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What are the different types of frequency extenders?

In the Test & Measurement field, there are three different types of frequency extenders. These include devices such as multipliers and mixers, and systems made from a combination of both. They have been described in more detail in ‘What is a Frequency Extender’.

When it comes to the type of the equipment and applications for which frequency extenders are employed, there are five different types:

  1. Signal Generator Extenders.

This group of signal generator extenders relies on a frequency multiplication as a principle of frequency conversion. In such devices, the output signal frequency is an integer harmonic (multiple) of the input frequency i.e. FOUT = N x FIN. A frequency extender operating based on that principle, can extend the frequency of a signal generator or any other frequency source by a fixed multiple. This multiple is also called a multiplier number and is determined by a type and number of cascaded multiplier devices that it consists of. A typical multiplier number can be as low as 2 and as high as 30 or higher. Higher order multipliers / extenders are achieved by cascading low order multipliers such as doublers (x2), tripplers (x3), quadruplers (x4) and quintuplers (x5).

Signal Generator Extender diagram.
Figure 1. Signal Generator Extender diagram.

 

  1. Spectrum & Signal Analyser Extenders.

These types of extenders operate on a principle of heterodyne mixing, in which the local oscillator signal (LO) is mixed with an input radio frequency signal (RF) to produce an intermediate frequency signal IF i.e. IF = RF ± LO, which is then used by the baseband analyser for spectrum and signal analysis.

In its simplest form, a frequency extender in such an application could be a stand-alone harmonic mixer. Devices like these often operate with at a multiple of a LO signal that is supplied by the analyser to produce the IF frequency in accordance with a formula IF = RF ± N x LO, where N = 1, 2, 3 … A typical diagram of a spectrum & signal analyser extender is shown in Figure 2:

Spectrum & Signal Analyser Extender diagram.
Figure 2. Spectrum & Signal Analyser Extender diagram.

Such mixers, although simple and relatively inexpensive, suffer from a high conversion loss that is associated with operating at a high number of LO harmonic. For cases and applications where combined sensitivity of the test equipment is of primary importance, more complex extender systems are required. Such extenders are aimed at achieving the lowest possible noise floor and preserving the sensitivity of the analyser, and typically are equipped with fundamental or sub-harmonic mixers which operate at the LO harmonic number of 1 and 2, respectively. These systems offer much lower conversion losses in comparison with their harmonic counterparts and are more suited to applications where the measurements require much higher order of fidelity i.e. radar signals, wireless communications etc. This is often achieved at the expense of additional LO multipliers that have to be used to provide the frequency and power for fundamental or sub-harmonic mixers.

 

  1. Vector Network Analyser Extenders.

This type of frequency extenders is designed to work with vector network analysers (VNAs) to measure complex scattering parameters (S-parameters) at frequencies that stretch beyond the standard range of the analyser. They are aimed at preserving the measurement precision and maximising the dynamic range and stability of the test setup. When it comes to the principle of frequency extension, these systems consist of both types of devices – multipliers and mixers. The latter often operating at high LO harmonics, although sub-harmonic mixers (2nd LO harmonic) are also used in high performance extenders for applications where dynamic range is essential i.e. antenna range measurements.

Typical VNA extension system comprises of two frequency extenders. Each extender consists of a multiplier chain for converting the RF signal to the required frequency range and typically two receivers / down-converters, which are driven by the LO source from the VNA, produce IF test and IF reference signals. The IF signals are fed into the VNA receivers to measure the S-parameters. While the actual RF test frequency could be in a range of several hundred gigahertz, the down-converted IF signals are in the range of tens of megahertz. The concept of the VNA extender is shown in Figure 3:

Vector Network Analyser Extender diagram.
Figure 3. Vector Network Analyser Extender diagram.
  1. Noise Figure Analyser Extenders.

This group of frequency extenders is designed to extend the range of noise figure analysers and spectrum or signal analysers with a noise figure measurement option. Typically, a set of extenders would consist of two modules – a noise source and a block down-converter. The purpose of the noise source is to generate a known and stable level of a broadband noise at the frequency of interest, while the purpose of the block down-converter is to convert the high frequency noise to the low IF frequency at which the analyser can measure. Such an extension system is aimed at achieving the highest possible Excess Noise Ratio (ENR) and lowest possible noise figure, which together allow for high level of measurement accuracy. The architecture of the block-down converter is largely similar to that of a spectrum analyser extender with particular attention paid to noise performance and spurious content.

 

  1. Communication Extenders.

Communication extenders are suited for applications that call for extremely high levels of accuracy when it comes to measuring modulation quality and performance of complex wireless devices and systems. They are designed specifically with signal fidelity in mind and to minimise the intrinsic Error Vector Magnitude (EVM) of a test system. EVM is a popular performance metric that helps to quantify a combined impact of all system impairments and present them in a form of one parameter.

The system EVM value is impacted by the noise figure, third-order intercept point and signal-to-noise ratio, phase noise and nonlinearities, to name just a few parameters.

Typically, communication extenders are used to generate and transmit, as well as receive and analyse modulated signals. They operate on the principle of heterodyne mixing where the fundamental or sub-harmonic mixers are setup to work in a linear region. The up- and down-converter operate as a transmit (Tx) and receive (Rx) module, respectively. The transmitters are often interfaced with a signal generator an arbitrary waveform generator (AWG) that provide LO and baseband IF modulation. The receivers are typically used with a variety of signal analysers and oscilloscopes that can perform a complex analysis of a down-converted IF signal. The simplified diagrams of Tx and Rx units are shown in Figure 4:

Figure 4. Communication extenders diagram – Tx (left), Rx (right).

 

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