The desensitization problem is a severely reduced receiver sensitivity caused by the presence of a strong local signal. Desensitization can occur across a wide frequency spectrum, not only in the band where the offending signal is located. The strong offending signal jnight not be heard, except when the receiver is tuned to or near its frequency.
It the signal is strong enough to drive the RF amplifier of the receiver into non-linearity then it might generate harmonics (integer multiples) of the strong signal. For example, if you live close to a 780-kHz AM broadcast band signal and the receiver overloads from that signal then you might be able to pick up the signal at 2/ (1560 kHz), 3f (2340 kHz), 4/(3120 kHz), and so on up throughout the shortwave bands. Notice that this problem is not the fault of the radio station (which must be essentially clean of harmonics because of FCC regulations) but rather is an inappropriate response on the part of your receiver.
This problem occurs when two signals of different frequencies (for example, F) and jFs) mix together in a nonlinear element to produce a third frequency (F^). The third signal is sometimes called a phantom signal. The frequencies involved might be (and probably are) the assigned fundamental frequency of a legitimate station or their harmonics. The nonlinearity can be caused by receiver overload, improper receiver design, or some other source (legend has it that rusted downspouts and corroded antenna connections can serve as a nonlinear PN-junction). As they say in the new science of Chaos Theory, ., nonlinearity can arise throughout nature in subtle ways."
The possible new frequency (Fi) will be found from F:i - mF} ± nF>, where m and n are integers, although not all possibilities are likely to occur in any given situation. Suppose one local ham is operating on 10,120 kHz (in the new 30-m band) and another is operating on 21,390 kHz (in the 15-m band). Both stations are operating normally, but at least one is close enough to overload your receiver and produce nonlinearity in the RF amplifier. When the second harmonic of 10 J 20 kHz (i.e., 20,240 kHz) combines with 21,390 kHz, the result is a third frequency at 21,3iH) kHz - 20,240 kHz =< 1,150 kHz—right in the middle of the AM broadcast baud.
With all of the signals that might exist in your locality, an extremely large number of possible *intermod" combinations can arise. A hill in my hometown is right in the middle of a densely populated residential neighborhood. On that hill are two 50-kW FM broadcast stations; a 5-kW AM broadcast station; scores of VHF and UHF landmobile radio base stations or repeater transmitters; and an assortment of paging systems, ham operators, medical telemetry systems, and ihe microwave towers of an AT&T long-lines relay station—all within a city block or two. Only a few radio receivers woVk well unassisted in that neighborhood!
There are two approaches to overcoming these problems. First, either reject or somehow selectively attenuate the offending signal (or one of the signals, in the case of an intermod problem). This is done using a wavetrap. Second, add a passive preselector vo the front end of the radio receiver between the antenna line and the antenna input of the receiver.
Figure 14-1 shows three wavetrap circuits that can be used from the AM band
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14-1 (.A) Series-parallel wave trap, (B) voltage tunable filter/trap, and (C) VHF/UHF half-wave shorted stub trap.
up through VHF/liHF frequencies. The circuit in Fig. 14-1A is an LC wavetrap based on inductors and capacitors. Two LC resonant circuits are shown in Fig. 14-J A. tn series with the signal path is a parallel resonant circuit (LJC:). These circuits have a very high impedance at the resonant frequency so they will attenuate signals of that frequency trying to pass through the line between Jt and At the same time, the impedance at all other frequencies is high, so those frequencies will f>ass easily from Ji to Jj. Shunted across ihe line on both ends of the signal path art1 a pair of series resonant circuits. These LC circuits have a low impedance at the resonant frequency and a high impedance at all other frequencies so thev will shunt only the offending signals to ground.
The wavetrap of Fig. 14-1A can be built using either fixed inductors and variable capacitors or vice versa, it can be built either for one fixed frequency or as a variable wavetrap that can t>e tuned from a front-panel knob to attenuate the offending signal.
Figure 14-1B shows the circuit of a sinular type of wavetrap, but it is built wil h voltage-variable capacitance diodes, also called l aractws. These diodes prodticr n capacitance across their terminals that is proportional to the applied reverse bias tuning voltage (Vr)< tn the circuit of Fig. 14-1B, only series resonant circuits are used,
Wa vet raps built from LC circuit elements are useful well into the VHF region, hi fact, many video and other electronics stores sell wavetraps. (Fig. 14-1 A) built especially for the FM broadcast band (88 to 108 MHz) because nearby FM broadcasting stations are frequent sources of interference to VHF television receivers.
.Another VHF/UHF wavetrap is shown in Fig. 14-10. This typ<* of wavetrap is called a half-wavetengtti shorted stub. One of the properties of a transmission line is that it will reflect the load impedance every ha If-wavelength along the line. When the end of the stub is shorted, therefore, a ' virtual" short-circuit will also appeal every itaif-waveiefigth along the line. This length is frequeucy-sensilive so the physical length of the line is a tuning factor for this wavetrap.
The length of the line is found from Lft = 402 V/Fsihz, where L is in feet and F is in megahertz. The Vterm is the velocity factor of the transmission line. For common coaxial lines, the value of [/ranges from 0.66 (polyethylene) to 0,82 (polyfoam). For example, assume that you need to eliminate the signal from a local FM broadcaster on 88.5 MHz. The shorted stub is made from ordinary coax (V - 0.60) and must. Iiavr a length of ((492)(0.66)/88,51 = 3.669 ft (or 44").
The half-wavelength shorted stub is connected in parallel vriih the antenna input on the receiver. In the case shown in Fig. 14-1C. the stub is collected to the receiver and antenna transmission line through a coaxial tee'' connector. Although UHF connectors are shown, the actual connector that you would select must match your receiver and antenna. Also, it is possible to use 30<i-ft twin lead transmission line, rather than coax, as long as that type of line is used on the receiver and antenna.
The other approach to solving the problem is to use a passive preselector abend of the receiver- Again, the preselector is inserted directly in the transmission line between the receiver antenna input connector and the antenna transmission lino. Figure 14-^ shows a typical circuit for this type of preselector. The tuning is controlled by C[A/L2 and i and are trimmed by C> and C.-i to permit tracking of the two LC circuits- The circuit should be built in a closed, shielded metal box.
Other interference 2S9
14-2 Passive tunable preselector circuit.
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