Low-priced shortwave receivers often suffer from performance problems that are a direct result of the trade-offs that the manufacturers make to produce a low-cost model. In addition, older receivers often suffer the same problems, as do many homebrew radio receiver designs. Chief problems are sensitivity, selectivity, and image response.
Sensitivity is a measure of the receiver's abibty to pick-up weak signals. Part of the cause of poor sensitivity is low-gain in the front end of the radio receiver, although the IF amplifier contributes most of the gain.
Selectivity is a measure of the ability of the receiver to (a) separate two closely spaced signals and (b) reject unwanted signals that are not on or near the desired frequency being tuned. The selectivity provided by a preselector is minimal for very closely spaced signals (that is the job of the IF selectivity in a receiver), but it is used for reducing the effects (e.g., input overloading) of large local signals . , . so fits the second half of the definition.
Image response affects only superheterodyne receivers (which most are) and is an inappropriate response to a signal at twice the receiver IF frequency that the receiver is tuned to. A superhet receiver converts the signal frequency (RF) to an intermediate frequency (IF) by mixing it with a local oscillator (LO) signal generated inside the receiver. The IF can be either the sum or difference between the LO and RF (i.e., LX3 + RF or LO - RF), but in most older receivers and nearly all low-cost, receivers it is the difference (LO - RF). The problem is that there are always two frequencies that meet "difference" criteria: LO — RF and an image frequency (F,) that is LO + IF. Thus, both F-t - LO and LO - RF are equal to the IF frequency. If the image frequency gets through the radio's front-end tuning lo the mixer, it will appear in the output as a valid signal.
A cure for all of these is a little circuit called an active preselector, A preselector can be either active or passive. In both cases, however, the preselector includes an inductor/capacitor (LC) resonant circuit that is tuned to the frequency that the reaver is tuned to. The preselector is connected between the antenna and the receiver antenna input connector (Fig. 7-1A). Therefore, it adds a little more selectivity to the Front end of the radio to help discriminate against unwanted signals. The inset to Fig. 7-1A shows the normal switching used in preselectors to allow it to be cut in or out of the circuit.
The difference between the active and passive designs is that the active design contains an RF amplifier stage, but the passive design does not. Thus, the active preselector also deals with the sensitivity problem of the receiver. This chapter looks at one passive and several active RF preselector circuits that you can build and adapt, to your own needs.
The passive preselector circuit is shown in Fig, 7-IB. This circuit contains only inductors and capacitors, with each tank circuit shielded from each other. The individual tuned circuits are coupled using the common reactance method (Le,, capacitors C2 and cany RF from one circuit to another). The input coil and the output
7-1 (A) Preselector is connected as an RF amplifier ahead of the receiver and (B) passive preselector circuit.
coil contain link coupling to transfer the RF signal in and out of the circuit respectively.
The tuning capacitors used in the passive preselector should he ganged to a common shaft, A proper unit would have three identical capacitors on the same shaft, with trimmer capacitors on each section (which allows the sections to be aligned to track each other). The trimmer capacitors are adjusted to track the high end of the band, and the slug-tuned inductors are adjusted for low-end tracking.
The active preselector circuits are based on either of two devices: the MPF-103 junction field-effect transistor (JFET) and the 40673 metal-oxide semiconductor field-effect transistor (MOSFET), Both of these devices are easily available from both mailorder sources and from local distributor replacement lines (for example, the MPF-IG2 is the NTE-312 and the 40673 is the NTE-222). These transistors were selected be-cause they are both easily obtained and are well-behaved into the VHF region.
Preselectors should be built inside shielded metal boxes in order to prevent RF leakage around the device. Select boxes that are either die-cast or are made of sheet metal and have an overlapping lip. Do not use a low-cost tab-fit sheet-metal box.
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