I did some additional measurements and I am now pretty sure, that the root cause for the spurs is intermodulation between the 40MHz IF signal @ CLK1 and harmonics of the transmit oscillator signal @CLK2.
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Measurement @ 10.1MHz:
Spurs are +/- 400kHz around 10.1MHz: 40MHz +/- 4*10.1MHz = +/- 400Khz
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Measurement @ 10.2MHz:
Spurs are +/- 800kHz around 10.2MHz: 40MHz +/- 4*10.2MHz = +/- 800Khz
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Measurement @ 7.0MHz
Main Spurs are +/- 2MHz around 7.0MHz: 40MHz +/- 6*7.0MHz = +/- 2Mhz
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Main Spurs are +/- 3.2MHz around 7.2MHz: 40MHz +/- 6*7.2MHz = +/- 3.2Mhz
The Spur #6 is @ 3.2MHz = 6*7.2MHz-40MHz
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Measurement @ 18.1MHz
Main Spurs are +/- 3.8MHz around 18.1MHz: 40MHz +/- 2*18.1MHz = +/- 3.8Mhz
and they are really bad: only 20dB below the carrier!
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...and so on and so forth...
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The initial impression, that there are spurs +/- 2.x MHz? were a result of the measurements at 7.y and 14.z MHz with intermodulation products in the 2.xMHz range.
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73, Joerg - DB2OO
