Choosing An Operating Frequency - Ramsey Electronics FM25A Assembly And Instruction Manual

CHOOSING AN OPERATING FREQUENCY

(A) It really is NOT sufficient to just "check" the FM band for an empty fre-
quency, using the FM portable radio closest at hand. It is your responsibility to
carefully research what FM stations can be listened to with a good system
within the transmitting range of your FM25A. This is especially important in the
low end of the FM broadcast band (88-92 MHz), where there are numerous me-
dium power National Public Radio stations. You may not be aware of these sta-
tions but your neighbors may be receiving them, using a good receiver and out-
door 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" re-
ceivers, whether portable, mobile or hi-fi, are designed to tune in 200 KHz in-
crements 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 opera-
tion will not cause interference to broadcast reception. Please study Appendix
A of this manual before using your FM25A.
ADJUSTING YOUR FM25A 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, program
DIP switches S3, S4, and S5 for the frequency desired and then momentarily
depress the 'Program' button. 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 S4 (1
MHz switch) is equal to 6 MHz (2+4=6, 6x1 MHz = 6 MHz), closing 4, 2 and 1
on S5 (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 between 88 and 108 MHz. To set the fre-
quency above 100 MHz, the S3 positions must add up to ten, you would close 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.
FM25A • 17