spurious and harmonic rejection anyone know about this?

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radionortheast
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spurious and harmonic rejection anyone know about this?

Post by radionortheast » Tue Mar 26, 2019 9:44 am

just wondering what these figures mean spurious harmonic rejection harmonics: 50db, spurious: -50dbc

i’ve just abit confused does mean it will put out carriers close to the carrier? not something out of the fm band? higher figure must be more desirable, i've often seen figures quoted with transmitters but no explanation. :)

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Re: spurious and harmonic rejection anyone know about this?

Post by Albert H » Wed Mar 27, 2019 2:11 am

It's quite simple. The wanted carrier is set to the top of the analyser screen. Other outputs should be much lower. There will usually be some products at twice and three times the wanted output frequency, but they should be very small compared to the carrier. You want a minimum of -60dBc (decibels referenced against the carrier level). The harmonics should be at least 1000 times (-60 dBc) smaller than the wanted carrier. If your transmitter is 1 Watt output, the harmonics should be a milliwatt or so.

Spurs and noise around the carrier should be even lower - a spec. that I get with my driver boards (when they're installed into their cases) has no in-band product greater than the "grass" level at the bottom of the analyser trace - around -96 dBc. This is exceptionally good. Most commercial rigs will be (typically) around -80 to -90 dBc.

One infamous attempt at simplifying an NRG PLL Pro III board (by cutting out the balancing circuitry in the oscillator and missing out lots of "unnecessary" supply filter capacitors) produced almost as much ½f as the wanted output. The sub-harmonic was only 12dB below the carrier level. The idiot also tried to make the amplifier stages "broadband" by over-driving one into the next - this gave rise to lots of spurs and harmonics. The "Easytune 3" actually produced more unwanted signals than it produced wanted carrier!

Take a look at some commercial equipment specifications and you'll soon see how they work (and which ones can be trusted!).

Never believe any antenna gain that's quoted in "dBi" - that's related to an "isotropic" radiator (which doesn't actually exist). A simple dipole has a notional "gain" of 2.15dBi - obvious nonsense - but you'll see people inflating their gain figures by quoting dBi (Paul Hollings always does this in an effort to make his aerials look "better").
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Re: spurious and harmonic rejection anyone know about this?

Post by thewisepranker » Wed Mar 27, 2019 10:46 am

Albert H wrote: Wed Mar 27, 2019 2:11 am The harmonics should be at least 1000 times (-60 dBc) smaller than the wanted carrier. If your transmitter is 1 Watt output, the harmonics should be a milliwatt or so.
-60 dBc is 1,000,000 times smaller than the carrier.

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Re: spurious and harmonic rejection anyone know about this?

Post by NOYB » Thu Mar 28, 2019 9:36 am

Beat me to it wisepranker!

I don't see anything wrong with quoting dBi figures. Equally if they want to quote relative to dipole, specify it as dBD to avoid ambiguity. Nothing wrong with either and people dealing with this stuff should understand the difference.

Personally I prefer dBi - seems more scientific/precise to me than relating a given gain to an "arbitary" antenna. Why not dBY (dB relative to a Yagi)?!!

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Re: spurious and harmonic rejection anyone know about this?

Post by Albert H » Thu Mar 28, 2019 9:07 pm

thewisepranker wrote: Wed Mar 27, 2019 10:46 am
Albert H wrote: Wed Mar 27, 2019 2:11 am The harmonics should be at least 1000 times (-60 dBc) smaller than the wanted carrier. If your transmitter is 1 Watt output, the harmonics should be a milliwatt or so.
-60 dBc is 1,000,000 times smaller than the carrier.
Depends if you're talking Voltage or power!
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Re: spurious and harmonic rejection anyone know about this?

Post by Albert H » Thu Mar 28, 2019 9:14 pm

NOYB wrote: Thu Mar 28, 2019 9:36 am Beat me to it wisepranker!

