TX Techniques TX FM1, Technical Manual

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TX-FM1/S/R/A Technical Manual www.txtechniques.co.uk
5.7 19kHz, 38kHz and 57kHz Frequency Trim
Using a frequency meter, monitor Pin 7 of IC6 and adjust VC1 for 19,000Hz. This will
also, in turn, adjust the 38kHz and 57kHz subcarrier frequencies.
5.8 RDS Injection Level
The RDS injection level can be adjusted using VR5 on the “FM1-RDS” PCB.
5.9 RDS Subcarrier Null
The DSSC generation of the RDS signal requires the 57kHz carrier to be nulled, for
correct operation. VR1 on “FM1-RDS” PCB controls this setting, and requires the use
of a spectrum analyser connected to the front panel “AF” monitor BNC socket
5.10 MPX Output Level
The overall MPX level (ie. audio, pilot and subcarrier) can be altered using VR6 on
the “FM1-RDS” PCB. This setting requires the use of a modulation monitor
connected to the front panel “AF” monitor BNC socket
5.11 MPX Pilot Injection Level
The level of pilot present in the stereo broadcast can be varied, independently of the
overall MPX setting, by adjusting VR4 on the “FM1-RDS” PCB. This setting requires
the use of a modulation monitor connected to the front panel “AF” monitor BNC
socket
5.12 MPX Subcarrier Null
The DSSC generation of the stereo multiplex signal requires that the 38kHz carrier is
nulled, for correct operation. VR2 on “FM1-RDS” PCB controls this setting. This
setting requires the use of a spectrum analyser connected to the “AF” monitor BNC
socket.
5.13 MPX Separation Level
VR1 on “FM1-RDS” PCB controls stereo separation, which should be set using an
oscilloscope connected to the front panel “AF” monitor BNC socket and a 1kHz tone
connected to either the left or right audio input only. Setting the RDS injection and
MPX pilots levels to minimum will improve clarity of the displayed waveform.
Maximum separation is achieved when the waveform exhibits a flat baseline.
5.14 Composite Clipping Level
VR7 and VR8 on the “FM1-RDS” PCB adjust the amount of clipping applied to each
polarity of the MPX waveform. A digital storage oscilloscope is required for this
adjustment. Both trimmers should be adjusted for exact symmetry, and so that
overshoots (above and below the average limited/clipped audio levels) are removed.