Just about every aspect of circuit design becomes more complicated as signal frequencies extend into the gigahertz range, and filters are no exception.
In low-frequency analog circuits, we can often accomplish a filtering task with nothing more than one resistor and one capacitor. When steeper roll-off is required, we often turn to active filters that take advantage of standard topologies such as the Sallen–Key (shown in the diagram below).
The Sallen–Key filter is a straightforward circuit that provides a two-pole response.
Op-amp-based filters are increasingly impractical as frequencies move into the radio-frequency range. RF engineers often return to passive components, which don’t suffer from the frequency limitations exhibited by amplifiers, but even passives become problematic when extremely high frequencies cause nonideal component properties to more seriously interfere with the operation of the circuit.
Variable-Frequency Filters for RF Systems
One task that is more difficult than designing a high-frequency filter is designing a tunable high-frequency filter. The term “tunable” refers to the fact that the filter’s frequency response can be adjusted. In my experience, tunable filters are somewhat rare in the low-frequency analog world, but they are a fundamental aspect of RF applications, because many RF receivers must be able to tune into various carrier frequencies within a specified range.
Two options for high-frequency tunable filtering are cavity tuned filters and switched filter banks. A cavity filter consists of a resonant metal cylinder that surrounds a movable tuning rod. Changing the position of the tuning rod affects the resonant characteristics of the structure, and this in turn alters the frequency response.
A switched filter bank is basically what the name implies—a bank of filters combined with input and output switches. The idea here is to have a single module that includes multiple filters, and the switches allow the user to select the desired frequency response.
As you might imagine, these solutions are not particularly compact. Furthermore, cavity filters do not offer convenient frequency adjustment, and switched filter banks provide multiple fixed frequency responses instead of a continuously adjustable range.
This new tunable low-pass filter from Analog Devices is a small-form-factor alternative to cavity filters and switched filter banks. Unlike components that incorporate some sort of mechanical functionality, it’s an MMIC—i.e., a monolithic microwave integrated circuit. As you can see in the functional diagram below, the interface is straightforward: all you need is an RF input signal and a control voltage, and the chip provides the filtered RF output signal.
Diagram taken from the HMC882A datasheet.
The cutoff frequency of the filter can be chosen from a continuous range extending from 3.95 GHz to 6.9 GHz. This frequency range corresponds to a control-voltage range of 0 V to 14 V.
All things considered, I would say that this is a convenient tuning method—it’s a purely electrical solution that provides dynamic adjustability. It’s true that 0 V to 14 V is a large range that exceeds typical supply voltages, but overall this seems like a fairly small price to pay.
The HMC882A’s roll-off is quite steep. It provides more than 20 dB of attenuation at a frequency that is equal to the cutoff frequency multiplied by 1.28. As a point of comparison, a single-pole filter provides 20 dB of attenuation at a frequency that is equal to the cutoff frequency multiplied by 10.
Selecting the Cutoff Frequency
The control-voltage input pin doesn’t require any significant amount of current, so you could bring the frequency-selection functionality into the digital realm by driving the pin directly from a high-voltage DAC.
Apart from the typical convenience of a microcontroller-based interface, this approach would allow you to introduce a bit of additional accuracy into the system. As you can see in the plot shown below, the relationship between the control voltage and the cutoff frequency is not perfectly linear. It is also influenced by temperature. If you incorporate a microcontroller into the system, you could compensate for these effects in firmware.
Plot taken from the HMC882A datasheet.
Do you have any experience with cavity filters or switched filter banks? Is there any reason why you would be hesitant to use a device such as the HMC882A? Feel free to share your thoughts in the comments section below.