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TOPIC: SunSDR2 PRO on 2m with high power FM BC stations

SUN2 SDR2 Pro - 2m overload in presence of high power FM BC stations 04 Apr 2018 13:45 #1

  • Charly
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Hi all,
I my location I get plagued not only by qrm from LED and switched power supplies etc but have to cope with a powerful local broadcast station in line of sight. My Sun SDR sees in worst case condx eight FM broadcast carriers with a combined power (power meter at antenna terminal) @ -30dBm (!) (or even more from a frequency hopping Tetra TX) nearby.

Such high signal level are well above average, fortunately I got some info from Roman (EE) and tested various filters in the 2m RF path.
I purchased a commercial @ 100 MHZ High pass filter and it did a good job already.
As a next step, I tuned a 8MHz wide VHF TV Filter to 141/149 MHz which gave even better results as visible in the bandscope screen shots further down.
(for both pics the 2m narrow band fuilte is activated - front end gain set to 10dB). With wideband filter full overload condx!)

Next step will be to use an even better 2m Band pass filter as designed and described by YU1LM.
Just waiting for the PCB board which I copied to quickly build such a filter.

With the present converted (8 MHz retuned TV) Bandpassfilter the wideband noisefloor already dropped significantly and most spurious and AM breakthrough disappeared completely.

The bad news: I discovered after this improvements, the big noise hump shown between 140 and 150MHZ is pickup from D4/D5 DAB+ broadcast (leaking from a local cable network).
In those directions I suffer from ta significant loss of sensitivity until the origin of such leakage is localised and stopped.
Weak DX signals and MS signals disappear completely in such a noisy environment.

Facit: The closer one get to the system noise floor, the more QRM can be detected.

If your SUN SDR suffers from similar problems, a highpass or better a bandpass filter may help a lot -
The dditional filter is to connect between RX out and ADC in.
Activate or disable the signal path in software by chosing the VHF antenna or the ADC input terminal.

!!! Adding a filter should not affect warranty and should not cause any harm if done properly with a set of short SMA interconnecting cables. !!!

I'll post new pics when the foreseen YU1LM filter is build, measured and inserted.

73,
Charly, DF5VAE

SUNBS-VHFinput.jpg



SUNBS-VHFinput.jpg
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SUN2 SDR2 Pro - 2m overload in presence of high power FM BC stations 04 Apr 2018 13:51 #2

  • Charly
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Sorry, twice the same plot -
so I try again to sent the correct one
SUNSDRBS-VHFADC-Highpassinput.jpg
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SUN2 SDR2 Pro - 2m overload in presence of high power FM BC stations 15 Nov 2018 16:58 #3

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Hello,

after being plagued by external qrm and very high noise level on 144MHZ I did quite a lot of tests and measurements to understand the cause of the huge noise hump you clearly see on the widenaband plot.
Shortly I will present a number of results.

73,
Charly
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SUN2 SDR2 Pro - 2m overload in presence of high power FM BC stations 20 Nov 2018 16:49 #4

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Hello all,

While SDR Trx have - no doubt about it, huge inband dynamic range, far better even than most analogue designs with roofing filters and high IF this new technology has its price if you do not live in a quiet rural area!
Plagued with far more than the typical 2m qrm I continued my investigation:
First some pictures which show what happens when my SDR is connected to a ham-band antenna in an area which I thought "rural" when I moved to here.

