What is an EMI Receiver?

An EMI Receiver is a test device similar to a Spectrum Analyzer that is used to observe the RF signals with respect to frequency. In EMC emission tests, the radiated electromagnetic energy is measured by using a spectrum analyzer or an EMI receiver with a suitable current probe (for conducted emission) or a suitable measuring antenna (for radiated emission).

Most EMC conducted and radiated emission tests belong to CISPR standards, which require Quasi Peak (QPK) and Average (Avg) values. EMC Testing for the MIL (Military) standard, also requires peak noise measurement. A spectrum analyzer does not have a Quasi Peak (QPK) detector. Hence, a spectrum analyzer is only suitable for basic pre-compliance testing; it cannot be used for conducting the full range of EMC tests for certification. This is why we use an EMI receiver.

To perform EMC emission measurement for certification, the measuring apparatus must comply with the CISPR 16-1-1 standard. The CISPR 16-1-1 standard describes the requirements such as detector type, selectivity, harmonic suppression, resolution bandwidth, input impedance, dynamic range, noise threshold, and so on, of voltage, current, and EM field measuring devices in the frequency range of 9 kHz–18 GHz. Commercial EMI receivers fully comply with CISPR 16-1-X standard family. 

An EMI Receiver is used along with transducers (for example, antenna, E/ H field probe, etc.) and correct cables to measure the radiated and conducted emissions emanating from electronic devices. It is also used to check the electric field uniformity during the EMC immunity tests, diagnose and repair EMI problems that arise very often during the early stages of the product development cycle. 

Figure 1: EMI receiver

Figure 2: A conducted emission graph

 An EMI Receiver is like an oscilloscope. However, oscilloscopes look at signals in the time domain, but the EMI receiver looks at signals in the frequency domain. Hence, the EMI receiver will display the amplitude of an RF signal on the Y-axis (usually in dbµV) and the frequency of the RF signal on the X-axis. The x-axis of the EMI receiver is linearly calibrated in frequency with the higher frequency being at the right-hand side of the display. The Y-axis of the receiver is calibrated in amplitude (either linear or logarithmic scale, for most applications, a logarithmic scale is preferred for most measurements). The EMI receivers indicate the measurement reading by using the peak detector, Quasi peak detector (QPK), average detector, and RMS detector.

Figure 3: Electromagnetic interference test set up

 Nowadays the EMI receivers come with FFT (Fast Fourier transform) technique, which offers various advantages such as reduced time for measurement when compared to traditional EMI receivers and features.

How does an EMI receiver work?

Figure 4: General block diagram of an EMI receiver

 A simple block diagram of the EMI receiver is shown in figure 4. Basically, an EMI receiver is a Super Heterodyne receiver that utilizes a frequency mixing (heterodyne) method to convert a received RF signal at a certain frequency to a fixed intermediate frequency (IF), which can be processed more conveniently than the original carrier frequency. Let's discuss the working of an EMI receiver.

The input signal from conducted emission test (via current probe) or radiated emission test (via antenna) is received by the RF preselection bandpass filter (BPF1) and amplified by the low noise amplifier (LNA). The preselection filter section prevents the unwanted signal from entering the receiver, thereby ensuring the selectivity of the receiver.

The output of the low noise amplifier (LNA) (i.e., SIN) is fed into the mixer M, where the signal from the local oscillator LO with frequency f(LO) is superimposed. By tuning the frequency of the local oscillator, the user can measure the signal at the desired frequency of interest to be measured. The output of the mixer (SIF) is in the intermediate frequency (IF), which can be more conveniently processed than the original carrier frequency. The mixer output (SIF) is fed to a bandpass filter which determines the bandwidth and the linear signal transfer characteristics of the EMI receiver.

In the demodulator (DEM), the IF signal SIF is rectified and fed to the detector (DET). The detector section of the EMI receiver consists of the peak detector circuit, Quasi peak (QPK) circuit, average detector circuit, and RMS detector circuit. The detectors further process the signal, and the output of the detector is fed to the display system.

Peak detector: A peak detector is an electronic circuit consisting of a diode, resistor, and capacitor to determine the peak of the input signal. A peak indication is achieved with an extremely short time constant for charging and a very long time constant for discharging.

Figure 5: Peak detector

Quasi-peak detector (QPK): A quasi-peak detector circuit weighs the signal according to the repetition rate (PRF) of the signal and outputs accordingly. A quasi-peak indication is achieved with a short time constant for charging (1 ms in Bands B/C/D, 45 ms in Band A), and a relatively long time constant for discharging (500 ms in Band A, 160 ms in Band B, and 550 ms in Band C/D).

Figure 6: Quasi-peak detector

 Average detector: Provides the average amplitude of each signal component across its period.

RMS detector: Forms the effective value of the demodulated IF signals.

 Key Specifications of an EMI Receiver:

Frequency range: Represents the useable frequency of the EMI receiver. The EMI receivers are available in a wide frequency range from Hz to GHz range.

Frequency accuracy: Represents the frequency accuracy of the EMI receiver in ppm.

Detectors: Represents the types of detectors used by the EMI receiver. For example, Peak, Quasi-peak, Average, RMS, and CISPR-Average detectors. 

Measurement accuracy (S/N 20 dB): Represents the measurement accuracy of the EMI receiver in ± dB.

IF Bandwidth: Represents the IF bandwidth of the EMI receiver. Usually, it is in the kHz range.

RF input impedance: Represents the input impedance of the EMI receiver. Usually, it is in the range of 50 ohms.

VSWR: Represents the measure of how efficiently the electrical signal (voltage) is transmitted from the current probe or receiving antenna into the input terminal of the EMI receiver. A low input VSWR is the indication of a good EMI receiver.

Pre-amplifier Gain: Represents the voltage gain of the inbuilt pre-amplifier in dB. 

Measurement Time (manual mode): Represents the time taken to measure in manual mode. Usually, it is in the range of ms to s.

Preselection (frequency ranges): Represents the operating frequency range of bandpass filter banks associated with the preselection. For example (six bandpass filters), 150 kHz to 5.67 MHz, 5.67 MHz to 11.19 MHz, 11.19 MHz to 16.71 MHz,16.71 MHz to 22.23 MHz, 22.23 MHz to 30 MHz.

Maximum CW RF Power: Represents the maximum CW RF power that can be measured by the EMI receiver.

Noise floor: Represents the noise floor level of the receiver in dBμV at various conditions like Noise level (preselector ON), 50 Ω termination, Input Attenuation 0dB, Preamplifier OFF, and 50 Ω termination, Input Attenuation 0dB, Preamplifier ON. 

A/D Converter Resolution: Represents the analog to digital converter resolution in bits. 

Sampling rate: Represents the number of samples per second. Usually, the EMI receiver sampling rate is in the range of MHz.

FFT frequency resolution:  Represents the FFT frequency resolution in Hz / kHz.

Power supply: Represents the power supply rating. For example, 230V, 50Hz/60Hz.

Power consumption: Represents the power consumption of the EMI receiver in VA.

Dimensions: Represents the dimension of the EMI receiver in L × W × H mm.

Weight: Represents the weight of the EMI receiver in Kg.

Operating temperature: Represents the safe operating temperature limit of the EMI receiver in °F (°C).

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