Carrier 42 HQX Manual de usuario descargar pdf (Pagina 34)

situation, but, it is the JT65 protocol's limitation, not something that just a bit of clever

programming can fix.

A subtle and less often realized implication of JT65 suffix/prefix rules is that it is totally impossible for a station

using a suffix or prefix to work another station using a suffix or prefix. There is not combination of reworking the

bit space that will allow for two 28 bit call signs and two 15 bit prefix/suffix allocations.

Having covered the contents of a JT65 frame I can now speak to the physical transport

method, or, more clearly put, its modulation format.

JT65 uses 65 tone AFSK modulation. Each tone is of a specific duration and sent singly

without overlap or phase changes between tones. That JT65 is FSK implies that it does not

require a linear (Class A/AB) but can use Class C. While this is true in theory it is less clear in

actual practice. It is unfortunate that JT65 has a reputation as being immune to the

consideration one must take when running modes (such as PSK31) where overdrive can

produce a severely distorted signal. While JT65 is far less prone to producing a 'dirty' signal

as seen with too much drive in PSK31 it is not immune to it.

To understand this you have to consider the A in AFSK. A = Audio, FSK = Frequency Shift

Keying.

Any modulation method that uses an AUDIO input to a transmitter can produce a

distorted signal regardless of it being FSK, PSK, MFSK or any other possible way to

modulate a transmitter.

While operating JT65 with an SSB transmitter you should always consider that you

could be producing a poor signal if you are over driving your transmitter or have

distortion/noise on the input line.

While it is, relatively, uncommon to see a distorted JT65 signal it is usually quite bad when

you do see one. In my own station I found, much to my own embarrassment, that for a short

time I was producing a poor JT65 signal. I checked my drive level and ALC indication finding

nothing wrong and after much checking I found that the ground shield had lost connection on

my audio input line... this created a very noisy input to the transmitter that then mixed with the

JT65 tones creating what can only be described as a nightmare on the air. So. Keep in mind

that no matter how many times you hear that JT65 is FSK and can't produce a dirty signal

that that is dead wrong. Mind your levels, signal integrity and listen if someone says you're

splattering over the entire pass band as it certainly can happen.

Another warning is to watch your transmitter when running at higher power levels. JT65 is

equivalent to 100% key down for the 47 second TX duration. You can easily overheat your

final if it can't handle this situation for the full duration. If you melt your finals you have only

yourself to blame.

The specific modulation for JT65 is, again, 65 tone AFSK. A JT65 transmission begins at 1

second into a new minute and lasts 46.811 seconds. Essentially a TX begins at 1 second into

a new minute and ends at 48 seconds. JT65-HF will key the transmitter at the top of a new

minute (second=0) but tone will not begin until second=1.

Of the 65 tones 64 convey data while the first tone conveys a synchronization chain that aides

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Carrier 42 HQX Manual de usuario Pagina 34