While the sun apparently has no problem to cope with near by 2m qro Stations the hardware is extremely sensitive to certain out-of-band signals.
The first interfering signals identified where AM broadcast stations transmitting around 15 MHz - (lower nyquist band)
A carrier at 15.82 is mixed to 144.174 used for FT8.
On the other side any signal transmitted on 175.82 MHz ( or 303.18 MHz) appears as well in the FT8 band on 2m.
So here a screenshot takes as follows:
SunSDR2 Pro - connected to well matched 2m Antenna (via 24m Ecoflex 10) using the internal wideband filter.
So there are various products - The 1,5MHz wide band just below the hamband is reciprocal mixing of a near by DAB+ signal (-49dBm at antenna cable end)
The lower 1.5 MHz wide signal is a higher order mixing product.
There are about 10 FM broadcast carriers (likely with mixing products in between)
In the actual 2m band are pickup signals (in band) from leaking (Vodafone) TV cable net with an FM-soundcarrier on 145.75 MHz.
Towards 160MHz mainly reciprocal mixing from local Marine band Transmitters (strongest carrier ~ 161.44 MHz) at -60dBm) plus intermod/mixing products with FM broadcast signals.
Except all inband qrm from cable TV/DSL pickup could be filtered - by carefully designed bandpassfileters and high Q components.
However therese parts are usually out of question from SMD equipped boards and small footprint Transceivers.

The second screenshot is taken with the same test setup - BUT - this time the narrow Band (SAW) filter is activated.
That would be the NOMINAL setup to operate 2m.
Pictures 3 and 4 show high resolution details of the DAB carrier mirrored just below the 2m band.

So this is my starting point - next time I will upload some more screenshots showing my painful progress and my attempts to remove unwanted out-of-band products without degrading the very good sensitivity of my SUN SDR.
You are lucky if you do not live on a rural island filled with man-made interference.
Next week more screenshots to follow.

73, Charly, DF5VAE
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Final cure of my SUN SDR in presence of high power out of band signals 12 Feb 2019 13:17 #5

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All readers of this thread may be aware that since I use my SUN SDR on 2m, I tested quite a number of homebrew filters as well as various retuned commercial surplus filters and dedicated ham band filters.
So far I never got all filter parameters in line: when I reached the wanted selectivity, I had too high insertion losses. most low loss filters had too wide bandwith and did not cure all problems.
I also tested the insertion of a 2m bandpass filter in between RX-out and DAC-in, but noted that the input MMIC in my heavily polluted area (JO64) contributed a good part to my problems as the MMIC was overloaded by strong out of band signals and mirror images .

As last resource I tested a large size helical filter manufactured by Juri, IZ4OSG.

This Filter Its more than half the size of my sun, but the plots Juri provided tempted me to purchase one of his helical filters.

In brief: the spects Juri publishes are met - insertion loss is below 0,4dB and the out of band selectivity is very good.

All strong out-of-band signals are attenuated to a level where they do not harm anymore.
No SW break through. I cannot barely measure any mirror signal around 175 MHz mixing into the 2m band.

There is still a 10dB increase of noisefloor (in other directions more than 15 dB), polluting the 2m band and all adjacent frequency bands.
This pollution is caused by high level leakeage from a local Vodafone cable net, carrying high level 256 Quam signals (Internet downlink - Eurodis) in the whoile VHF band polluting also airband frequencies.
The presence of this large band pollution was confirmed by the German authorities (Bundesnetzagentur) but it appears they lack manpower to take action.

The screenplots show worst case situation Antenne turned to highest out of band signals.

Setup: Sun LNA on, #### internal sun wideband filter DEACTIVATED (!!!) ####

IZ4OSG Helical Filter between 2m Yagi and RX intput.
I use an additional 2m Bandbass between RX-out and DAC in (The helix input filter is activated/deactivated via a transferswitch for this test setup)

The signals visible at 90-100-110 120 MHz etc are internal or external leakeage from the 10MHz GPS timebase (or its distribution amplifier).

I propose the use of cheap surplus ferrite cores as choke on the antenna cables and recommend good DC filters in in the 15 V DC supply.

Wishing you good reception
73, Charly, DF5VAE



sun_nohelical.jpg


sun_helical.jpg
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SUN2 SDR2 Pro - 2m overload in presence of high power FM BC stations 12 Feb 2019 13:22 #6

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Typo - the SUN Wideband filter was activated during the above test. The SAW narrobandfilter was off!
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