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AF1 • 4
INTRODUCTION
Radio men know that the CW signal is the most reliable mode of
transmission when operating on the amateur radio bands. There are,
however, some inherent problems that exist in CW reception.
When listening to a carrier wave, or CW signal, our receiver is simply letting
us hear the difference between the local oscillator inside the radio and the
desired receive frequency. While this is the simplest form of reception (no
demodulation of the signal is required) it is also prone to receiving multiple
signals at one time making the desired carrier quite difficult to copy.
In an attempt to limit the number of carriers heard through the receiver one of
the easiest solutions is to filter the received audio to only allow a single CW
tone to be heard. That's where the Ramsey AF1 filter comes in to help.
The audio filter board creates an audio frequency bandpass filter to remove
the undesired tones. It also has a selectable bandwidth (the width of the
bandpass filter) of 100, 250, 500, or 750 Hz. The center frequency of this
filter is also tunable from 400 - 1000 Hz, so you can tune the radio to
frequency that you would like to hear, not to whatever frequency the filter
wants to “listen to”.
When designing the new Ramsey SX series of HF frequency transceivers,
our engineers developed a superior audio filter using the latest in switched
capacitance filter technology integrated circuits. In fact, the filter performed
so well that our AF1 kit is a "spin-off" from the HF radio project.
AMPLITUDE
FREQUENCY (Hz)
0
250
500
750
1000
1250
1500
1750
DESIRED SIGNAL
BANDPASS
FILTER
UNDESIRED
SIGNAL