Three Digitally-Controlled Tunable Filters for VHF to 18 GHz Applications
Tunable filters have played an essential role in many RF/microwave signal chains, whether employed in a system front-end or as harmonic rejection for local oscillators. The new digitally-controlled tunable filter IC product offering from Analog Devices (ADI) provides leading performance with enhanced digital functionality to address many of these applications.
In today’s multi-channel, wideband, multi-octave tuning systems, it is necessary to filter out unwanted blockers to preserve the signals of interest. Common ways of implementing a filter in many systems is to either use discrete switched filter banks or analog tunable filters. Discrete switched filter banks often increase board real estate, filter development time and cost - opposite the goals of lowering size, weight, power and cost. Analog tunable filters, while small and compact, have their own compromises: tuning voltage generation, phase noise and switching speed. These solutions are often designed for a specific application, requiring significant system engineering development time.
To address these compromises and reduce development time, ADI has developed three digitally tunable filter products that combine an enhanced semiconductor process with common industry packages. This results in compact filters, highly configurable by standard serial-to-parallel interface (SPI) communication and with fast RF switching speeds. ADI has incorporated a 128 state lookup table within each IC to quickly change filter states for fast frequency hopping systems. The latest products introduced by ADI are the:
- ADMV8818, with four highpass and four lowpass filters, operating from 2 to 18 GHz
- ADMV8913, with a highpass and lowpass filter, operating at X-Band
- ADMV8052, with three bandpass filters, operating at UHF and VHF frequencies
The ADMV8818 is a flexible filter that provides tunable bandpass, highpass, lowpass, bypass or all-reject responses between 2 and 18 GHz. The IC comprises two sections: input and output (see Figure 1). The input has four highpass filters and an optional bypass, selected by the two RFIN switches. Similarly, the output section has four lowpass filters and an optional bypass, selected by the two RFOUT switches. Each highpass and lowpass filter is tunable with 16 states (four-bit control word) to adjust the 3 dB frequency (f3dB). This flexible architecture provides full frequency coverage without any dead zones.
Any wideband defense or instrumentation application where high speed data converters are used is an application for the ADMV8818. Electronic warfare (EW) is one important application, as the filter provides rapid reconfigurability and a small form factor. This enables multi-channel systems to scan the full 2 to 18 GHz spectrum, selecting a frequency of interest to be digitized. The flexibility of the filter expands the design possibilities for future EW systems.
The ADMV8913 a combination highpass and lowpass filter designed for X-Band applications (see Figure 2). It has low insertion loss of 5 dB. The highpass and lowpass filters are also tunable with 16 states (four-bit control word) to adjust f3dB. The ADMV8913 also has a parallel logic interface for setting the filter states without using the SPI interface. A parallel interface is advantageous for systems requiring fast filter response time because it eliminates the delay of the SPI transaction.
X-Band radar systems, whether mechanically steered or high channel-count phased arrays, often require filtering solutions that must be compact, easily configurable and have low insertion loss. The ADMV8913 is well suited for this, with low insertion loss, a small form factor and either SPI or parallel digital interface options. These features enable it to be placed close to the front of the systems to optimize performance and reduce integration complexity.
The ADMV8052 integrates three bandpass filters that span bands from 30 to 520 MHz, with a typical 3 dB bandwidth of 9 percent and insertion loss of 4 dB (see Figure 3). The bandwidth can be adjusted by ±2 percent, and the insertion loss will vary by ±1 dB. Depending on the location of the filter in a system, the bandwidth and insertion loss can be traded. The ADMV8052 uses a patent-pending interpolation technique enabling a simple method to adjust the filter states with an eight-bit value (i.e., 256 states). Using this, each filter has approximately 0.5 percent resolution in setting the center frequency. The ADMV8052 includes a mode where the filters can be bypassed.
The VHF and UHF coverage of the ADMV8052 is tailored to next-generation radios in military and civilian communication systems - manpack, handheld and public safety - where undesired signals must be filtered. Its low insertion loss, bandpass filtering performance and digital reconfigurability simplify transceiver design.
In summary, the new digitally-controlled tunable filter IC product offerings from ADI provide leading performance with enhanced digital functionality, offering advantages for many applications. These three products are the first in ADI’s digitally tunable filter portfolio.
Analog Devices, Inc. Wilmington, Mass.