FM25B • 20
stations but your neighbors may be receiving them, using a good receiver and
outdoor antenna. Interfering with such reception is a direct violation of federal
law. The most reliable way of finding a truly open frequency on the FM band is
to check the band with a very good FM receiving system using an external an-
tenna. If you do not have access to such a radio, most modern car radios (with
exterior antenna) are very sensitive and usable to help you know what stations
your neighbors really can be receiving on a particular frequency.
(B) In choosing an operating frequency, remember that most "digital-tuning"
receivers, whether portable, mobile or hi-fi, are designed to tune in 200 KHz
increments and therefore might not receive well a signal operating between
these pre-tuned standard broadcasting frequencies. In order to comply with
Part 15 of FCC regulations, it is your responsibility to determine carefully that
your operation will not cause interference to broadcast reception. Please study
Appendix A of this manual before using your FM25B.
ADJUSTING YOUR FM25B TRANSMITTER
Keep all tests very brief until you have carefully chosen an open operating fre-
quency in the FM broadcast band.
1. Transmitting Frequency
After finding a suitable "open" frequency in the 88-108 MHz FM band, pro-
gram DIP switches S3, S2, and S1 for the frequency desired. The DIP
switches are programmed in BCD, a form of binary that is very easy to use.
Look at your circuit board or parts layout diagram; you’ll see the numbers 1, 2,
4, 8 printed next to each DIP switch. On these switches you add up the closed
positions to make to make any number between 0 and 9. For example: closing
position 1 and 8 on S3 (10 MHz switch) is equal to 90 MHz (1+8=9, 9x10 MHz
= 90 MHz), closing 2 and 4 on S2 (1MHz switch) is equal to 6 MHz (2+4=6,
6x1 MHz = 6 MHz), closing 4, 2 and 1 on S1 (0.1 MHz switch) is equal to 0.7
MHz (4+2+1=7, 7x .1 MHz = .7 MHz). This makes the final frequency equal to
96.7 MHz (90+6+.7=96.7). These switches may be set to any frequency be-
tween 88 and 108 MHz. To set the frequency above 100 MHz, the S3 posi-
tions must add up to ten. To do this you would close positions 8 and 2
(8+2=10). Any switch setting greater than 9, with the exception of S3, is invalid
and will be read as 0.
After setting your DIP switches, recheck your ‘math’ and listen on a nearby FM
radio for the carrier frequency. No audio input should be applied while making
this first adjustment. You can simply listen for a "quieting" on an unused chan-
nel instead of the normal background "static" you would normally find. If you
don’t hear anything yet, don’t worry. We need to adjust L1 for the correct tun-
ing voltage at TP1 to ensuring proper operation across the band. TP1 is lo-
cated just off the exposed lead of R21 that is closest to the outside of the
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