Remotely-tunable Notch and Bandpass Filters for Evaluating System Performance
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 notch or bandpass frequencies. The alternative is to have available a large number of filters for discrete frequencies that must be manually swapped in and out, which is inconvenient, expensive and cumbersome. The DCnVNF series digitally-controlled tunable notch and DCnVF digitally-controlled tunable bandpass filters provide a tunable solution that allows a reject band or passband to be placed 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 interface.
The DCnVF series filters are microprocessor-controlled, precision filters that are designed for use in bench-top applications as well as in the field. They can be ruggedly built, using shock-mounted components that allow them to withstand a vibration of 2 G and shock of 5 G (non-operating). A typical three-section model measures 10 in. wide x 3.4 in. high x 14 in. long, and weighs approximately 12.5 lb. The filters operate from 11 to 17 VDC at a maximum of 3 A.
Standard models in the DCnVF series cover 100 to 200 MHz, 200 to 400 MHz and 250 to 500 MHz. Other notch filter models are available for frequencies up to 2 GHz, and the bandpass versions operate to 4 GHz. The standard step size within these ranges is 1 MHz with a typical accuracy of ±300 kHz. Acquisition time is less than 10 s, which represents a frequency change from one end of the band to the other. In actual operation, acquisition time is typically much less than 1 s, since the frequency changes tend to be far less than the full filter tuning range.
The units can be operated either from the front panel via an optional 16-key numeric keypad and LED display, or via remote control through the filter’s serial communications interfaces (RS-232, RS-432, RS-485) or GPIB port. 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 frequencies can be changed with simple commands. This is a much more convenient approach than creating a remote-controllable switched bank of discrete filters that would not afford the versatility of the DCnVF series, where a single filter can cover an entire octave range. Table 1 lists typical DCnVNF series tunable notch filter specifications.
A DCnVF 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 tuned to a specific frequency, allowing reception only on a single channel. Signals outside the passband are rejected. The passband is specified when the DCnVF series unit is purchased. A DCnVNF series tunable notch filter can be used for harmonics testing by attenuating the carrier frequency so that only the harmonics are present.
A bandpass version of the DCnVF series tunable filters can also be useful when making measurements at co-located transmitter sites, where there are many RF emitters. Each emitter contributes interference that makes it difficult to extract the desired signal from the clutter. In this case, the DCnVF series filter can be used to attenuate potentially interfering signals while passing only the one desired.
One of the important attributes of the DCnVF series is its ability to eliminate the hysteresis that occurs when a filter is tuned from one frequency to another. The DCnVF 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 always starts at zero and recalibrates from that point. As a result, there is no need to calibrate the DCnVF series filters.
While most DCnVF 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 DCnVF series can handle up to 10 W CW, and the bandpass version up to 50 W CW. When used in the transmission path, a DCnVNF series notch filter allows specific frequencies to be significantly attenuated to evaluate component or subsystem performance. In addition, the bandpass configuration allows a specific band of interest to be passed with minimal attenuation, while rejecting a certain band of frequencies, depending on the number of sections.
DCnVNF 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 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 DCnVF series bandpass filters can be specified with a 3 dB bandwidth between 1 and 10 percent. Constant bandwidth designs are also available. Table 2 lists typical DCnVF series tunable bandpass filter specifications.
Various characteristics of both the notch and bandpass filters can be tailored to specific applications. For example, 50 or 75 W input and output impedances can be chosen, the 3 and 40 dB bandwidths of the notch filter can be varied from the standard specification, and three- or five-section responses can be specified. The standard tuning 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.