LP-500 meter and PS

VK3ICM
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LP-500 meter and PS

Postby VK3ICM » Wed Sep 22, 2021 3:37 am

I have an LP-500 meter that has a spectrum display. I noticed that it does not seem to display the IMD improvement from a 2 tone test with PS on vs off. Apparently the LP-500 coupler needs a 200hz sub-tone to properly display a two-tone test on the spectrum meter. The 2 tone output on the meter provides this so I have it connected to the line-in on the 7000. On TX, I can see PS correcting, and the IMD improvement in Thetis, just not on the meter.. curious if anyone has one that could shed any light?

Cheers - Chris
VK3ICM
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w-u-2-o
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Re: LP-500 meter and PS

Postby w-u-2-o » Wed Sep 22, 2021 1:40 pm

I don't own any Telepost products, but I was intrigued by this question and thus I took a look at the Telepost website and also the LP-500 manual.

The LP-500 is showing you a spectrum display of the demodulated audio on your transmitted signal. Thetis is showing you a spectrum display of the RF output of the radio. The two are completely different measurements. There is no way they would even be close to the same in appearance, much less in meaning.

Telepost's claims that the spectral content of the demodulated audio is "meaningful" (their term from the manual) in terms of transmitted IMD products is very, very weak. There will be some correspondence, of course, as RF IMD products are a source of distortion and the effects of such distortions will be present, and heard, in the received audio. Indeed, that is why our radios, when running PureSignal, sound better than all other radios on the planet.

Telepost makes some claims regarding their formation of a special test signal to make the resulting audio spectrum useful for RF IMD measurements. Without actually using it for myself I have to seriously question this approach. Certainly it is far less accurate and easy to understand compared to the standard and straightforward spectrum analysis obtained using Thetis in DUP mode with an appropriately configured coupler. And, as you've noted, it does not provide anywhere near accurate results.

If you want to know how good or bad your RF IMD and harmonic products are, you must use an RF spectrum analyzer of some type. The one built into our radios, aka the panadapter, is a very good one. That is the tool that you should be using and paying attention to for such measurements.
VK3ICM
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Joined: Tue Nov 12, 2019 1:44 pm

Re: LP-500 meter and PS

Postby VK3ICM » Wed Sep 22, 2021 7:53 pm

Thanks for digging into that Scott. I typically only use it as a large screen power meter as it sits some distance away from my desk, but stumbled on 'spectrum display' and got curious.
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DL2XY
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Re: LP-500 meter and PS

Postby DL2XY » Wed Sep 22, 2021 10:56 pm

No LP-500 but a Microham SMORF.
It can measure IMD quite good, along with many other things.

Smorf PS On.png
Smorf PS On.png (6.77 KiB) Viewed 2451 times
Smorf PS Off.png
Smorf PS Off.png (6.79 KiB) Viewed 2451 times

73 Walter
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w9ac
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Re: LP-500 meter and PS

Postby w9ac » Wed Sep 29, 2021 1:05 am

VK3ICM wrote:I have an LP-500 meter that has a spectrum display. I noticed that it does not seem to display the IMD improvement from a 2 tone test with PS on vs off. Cheers - Chris VK3ICM

Scott is correct. I own an LP-700. Toggling PS on/off shows zero difference on the LP-700 spectrum display for the reason Scott described.

I use the LP-700 primarily as a wattmeter but also use it to show CW keyed waveform performance. It's still not as a valuable as a multi-trace oscilloscope when trying to make system timing measurements. It also doesn't have external triggering but does do a reasonably good job of stabilizing the waveform on the screen without triggering.

With a pair of couplers, the LP-500/700 can show amplifier linearity with a trapezoid waveform. That's not as precise an RF analysis where we're attempting to see better than -50 dBc IM products. At that level of measurement, tiny visual changes in the trapezoid slope can equate to large dB changes on a spectrum analyzer. The trapezoid test is still a useful indicator of IMD under dynamic conditions, like tweaking amplifier loading while talking on SSB.

Paul, W9AC

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