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High-band QMX rev2 (2024), 9V version, Firmware 017.? Having fun learning about this radio. LPF cutoffs are good, though LPFs 1 and 2 show stopbands a little high, about –30dB.? I get 4W out on 20m and 17m, less than 3W on 15m, 12m, and 10m.??

I thought I'd take a look at the output spectra, after reading several posts here about harmonics.? I don't have a tinySA, but I do have a 200MHz oscilloscope with good FFT capabilities.? I made the following setup:? QMX --> Dummy load --> 10X probe --> Scope.? The second harmonics on 15m, 12m, and 10m are higher than what I would expect, between 39dB and 42dB below the fundamental.? 20m and 17m are OK, 63dB and 52dB down, respectively.

My understanding is that the push-pull Class D amplifier is effective at suppressing even harmonics, prior to any filtering, so I'd like some ideas about where the high levels might be coming from and what I might be able to do to reduce them.

Thanks!
Eric

Thanks, Hans.? The tinySA has been on my list for a while.? Now I have a good reason to get one!

Eric

Consider the TinySA Ultra instead:??
It is quite advanced in features and functions compared to the previous TinySA model; I felt that it was well worth the cost increment.
73, Don N2VGU

Be sure to also buy attenuator(s), since the SA can be catastrophically damage? if connected directly to even a QRP rig.
I use a combination of a 10W dummy load with a built in 30dB attenuator followed by a 30dB SMA attenuator.

Thanks. ? I am getting a QRP Kits 41dB step attenuator, recommended in this forum.? What isn't clear to me, from the tinySA Ultra specifications, is whether its "recommended input power below -25dBm" is before or after the SA's built-in attenuator.? How do you read it?

Eric

Hi Eric,

For best results, keep the input at the TinySA under -25DBm.? That means for 5 watts, a total of 36-(-25) = 61 DB.? An additional power attenuator of either 20 or 30 DB would be a good addition.? Get one rated for the highest power that you will be measuring.? If you intend to measure 100-watt QRO rigs, use a 40-db attenuator.


A Tap would be a lower-cost choice if you already have a 100-watt dummy load.? Here are directions on how to build one:


I built a slightly different tap from a no longer available webpage.? I have attached the PDF that I got from the site.

73
Evan
AC9TU

Thanks, Evan,
I do have two dummy loads, the larger of which can handle up to 150W.? So I'll plan to build either the Salas or Hayward 40dB pickoff and follow it with a step attenuator.

Eric

On Fri, Mar 1, 2024 at 05:08 PM, Eric W. Hansen wrote:

What isn't clear to me, from the tinySA Ultra specifications, is whether its "recommended input power below -25dBm" is before or after the SA's built-in attenuator.? How do you read it?

Tiny SA also has a 20db LNA.

If you accidentally leave it it on then you'd exceed the input spec, so -25dbm should be your final target
just in case.
?
--
- 73 de Andy -

On Fri, Mar 1, 2024 at 12:59 PM, Evan Hand wrote:

For best results, keep the input at the TinySA under -25DBm.?

That is approximately -25dBm total power into the tinySA mixer; and gain/loss before the mixer, whether internal or external, will need to be accounted for.? -25dBm is not a hard and fast number and will vary with instrument and measurement.
One way to check if the level is OK or too high is to add some more attenuation and see if the carrier to intermod/harmonic measurement changes.? If no change then the IM/harmonic is real and not internally-generated.? If it changes, add attenuation until it doesn't.??
The "sweet" spot between distortion and noise can be narrow; some commercial SAs have 1dB (or less) step attenuators for fine level control for this reason.? An external step attenuator can be handy, but watch its power rating.....

I suspect that the tinySA designer made the internal LNA design such that it will saturate before it damages the mixer; that is what I would do.? What you will get instead is a screen full of massive distortion products, and, I expect, a warning from the instrument; I've never driven mine hard enough to find out.??
73, Don N2VGU

On Sat, Mar 2, 2024 at 04:07 PM, Da Amazin' man G0FTD wrote:

If you accidentally leave it it on then you'd exceed the input spec

Unless you have a specific reason to turn on the LNA, leave it off.? It will reduce the upper limit of the dynamic range of your instrument at the same time that it increases sensitivity.? Always turn it off if you are using external attenuation.? Only use it if your measurement is front-end noise limited.
This is a powerful instrument and as such it requires some thought and understanding to make good measurements with it.??
There is a lot of good information on spectrum analyzer use at Keysight, Rohde & Schwarz, Anritsu and other manufacturers.??
Here are a couple to get started.


73, Don N2VGU

Borrowed a tinySA Ultra and a 40dB attenuator from a friend today, and got good results using that and 10dB of attenuation inside the tinySA.? All harmonics in the radio's output are in compliance with the FCC limit.? I declared victory and buttoned the QMX into its case. Looking forward to warmer weather and taking it into the field.

73, everyone,
Eric