1. Frequency Tolerance (Accuracy and Resolution)
Frequency tolerance describes the frequency accuracy of a center frequency or carrier signal. Specifically, frequency tolerance is the maximum deviation in hertz from the desired center frequency. This specification generally applies to both vector signal generators and vector signal analyzers. You can use a frequency counter to measure frequency tolerance.
A variety of components can determine frequency tolerance, but the local oscillator (LO) of an instrument has the greatest effect. RF synthesis of an LO is based on phased-locked-loop (PLL) circuitry, so the precision of the crystal oscillator, which is usually a voltage-controlled crystal oscillator (VCXO) or oven-controlled crystal oscillator (OCXO), can significantly affect the frequency accuracy of the LO. Thus, the accuracy of the reference source is also typically specified in parts per billion (PPB).
For RF signal generators that implement direct upconversion, frequency tolerance is solely dependent on the frequency accuracy of the LO. Figure 1 shows both the LO and the frequency accuracy of the intermediate frequency (IF) signal can affect frequency tolerance in an RF signal generator.
Figure 1. The IF Signal and LO Can Affect Frequency Tolerance
For some applications, the frequency resolution of the RF instrument is more important than the absolute frequency accuracy. For example, the frequency accuracy of many wireless devices is often 100 Hz or more. The frequency accuracy of the RF instrument is not always essential because these devices use digital signal processing (DSP) to remove carrier offset.
Unlike frequency tolerance, frequency resolution is the ability of an instrument to hit exact frequencies rather than coercing a signal to the closest frequency achievable. For RF continuous wave generators and direct upconversion RF vector signal generators, the frequency resolution is based entirely on the frequency resolution of the LO. More precise frequency resolution can be obtained for RF vector signal generators with a superheterodyne architecture. On some of these instruments, signal processing techniques, such as direct digital syntheses, enable these instruments to achieve a frequency resolution of less than 1μHz.
2. Output Level Accuracy
Output level accuracy characterizes the amplitude error of an RF generator across a broad frequency range. Output level accuracy is measured in dB as the deviation from the desired output level. Figure 2 shows that the level accuracy of a vector signal generator is affected by a variety of sources, including linearity of the digital-to-analog converter (DAC), attenuators, mixers, and even temperature.
Figure 2. Level Accuracy is Determined by the DAC, Mixer, and Filter
Deviation of these components from ideal performance is often predictable, and many RF signal generators can achieve better output level accuracy through sophisticated calibration schemes.
Understanding RF Instrument Specifications Part 1 of this series details VSWR, which is one of the most interesting contributors to output level accuracy. VSWR results from small impedance mismatches (from 50 Ω) in the system. Impedance mismatch causes signal reflections that reduce output level accuracy because reflections can significantly affect the amplitude of the RF output.
Output level accuracy is particularly important to consider in wideband applications. Varying power levels across frequency bands can distort a modulated signal and increase error vector magnitude (EVM) measurements. For example, a wideband code division multiple access (WCDMA) receiver requires 5 MHz of bandwidth. If the power level across this band are inconsistent, the demodulation of the symbols may be adversely affected. In fact, any higher order modulation scheme, such as 256-quadrature amplitude modulation (QAM), may be affected by small variations in amplitude over the bandwidth of interest. Thus, EVM degradation for wideband, high-order modulation schemes is the result of poor output level accuracy.
3. Output Power Range
Output power range indicates the available vertical range of powers for generated signals. Output power is the transmitted power generated, measured in dBm, from the RF instrument before the signal travels across any medium. Thus, the output power range does not account for any lossy transmission channel. You can apply transmission channel gain to the output power to calculate the effective radiated power (ERP). Figure 3 illustrates that the output power range is a result of the IF power generated by the DAC and any gain that is applied during upconversion.
Figure 3. Output Power Range is Determined by the DAC, Mixer, and Gain Amplifier
Both the maximum and minimum output power of an RF signal generator are important specifications. When specifying minimum power, the noise floor of the instrument and any low-level spurs determine the minimum usable signal power. For example, if an instrument has a noise floor of –140 dBm/Hz, a user can only achieve an effective dynamic range of 80 dB at power levels of –60 dBm and higher. Thus, when determining the minimum output power, the noise floor of the device should also be considered. As an example, consider global positioning system (GPS) devices. Because these devices use low-level signals, they require an instrument with the lowest noise floor possible.
Maximum output power is typically a function of the linearity of the gain amplifier. Because RF amplifiers begin to introduce distortion products the closer they come to saturation, maximum output power specifications are most useful when considered along with distortion specifications. For most applications, the required output power depends greatly on the requirements of the device under test (DUT). For example, devices such as Low Power Device 433 MHz transceivers are short range devices that broadcast around 10 mW. To test these devices, the required maximum output power is –20dBm for a direct connection to the DUT. On the other hand, the electronic product code specifications allow radio frequency identification transmitters to transmit with up to 1 watt of power (+30 dBm). This level is outside the range of most vector signal generators, so external amplification is often required.
