Signal generator for the AM and shortwave bands

The same general type of circuit can be used for the AM band (530 to 1610 kHz) or for the shortwave bands (1610 kHz to 30 MHz). The version shown in Fig- 10-7 uses a standard transformer that has a 217-(mH inductance in the primary winding, Tliis particular example uses a Toko coil from Digi-Key (Part No. TK-I903) in conjunction with a 365-pF broadcast variable capacitor to cover the AM broadcast band. For other frequencies:

or if F and L are known but C is unknown:

or if F and C are known but L is unknown:

Bekende Circuits

In all three cases, F is in hertz, C is in farads, and L is in heniys. For frequencies at which there is not an easily available coil, select a toroidal core and wind a coil yourself. In either case, allow about 20 pF for stray capacitances in the circuit (the actual amount might be moTe or less, depending on the layout).

A VHF voltage-tuned oscillator circuit is shown in Fig. 10-8A. I've used this circuit at frequencies from 20 to 150 MHz. The circuit uses a feedback capacitor (Ca) across the collector-emitter circuit of the transistor. This capacitor is critical, and the circuit probably wont oscillate without it. The tuning network consists of inductor, Limt dc-bloc king capacitor, Ci; and varactor, Dt, Varactors are voltage-variable capacitor diodes. The inductor is wound on either a VHF toroidal coil or an air form about %* in diameter. If no. 20 or so solid wire is used for Li then the coil will be self-supporting after it is removed from the coil form.

The VHF oscillator of Fig. 10-8A is tuned with a dc voltage (V,) applied through a 150-kii resistor (if4). This voltage is positive in order to reverse bias Dj. A problem with this circuit is that the frequency is a nonlinear function of frequency because of the characteristics of D\. Figure 10-8B shows a somewhat more linear version of this same circuit. In this case, the dc-blocking capacitor is replaced with a second varactor diode that is identical to the first. The total capacitance of the combination is half the capacitance of one diode, but the voltage-vs-frequency characteristic is more linear.

Both of these circuit variations can be fixed-tuned with a dc voltage, variable-tuned through a potentiometer, or swept across a band of frequencies with a sine-wave (FM) or sawtooth signal. In the case of the sawtooth, use a 10- to 60-Hz signal.

Signal generator/or the AM and $!wrtu>ave ixtnrts

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