The Axino low power FM transmitter
This is a description of the development of a prototype low power FM transmitter. It has been used for short term lease within New Zealand for one-off events, shows, schools, open days and church broadcasts for the hard of hearing. It was even useful to a low power FM (LPFM) broadcaster who needed cover while their own equipment was under repair.
The transmitter is operated under the N.Z General User Licence for low-power FM broadcasting, and is set to emit less than 1 watt RF power. I do not generally hire this out now but this might still be possible upon enquiry.
Transmitter Features
INPUT MODES
- Analogue audio unbalanced (2*RCA)
- Analogue audio balanced (2*XLR male)
- Digital audio SPIDF coax (RCA)
- Digital audio SPIDF optical (toslink)
- PC USB (driverless)
- Internal test tone or wav audio file on microSD
GENERAL
- RF Output: N-type (f) 50 ohm.
- AC 230V (max 85VA) or DC 12V-13.8V (~2.5A) reverse protected.
- Frequency range: 87.6MHz to 108MHz in 50kHz steps.
- Dimensions: Width 420mm; Depth incl connectors 370mm; Height 120mm.
- Pre-emphasis: 50us.
- Stereo OR Mono operation
- Over-deviation limiter and clipper.
- External MPX input available.
SPECIFICATIONS
- RF OUTPUT: 0.5W to 5W (internally set).
- Analogue audio sensitivity: 0dBu (0.775Vrms) for 75kHz deviation.
- Pilot frequency: 19kHz within 2Hz.
- Carrier frequency accuracy better than +/-2kHz.
- RF harmonics: >56dB below carrier.
Limitations:
- There are no external frequency or power controls.
- The transmitter has no microphone input and accepts line levels only.
- Although crystal controlled, the transmitter cannot be synchronised to an external reference.
- There is no RDS encoder although external MPX can be fed in via an MPX I/O link. 19kHz reference frequency output is available.
- The internal limiter acts only to prevent gross over-deviation and is not intended to be a compressor or audio processor.
