First RF project - Page 1
Forum section: electronics / projects

author: djacobow
author_rank: Super Contributor
timestamp: September 06, 2015, 06:55:56 pm
content: I'm a pretty experienced digital guy, but have never done anything RF and am looking for tips and guidance on how to start. Perhaps you guys can point me in the direction of the best reading.I have a project in mind, based on the failure of an earlier software-only project using RTL-SDR dongles as the receiver.Basically, I would like to round-robin tune VOR's quickly. VORs are a ground based navigation aid for aircraft. They have a simple, narrowband signal whose format you can read about here:https://en.wikipedia.org/wiki/VHF_omnidirectional_range. By quickly (or simultaneously) scanning all the VOR channels, I can determine which are present, can determine the station from their morse identifier, and combining a database of station locations and the calculated azimuths, calculate a position. The more stations I can collect, the better the position estimate can be. There will be cases when I can't show a position but I can show, for example, that the aircraft is on a line between two stations.A few years ago I wrote the DSP, first in GNUradio, and later in fixed-point C, to decode such signals. That worked. I was able to determine the azimuth to the station as well as decode the morse identifier. I also wrote some preliminary code for calculating positions from the azumuths.However, using the RTL-SDR I ran into some problems.First, if I reduced the sample frequency to 250ksps, I could tune in a station, but I would get images of nearby stations because the RTL-SDR has almost no front-end filtering. Certainly nothing tight enough to block out adjacent stations. (For reference, VORs are between 108MHz and 117.95MHz, with 50kHz spacing). I could not generally get clean input data for each station.An alternative approach was to increase the sample rate quite a bit and put a front-end filter that just captures the entire VOR band. I ordered some filters from mini circuits for this purpose, but they were not particularly steep, and in any case, the highest sample rate I could achieve was 4Msps, so I would have to survey the band in halves, but worse than that, I "discovered" that distance and weak VORs were much weaker than nearby powerful ones, and so the 8b dynamic range made the RTL-SDR not a good tool for wideband sampling of a bunch of different narrowband signals.Other complicating factors are that immediately adjacent to this band on one side is VHF aviation comunications, and on the other side, ridiculously-high-power commercial FM. So filtering is just something I couldn't avoid.So, I let the whole project drop for a couple of years while I worked on other things.Now I'd like to get back to it, and am thinking that the right path is perhaps custom hardware. So, where to start? My inclination is to try the low-sample rate narrowband approach again, but somehow with tunable input filters that I can center on each channel under electronic control? Is there any easy/cheap way to do that? I also would like to stick with the simple front end doing all the decoding in DSP rather than use a more complete analog radio, but if there are chips that do everything, I'd be interested.I have not decided yet, but if this project works I will probably open-source it.PS -- why not GPS? I like the idea of a terrestrial backup to GPS, and despite certain rhetoric, the VOR network will be with us for awhile.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 06, 2015, 10:37:19 pm
content: Yeah, can't be done without filtering. Sampling produces aliasing (mixing) at harmonics, so a sample rate of 250kHz pulls in 0-125kHz (baseband), 250-125kHz (first alias, reversed), 250-375kHz (first alias, forward), and so on. You don't necessarily need to filter tighter than that, but to resolve individual channels without getting others aliased into the sample band, you will.The trick with Nyquist is it works in any band, as long as the total bandwidth meets the criteria. When you reconstruct the signal, you can bandpass it into any final band. Or use equivalent time sampling to put it in baseband. In which case, it's not exact reconstruction, but band shifted so that the frequencies within the band are still there, with the same distribution and phase (or in the case of ETS, a scaled distribution).Filters aren't so bad to make. If you have a well shielded, consistently 50 ohm system, then you can control what all goes into it. Put a standard filter on there, look up the LC values from a calculator such ashttp://www-users.cs.york.ac.uk/~fisher/lcfilter/and calculating coils with e.g.http://hamwaves.com/antennas/inductance.html. Capacitors can be as normal (pref. C0G ceramic, disc or MLCC, perhaps with adjustables in parallel to fine tune; the coils can be rigid wire so they can be tuned by bending).The main difference you get for badly tuned vs. perfect filters is how flat and sharp the passband is. If you only care about the asymptotes (high attenuation very far from the passband), you don't really need to do anything picky.Tuning a filter of course assumes you have some means to check; a wideband white noise generator followed by spectrum analysis at a high sample rate should do well enough.Tim

author: tggzzz
author_rank: Super Contributor
timestamp: September 06, 2015, 11:05:20 pm
content: Quote from: T3sl4co1l on September 06, 2015, 10:37:19 pmWith some limitations, that can be done with the OP's now-unused SDR dongle. Seehttps://entertaininghacks.wordpress.com/2015/08/10/measuring-rf-filters-with-a-homebrew-1-5ghz-scalar-network-analyser-costing-3248/for a few results. When I get the time I'll publish the software I use to post-process the output from rtl_power and produce those graphs.

