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Marti 950 MHz STL-10 spurious emissions

Posted: Sun Nov 23, 2014 10:39 am
by kcbooboo
I've got a pair of Marti STL-10 / R10 systems here around 949 MHz. Making 10 watts into a good dummy load.

I connected the RF output to a 50w 30dB coaxial attenuator followed by a 2w 20dB coaxial attenuator, to the input of my spectrum analyzer. The SA claims the 948 MHz carrier is at -9.75dBm, which is close enough for me. 10w is +40dBm and I've got 50dB of attenuation so as to not overload the SA. -10.0dBm would be the target.

These units generate and amplify the signal at 1/2 the desired output frequency. They then run it through a varactor doubler to come up with the 950 MHz signals.

There's a sub-harmonic at 474 MHz that's at -66dBm, which is about -56dBc. The Marti specs call for -60dBc. I also checked up around 1422 MHz (3 times 474 MHz) and found a similar -66dBm spike. My SA is only good to 1.5 GHz so I can't check any higher.

Both units have the same levels at the output frequency and +/- 1/2 the output frequency. Either they're both tuned wrong or that's just how they are.

Is this normal? Suggestions to minimize the out-of-band signals besides some sort of bandpass filter? Does anyone have anything to compare this to?

Bob M.

Re: Marti 950 MHz STL-10 spurious emissions

Posted: Sun Jan 14, 2018 7:15 pm
by DaleNX8J
kcbooboo,

Here it is 3 years after your post.... but this is still relevant.

If an XMTR does not make published spec, it's either your test equipment/setup, a problem with the XMTR, or specmanship. We use STL-10's for an AM link but I've never put them on an analyzer.

To allow folks to sleep easier, consider that the antenna likely shows loss at half-freq, so the sub at 474 MHz is likely radiating beneath allowable spec limit. Could be the same for the 2nd harmonic, if the antenna is selective enough.

If I had this unit on the bench, I'd look closely at tuning around the doubler and PA, while operating into a known 50 Ohm dummy. Lots of things can cause a dirty signal; poor connections, bad power supply bypass, RF feedback, loose screws on a shielded enclosure, etc.

Re: Marti 950 MHz STL-10 spurious emissions

Posted: Mon Jan 15, 2018 9:22 am
by KPJL FM
In order to check any RF generator for spurs on a SA, you need to kill the Fo completely at the input to the SA. Otherwise, the SA will show harmonics up the wazoo.

Re: Marti 950 MHz STL-10 spurious emissions

Posted: Tue Jan 16, 2018 6:41 am
by kcbooboo
I feel reasonably confident that with the 50dB of attenuation ahead of the SA, the harmonics I was seeing are truly present. When I increased the input signal by removing some attenuation, the fundamental and all harmonics increased by exactly the same amount. Usually SA overload or internally-generated spurs will not change by the same amount as the fundamental when the signal level is changed, especially by 10-20dB.

Bob M.

Re: Marti 950 MHz STL-10 spurious emissions

Posted: Mon Jan 22, 2018 9:30 pm
by W2XJ
Mostly true but 50 DB is not enough attenuation. A 50 DB directional coupler AND another 20 DB pad would be much better.

Re: Marti 950 MHz STL-10 spurious emissions

Posted: Mon Feb 12, 2018 6:34 pm
by RodeoJack
If all else fails & you don't have the time to bench it, I'd recommend Bohn Broadcast. I sent 3 pairs of those STLs out to them & they came back like new... better power supplies, replaced rotary switches, LEDs for the meter lamps & a bunch of updated parts.

I know there are others who do this kind of work. In this case though, I couldn't have found the time to do the work these guys did for the price.

I didn't have enough loaner gear to take all 3 out at the same time, but a couple of companies out here have been replacing their 950 gear with internet transports and had extras on their shelves. Good to have friends.

Re: Marti 950 MHz STL-10 spurious emissions

Posted: Tue Feb 13, 2018 7:35 am
by kcbooboo
The unit was on my bench over 3 years ago. It's either back in service working fine or sitting on a shelf until something else breaks and it gets put into service. I'm not losing sleep over a few dB which will probably be attenuated by the antenna system.

"These aren't the 'droids we're looking for. Move along. Move along."

Bob M.