Low power NBFM transmitter

The transmitter is a crystal-con-12 trolled, battery-powered, one-chip narrowband frequency modulation v(NBFM) model for operation in the 27- m band, primarily as a wireless microphone*.

The circuit is an application of Motorola’s MC2833 single-chip VHF narrowband FM transmitter IC, designed to work in the 27-MHz band. The transmitter output power is about 10 mW (+10 dBm), which, owing to the expected low efficiency of the antenna used, will result in a typical effective radiated power (ERP) of less than 1 mW. The range of the transmitter, therefore, is limited to 10-20 meters (33-66 ft).

Low power NBFM transmitter Schematic diagram

The active circuitry contained in the MC2833 includes a microphone amplifier, a voltage controlled oscillator, and two auxiliary transistors which are used for frequency multiplication or RF amplification, depending on the desired output frequency.

Preset P1 is used to adjust the microphone gain, and preset P2 to adjust the deviation. Remember, the transmitter will produce NBFM only, with a maximum deviation of 5 kHz. This means that a narrow-band receiver (such as a typical 27 MHz CB unit) is required for sufficient audio output.

The quartz crystal, X1, resonates in fundamental mode (here, 9 MHz), calibrated for parallel resonance with a 32 pF load. The final output frequency is generated by frequency multiplication (here, x3) within the MC2833.

Construction of the transmitter follows the rules of RF design: keep all com-
ponent leads as short as possible, fit the screening as indicated on the printed circuit board, and do not use an IC socket for 1C1.
The transmitter is fairly simple to adjust: simply peak the three trimmers, Cg. Cg and C18. for maximum output power delivered to a 50 S/ dummy load. Alternatively, connect the antenna and an oscilloscope to the transmitter output, and adjust the trimmers for maximum RF voltage. Next, listen to the transmitted signal on a 27 MHz receiver, and adjust the two presets until the best possible modulation is achieved. Do not set the microphone gain too high, since this will easily
cause clipping.

Low power NBFM transmitter Schematic diagram

If the transmitter is used as a wireless microphone, the antenna will typically be a piece of flexible wire with a length of about 1 m. The transmitter draws a current of about 7 mA, so it is good practice to keep an eye on the state of the battery.
* This transmitter is not licensable as a wireless microphone in the UK.
This transmitter is not licensable as a wireless microphone in the UK.

Low power NBFM transmitter Schematic diagram
Parts list
Resistors:
R1 , R2 = 100 kΩ
R3 = 2.7 kΩ
R4 = 47 Ω
R5 = 470 kΩ
R6= 1 kΩ
R7 = 390 kΩ
R8 = 1.5 kΩ
P1. P2 =1O0 kΩ  preset, H
Capacitors:
All fixed capacitors are ceramic, unless otherwise indicated
C1= 4.7 nF
C2. C3, C16 = 10 nF
C4 = 1 pF MKT
C5 = 2.2 nF
C6, C12 = 56 pF
C7 = 82 pF
C8, C9, C18 = 60 pF foil trimmer
C10 = 220 pF
C11 = 1 nF
C14, C15 = 39 pF
C17 = 47 pF
C19 = 1 pF, tantalum
C20 = 47 nF
Inductors:
L1, L2= 1 uH
L3, L4 = 330 nH
L5 = 2.2 uH
Integrated circuits:
IC1 = MC2833P (Motorola)
Miscellaneous:
K1 = BNC socket
X1 = crystal. 27.005 MHz
MIC1 = Electret microphone

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