If you’ve ever used an RF signal analyzer, you’re probably used to seeing displays like the spectrum graph or constellation plot. Of course, trusting the answers you get from the instrument often requires understanding what’s inside-mixers, filters, oscillators, amplifiers, and more-and how it all works together to make up an RF signal analyzer (or receiver, for that matter).
Those who are new to the RF world need to understand the theory behind how a mixer operates. Fundamentally, a mixer is a three-port frequency-translating component that mathematically multiplies two signals. A downconverting mixer has two input ports: one for the input RF signal and one for a second signal from a local oscillator, a component we’ll talk more about later (Fig. 1).
If we think of the mixer in purely mathematical terms, it’s much easier to comprehend its frequency-translating nature. For example, when multiplying sine and cosine waves of different frequencies, the result is the sum and difference of the two frequencies:
Therefore, multiplying sine waves at frequencies f1 and f2 would yield resulting sinusoids at f1-2 and f1+2. We then can use this frequency-translating characteristic to predictably translate RF signals to a desired intermediate frequency. Within an RF signal analyzer, this enables that signal to be digitized with higher-resolution analog-to-digital converters (ADCs).
The second key element of an RF system, and one that is used to drive the second port of a mixer, is the synthesizer. In this context, we often refer to the synthesizer as the local oscillator (LO). Today, most RF signal analyzers use either a voltage-controlled oscillator (VCO) or a yttrium-iron garnet (YIG) as their underlying synthesizer technology.
In general, VCOs tend to be faster-tuning and YIGs tend to provide the best phase-noise performance. Often, “production-test grade” equipment (where tuning time is comparatively more important) will use a VCO as the LO, and “R&D grade” equipment will use a YIG at the LO.
With a synthesizer driving the mixer’s LO port, the RF signal is translated to a new frequency based on the frequency of the LO. For example, suppose the RF input signal was a modulated signal at 8 GHz, and the synthesizer was configured to generate a sine tone at 7 GHz. As one might expect, the output port of the mixer would contain two “copies” of the original signal, one at 1 GHz (8 – 7 GHz) and one at 15 GHz (8 + 7 GHz).
After any mixer, signal content will generally exist at two different frequencies separated by two times the LO frequency. Thus, an RF filter is used to filter out the unwanted copy of the signal. Most RF signal analyzers use a fixed IF frequency, which means that no matter the center frequency of the instrument, the LO frequency is adjusted so the RF signal is always mixed to the same IF.
A simple bandpass filter easily accomplishes this task, a component that not only removes the unwanted mixer image but must also preserve the integrity of the desired signal in the passband (Fig. 2). Even though the “unwanted” mixer product is generally at a much higher frequency, it’s still important to filter out this image because mixing products can alias into the passband when sampling with an ADC.
Once the RF signal has been translated to a lower frequency, the RF signal analyzer’s internal ADC digitizes the signal. When digitizing a signal, the important signal characteristics are confined to a specific band of frequencies, notably higher than dc. So given that the ADC can ignore all dc content, this allows us to sample the signal in one of several Nyquist “zones.”
Many RF signal analyzers utilize either the second or third Nyquist zone of an ADC because it enables the instrument designer to use a higher intermediate frequency. For example, suppose our digitizer had a maximum sampling rate of 500 Msamples/s. Given the Nyquist criteria, we can capture signals from 0 Hz to 250 MHz within the first Nyquist zone.
However, if we sampled a signal that was higher in frequency-perhaps a tone at 300 MHz-this signal would “fold” over the 250-MHz Nyquist edge and appear at 200 MHz (300 MHz – 250 MHz). In this scenario, the tone is in the second Nyquist zone of the ADC, which ranges from 250 MHz to 500 MHz. As we observe in Figure 3, each Nyquist zone appears at multiples of 250 MHz (fs/2).
Amplifiers And Attenuators
So far, we’ve ignored the role these components play in a signal analyzer. But it’s important to remember that any time a signal passes through a passive component such as a mixer or filter, it is attenuated according to the insertion loss of the device. In practice, various amplifiers are used throughout an RF signal analyzer to optimize the signal level at each stage.
The bulk of attenuation in an RF signal analyzer happens at the very first stage with a programmable step attenuator in front of the first mixer. This attenuator is used to control signal power so the power at the first mixer (called mixer level) does not drive the mixer into compression, introducing distortion. Of course, many RF signal analyzers use many small attenuators (or “pads”) between mixing stages. These attenuators are usually on the order of 3 to 4 dB and improve impedance matching between stages.
Similarly, various amplifiers are often used between mixing stages as well. The amplifiers are also used to maintain signal level throughout the signal analyzer’s multiple stages, ensuring that a sufficient signal-to-noise ratio is preserved.
In practice, modern RF signal analyzers often use multiple mixing stages when downconverting a signal from RF to IF. In the classic three-stage downconverter, three mixing stages are used, and the signal is first translated to a higher frequency before being downconverted in two subsequent stages to the final intermediate frequency.
For many people new to RF measurements, mixing and downconversion might seem like black magic at first glance. But with a little background on RF components such as mixers, synthesizers, amplifiers, and attenuators, an RF signal analyzer isn’t quite as difficult to understand as you might think.