disappointing performance so far

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gteague
Posts: 6
Joined: Wed Jul 22, 2015 6:32 pm

disappointing performance so far

Postby gteague » Tue Aug 04, 2015 5:54 pm

hi folks! i've had my sdrplay a few weeks but it arrived with at least two other radios and i've been distracted and not had a chance to really use it yet. i'm trying to get an outside antenna up, but for now all i have is about 100' of speaker wire running around the house and a backpackers 40-10m also strung inside the house.

i used it for about 4 hours last night with sdhdr and sdrconsole and i'm the first to admit i don't understand the first thing about sdr software. as an oldschool radioman, the settings mostly baffle me and in some cases (such as that infernal direct frequency form with the blanks and way too many decimal points in sdrconsole), even entering a frequency is a struggle.

but sdhdr has an outstanding feature with the table you can bring up with ham and sw bands and just double click. i added the time reference stations to the user1 list as well.

i was basically unable to receive wwv at audible levels on either app all of last night on their freqs from 2.5-25 mhz. i have an icom718 sitting right beside the sdrplay using the same antennas and wwv was booming in on at least 3 of the freqs the whole time.

i tuned into the 20m ham band and found some great qso's using the icom, but they couldn't be heard on the sdrplay with either app.

next i tried some sw bcst stations and late last night i was able to receive r. habana, but with much fading. this station was crystal clear on the icom.

is there a chance i'm missing critical settings in the software? or maybe overloading the front end with 100' or 45' of wire connected? i'm getting in a 18" telescoping whip this week and i plan to try it out under the same condition.

is there a test i could run to determine whether my radio is working properly?

thanks for any help. /guy (73 de kg5vt | wqpz784)
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Tech_Support
Posts: 202
Joined: Mon Jun 01, 2015 7:00 pm

Re: disappointing performance so far

Postby Tech_Support » Tue Aug 04, 2015 9:04 pm

Hello Guy,
Getting the best out of a SDR platform can take a bit of time and effort, partly because these devices are designed to be highly flexible to cope with a very wide range and variety of signals and also because as you have already intimated, the application software can be very complex.
When using the RSP, there are a few key points to note. Firstly, when starting off, we would recommend that you use one of the simpler application packages such as SDR sharp or HDSDR. SDR Console is very powerful and is favoured by many users, but it is also rather more complex and takes a bit more getting used to. The second point to note is that when using SDR Sharp or HDSDR, the default settings for the RSP should not be viewed as being optimal for any specific signal types or reception conditions. This is because it is impossible to come up with a set of default settings that will work well for all signal types in all bands. What works well for a Digital TV signal at 800 MHz is unlikely to work well for an AM signal at 1 MHz. Please view the default settings as a 'starting point' from which you will need to trim the receiver to get the best possible performance. We find that most users seem to prefer to turn the AGC system for the RSP off. The RSP AGC system is simply designed to prevent signals from overloading the receiver ADCs. However, the ADC 'set-point' you would need will depend upon the type of signal you are trying to receive. A signal with a large peak to average ratio might require a lower set-point relative to the ADC full-scale than say an FM signal. Manually adjusting the gain, will give you more flexibility and prevent the performance from being constrained by what the in-built algorithm decides is the right thing!
Another point to note is that unlike conventional receivers or even other SDR platforms, the RSP does not specify gain. Instead it specifies gain reduction from the maximum gain setting. This is quite fundamental to how the chipset that underpins the RSP operates and has caused some confusion. Also the LNA does not have continuously variable gain. Instead, it has a fixed 24 dB gain step (reduction). The point within the overall gain reduction curve that this step is applied is controlled by the 'LNA trip threshold' in the ExtIO window. In the HF bands, this can be adjusted to be between 24 and 59 dB below maximum gain. With the LNA at its minimum gain setting, the receiver overload performance will be enhanced, but the receiver sensitivity will also be degraded somewhat. Try playing about with this setting to see whether performance is better or worse with the LNA at max or at min gain.
If you are interested, the following document: http://www.sdrplay.com/docs/SDRplay_AGC ... e_r2p2.pdf explains in a lot more detail how the gain control system for the RSP works and how an AGC loop might be constructed.
The final parameter that you can adjust is the receiver bandwidth. The RSP has programmable high dynamic range continuous time active filtesr which can be adjusted to sit at low as 200 KHz or as wide as 8 MHz bandwidth. With an 8 MHz bandwidth you can capture a large chunk of spectrum and monitor many signals simultaneously. However you are also more likely to have problems receiving very weak signals if you also have very strong signals within the same bandwidth, particularly if you are working with the RSP AGC enabled, where the AGC loop will be acting on the strong interfering signal and not the weak one that you are trying to receive. Try reducing the receiver bandwidth to 600 KHz or even 300 KHz to see if that helps.
Finally, as well as this forum, if you are a Facebook user, there is a very active and friendly open Facebook group for the RSP at: https://www.facebook.com/groups/sdrplay/?fref=ts
Here you will find people that can give you a lot of tips and advice about how to get the best out of the RSP for a particular application.

