High-speed analog-to-digital converters (ADCs) have changed most approaches to receiver front ends. An ADC allows a good portion of the analog signal chain leading from the antenna to be eliminated. Similarly, in an instrumentation system, a high-speed ADC allows signals from a sensor to be quickly digitized with a minimum of analog processing. To demonstrate effective practices in directly digitizing RF signals, the RF subsystem for an instrumentation product used in high-energy physics will be examined. The instrument, which consists of an RF chain and a digitizer, can directly process and sample RF signals to 500 MHz. Although it is only one subsystem, it plays a crucial role for the final system since it defines the limitations of the test instrument.

The RF subsystem has several demanding performance requirements, including high signal-to-noise ratio (SNR) and good linearity. To achieve these requirements, an investigation was launched into digitizer performance limitations imposed by noise. A mathematical model for the system's noise sources was developed in order to predict output SNR and compare this to actual measurements. Figure 1 shows a simplified block diagram of the RF subsystem.

When sampling high-frequency signals, it is necessary to fulfill two criteria in order to avoid aliased components. The first criterion demands that the bandwidth of the signal to be sampled should not exceed one half of the sampling rate, or f_s/2. To fulfill the second criterion, the maximum and minimum frequencies of the sampled signal should be within the same Nyquist zone. Equation 1 can be derived by using both criteria:

where:

N_Z = the Nyquist zone number, and

f_RF = the center frequency of an arbitrary RF input signal.

If the signal with center frequency f_RF and bandwidth not exceeding fs/2 lies in the first Nyquist zone (NZ = 1), then a sampling frequency of f_S = 4f_RF is optimal. But under such conditions, modern ADCs would not allow sampling of signals with center frequencies higher than a few hundreds of MHz. Still, this limitation can be overcome if the Nyquist zone number is properly chosen. As seen from Eq. 1, increasing N_Z means that a high-frequency RF signal can be sampled with a relatively low sampling frequency, an approach known as under-sampling. For example, sampling an arbitrary signal with center frequency f_RF = 500 MHz, bandwidth of f_S/2, and Nyquist zone of N_Z = 9 requires a sampling frequency of f_S = 117 MHz.

An AD9433 ADC from Analog-Devices (www.analog.com) was selected for the RF subsystem. It is specified with a maximum sampling frequency of 125 MHz and therefore fulfills the need for a high sampling frequency of f_S = 117 MHz. Figure 2 shows the spectrum of an arbitrary signal and the frequency translation effect of the under-sampling technique.

A major problem of the under-sampling technique is the additional components that appear outside the signal bandwidth of interest. In order to limit the signal bandwidth to less than f_S/2, an anti-aliasing bandpass filter was added to the RF chain. The anti-aliasing filter has an 8-MHz bandwidth, a center frequency of 498 MHz, and passband insertion loss of 3.5 dB. It is a surface-acoustic-wave (SAW) bandpass component with more than 50-dB suppression of signals outside the chosen Nyquist zone. It is located at the input of the RF subsystem to suppress unwanted frequency components that would otherwise be amplified in the RF chain.

One of the functions of the RF signal chain is to keep the amplitude at the input of the ADC constant whenever the input signal is within the specified dynamic range, requiring high gain and programmable gain control. The RF chain developed for the instrumentation product features four amplifiers with gain of about 20 dB. Two variable attenuators with maximum attenuation of 31 dB and 1-dB steps each facilitate programmable gain control.

When designing the RF subsystem, some generic rules were followed. The first amplifier was chosen to be a lownoise amplifier with considerable gain to set the noise figure of the system. Similarly, the last amplifier has the highest third-order intercept point in order to set the dynamic range of the amplifying chain. A lowpass filter is placed between to prevent out-of-band oscillations. At the end of the RF chain, another SAW bandpass filter is used to eliminate unwanted frequency components possibly generated by the RF chain. A 3-dB attenuator in front of each SAW bandpass filter improves the filter's return loss (S₁₁).

Once the topology of the RF chain is set, its basic limitations can be studied. For high-level input signals, nonlinearity is a major problem. The variable attenuators must be properly set to achieve good linearity performance in the RF chain. The ADC's linearity is measured by comparing two signals coming from the same analog source and sampled on the same ADC, but one passed through a 6-dB attenuator before being sampled. Figure 3 shows the ADC's decibels full scale (dBFS) as a function of the input signal power for both signals and the ratio between them. In Fig. 3, it can be seen that the ADC at high power input levels has a nonlinearity of 0.1 dB per 15-dB change of input signal power, which cannot be neglected for a wide dynamic range. Maintaining signals to the ADC at a constant input level would solve the problem. For the RF subsystem, the input level to the ADC was maintained at about -5 dBm, for a tradeoff between RF chain and ADC linearity levels.