I recently was puttering-around with an Si5351A and was reminded it is a
fractional-N synthesizer. One of the features of this is fairly prominent jitter
(phase modulation) on most frequencies. I looked Si5351A up on the list
and found the following note from 2022:
On 9/19/2022 10:27 AM, Joe Taylor via wsjt-devel wrote:
Hi Alan,
On 9/19/2022 1:01 PM, Alan Altman AG7XS via wsjt-devel wrote:
Greetings,
Thank you for all the great work on creating and improving WSJT-X.
You are probably aware of the ADX transceiver designed by WB2CBA. It is a simple open source digital HF rig designed for home
brewing. We are working on replacing the Arduino with a RPi PICO. Due to differences in internal peripherals and timers, it is
more complicated to measure the TX frequency in order to send it to the Si5351 DDS we use.
[Note: Si5351 is not a DDS; it is a PLL, normally using a fractional-N divider]
In rethinking the problem, it occurred to me that WSJT-X must first generate a digital list or table of tones for each
transmission before they are sent to the sound card and on to the rig as an analog signal.
FT8 signals are not generated from a list of tones. Abrupt frequency changes from one tone to the next would generate unwanted
sidebands with amplitudes far above those of a properly generated FT8 signal. For relevant details see Figure 3 and the
associated text in our QEX paper describing the FT4 and FT8 protocols:
https://physics.princeton.edu//pulsar/k1jt/FT4_FT8_QEX.pdf
In addition to Joe's admonishment here, I note that the Si5351 spurs due to
jitter can easily
be strong enough (~ -45dBc) to interfere with adjacent FT8 signals.
I'm not sure it's a good idea to use an Si5351 to generate a transmit signal of
significant power (> 10dBm
perhaps) without special care.
73,
Dana K6JQ
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