RF Measurements Tutorial: RF Device Test Basics

This tutorial covers RF device testing basics using RF equipment. It delves into RF tests and RF measurements parameters such as power, gain, spurious signals, harmonics, P1dB, noise figure, image rejection, return loss, phase noise, group delay, frequency stability, TOI (Third-Order Intercept Point), AM-PM conversion, and more.

It covers both RF transmitter and RF receiver measurements. This tutorial aims to help RF engineers understand how to test and measure various RF specifications of RF power amplifiers, RF LNAs (Low-Noise Amplifiers), and RF transceivers using RF test and measurement equipment like spectrum analyzers, signal generators, and sweep oscillators.

Gain Measurement

Gain is the ratio of output power to input power, expressed in dB (decibels). For example, if the gain of an amplifier is specified as 20dB, it means that if the input to the amplifier/DUT (Device Under Test) is -15dBm, the output power will be +5dBm.

Gain Flatness Measurement

Gain flatness, or gain response, measures the variation in gain across the operating frequency range of RF systems like RF transceivers and RF power amplifiers.

Gain Flatness Measurement

Spurious Measurement

Spurious frequencies are unwanted frequencies present at the output of a DUT, typically devices containing mixers, such as RF frequency converters. These frequencies are usually at non-integer multiples of the input frequency fed to the DUT.

Harmonics Measurement

Similar to spurious signals, harmonics are also unwanted frequencies at the output of the DUT. However, they occur at integer multiples of the input frequency. The same setup as in Figure 2 can be used for measuring harmonics.

RF Harmonic Measurement and test setup

Spurious vs Harmonics

Spurious and harmonics are both unwanted signals in RF and communication systems, but they differ in origin and characteristics. Harmonics are integer multiples of a fundamental frequency, naturally occurring due to non-linearities in amplifiers and oscillators (e.g., if the fundamental is 1 GHz, harmonics appear at 2 GHz, 3 GHz, etc.). In contrast, spurious emissions are unintended signals at non-harmonic frequencies, arising from factors like intermodulation, mixing products, or oscillator phase noise. While harmonics are predictable and usually mitigated with filters, spurious signals are more unpredictable and can cause interference across a broader frequency range.

1dB Compression Point Measurement

Also known as the gain compression point, this refers to the output power level at which the DUT starts to saturate. To measure it, gradually increase the input power and measure the corresponding output power. The 1dB compression point is the input power level where a 2 dB change in input results in only a 1 dB change in output.

Noise Figure Measurement

Noise figure is a measure of the amount of noise generated by the amplifier/DUT. It’s related to noise temperature as follows:

$NF = 10 \log(1 + \frac{T}{290})$

Where:

• NF is the noise figure in dB
• T is the noise temperature in Kelvins

Noise Factor versus Noise Figure

Noise Factor (F) and Noise Figure (NF) both quantify the degradation of a signal-to-noise ratio (SNR) in a system but differ in representation. Noise Factor (F) is a linear ratio that defines how much noise a device adds compared to an ideal noiseless system, given by F = SNRin/SNRout.

Noise Figure (NF) is simply the logarithmic (dB) form of the noise factor, calculated as NF = 10log10(F). While Noise Factor is used in theoretical calculations, Noise Figure is more practical for engineers as it directly indicates signal degradation in dB, making it easier to interpret in RF and communication systems.

Noise figure vs Noise Temperature

Noise Temperature (Tn), measured in Kelvin, represents the equivalent temperature at which a resistor would generate the same amount of noise power as the device. The relationship between them is given by Tn = (F-1)T0, Where T0 is standard reference temperature usually 290K. While NF is commonly used in RF system design, Noise Temperature is preferred in radio astronomy and satellite communications.

Image Frequency Rejection Measurement

This specification is relevant for the receiver part of an RF transceiver. For instance, to measure the image frequency rejection of a C-band downconverter operating at an input frequency of 3700-4200 MHz and an output frequency of 52-88 MHz, we need to identify two input frequencies (f1 and f2) that produce the same output frequency (e.g., 70 MHz). These two frequencies are considered images of each other.

