SDS100/SDS200: Weird anomaly receiving civil aviation with SDS100

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GROL

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I have only seen this on one frequency so far. Local airport AWOS is 118.175. I normally have any AWOS or ASOS frequencies locked out for scanning. When scanning or sitting on 118.175, the beginning of the AWOS broadcast volume is very low with an obviously incorrect RSSI -60dBm. After about 3 seconds, the volume goes way up to what I would expect monitoring the AWOS and the RSSI reads around -110dBm which is what I would expect compared to other radios I monitor that AWOS with from home. Modulation is set to AM and Audio option set for Analog. I tried all the filters, but the behavior did not change. If I turn off filters, the issue goes away, but the RSSI drops to about -120dBm. No other transmissions on other aviation frequencies exhibit this behavior and there are plenty with low RSSI near -110dBm, so it doesn't seem to be associated with weak signals. I can still receive the AWOS with no filter, but it is of course much weaker and not to the level received by my BCD395XT or BCD325P2. All with the same external antenna. No other radios I have exhibit this behavior receiving the local AWOS including my Yaesu FT-250A transceiver and a Radio Shack PRO-164. This is not of course a major issue, but it is a bit of an annoyance. Has anyone else observed this odd behavior? I was going to try IFX this evening, but the AWOS is apparently shut down now.

I plan to use the BCD396XT and the BCD325P2 for aviation instead of the SDS100, but it's also nice to only need to carry one scanner. The BCD396XT is likely all I will take to air shows. The SDS100 performs very well with all the digital trunked systems I monitor especially the Simulcast systems, and it performs well enough with analog channels. But, I still prefer the BCD396XT or BCD325P2 for civil and military aviation.
 

Ubbe

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" The receiver chip has its own amplifier that are self serving and cannot be controlled and will try and reduce interferencies by reducing its gain. The sensor for that detects signals up to 7Mhz away so that a strong signal that wouldn't affect another scanner will in a SDS scanner reduce the overall sensitivity so that you might loose the signal you are monitoring. The sensor and amplifier sits before the filters, to the front end of the receiver, so any setting of filters will not affect that phenomenon. It's one of the results of using a receiver chip designed for 5-10MHz wide channels. "


"I injected a -50dBm signal and at a frequency 7MHz lower than the monitored one. The RSSI changed from showing -100dBm to -60dBm.
When I got within 3MHz it started to show -50dBm.

The desensening of the receiver, how the RF AGC adjust the gain by the result of signals off frequency, where that the reciever lost 5dB of the signal when injecting -80dBm signal 100KHz from the monitored frequency.
Injecting a -60dBm signal gave 20dB loss and reception where almost completly lost and could only be heard faintly with the squelch open. "


/Ubbe
 

GROL

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Thanks, Ubbe, for the detailed information. This radio definitely exhibits some odd behavior. If I enable IFX for this AWOS on 118.175, the audio is much louder. RSSI is -60dBm but that cannot be correct. With IFX enabled I hear traffic on an adjacent channel mixing with it when a pilot is transmitting. It definitely reinforces the fact you should have a non SDS scanner for conventional frequencies in some circumstances. I have not had problems with public safety conventional channels so far. Maybe due to the difference between AM and FM. Not sure yet about that, I am still thinking about it.

In reality, the SDS100 will be fine much of the time for monitoring aviation, but I will certainly use one of my other radios for dedicated aviation monitoring. I have a good livery of radios for aviation monitoring. Uniden BCT15X(my top choice for aviation), BCD996P2, BCD396XT, BCD325P2, BCD436HP, Radio Shack Pro-164 and for VHF only a Yaesu FTA-250 transceiver.

It would be nice to have a block diagram of the SDS100! I could better wrap my head around what is actually happening, but what you provided is a very good start. I am way more experienced with conventional receivers, and I do have experience with QPSK. Before I retired from the Air Force/Air National Guard ( about 9 years total active duty time) I had three electronics career fields in 24 years. Ground Radio systems, Wideband/Satellite systems and Air Field systems(ATC radio and air navigation). I will be studying this SDS100 as much as I can figure out. It is certainly an interesting application using the Rafael R836 chip
 

Ubbe

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Maybe due to the difference between AM and FM.
No difference between modes and frequency bands. It works in the same way regardless of modes and frequency bands. I measured using a 800MHz frequency as that are what people use the most, and it's just as bad in that band as in any other band. When you receive in digital mode you are not aware of the problem, you are only monitoring the demodulated signal and the RF interference problem are masked. There are usually higher RF powers in lower bands like in VHF, signals attenuate less, and you are closer to FM broadcast transmitters and NOAA and pager systems and you are usually monitoring in analog mode where the problems are easier to hear. When signals mix in digital reception you only miss a conversation without you noting it, if you don't have another receiver monitoring the exact same thing.

It would be nice to have a block diagram of the SDS100!
If you google the R836 you will find the block diagram for it. It outputs something like a 1,6MHz wide spectrum that the DSP handles and use software filters for different demodulation modes and width. The 1,6MHz spectrum can be viewed using a bandscope program on a PC and a USB cable connected to the scanners USB micro port.

