Remotely-tunable Notch and Bandpass Filters for Evaluating System Performance
By Sam Johnson, Filter Product Manager, Trilithic, Inc.

Tunable notch and bandpass filters can be very cost-effective in high-frequency measurement and communications applications, especially when it is necessary to frequently change bandpass or notch frequencies. The alternative is to have available a large number of filters with discrete passbands that must be manually swapped in and out, which is inconvenient, expensive, cumbersome, and does not lend itself to automation. The DCVNF and DCVF Series digitally-controlled tunable notch and bandpass filters from Trilithic provide a solution that allows a filter’s notch frequency or bandpass center frequency to be tuned anywhere within an octave range, such as 1 to 2 GHz, by entering the desired value from the unit’s front panel or remotely via a serial bus or GPIB.

The DCVNF and DCVF Series filters are microprocessor-controlled precision filters that are designed for use in both bench-top applications and in the field. They can be ruggedly built, using shock-mounted components that allow them to withstand vibration of 2 G and shock of 5 G (non-operating). A typical model measures 10 in. wide x 3.4 in. high x 14 in. long, and weighs about 12.5 lb. The filters operate from 12 to 15 VDC at a maximum of 3A, with an option of 115 VAC available upon request.

These units can be operated either from the optional front panel control via a 16-button numeric keypad and LED display, or via remote control through one of the filter’s available serial communications interfaces (RS-232, RS-432, and RS-485), Ethernet, USB, or GPIB. The remote control ability can be useful when the notch or bandpass filter precedes the input of a multi-channel receiver that is located away from the operator. Notch or bandpass center frequencies can be changed with simple commands. This is a much more convenient approach than creating a remotely controlled switched bank of discrete filters that would not afford the versatility of the DCVNF and DCVF Series, which can be tuned to any center frequency within their octave range.

A DCVF Series tunable bandpass filter can be used to test a receiver to verify its calibration at a specific frequency or range of frequencies. The filter is connected between the transmission line from the antenna and the input of the receiver, and is tuned to a specific frequency, allowing reception only on a single channel. Signals outside the passband are rejected. The passband is specified when the DCVNF and DCVF Series unit is purchased. A DCVNF Series tunable notch filter can be used for harmonic testing, attenuating the carrier frequency so that only the harmonics are present.

A bandpass version of the DCVF Series tunable filters can also be useful when making measurements at co-located transmitter sites, where there are many emitters. Each emitter contributes interference that makes it difficult to extract the desired signal from the clutter. In this case, the DCVF Series filter can be used to attenuate potentially interfering signals while passing only the one desired.

One of the important attributes of the DCVNF and DCVF Series is the ability to eliminate the hysteresis (or backlash) that occurs when a filter is tuned from one frequency to another. The DCVNF and DCVF Series filters return to exactly the same position because the stepper motor and optical encoder always tune from the same direction. When the filter is initially powered-on, it starts at zero and recalibrates from that point, then returns to the last known valid point. As a result, there is no need to calibrate the DCVNF and DCVF Series filters. However, organizations that require NIST-level traceability can avail themselves of Trilithic’s calibration service.

While most DCVNF and DCVF Series applications focus on the receive side of a system, the units can be used in low-power transmit applications as well. The notch filter version of the DCVNF Series can handle up to 10 W CW, and the bandpass version up to 50 W CW. When used in the transmission path, a DCVNF Series notch filter allows specific frequencies to be significantly attenuated to evaluate component or subsystem performance. Conversely, the bandpass configuration allows a specific band of interest to be passed with minimal attenuation, while all other frequencies are rejected by up to 65 dB.

DCVNF Series tunable notch filters have 40 dB bandwidths of at least 300 kHz, defined as the frequencies on each side of the notch at which the 40 dB rejection is achieved. The actual depth of notch at the filter’s center frequency is generally much deeper, typically 70 or 80 dB. The DCVF Series bandpass filters can be specified with a standard 3 dB bandwidth of 5% and 30:3 dB shape factors of 3.5:1 for a three-section filter and 2.2:1 for a five-section filter. Other percentage bandwidths are available upon request.

Various characteristics of both the notch and bandpass filters can be tailored to specific applications. For example, 50 Ohm or 75 Ohm input and output impedances can be chosen, the 3 dB and 40 dB bandwidths of the notch filter can be varied from the standard specification, and the required shape factor can be specified. Standard resolution is 1 MHz, and values from 500 kHz to 2 MHz can also be chosen. Power handling ability, resistance to shock and vibration, connectors, power supply requirements, physical configuration, and front-panel control can be customized as well.

About Trilithic
Trilithic, Inc. is an industry leader providing telecommunications solutions for major broadband and RF and microwave markets around the world. Comprised of three major divisions: Broadband Instruments and Systems offers test, analysis and quality management solutions for the major cable television systems around the world. The RF and Microwave Components Division provides various components and custom subsystems for companies specializing in cellular, military and other wireless applications. The E.A.S. Division is a leading supplier of government-mandated Emergency Alert Systems used by HFC service providers in the United States.

As an ISO 9001:2000 certified company with over 40 years of collective expertise in engineering and custom assembly, Trilithic is dedicated to providing quality products, services and communications solutions that meet and exceed their customers’ expectations.

Trilithic, Inc.
www.trilithic.com
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