13 May, 2019

More spurs than I had hoped for

My Just good enough 10 MHz reference based on the direct output of a Neo-7 GPS module, described in detail before on this blog, is good in keeping long-term frequency stability. Short-term stability and phase noise is as expected not so good. When connected to the reference input of my Elecraft K3 and listening to an outdoor antenna, I get quite a substantial amount of spurs around 10 MHz radiating from the GPS module. But since the K3 itself does averaging over a second or so, this is fine from the point of view of keeping accurate frequency.

The first plot, obtained from the Elecraft P3 Panadapter, shows the spurs. The strongest sidebands occur at 10 MHz +/- multiples of 100 Hz. There isn't much one can do about that. But since the amateur band starts at 10.1 MHz, the level of the spurs are enough reduced so it is OK.

It is worse with what the next plot shows, at a center frequency of 14 MHz. At this frequency there is no carrier, only sidebands spaced at what seems to be 240 Hz. This creates noise right at the beginning of the lower part of the 20 m band, and means that the reference cannot be used when operating CW on 20 m.

I'm sure this phenomenon can be understood and interpreted in light of the internal 48 MHz clock of the Neo-7 GPS and its mixing of divisions by integer numbers in order to get 10 MHz on average.

Something similar occurs at twice that frequency also, at 28 MHz, where sidebands are spaced 220 Hz apart, making the lower part of the 10 m band full of noise also when using the 10 MHz reference.

So I guess it is time to move on to something better.

This blog post first appeared on the LA3ZA Radio & Electronics Blog.

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