I don't see anything wrong with quoting dBi figures. Equally if they want to quote relative to dipole, specify it as dBD to avoid ambiguity. Nothing wrong with either and people dealing with this stuff should understand the difference.

Personally I prefer dBi - seems more scientific/precise to me than relating a given gain to an "arbitary" antenna. Why not dBY (dB relative to a Yagi)?!!
dBi is just used to inflate antenna gain figures. Remember - most consumers are technically naive!

dBD is a fair measure - a dipole can actually exist, and can be used for comparison. I had a customer recently who couldn't understand that an aerial could actually give (omnidirectional) gain. To persuade him to part with some money for a two-slot colinear, we set up the aerial we were offering and a dipole. Both were bolted to poles on the top of his studio building. We demonstrated using a little 5 watt driver, and his own receiver that had a field strength meter. He was astonished to see the difference in field strength between to two aerials, and he bought the colinear as it was a "cheap amplifier"!
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Re: spurious and harmonic rejection anyone know about this?

Post by Albert H » Thu Mar 28, 2019 9:16 pm

Incidentally - my latest valved PA produces 750 Watts RF out on the intended carrier frequency, and just 40mW of second harmonic! I'm quite pleased with that.
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Re: spurious and harmonic rejection anyone know about this?

Post by thewisepranker » Thu Mar 28, 2019 9:37 pm

Albert H wrote: Thu Mar 28, 2019 9:07 pm
thewisepranker wrote: Wed Mar 27, 2019 10:46 am
Albert H wrote: Wed Mar 27, 2019 2:11 am The harmonics should be at least 1000 times (-60 dBc) smaller than the wanted carrier. If your transmitter is 1 Watt output, the harmonics should be a milliwatt or so.
-60 dBc is 1,000,000 times smaller than the carrier.
Depends if you're talking Voltage or power!
dBc is exclusive to power ratios to avoid this confusion.

40 mW of 2nd harmonic with a 750 W carrier is -42.73 dBc.

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Re: spurious and harmonic rejection anyone know about this?

Post by NOYB » Fri Mar 29, 2019 11:14 am

Albert H wrote: Thu Mar 28, 2019 9:07 pm Depends if you're talking Voltage or power!
When have you ever seen spurious output quoted as a voltage?!

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Re: spurious and harmonic rejection anyone know about this?

Post by Albert H » Sat Mar 30, 2019 7:25 pm

NOYB wrote: Fri Mar 29, 2019 11:14 am When have you ever seen spurious output quoted as a voltage?!
On my Anritsu and HP spectrum analysers and on my Schlumberger Stabilock Radio Test Set! All receive and log spurs and harmonics as voltage peaks. These are then translated to power (if you want) into a known load impedance by the software in the test gear!

Voltage ratios are actually useful when you're doing the maths to design filters and combiners.
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Re: spurious and harmonic rejection anyone know about this?

Post by thewisepranker » Sun Mar 31, 2019 1:02 pm

Well yes but not at an absolute value. A calibration table will be essential because your instrument front end isn't flat, although they would love for you to think that it is.

With that logic, your "750 W of forward power" is actually a real measurement around 39 uA on your moving coil meter... Can I have a 14.66 uA rig please?

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Re: spurious and harmonic rejection anyone know about this?

Post by Albert H » Mon Apr 01, 2019 2:22 am

Pranker - you're right of course. Interestingly, the Anritsu is reasonably flay up to a bit over 1 GHz, then all sorts of bizarre "fiddle factors" have to come into play. The Schlumberger 4031 only goes up to 1 GHz, and has the most complicated compensation circuitry I've ever seen in anything! Again, it's pretty flat, but has some peculiarities at the low end.
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Re: spurious and harmonic rejection anyone know about this?

Post by NOYB » Mon Apr 01, 2019 8:32 am

Albert H wrote: Sat Mar 30, 2019 7:25 pm On my Anritsu and HP spectrum analysers and on my Schlumberger Stabilock Radio Test Set!
Well you learn something new everyday. I've never seen them quoted that way in equipment specs though. 1:1 score draw!

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