For example, the image frequency of 3700 MHz is 5785 MHz. This is derived as follows:

$F_{image} = 3700 + 2 * (1042.5) = 5785 MHz$

Here, an IF (Intermediate Frequency) of 1042.5 MHz is used, as depicted in the heterodyne downconverter diagram below.

$F_{image} = F_s + 2 * IF$

To measure the rejection, feed f1 (3700 MHz) and record the output power level (P1). Then, feed f2 (the image frequency, 5785 MHz) and record the power level (P2). The difference between P1 and P2 represents the image frequency rejection, usually specified in dBc.

Return Loss Measurement

Return loss is measured at all ports of the DUT. It indicates the accuracy of impedance matching at the ports.

$RL = 20 * Log (\frac{VSWR+1}{VSWR-1})$

Phase Noise Measurement

Phase noise is the ratio of signal power to noise power, measured in a 1 Hz bandwidth, typically expressed at a specific frequency offset from the RF carrier. The unit of measurement is dBc/Hz. For example, the phase noise at a 1 KHz offset from the RF carrier might be -60dBc/Hz.

The same setup as in Figure 2 can be used for phase noise measurements.

Group Delay Measurement

Group delay represents the time it takes for a signal to travel from the input to the output of the Device Under Test (DUT).

In the figure, an MLA (Microwave Link Analyzer) is used to measure the group delay of an RF Upconverter.

Group delay of upconverter = measured group delay of the setup - group delay of the mixer module.

Similarly, the group delay for an SSPA (Solid State Power Amplifier) or downconverter can be measured by replacing the upconverter in the setup with the DUT.

Frequency Stability Measurement

Frequency stability refers to the change in frequency of the DUT due to aging and temperature variations. There are two types: short-term and long-term, measured in PPM/day (parts per million per day) or PPM/year, respectively.

ppm means parts per million.

3rd Order Intercept Point RF Measurement

The Third-Order Intercept Point (TOI) measures linearity, indicating the amount of third-order harmonic distortion a device produces. It can be referenced to the input (IIP3) or output (OIP3) of a module.

$TOI \text{ in dBm} = (\text{Input signal levels in dBm}) + (\frac{\text{distortion products (dBc)}}{2})$

P1dB versus TOI

P1dB (1 dB Compression Point) and TOI (Third-Order Intercept Point) are key parameters in RF system linearity, but they measure different aspects of performance. P1dB is the output power level at which an amplifier’s gain drops by 1 dB due to compression, indicating the onset of non-linearity and saturation. TOI (Third-Order Intercept Point), on the other hand, predicts the point where third-order intermodulation distortion (IMD) products would hypothetically reach the same power level as the fundamental signals if linear amplification continued. TOI is typically much higher than P1dB and is used to assess an amplifier’s ability to handle strong signals without generating excessive distortion. While P1dB indicates power handling capability, TOI is crucial for understanding intermodulation performance in multi-signal environments.

AM-PM Conversion Measurement

AM-PM conversion measures the amount of undesired phase deviation (PM) caused by amplitude variations (AM) in the system.

The measurement setup for AM-PM and TOI point is shown below:

$AM-PM \text{ conversion}, Kp=13.2 \times 10^{-(\frac{P_{int}-P_i-G}{10})}$

Where:

• $P_{int}$ is the Third Order Intercept Point (TOI)
• $P_i$ is the input level in dBm
• G is the Gain of the Upconverter

For example, if $P_i = -33 \text{ dBm}$, $Gain = 30 \text{ dB}$, and $TOI = +17.41 \text{ dBm}$, then:

$Kp=0.1203 \text{ degree/dB}$

Fig. AM-PM conversion and TOI measurement

As shown in the figure:

1. Set Signal Source 1 to, say, f1 (70 MHz) and feed this signal, adjusting the power level to provide an output power of (P1dB - 3) dB.
2. Switch off Signal Source 1 and set Signal Source 2 to 71.001 MHz. Adjust the power level in Signal Source 2 so that the spectrum analyzer shows (P1dB - 3) dBm as the output.

Now, simultaneously feed both signals with the power levels just derived, and note down the TOI as shown in the figure above.