devices are built primarily with passive electrical components such as diodes, and active devices are built with active components such as transistors. The type of mixer used largely depends on the application, as there are relative advantages and disadvantages to each. For example, passive mixers that take advantage of diodes that typically results in a conversion loss which can impact the noise performance of the system.

Passsive mixers are often the most widely used type due to their simple construction and operation, their relatively large bandwidths, and good distortion performance. Active mixers are primarily used for applications where conversion gain, solid isolation of the input signal ports, and require little power to drive the output port. These active devices can be integrated with other signal processing systems and are less sensitive to the effects of matching signal loads.

Some parameters and specifications you will want to pay attention to, if you are looking for an RF downconverter, are the following:

1. Conversion Loss or Conversion Gain – This measure refers to the gain of the signal in an active mixer, or conversion loss which is a measure of the loss at insertion of the signals into a passive mixer. These are both typically measured in decibels.

2. Input Intercept Point – On a spec sheet this may be referred to as IIP3 and is a measure of the RF input power that results in the output power of unwanted intermodulation and the desired IF output being equal.

3. Spurious – A spurious external signal generates undesired frequencies may fall into the IF-band you are trying to capture. Spur tables will be provided by the manufacturer to show relative amplitudes of each response under the given signal drive conditions.

4. Isolation – This term refers to the amount of power that is leaked from one of the ports to the other. When this measure is high, the leakage between those ports will be small.

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