Rhys wrote:

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).

Hi

If you look at the FCC transmitter regs, -60 dbc is “ok” for many transmitters.

Bob

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?

Dave

manuals@artekmanuals.com

--
Dave
Manuals@ArtekManuals.com
www.ArtekManuals.com

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On 1/18/17 2:21 AM, Charles Steinmetz wrote:

9th Euro Freq & Time Forum
1995
Besancon

http://tf.nist.gov/general/pdf/1437.pdf

The OP attached a spectrum analyzer screen shot to his initial post --
go back and have a look.

It shows the harmonic series from 1 to 7, with H6 close to the noise
level.  Loudest harmonic is H2 at ~ -60dBc.

Charles

On 1/18/17 4:33 AM, Artek Manuals wrote:

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.

HI

So even a pretty expensive signal generator still has “loud” harmonics if judged at the -60 dbc level ….

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.

Bob