[time-nuts] Thunderbolt Harmonics
kb8tq at n1k.org
Wed Jan 18 11:56:44 EST 2017
> On Jan 18, 2017, at 9:44 AM, jimlux <jimlux at earthlink.net> wrote:
> On 1/18/17 4:33 AM, Artek Manuals wrote:
>> 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.
So even a pretty expensive signal generator still has “loud” harmonics if judged at the -60 dbc level ….
> 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.
> 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
> loss at 20MHz is 26.84
> loss at 40MHz is 41.22
> loss at 50MHz is 46
> est loss at 30 is 35?
> 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
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, 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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