spurious and harmonic rejection anyone know about this?

[radionortheast]

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.

[Albert H]

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").

[thewisepranker]

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.

[NOYB]

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)?!!

[Albert H]

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!

[Albert H]

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"!

[Albert H]

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.

[thewisepranker]

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.

[NOYB]

Depends if you're talking Voltage or power!

When have you ever seen spurious output quoted as a voltage?!

[Albert H]

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.

[thewisepranker]

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?

[Albert H]

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.

[NOYB]

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!