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Low Power NBFM Transmitter Schematic Circuit Diagram

Crystal-Controlled, Battery-Powered NBFM Transmitter

The transmitter functions as a crystal-controlled, battery-powered, one-chip narrowband frequency modulation (NBFM) model, specifically designed for operation in the 27m band, primarily serving as a wireless microphone.

Application of Motorola’s MC2833 for VHF NBFM Transmitter

The circuit represents an implementation of Motorola’s MC2833 single-chip VHF narrowband FM transmitter IC, tailored to operate within the 27-MHz band. With an output power of approximately 10 mW (+10 dBm), the expected low efficiency of the antenna used leads to a typical effective radiated power (ERP) of less than 1 mW. Consequently, the transmitter’s range is confined to 10–20 meters (33–66 feet).

Low power NBFM transmitter Schematic diagram

Active Circuitry in MC2833 for NBFM Transmitter

Contained within the MC2833 are active components comprising a microphone amplifier, a voltage-controlled oscillator, and two auxiliary transistors. These auxiliary transistors serve different purposes: frequency multiplication or RF amplification, depending on the desired output frequency.

Adjustments and Specifications of the Transmitter

Preset P1 facilitates the adjustment of microphone gain, while preset P2 is employed to modify the deviation. It is crucial to note that the transmitter exclusively produces narrowband frequency modulation (NBFM) with a maximum deviation of 5 kHz, necessitating a narrow-band receiver, like a typical 27 MHz CB unit, for adequate audio output. The quartz crystal, X1, operates in fundamental mode at 9 MHz, calibrated for parallel resonance with a 32 pF load. The MC2833 achieves the final output frequency through frequency multiplication (x3).

Construction and Adjustment of the Transmitter

Adhering to RF design principles during construction involves keeping component leads as short as possible, following the specified screening on the printed circuit board, and avoiding the use of an IC socket for 1C1. The adjustment process is straightforward: peak the three trimmers, Cg, Cg, and C18, for maximum output power to a 50 Ω dummy load. Alternatively, connect the antenna and an oscilloscope to the transmitter output, adjusting the trimmers for maximum RF voltage. Fine-tune by listening to the transmitted signal on a 27 MHz receiver and adjusting the two presets for optimal modulation. Caution is advised not to set the microphone gain too high, as it can easily lead to 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 licensed as a wireless microphone in the UK.

Low power NBFM transmitter Schematic diagram

Parts list

  • 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
  • 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, and 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
  • L1, L2= 1 uH
  • L3, L4 = 330 nH
  • L5 = 2.2 uH
Integrated circuits:
  • IC1 = MC2833P (Motorola)
  • K1 = BNC socket
  • X1 = crystal. 27.005 MHz
  • MIC1 = Electret microphone

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