4. Intermodulation Distortion
Measuring the third-order distortion products of an RF instrument yields the intermodulation distortion (IMD3) specification that characterizes the linearity of an RF instrument. In an RF system, components such as mixers and amplifiers typically introduce distortion products. These distortion products are more pronounced as components approach levels of saturation, and they are often still present even at lower power levels. Figure 4 highlights each of the components responsible for IMD3.
Figure 4. Mixers and Amplifiers Introduce Third-Order Distortion Products
Notice in Figure 4 that distortion is most prevalent in the RF signal chain where mixers and amplifiers are used.
One of the simplest ways to specify third-order distortion products is with a two-tone intermodulation test. With this test, two tones of the same power level and at different frequencies (usually within a few hundred kilohertz) are generated with a vector signal generator. You can observe both the distortion products and the two tones of interest upon generation. Figure 5 illustrates the products of an IMD3 test.
Figure 5. Second- and Third-Order Distortion Products of a Two-Tone Signal
As Figure 5 illustrates, second-order distortion products (f2 - f1, 2f1, f1 + f2, and 2f2) are generated far from the signal of interest. As a result, these distortion products can be easily filtered without affecting the desired signal. However, second-order distortion products also produce distortion with the fundamental tones. The result produces third-order distortion products that are much more problematic.
Figure 5 shows third-order distortion exists both far from the signal of interest (3f1, 2f1 + f2, f1 + 2f2, and 3f2) and close to the signal of interest (2f1 - f2, 2f2- f1). Many of the third-order distortion products can be filtered, but the products close to the signal of interest cannot. As a result, these distortion products are most commonly used as a measure of the output linearity of the system. Thus, IMD3 specifies the amplitude difference in dB between the fundamental tones and the third-order distortion products. Because distortion is more prevalent at higher power levels, specifying the output power of an IMD3 measurement is also important.
IMD3 is both an interesting and important RF specification. Intermodulation products cannot be easily filtered because they exist close to the signal of interest. As a result, distortion products can significantly affect the accuracy of a modulated waveform. These effects are commonly observed in degraded EVM performance of the instrument. Thus, IMD3 distortion is most significant in systems with strict EVM performance requirements. Testing receivers with higher-order modulation schemes (such as 64-QAM) results in stricter EVM requirements. For these devices, select an instrument with excellent distortion specifications.
5. Modulation Bandwidth (I/Q Rate)
Another RF signal generator specification that is critical to vector signal generators is the RF modulation bandwidth. Modulation bandwidth is defined by the maximum baseband sample rate, or I/Q rate. This bandwidth is a derivative of the Shannon Sampling Theorem, which states that a digital waveform must be updated at least twice as fast as the signal bandwidth for the digital waveform to be accurately generated. Modulation bandwidth is directly controlled by the arbitrary waveform generator (ARB) of the RF equipment highlighted in Figure 6. On some instruments, particular older ones, the RF front-end of the instrument can actually be wider than the modulation bandwidth. Conversely, the passband bandwidth of the RF front-end can affect modulation bandwidth if the bandwidth of the upconverter is smaller than the bandwidth generated by the ARB. When generating modulated signals with a vector signal generator, the usable bandwidth of the instrument is only as wide as the modulation bandwidth of the instrument.
The maximum baseband sample rate (I/Q rate) defines the modulation bandwidth and is largely dependent on the processing capabilities of the instrument. Figure 6 highlights the digital back-end of an RF vector signal generator block diagram. The bandwidth of the IF signal (or baseband signal, depending on the method of upconversion) determines the modulation bandwidth of the RF output.
Figure 6. Modulation Bandwidth is Determined by the Digital Interface and DAC
For many applications, the modulation bandwidth is often a non-negotiable specification that is determined by the particular communication standard. For example, WiFi (IEEE 802.11g) signal generation requires a modulation bandwidth of up to 20 MHz. By contrast, generation of WCDMA cellular channels requires only 5 MHz of modulation bandwidth. In each of these applications, it is valuable to have a modulation bandwidth that is much wider than the signal being generated. Sample-and-hold DACs produce aliases at multiples of their sample rate. Consequently, oversampling the signal can push baseband or IF images farther from the bandwidth of interest.
A variety of strategies can be used to increase the modulation bandwidth of an instrument. For example, vector signal generators that use direct upconversion often support external baseband I and Q inputs to increase the modulation bandwidth of the instrument. In addition, other instruments use signal processing techniques to maximize the baseband or IF sample rate without requiring additional waveform storage. Several NI PXI and PXI Express RF instruments support continuous data transfers from digital data drives at the full RF bandwidth of the instrument.
The specifications outlined in this article are featured because of their importance in signal generator characterization, as well as all RF systems. In Part 3 of this series, we explain common specifications used to characterize RF vector signal analyzers, such as dynamic range, resolution bandwidth, and noise floor.
Refer to the National Instruments RF Developer Network for more information about making RF measurements.