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Clock adjustment of PC2020年11月03日 14時21分29秒

Hello, FT8/FT4 enthusiasts.

Do you synchronize your computer's clock exactly? Do you synchronize your PC with the NTP server at startup, before and during FT8 operation, etc.?

And as I'm sure many of you have already done, I've been using GPS signals to constantly adjust my clock.

Here is a transcription of a previous article written by Igor from the JTDX development team.

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In JTDX, when the DT (time difference) delays for more than 0.18 sec, the FT8 decoder of QSO messages on matched filters is turned off, which leads to a decrease in decoder sensitivity by about 6 to 8 dB (!) In some cases, the number of transmitted messages increases to complete a QSO, in some cases the QSO is simply broken.

DT is calculated relative to an offset of +0.5 seconds from the start of the interval. If an FT8 signal is transmitted from DT -1.0 sec, then on the receiving side with accurate time synchronization, half a second of signal transmission will be lost, as a result, the probability of decoding such a signal at low SNRs will decrease. For FT4, such a spread in DT is unacceptable.

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I think this 6 to 8 dB loss would have a huge impact on QSO with weak signal stations. It could be a fatal loss, especially on 6m band multi-hop Es. I'm 6m crazy so I'm particularly nervous about adjusting my clocks.

If you are a WSJT-X, MSHV user, but your QSO partner is a JTDX user, the time discrepancy means that the decoding level of your signal on the other side of the QSO will be lowered.

There are lots of small, inexpensive GPS receivers out there if you search for them. What do you think? Why don't you try to use a GPS receiver to adjust your computer 's clock?

Well, it doesn't matter which method you use. The key is to make sure that your computer clock is exactly right.

How does a computer clock keep track of time? The answer is a crystal.

On the motherboard of a PC, there is a silver component (the one in the center of the picture) that contains this crystal.




But there is a problem with the crystal.

Interestingly, when a positive current is applied to one side and a negative current is applied to the other side, the negative side shrinks and bends into a U-shape. If the current flowed to both sides of the crystal was periodically switched between positive and negative, the crystal would vibrate. In other words, time ticks along with its vibrations.

The quartz crystal in a personal computer vibrates 14,148,180 times per second. In fact, quartz watches work in the same way.

The problem is that quartz crystals vary in quality. If the quality is poor, it is difficult to make the watch vibrate in precise numbers.

Natural crystals have varying amounts of impurities and shapes, so they have to be artificially created, but even so, it is difficult to manufacture them uniformly. In the case of a good crystal, the deviation is within plus or minus 1/100,000th of a second, and in the case of a bad crystal, the deviation can be as much as 1/10,000th of a second.

Although 1/10,000th of a second may seem small, it means 8.64 seconds per day, or a little over four minutes per month. This is the reason why a watch goes wrong. The trouble is that crystals tend to get even more crazy when the temperature is high.

PC's are full of heat-generating components, and PC watches are far more prone to malfunction than wristwatches.

8.64 seconds per day means 0.36 seconds per hour. I don't need to tell you what this means for us FT8/FT4 enthusiasts. So, I think it is necessary to adjust the clock at least once every 30 minutes if we are going to adjust the clock using an NTP server.



JP1LRT Yoshiharu


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