Thunderbolt Harmonics
Harmonic traps are another avenue to explore since the frequency is "fixed"
at well below ppm. Which leaves the fundamental untouched.
On Wed, Jan 18, 2017 at 5:21 AM, Charles Steinmetz csteinmetz@yandex.com
wrote:
Rhys wrote:
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.
Welcome to the world of RF. Loudest harmonic at ~ -60dBc (dB with respect
to carrier) is actually pretty good for a commercial product. Very few
distribution amplifiers do this well. For that matter, many good
laboratory RF generators are specified with harmonics only below -35 to -45
dBc. We do not generally expect RF sources or amplifiers to get down to
the -80 to -90 dBc range (although amplifiers with harmonics < -80dBc at
10MHz/1Vrms/50 ohms are possible), and certainly not the -100 to -120dBc
that we expect from high fidelity audio sources and amplifiers.
Even harmonics (which make the carrier asymmetrical) can cause phase
errors that are harmful in high-precision systems [1], so I am a vocal
supporter of distribution amplifiers with harmonics < -80dBc.
Note that cleaning up the Tbolt output to < -80dBc would probably require
a crystal filter (a filter with a sharp corner very close to 10MHz, in any
case), which means its phase response changes very rapidly with the filter
frequency. Sharp filters shift frequency with temperature, which causes
temperature-dependent phase shifts. Unless the filter is maintained in an
isothermal environment (like a good oven), this can cause problems in
sensitive applications.
Best regards,
Charles
[1] F.L. Walls (NIST), F.G. Ascarrunz (SpectraDynamics), The Effect of
Harmonic Distortion on Phase Errors in Frequency Distribution and Synthesis
(year unknown, probably late '90s).
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On 1/18/17 8:56 AM, Bob Camp wrote:
HI
the first time we ran the test (using a Keysight 33622 signal
generator) we saw significant 2nd and 3rd harmonics (50-60 dB down,
but easily detectable). A quick review of the data sheet.. Oh, the
signal generator spec is only -43 dBc for frequencies above 10
MHz.
So even a pretty expensive signal generator still has “loud”
harmonics if judged at the -60 dbc level ….
that's actually an inexpensive ($7.4k) function generator (with good
performance for a function generator that does a lot of stuff, except
generate chirps at the right rate)..
A $8k Keysight N9310 is -30dBc
A R+S SMA100 specs -30dBc (actually does better)
A $23k Keysight 8663D specs -55dBc (at 1 GHz)
Another case where low harmonic content is when doing two tone IMD
tests - if the sources have significant harmonic content, you might
be seeing intermod between the harmonics of the source, rather than
intermods between the fundamental of the source.
Which is one of the reasons a lot of IMD test setups have a variety
of filters in them.
These also are a pretty common item on eBay, at ham fest, and in your
typical RF junk box. 10.7 MHz IF filter cans can fairly easily be
tuned down to 10 MHz to custom roll bandpass filters.
Yeah, but it's easier (cheaper if you're paying for labor) just to buy a
box of 10 filters at $30/each and stack them
Thanks all for the extremely educational replies.
On 19 January 2017 at 07:08, jimlux jimlux@earthlink.net wrote:
On 1/18/17 8:56 AM, Bob Camp wrote:
HI
the first time we ran the test (using a Keysight 33622 signal
generator) we saw significant 2nd and 3rd harmonics (50-60 dB down,
but easily detectable). A quick review of the data sheet.. Oh, the
signal generator spec is only -43 dBc for frequencies above 10
MHz.
So even a pretty expensive signal generator still has “loud”
harmonics if judged at the -60 dbc level ….
that's actually an inexpensive ($7.4k) function generator (with good
performance for a function generator that does a lot of stuff, except
generate chirps at the right rate)..
A $8k Keysight N9310 is -30dBc
A R+S SMA100 specs -30dBc (actually does better)
A $23k Keysight 8663D specs -55dBc (at 1 GHz)
Another case where low harmonic content is when doing two tone IMD
tests - if the sources have significant harmonic content, you might
be seeing intermod between the harmonics of the source, rather than
intermods between the fundamental of the source.
Which is one of the reasons a lot of IMD test setups have a variety
of filters in them.
These also are a pretty common item on eBay, at ham fest, and in your
typical RF junk box. 10.7 MHz IF filter cans can fairly easily be
tuned down to 10 MHz to custom roll bandpass filters.
Yeah, but it's easier (cheaper if you're paying for labor) just to buy a
box of 10 filters at $30/each and stack them
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Hi
On Jan 18, 2017, at 1:08 PM, jimlux jimlux@earthlink.net wrote:
On 1/18/17 8:56 AM, Bob Camp wrote:
HI
the first time we ran the test (using a Keysight 33622 signal
generator) we saw significant 2nd and 3rd harmonics (50-60 dB down,
but easily detectable). A quick review of the data sheet.. Oh, the
signal generator spec is only -43 dBc for frequencies above 10
MHz.
So even a pretty expensive signal generator still has “loud”
harmonics if judged at the -60 dbc level ….
that's actually an inexpensive ($7.4k) function generator (with good performance for a function generator that does a lot of stuff, except generate chirps at the right rate)..
A $8k Keysight N9310 is -30dBc
A R+S SMA100 specs -30dBc (actually does better)
A $23k Keysight 8663D specs -55dBc (at 1 GHz)
Another case where low harmonic content is when doing two tone IMD
tests - if the sources have significant harmonic content, you might
be seeing intermod between the harmonics of the source, rather than
intermods between the fundamental of the source.
Which is one of the reasons a lot of IMD test setups have a variety
of filters in them.
These also are a pretty common item on eBay, at ham fest, and in your
typical RF junk box. 10.7 MHz IF filter cans can fairly easily be
tuned down to 10 MHz to custom roll bandpass filters.
