On 1/19/17 8:48 PM, Rhys D wrote:
Thanks for the detailed post Bill,
I'm learning a lot here!
So the spectrum analyser is indeed a "trap for young players"
As you guessed, it is a Siglent SSA3000X series analyzer.
I just looked at the same signal again with varied attenuations dialed in
on the instrument (I am using an external 20dB attenuator from minicircuits
as well)
Here is what I saw:
Attenuation - Fundamental - 1st Harmonic - 2nd Harmonic
15 dB - 11.40 dB - 49.13 dB - 51.12 dB
20 dB - 11.40 dB - 48.84 dB - 56.48 dB
25 dB - 11.28 dB - 48.32 dB - 49.15 dB
I guess these numbers mean I can't really trust what I can see on the
instrument screen?
Actually, that's fairly good. Most spectrum analyzers are good to about
1/2 dB with a moderate level signal (your fundamental).
The variation you're seeing is probably some combination of:
1) the mismatch between the source impedance and the spectrum analyzer
input impedance - the latter of which almost certainly changes with
attenuation setting
2) The calibration of the step attenuator.
3) maybe some change in harmonic production in the SA front end... in
your case, though the harmonic levels go DOWN as the attenuation is
decreased, which is the opposite of what happens with harmonics
By the way, I should just you know that I am not trying to solve a specific
timing problem here, I'm more using it as learning opportunity and making
sure that my setup is the best it can be.
You can have a lot of fun with a couple signal generators, or a sig gen
and a discrete oscillator, a box of filters and pads ,a mixer or two,
and an amplifier. You can look at mixer output spurs, compression in an
amplifier, spurs created inside the test equipment, etc.
An inexpensive clock oscillator (like used everywhere..get one of those
16 Mhz ones for an arduino or something) and a signal generator some
resistors and a T connector is fascinating. You can see what happens
when a signal goes "in" on the oscillator output and causes issues with
the oscillator output buffer, or when you put some RF on the power supply.
Thanks again for the input.
On 20 January 2017 at 12:26, Bill Byrom <t...@radio.sent.com> wrote:
You can't trust such low harmonic spurious measurements from a spectrum
analyzer unless you know how the spurs change with input level. The
second harmonic spur created in an amplifier or mixer inside the
spectrum analyzer input will typically increase by 2 dB for every 1 dB
of input level increase. Anytime you see a frequency converting RF
component (such as the mixer in the input of a spectrum analyzer), it is
nonlinear and will generate harmonics and intermodulation products. All
you need to do is to keep the input level low enough so that the
distortion products generated in the analyzer are below the signals you
are measuring. The best and easiest technique is to increase the input
attenuation by the smallest step available (such as 5 dB or 10 dB) and
checking how the spurious components change.
** If the harmonic or other spurious signal is coming from an external
source, it should not change as the input attenuation changes.
** If the harmonic or other spurious signal is generated inside the
analyzer, it should change relative to the fundamental signal as the
input attenuation changes.
** I'm talking about the harmonics or other spurious signals relative to
the fundamental frequency being displayed. If you remove the input
signal and still see the spur, it's a residual spur created inside the
analyzer unrelated to the input signal.
If you graph fundamental signal displayed amplitude vs changing input
level, you will typically see the following for spurious signals created
by most mixers or amplifiers:
(1) Fundamental signal = slope of 1
(2) Second harmonic signal = slope of 2
(3) Third order intermodulation (sum or different frequencies caused by
mixing of two signals) = slope of 3
For more background, see:
https://en.wikipedia.org/wiki/Third-order_intercept_point
If that is a SiglentSSA3000X series analyzer, here are the spurious
specifications from the datasheet:
** Second harmonic distortion: -65 dBc (above 50 MHz input with
preamplifier off)
Note that the second harmonic distortion is only specified at 50 MHz
input and above and at a -30 dBm input power level with the preamplifier
off. For comparison, here are the specifications of a Tektronix RSA507A
portable spectrum analyzer. Disclosure: I work for Tektronix.
** Second harmonic distortion: - 75 dBc (above 40 MHz input,
preamplifier OFF)
** Second harmonic distortion: - 60 dBc (above 40 MHz input,
preamplifier ON)
I'm sure that the reason for a lower limit on the second harmonic
specification is that the results are worse at lower frequencies. So
it's quite possible that the harmonics you see are mainly coming from
the spectrum analyzer input mixer or preamplifier. As I suggested
earlier, try lowering the input level by 5 or 10 dB and see if the
harmonics go down linearly.
--
Bill Byrom N5BB
On Tue, Jan 17, 2017, at 08:40 PM, Rhys D wrote:
Hi all,
Before I start, let me say I'm rather a newbie at this sort of
stuff so
please be gentle.
I was looking at the output of my Trimble Thunderbolt GPSDO and
was rather
surprised to see really "loud" harmonics in there. ~ 60dB down
from the
10Mhz signal.
Can anyone here shed some light on what I am seeing here?
Surely this isn't what it is supposed to look like? Should I be
trying to
filter these before going to my distribution amplifier?
Thanks for any light you can shed.
R
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