Silence from the sands
Quartz creates quiet: A synopsis of how crystal filters can be used to protect VHF receivers from adjacent and spurious radio interference.
It is as marvelous to contemplate the thousands of years man trod the globe, blind to the riches right under his feet, as it to list the variety of advances coming from those sands.
But before the births of the integrated circuit and Silicon Valley, there was the quartz crystal.
In the long history of modifying natural materials, the discovery of piezoelectricity (the electric charge generated by a crystal under proper compression) occurred just over a century ago. It has only been within a single lifetime (spurred by electronics research in World War II) that improved crystal manufacturing techniques have allowed true mass-production for radio work. (An excellent history of the U.S. quartz crystal industry written by Virgil Bottom, available on the Internet, is included in the references section.) “Mass-produced” may be a misnomer because crystals are as individual as snowflakes. These radio-assistive flecks are, in a real sense, man-made sands because once raw quartz of an electronic grade is located, cutting, lapping, polishing, etching, plating, aging and sealing processes still have to be performed under highly controlled conditions before a crystal ends up as part of a receiver chain or other device.
About 50 to 60 crystal manufacturers are left in the United States. Space does not permit listing them all, but a short list of prominent ones for the mobile radio industry is given in the box on page 45.
Filtering out interference The proliferation of radio systems at VHF highband has increased the incidence of adjacent channel interference, particularly for public safety agencies, which account for a large portion of highband use. One of the uses of filters is between the antenna and the receiver to reduce adjacent channel signals. Crystal filters are used in devices or situations where cavity filters are too large for practical purposes. They are also used when cavities have too broad a bandwidth to resolve interference caused by signals close to the desired channel frequency.
A crystal filter can be configured as a high-pass or as a low-pass notch filter (band-stop-designed to attenuate a specific frequency band) with low insertion loss. With the addition of a second crystal, a symmetrical bandpass filter can be created. With the proper equipment, this can be manufactured on the benchtop. A homemade filter of this type can reduce by 25dB a VHF highband signal that is only 15kHz from the desired signal, with only 1.0dB of insertion loss. (See Buller under references.)
Crystal filters can approximate a rectangular passband characteristic at the receiver’s intermediate frequency (IF), restricting detection to the part of the signal containing the wanted information. In the path of a low-level signal, a properly terminated crystal filter passes only the fundamental frequency and a small segment of the frequencies bracketing the fundamental.
Crystal filters can be manufactured for frequencies from 10kHz to 300MHz, with bandwidths ranging from 0.001% to 10% of the fundamental frequency. A receiver crystal filter’s bandwidth usually falls between 0.02% and 0.05% of the crystal frequency, depending on what attributes were designed into it in the manufacturing process. Several crystals of the same frequency can be cascaded to provide a narrower bandpass.
Crystal filters provide higher Q (economy of energy loss resulting from resistance) in narrow channel-spacing situations. There are trade-offs. Although crystal filters are convenient, they generally have higher attenuation and insertion loss than cavity filters. The limiting factor on system sensitivity, however, is site noise level. Site noise can be caused by nearby transmitters, power lines or other ambient conditions. (Antenna noise figure is determined by the level of ambient site noise.)
Dealing with insertion loss There are ways to minimize the impact of insertion loss when using crystal filters. Placing an amplifier between the filter and the receiver, or between the antenna and the filter, reduces system noise figure. At low antenna-noise figures, the amplifier is better placed closer to the antenna (such as with tower-top amplifiers). As site noise increases, and takes the antenna noise figure up with it, the relative position of the amplifier in the receiver chain diminishes in importance. An amplifier placed ahead of the crystal filter is more susceptible to interference. Strong, off-channel signals may increase enough to cause receiver desense, and intermod becomes a real possibility. Noise and interference levels must be factored into receiver system chain design. The setup that produces the best system noise figure may not be practical when certain interfering signals are present. A decision to add a crystal filter is probably spurred by a site already thick withnoise and interference.
Different filters have varying amounts of insertion loss. As a rough guide, the lower the insertion loss, the fewer the number of poles in the filter, but fewer poles means poorer selectivity. Two-pole monolithic filters can be cascaded to create four, six, and eight-pole filter responses by adding coupler capacitors between two-pole sections.
Matching the crystal to the system The choice to use a crystal filter should not be based solely on insertion loss, but should be made in the context of the whole receiver chain and the system noise figure, including the ambient site noise or antenna noise figure. System requirements for the crystal need to be specified. These specifications can include operating frequency, desired insertion loss, attenuation, passband width, spurious response, terminating impedance, operating temperature range, resonance factors, fixed or pullable placement and holder size.
Trade-offs have to be considered among interacting factors, including passband width, stopband width, selectivity, number of poles, VSWR, etc. When specifying crystal filters for a receiver application, it is important to discuss your needs and goals with the engineering staff of the manufacturer.
Selected references Buller, Patrick E., “Build a Crystal Filter to Solve VHF Interference,” Mobile Radio Technology, May 1995. Bottom, Virgil, “History of the Quartz Crystal Industry in the USA,” Oak Frequency Control, www.ofc.com/history/vbottom.html. Kinley, Harold, “Technically Speaking: Site Noise and Its Effects,” Mobile Radio Technology, May 1995. Kinley, Harold, “Technically Speaking: Radio Frequency Filters,” Mobile Radio Technology, November 1993. Kinley, Harold, “Technically Speaking: Using Crystal Filters at VHF High Band,” Mobile Radio Technology, June 1998. Ludvigson, David, “Servicing Pagers: Tales Crystal Filters Tell,” Mobile Radio Technology, October 1994. Singer, Edward, Land Mobile Radio Systems, Prentice-Hall, Englewood Cliffs, NJ, 1989.
