Re: LPF frequency curves #lpf

Bob ZL1RS
 

Some measurements just for information (as measured on 14 MHz with a HP8592B spec an).

The QRP-Labs Si5351a synth board 'raw output' contains the fundamental and fairly strong odd harmonics only.  The even harmonics are all well suppressed, by about -50dBc  (perhaps by a push-pull output stage in the IC?)

Si5351a output via a 0.1uF capacitor:
2nd harmonic less than -50dBc
3rd harmonic -10dBc
4th harmonic about -50dBc
5th harmonic -16dBc
6th harmonic about -50dBc
7th harmonic -20dBc

The PA stage does add some non-linearity itself.  The 'raw' output from a pair of BS-170's biased just at their 'turn on' point (more or less Class B) in a single band U3S (i.e. at the input socket of the LPF):
2nd harmonic -24dBc
3rd harmonic -15dBc

After plugging in a standard QRP-Labs 20m LPF, the harmonic content at the U3S RF output is:
2nd harmonic -54dBc
3rd harmonic -52dBc

73, Bob  ZL1RS

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