If Centeh Frequency

SPACE StGNAÜ

Fig, K Tuning of RTTY signals. In A, the RTTY FSK signaf is correctly tuned in a receiver with the ideal tf passbond and the BFO is properly set. In B, the BFO is improperly set with the mark signal in the if passband and the space signal out of it. The distance from the BFO to the if center should be 2550 instead of 2250 Hz.

trum. Obviously, the tuning is most critical for this bandwidth. Note that the BFO should be set 2550 Hz away from the center of the if passband. It doesn't make am difference whether the BFO is above or below the if frequency—this will only swap the MARK and SPACE tones. The incorrectly tuned example shows the BFO set too close to the if frequency. Here, the SPACE frequency will "fall off" the edge of the if curve causing it to be weaker than the MARK signal. This will result in errors, particularly when noise or fading is present. II your receiver has a wider passband than 1200 eps, the tuning will not be as critical. However, it is very desirable to have the tSK signal centered in the passband. We then have the maximum tolerance for drifts, mis-tuning, etc.

So we see that we need to do two things, First, get our BFO settings correct and, second, be able to accurately tune in the RTTY signal so that it is in just the right place in the if passband. The answer to both these requirements is a good tuning indicator. An additional advantage of a good indicator is that we can set the shift on our transmitter correct ly. *

Types of indicators

Many different devices have been used for RTTY tuning indicators, A list might include: Zero-center meter across discriminator load

Neon bulbs on keyer output Eldctron-eye tubes on die MARKSPACE detector output Variable-angle scope display Flipping-line scope display Detected-puke scope display Scope cross-pattern.

Most RTTY'ers have their favorite indicator. However, the display which seems the easiest and fastest for the newcomer to learn to use is the cross-pattern oscilloscope. This method was originated by Merrill Swan W6AEK. one

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