Wireless Microphone

 

Probably the most basic transmitter is a simple wireless microphone. This device may be used to connect a microphone to an audio or PA system without the interven-ing cable, which is a hazard in many situations. The wireless mike is a simple transmitter that acts as a one-way radio link to a nearby FM receiver. The output of the FM receiver then feeds the audio or PA system, replacing the wired microphone. Professional-grade mikes of this sort use a crystal-controlled receiver and transmitter operating outside the FM broadcast band at frequencies specifically intended for this

service. (Frequencies around 170 MHz are commonly used for this purpose.)A typical circuit of a wireless microphone is shown in Figure 3-1. It consists of an audio amplifier that feeds audio into the bias network of a free-running oscillator cir-cuit operating in the FM broadcast band. An electret microphone feeds audio into audio amplifier stage Q1. R1 biases the mike and may be varied to suit the mike used. This value is normally specified in the manufacturer’s data sheet, but 4.7 K is a good “generic” value if no data are available for the microphone. Audio is coupled via C2 to the base of Q1, which is biased by R2, R3, and R4 to about 4 volts and 0.5 milliamps. A low-noise, high-gain audio transistor, such as the 2N3565, should be used, but most high-gain, low-current transistors of this nature should work fine. Amplified audio at the collector is coupled through the RC network C3 and R5 to the base of oscillator Q2. Q2 acts as a grounded base oscillator, with feedback provided by C8. R5 and R6 provide starting bias for the oscillator transistor, and R7 provides emitter bias.

A VHF transistor, such as a 2N3563, 2N5179, or MPSH10, can be used for the oscillator, and any good 500-MHz or better NPN transistor should work, although the modulation characteristics may vary somewhat. Transistors with larger geometries or lower frequency ratings tend to have larger capacitances, with possibly somewhat better modulation capabilities, so do not think that a higher-frequency transistor than you really need will perform better. Tank circuit L1 C6, along with stray circuit and transistor collector-to-base capacitance, determines the frequency of oscillation. The collector-to-base capacitance is a function of collector-base voltage, and this voltage is modulated by the applied audio from R5, causing directfrequency modulation of the oscillator frequency. Because the oscillator power out-put also varies with collector voltage, some AM component will also be present, but this creates little harm in this application and is the price paid for simplicity. L1 is

typically tapped at 10–30 percent total turns from the RF ground end (Vcc rail or C9 in this case). The tap should be as close to ground as possible consistent with good signal output because the closer the tap is to the collector, the more effect the antenna will have on pulling the transmitter frequency. This effect is undesirable and can make the transmitter difficult to set on frequency. C6 is typically a 5- or 7.5-mm polyethylene or Teflon trimmer and is used to set the oscillator frequency. Its value is typically 3–5 pf per meter of operating wavelength.Because the FM broadcast band is approximately a 3-meter wavelength (100 MHz), C6 would be approximatley 9–15 pf. A 10 pf capacitance would be about right because the transistor and stray capacitances will be approximately 2–5 pf. Alternately, C6 can be made fixed and L1 varied via a slug or by stretching and squeezing turns, but this technique might be awkward for some applications, so the variable capacitor may prove to be a more practical method of frequency setting. The antenna is usually made about one-tenth of a wavelength at the operating frequency,

and at 100 MHz, is about 30 cm (approximatley 1 foot) long, but length also depends on application, mechanical constraints, allowable signal strength for meeting legal or FCC regulations where applicable, and the transmitting range desired. C7 is used as a DC-blocking capacitor and is generally around the same value as the tank-tuning capacitor. These values are only rules of thumb and provide working values for components, but final values should be calculated for optimum performance or determined empirically on working models.