Yeah, but it's easier (cheaper if you're paying for labor) just to buy a box of 10 filters at $30/each and stack them
Be very careful cascading those Min-Circuits filters without putting some sort of isolation between them. You can get all
sorts of wonky results as the reactances in one mis-terminates the reactances in another.
Bob
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Jim Lux...
Note that the Keysight 33622 is basically an ARB, and not really intended to
be an RF signal generator. It's output is DAC based, not oscillator based.
Hence it will tend to have higher distortion products than a good RF
generator. As you point out, a suitable filter for the intended frequency
certainly can clean that up, of course.
Tom Holmes, N8ZM
-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces@febo.com] On Behalf Of jimlux
Sent: Wednesday, January 18, 2017 9:45 AM
To: time-nuts@febo.com
Subject: Re: [time-nuts] Thunderbolt Harmonics
On 1/18/17 4:33 AM, Artek Manuals wrote:
R
Is what your seeing a harmonic (2nd? 3rd?) or a spur i.e what frequency
is the "harmonic" ?
How are you measuring this ? (Spectrum analyzer ? make/model?)
More importantly and at the risk of displaying my naivety, what is the
application that you are using the 10MHz source to feed and why if this
harmonic is 60db down (or even only 40db down, the quoted spec) why
would one care? What is the predicted error you will get in your
application as a result?
One application that needs low harmonic content is where you are
measuring the harmonic generating (or lack thereof) of a downstream
component.
I have an application where we're measuring the performance of a RF
chain followed by a digitizer. An easy test is to feed in a nice sine
wave (at a frequency that is NOT a submultiple of the samplerate) and
look for harmonics in the power spectrum of the sampled data stream.
the first time we ran the test (using a Keysight 33622 signal generator)
we saw significant 2nd and 3rd harmonics (50-60 dB down, but easily
detectable). A quick review of the data sheet.. Oh, the signal
generator spec is only -43 dBc for frequencies above 10 MHz.
Another case where low harmonic content is when doing two tone IMD tests
- if the sources have significant harmonic content, you might be seeing
intermod between the harmonics of the source, rather than intermods
between the fundamental of the source.
For 10 MHz, you can get minicircuits filters for 10.7 MHz that are
fairly wideband and work pretty well... about 20-30 dB of harmonic
suppression per filter I'd use the low pass flavor
SBP-10.7
loss at 20MHz is 26.84
loss at 40MHz is 41.22
loss at 50MHz is 46
est loss at 30 is 35?
SLP-10.7
loss at 10 is 0.65 spec
loss at 20 is 31.35 spec measured -33
loss at 30 measured -60
loss at 34 is 47.26 spec
loss at 40 measured -77
loss at 67.5 is 69.85 spec
Yeah, they might have a significant tempco, but you're running all this
stuff in an underground lair with small temperature variations, right?
It only looks like a small volcano from the outside.
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Am 18.01.2017 um 22:12 schrieb Bob Camp:
Yeah, but it's easier (cheaper if you're paying for labor) just to buy a box of 10 filters at $30/each and stack them
Be very careful cascading those Min-Circuits filters without putting some sort of isolation between them. You can get all
sorts of wonky results as the reactances in one mis-terminates the reactances in another.
Bob
But you can be quite lucky:
1 SLP15+ filter (15 MHz Low pass):
<
https://www.flickr.com/photos/137684711@N07/31554251684/in/album-72157662535945536/
3 filters cascaded
<
https://www.flickr.com/photos/137684711@N07/32019253490/in/album-72157662535945536/
Not bad.
73, Gerhard
Yeah, but it's easier (cheaper if you're paying for labor) just to buy a box of 10 filters at $30/each and stack them
Be very careful cascading those Min-Circuits filters without putting some sort of isolation between them. You can get all
sorts of wonky results as the reactances in one mis-terminates the reactances in another.
that's what a VNA is for.
On 1/18/17 6:25 PM, Tom Holmes wrote:
Jim Lux...
Note that the Keysight 33622 is basically an ARB, and not really intended to
be an RF signal generator.
Yes, but it has performance that is as good as the run of the mill
signal generator.
The big defect in the 33600 series is that it cannot be phase locked to
an external reference. If you feed it a 10 MHz ref, it has a frequency
locked loop that pushes the internal oscillator around to match the
external reference.
It's output is DAC based, not oscillator based.
Hence it will tend to have higher distortion products than a good RF
generator.
As good as, if not better, than an inexpensive generator (e.g. the low
end Keysight sig gens)
phase noise at non-harmonic frequencies is not as good.
As you point out, a suitable filter for the intended frequency
certainly can clean that up, of course.
Tom Holmes, N8ZM
On 1/18/17 8:04 PM, Gerhard Hoffmann wrote:
Am 18.01.2017 um 22:12 schrieb Bob Camp:
Yeah, but it's easier (cheaper if you're paying for labor) just to
buy a box of 10 filters at $30/each and stack them
Be very careful cascading those Min-Circuits filters without putting
some sort of isolation between them. You can get all
sorts of wonky results as the reactances in one mis-terminates the
reactances in another.
Bob
But you can be quite lucky:
1 SLP15+ filter (15 MHz Low pass):
<
https://www.flickr.com/photos/137684711@N07/31554251684/in/album-72157662535945536/
3 filters cascaded
<
https://www.flickr.com/photos/137684711@N07/32019253490/in/album-72157662535945536/
And the SLP10.7+ is very similar, except the 3dB down is around 13.5
MHz, and it's down 33 dB at 20 MHz.
As a practical matter, a chain of two SLP10.7 knocks the harmonics from
a Wenzel streamline down below the broadband noise floor when measured
with a system with 4 dB NF.
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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