Behind all these applications you will find technologies like LTE, 3GPP, Wi-Fi, Bluetooth, ZigBee, RFID, NFC, and DECT.Regardless of the protocol, the hardware is similar; a receiver, a transmitter, or a transceiver which typically use an amplifier module to help increase the signal amplitude.
Each application demands different specifications and requirements. For example, the amplifier which is located in the receiver path has to be frequency selective and have low noise.Alternately, a power amplifier in the transmitter path has to be optimised for maximum output power. Regardless of the end application, all amplifiers have the same basic parameters like frequency range, gain, output power, linearity, matching and stability.
Operating frequency range and gain
RF amplifiers have performance that varies with the input frequency. Two of the most basic parameters are the frequency range and the gain for defined frequencies or over a defined frequency range. A spectrum analyser with a built-in tracking generator like the Rigol DSA800-TG or the DSA1000-TG is suitable for these types of measurements.
The first step is to normalise the output power of the tracking generator. Normalisation is the process of mathematically subtracting the output spectrum of the tracking generator, through the cables and adapters, from itself. This removes cable and adapter losses as well as nonlinearities in the tracking generator output.
The next step is to connect the amplifier between the generator output and the spectrum analyser input. With this information, it is possible to determine the frequency range that provides the best performance as well as the maximum gain.
1dB compression point
Data sheets for amplifiers typically report the output power measured at the 1dB compression point. This is the point where the measured output power deviates 1dB from the theoretical, ideal output power. This deviation originates from the saturation of the amplifier. Above the 1dB compression point, the gain decreases significantly. Additional input power will not increase the output power.
The 1dB compression point can be determined by sweeping the output power at a specific frequency and then measuring the resultant output of the amplifier. A typical test setup requires an RF source with a wide dynamic range, like the Rigol DSG3000 series, and a spectrum analyser like the Rigol DSA800 series.
What is the significance of the 1dB compression point?
With digitally modulated signals like the QAM-modulated carrier from a digital TV signal, we are usually concerned with the average power of the channel. But the ratio between peak and average power with a QAM signal can be between 6 and 13dB.If this ratio is large enough, the peak power could exceed the 1dB compression point. Applying such a signal to the amplifier will lead to a distorted output. Therefore it is crucial to select an amplifier which has enough space between maximum peak power and the 1dB compression point to accurately amplify the entire range of the input signal.
Third order intermodulation
Another specification which contains information about the linearity of the device is the third order inter-modulation point, called TOI or IP3.
There are two ways to determine the IP3-point: One is graphically, where the IP3-point will be found from linear interpolation of transmission graphs. The other method is to measure the power of the original sine wave signal and the inter-modulation product third order within the small signal range and then calculate the theoretical point. This method is the technique used with Rigol spectrum analysers that have been updated to include the Advanced Measurement Kit firmware option (DSA800-AMK/DSA1000-AMK).
Datasheets can help designers select the proper elements for a particular circuit design. But, sometimes, components can perform outside of the design parameters. Verification of specific part performance can be made using a few simple tools and test setups. With the use of the RF signal generator series (DSG3000, 3 or 6GHz, with or without IQ modulation option) and the Spectrum Analyser portfolio Rigol is able to address applications within the communications market. To complete the test setup you can add one or more remote controllable power supplies (DP800 series) to power up your device. Additionally Rigol can offer a 5½ or 6½ digits multimeter to measure the power consumption or quiescence currents.
This is an edited version of the original article, which can be viewed via the pdf link below.
Thomas Rottach is an applications engineer with RIGOL Technologies EU