Wireless Microphone: Audio in the ISM band 4

Published:2011/8/19 2:15:00 Author:Amy From:SeekIC

Ton Giesberts

Interface

Thanks to these complex modules that include almost all the necessary elec­tronics, what remains is straightfor­ward. Figure 4 shows the two parts of the circuit. In the transmitter we can either connect a microphone or any other desired audio source with a max­imum output level of -15 dBV. In most cases, however, an electrets micro­phone will be used. There is a small offset voltage present at the AF input with PI adjusted to maximum: in our prototype we measured about 0.15 V If a dynamic microphone is to be con­nected, it is essential to add a coupling capacitor. Alternatively, omit Rl and connect the dynamic microphone In place of the electret microphone. PI can be used to attenuate microphone signals that are too high, so that the radio module is not overdriven.



The receiver circuit is slightly less sim­ple. The receiver module has two out­puts, one for the signal itself and one which indicates whether the signal strength is adequate or whether the muting circuit has been triggered. Since we have plenty of power to spare at the receiver (battery operation here is not essential) we can afford an extra indicator in the form of LED Dl.

In order to amplify the output of the receiver module (which, at 10 kΩ, is not exactly low impedance), we have added a buffer amplifier. This is a classi­cal non-inverting AC amplifier built around a rail-to-rail opamp which can operate from a voltage of between 2.7 V and 12 V, almost the same range as for the module. We have shown a supply voltage of 5 V. although 3 V or 12 V would do just as well. If a different (non-rail-to-rail) opamp were used, a supply voltage of 5 V would be required- Many opamps will only operate correctly with a sympatric power supply of ±5 V or with an asymmetric 10 V supply. A fur­ther advantage of the TS921 used he>e is its high output drive capability: it can directly drive headphones or even two 3211 headphone transducers wired in parallel although in this case C6 should be replaced by a 100 pF 10 V type. The 47 n output resistor protects the opamp from the inductive load of a shielded cable and from short circuits. Trimmer potentiometer P2 allows the gain to lie adjusted from unity (P2 at minimum resistance) to 10 dB (P2 at maximum resistance. C6 removes any DC compo­nent from the output and R7 ensures that there is always a load at the out­put. Since the opamp has asymmetrical supplies, a capacitor (C5) is also required in the feedback circuit. R3 and R4 set the operating point of die opamp at half the supply voltage. C7 and CB provide extra power supply decoupling. At higher supply voltages it is neces­sary to increase the current-limiting resistors for the low-current LEDs so that the current through them does not exceed about 2 mV

We have designed a two-part printed circuit board to accommodate the radio modules and the few external compo­nents (Figure 5). The layout is designed for optimum audio performance, The components should be fitted to the board, observing that the transmitter module can only be fitted to the copper side. An ordinary 3.5 mm jack socket provides the audio output.



All that remains are the antennas. In principle a stiff piece of wire with length 1/4 X (78 mm at 860 MHz) will do the job; mote professional antennas can be found on the Circuit Design website at http://www.cdt21.com/.

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