Sincerely

Tech_Support
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gteague
Posts: 6
Joined: Wed Jul 22, 2015 6:32 pm

Re: disappointing performance so far

Postby gteague » Tue Aug 04, 2015 9:24 pm

many thanks for the detailed response. i have already played with the bandwidth settings and tried from 200khz to 8mhz and couldn't really remember any differences. to be honest, i thought that only applied to how much spectrum i could see on the display and i still can't figure out why i can't see 8mhz of spectrum as i've seen in the pictures and videos.

i've also played with the noise blankers and rf and if nr and agc settings and nothing seems to make much of a difference.

maybe if you could just tell me what settings to concentrate on so that i can receive wwv or sw bcsts. i can't imagine anything more simple than that and, as i mentioned, i have at least 3 portable sw radios with whip antennas that total didn't cost what the sdrplay did and are far superior at such reception. as you say, the settings do not always compare and it's unfortunately not as simple as me using the settings from one of those radios and applying those to the sdrplay.

is there any definite way i can tell if my radio is defective or not? i've rarely experience such poor performance since i left a high qrm/qrn/intermod major metropolitan area. my radios were all useless there--i could barely get fm radio or ota tv! but i'm not 15 miles from the nearest town and i'd expect to receive signals even inside the house.

tks! /guy
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gteague
Posts: 6
Joined: Wed Jul 22, 2015 6:32 pm

Re: disappointing performance so far

Postby gteague » Tue Aug 04, 2015 10:32 pm

that's an interesting and helpful document link you sent. i missed that when going over the documentation on the site.

i decreased the gain by 25db and noticed an immediate improvement in received signal readability. i reset all the sdhdr settings to factory default so i could start over from scratch. also, the sdrplay extio settings allowed me to finally find the way to display the full 8mhz of bandwidth. wish there was a setting in-between 1.5xxx and 5mhz though--2 and 3mhz would both be useful.

so what is the pitfall of checking the box to lock the agc to the application software settings? does that not allow the application to do the adjustment i'm doing manually?

oh well, i have a lot more reading and experimenting to do. but things on the receiving front are already looking better. using both a 21' and a 100' indoor antenna wasn't much difference, so guess the overload fear is reduced.

yeah, i doubt i'll need much more than the sdhdr for awhile and in any case i love the sw bcst schedules and the ease of entering my own frequencies although the sorts don't 'stick' very well.

thanks again for the detailed response and i need to take it line by line. i did find 3 sdhdr doc sources and i'm going to read them as well.

/guy (73 de kg5vt | wqpz784)
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Tech_Support
Posts: 202
Joined: Mon Jun 01, 2015 7:00 pm

Re: disappointing performance so far

Postby Tech_Support » Wed Aug 05, 2015 2:38 am

Hello again Guy,
I am glad that you have managed to make some progress. Regarding AGC, when using SDR Sharp or HDSDR, there are two independent AGC systems that you can access and they operate completely seperately from each other. The one that you can see in the ExtIO window, actually controls the gain of the Tuner after sensing the power level at the ADC inputs. This is not a panacea for all signal conditions because whether it is effective or not depends upon the interference environment. The AGC will only correctly act upon signals that it sees within the bandwidth of the detector. This is effectively constrained by the IF filters. So for example, supposing hypothetically you are looking for a fairly weak signal at 17 MHz and you have a very strong interfering signal at say 7 MHz. Supposing you also have the AGC set point at -15dBfs, the AGC will see that the signal is weak and try to turn the gain up to get the overall levels at the ADC input to be 15 dB below full scale. However, in doing this, the 7 MHz signal might well now overload the front end and degrade the SNR substantially. As the AGC loop is self contained within the tuner, it cannot measure SNR, it can only measure signal level. You have 12 bit ADCs which theoretically give you around 72 dB of dynamic range and so with a single weak signal inside the receiver bandwidth, you don't need to be at -15 dBfs. Backing off to -40 dBfs would still give you decent headroom above the quantisation noise of the ADCs and may well help pull the front end out of compression and improve the fidelity of the wanted signal. If the interferer is within the bandwidth of the IF filter though it is a different matter as the AGC will act to prevent the interferer from overloading the ADCs by turning the gain down. Manually adjusting the gain and tweaking the LNA trip threshold gives you a greater degree of flexibility for handling a wide range of interference conditions. These are simplistic examples, but hopefully they illustrate the point.
If the receiver is only designed to cope with a relatively narrow range of signal types (AM/FM etc) and crucially a more limited signal bandwidth, it is possible to design AGC loops that are tailored to these specific signal types. The Tuner AGC system works very well for 'managed' broadcast networks such as TV systems where system is planned to give a very specific worst case interference environment. It will not work so well in an unmanaged environment such as what you might typically find in the amateur bands, where your nearest neighbour may be transmitting 100 Watts only a hundred metres from your receiving antenna!
The AGC that is within the application itself does not control the gain of the tuner in any way. It simply acts upon the signal as presented to the application. There is no reason that we are aware of not to use this as it has no impact on the tuner performance whatsoever.

I hope that this helps.

Sincerely

Tech_Support
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gteague
Posts: 6
Joined: Wed Jul 22, 2015 6:32 pm

Re: disappointing performance so far

Postby gteague » Wed Aug 05, 2015 2:57 am

tks again. i'm collecting your posts into an evernotes file.

i've found a very useful page at:

https://sites.google.com/site/g4zfqradi ... sing-hdsdr

which starts with an install and setup guide aimed squarely at the beginner and seems to explain every window and control. and i've re-located the sdrplay further away from my computer to try to improve the s/n ratio and lower the noise floor..

your explanation about the agc was very clear.

i was just totally caught by surprise how totally foreign sdr turned out to be upon first exposure. i've been making my living with either radio or computers or both for 45 years and you'd think i'd have seen everything by now. i was swl'ing with a beat up hallicrafters someone rescued from the trash when i was ten years old. i remember sitting exams in the navy and having to draw block diagrams of receivers, for example. evidently, none of that prepared me! [g]

/guy (73 de kg5vt | wqpz784)
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