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Mike

I do see it. It is much lower level and it disappears sooner when signal
is added. I am thinking it is simply a harmonic if the 300 MHz transient.
Since neither one has any meaningful effect on the instrument’s usefulness,
I guess my intellectual curiosity does not extend to researching it further.

On Aug 26, 2019, at 7:31 AM, Mike Brown <mbmail@...> wrote:

Looks like my 300 MHz spike is a tad worse than yours as there is still

5dB

or so visible when measuring 50dB of attenuation. And I can now see it
flips negative at times which as you say seems to confirm it is boosted
noise.

I take it you're not seeing a 600 MHz spike. I'm still bemused about its
cause on mine.

73

Mike

On Mon, 26 Aug 2019, 12:00 Warren Allgyer, <allgyer@...> wrote:

So it turns out my "worse" unit indeed does have a 300 MH spike and I

just

never bothered with it until I went looking for it. Here is why.

The spike is actually immediately after the 300 MHz point and, as I
suspected, it seems to be a product of the mode-switching of the Si5351

at

that point where it transitions from working with the fundamental

frequency

to the 3rd harmonic with boosted exciter level. Once calibrated, the

bins

occupied by the 3rd harmonic (above 300 MHz) contain different

calibration

offsets and cause the noise floor to rise. As noted elsewhare, the spike
intermittently flips polarity and shows up sometimes as negative going.

It

indeed seems to be more of a noise component than a spurious emission.

So I placed a variable attenuator in the S21 path to see what happens as
that path goes from open to zero attenuation where it was calibrated.

At 70

dB attenuation the exciter signal is actually below the noise floor and

all

we see is the noise floor with the spike. When we reduce the

attenuation to

50 dB the signal overcomes almost all of the noise floor in the

fundamental

region and much of the higher noise floor in the harmonic region. The

spike

is essentially gone even at this very low signal level.

Subsequent changes to 40 and 10 dB of attenuation still show the noise
transition a bit but there is no spike and the measurement is valid

across

both ranges.

So YES, the "spike" angel is indeed on the pinhead........ and it

affects

nothing with regard to the usefulness of the instrument.

WA8TOD