/Ubbe
 

GROL

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No difference between modes and frequency bands. It works in the same way regardless of modes and frequency bands. I measured using a 800MHz frequency as that are what people use the most, and it's just as bad in that band as in any other band. When you receive in digital mode you are not aware of the problem, you are only monitoring the demodulated signal and the RF interference problem are masked. There are usually higher RF powers in lower bands like in VHF, signals attenuate less, and you are closer to FM broadcast transmitters and NOAA and pager systems and you are usually monitoring in analog mode where the problems are easier to hear. When signals mix in digital reception you only miss a conversation without you noting it, if you don't have another receiver monitoring the exact same thing.

If you google the R836 you will find the block diagram for it. It outputs something like a 1,6MHz wide spectrum that the DSP handles and use software filters for different demodulation modes and width. The 1,6MHz spectrum can be viewed using a bandscope program on a PC and a USB cable connected to the scanners USB micro port.

/Ubbe
Thanks Ubbe. I have seen the R836 block diagram. Are there no RF filters on the RF input before it in the SDS scanners? That can make a huge difference. I may be in a good signal environment, but I hear more traffic with the SDS100 in P25 than I ever did before, especially of course the local simulcast systems. As far as conventional, I have not had extensive use of it there but so far it is good. What I would like to see is a block diagram of the whole radio. I do not need to see detailed schematics that supposedly the US FCC does not want you to see, I suppose due to the 1986 law about altering to receive cell calls, which is of course long outdated since you can't decode it anyway. Back in the 1980's some politicians got caught saying certain things then when it was analog. And so they decided making it illegal would stop it. Technology stopped it.
 

Ubbe

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It's probably not much to see. It's the standard switchable bandfilters at the front end, the same as in Unidens other scanners, they switch filter at the same frequency as Unidens x36 scanners when measured. Then there's a gain programmable amplifier, probably a 2 transistor design followed by a programmable attenuator that also operate as the 20dB attenuator to save components. The 836 receiver then follows and after that the DSP and the audio section. The main CPU are connected to each module. They probably use the same technology for Close Call as in other scanners, a PLL IC that only use the frequency counter part of it, and that gets obsolete after a couple of years and are a different IC in almost each scanner model and sometimes also in the same model, needing firmware upgrades to handle the new IC.

That RF amplifier can give a better sensitivity than other scanners but will then also be more suspect to interferencies. If you have a SDR dongle and something like SDR# to display a bandscope you'll see what happens if you increase the gain too much in a SDR receiver chip. The "smart power detector" in the R836 are supposed to handle some of the overload issues but that receiver chip can be bought for 85 cent from china so it's easy to understand what type of quality you can expect from it.

/Ubbe
 

GROL

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Thanks again, Ubbe. I would not want to open mine to investigate all this. Good to know there is switched filtering on the front end, otherwise I would suspect things would be far worse with interferences. If the attenuator is in fact programmable, I surely wish they would do a firmware update to allow selection of some lesser levels. 20 dB is way too much to be useful. 20dB is 1/100 original signal. Not much left. I would be happy to also have 10dB, but a few choices would be awesome. Maybe 6, 9, 12, 15.

I do have an SDR dongle and have observed exactly what you describe with gain which of course makes sense. The price of the R836 doesn't surprise me or bother me. I can imagine there are hundreds of millions produced since they are in so many TV receivers. Mass produced components get pretty cheap, but of course purpose built for one brand or model can be expensive. I have bought common digital chips for as little as 5 cents each in small quantities. And that was of course through a third party seller. Not direct from the Chinese plant. Back in the old days of Radio Shack when we could not just buy them on eBay, many of those 5 cent chips were around two dollars for a pack of two. 5 resistors were $1.29. Now you can get resistor kits from third party sellers on eBay that are 600 pieces, 30 values for $8, and capacitor kits too that are pretty cheap. TO-92 transistor assortment kit, 600 pieces, $10. Very mass produced components.
 

Ubbe

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20 dB is way too much to be useful. 20dB is 1/100 original signal. Not much left. I would be happy to also have 10dB, but a few choices would be awesome. Maybe 6, 9, 12, 15.
Uniden where experimenting with different values, changing the total gain, at the beginning of the SDS100 release based on user feedback. A user selectable gain figure, as well as a less attenuating attenuator setting, are on the wish list.

Unidens think tank will sit in a meeting and look at the wish list and note for each suggestion how costly it might be. Then they try to figure out if there's a profit to be made or if it's just gonna be an additional expense, I.E. will they sell more scanners with that feature added. Will we still buy Unidens next scanner if we don't get that variable gain feature? Sure we will, as they have no serious competitors and they know it.

/Ubbe
 

GROL

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Well, that is the problem. No serious competitor. GRE was a competitor for a long time until recently. I suppose GRE went down with the Radio Shack ship. Even the Brand name "Whistler" turns me off! At least my UNIDEN scanner doesn't just have Bearcat on it.
 
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