author: djacobow
author_rank: Super Contributor
timestamp: September 07, 2015, 12:07:03 am
content: Quote from: T3sl4co1l on September 06, 2015, 10:37:19 pmWell, one or two don't seem so bad, but I either need to figure out how to make one I can tune electronically, using varactors and other black magic, or I need to mux between one for each potential channel. Figuring out how to do that is where I'm stuck.I have a handheld aviation receiver as well as a handheld scanner and both of them are able to tune in these frequencies without pulling in their neighbors, which tells me that they have the front-end filtering in place, and based on the size of these devices alone, I don't think there is a filter for every "channel" of interest, so they are doing something cleverer.

author: Fank1
author_rank: Regular Contributor
timestamp: September 07, 2015, 12:47:45 am
content: I thought about this also but I just can't program that well.As for the RF side, use a block converter say a SRA-1 mixer and a 100MHZ canned oscillator with a low pass diplexer on the output.Convert the 117 MHz to 17MHZ and use the rtl on it. The channel spacing will then be a much larger percentage of the carrier frequency.Jerry

author: retrolefty
author_rank: Super Contributor
timestamp: September 07, 2015, 01:00:04 am
content: QuoteThey use the classic superhetrodyne design where any channel selected is converted to a single common I.F. frequency. Then a single filter designed for the bandwidth desired can be easily implemented. One can use discrete LC, crystal, mechanical resonator, ceramic resonator, etc. The 'front end' filter (usually a simple LC bandpass) need only cover the whole band in question and the IF filter will handle the channel to channel interference filtering.

author: rfeecs
author_rank: Frequent Contributor
timestamp: September 07, 2015, 03:34:10 am
content: Here's one approach:http://www.ultman.com/KnowledgeBase/dsPIC_sw_receiver_Rev1.1.pdfIt uses some of the techniques already discussed. Fixed pre-selector filter, down convert and sample and DSP.I noticed the block diagram in figure 4 says AD9944, but should be AD9954 direct digital synthesizer.It strikes me that there might be some integrated receiver chip that could do this. But finding the right one is a challenge.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 07, 2015, 03:50:31 am
content: If you have the sample rate available to pull down the entire band, and process it in DSP, then you can take the entire band, or suitable fractions of it (in which case you'll need pretty sharp, precise filters to avoid interference from channels just past each band edge).You are correct, if you can only pull down a channel at a time, you need N filters for N channels, and they need to be very good filters indeed (50kHz width out of 115MHz is 0.05% bandwidth, or a Q of over 2000!). Which you could only possibly pull off with very nice resonators (say, silver plated helical resonators), or crystal filters (which are rather more economical and much narrower).Otherwise, as mentioned, the best approach is a converter, which effectively lends you the bandwidth of that one, narrow, precise IF filter (and amplifier, if needed), for any frequency within the RF band. But you have to convert the frequency carefully, otherwise you unnecessarily introduce noise, reduce dynamic range, generate images (and harmonics thereof), and so on.Tim

author: rfeecs
author_rank: Frequent Contributor
timestamp: September 07, 2015, 03:58:49 am
content: This ain't rocket science. Every AM and FM radio does this for the past 100 years.

author: djacobow
author_rank: Super Contributor
timestamp: September 07, 2015, 04:27:11 am
content: Quote from: rfeecs on September 07, 2015, 03:58:49 amDid someone say that it is rocket science? I introduced the project from a noob's perspective.

author: djacobow
author_rank: Super Contributor
timestamp: September 07, 2015, 04:36:34 am
content: Quote from: T3sl4co1l on September 07, 2015, 03:50:31 amPretty sure this won't work, at least with cheap off-the shelf stuff because of the variability in signal strength. Say you have 12b samples, that's 72dB. It's altogether possible (actually, likely) that stations are more than that apart in relative strength.This would have been my go-to option, as there are a handful of projects and a bunch more commercial products that have the bandwidth. But capturing and decoding one signal spread over a big chunk of bandwidth is a different animal than capturing a bunch of of widely varying strength from different sources.

author: djacobow
author_rank: Super Contributor
timestamp: September 07, 2015, 04:40:49 am
content: Quote from: rfeecs on September 07, 2015, 03:34:10 amYeah, I've seen this paper; it's been up for a long time. Putting together my own front end, LO, and mixer is a bit outside my comfort zone, but I think it is really the only way forward unless I can find a part that does it all. The two mixers he used (SA602) are cheap, but the AD9954 is a $35 part and he used two to create the quadrature LOs to drive the mixers.The paper does not mention any gain stage prior to the mixer. Is that common and/or reasonable?

author: djacobow
author_rank: Super Contributor
timestamp: September 07, 2015, 04:52:08 am
content: Quote from: retrolefty on September 07, 2015, 01:00:04 amYes, with your comment and those from rfeecs, this is coming into focus. This is not an easy "SDR" (ie, direct conversion) project. The right way to do it is with a traditional superhet approach.

author: rfeecs
author_rank: Frequent Contributor
timestamp: September 07, 2015, 04:53:37 am
content: Quote from: djacobow on September 07, 2015, 04:27:11 amI was referring to N filters for N channels with a Q of 2000. Let's just say it sounds needlessly complex.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 07, 2015, 05:08:03 am
content: Quote from: retrolefty on September 07, 2015, 01:00:04 amA "Superhet" is a very hard act to follow!I would also like to point out a few other things:-(1)Classical "direction finding" only uses 3 stations & works pretty well.(2)I really wonder,if you idea is "a goer",why Avionics companies haven't done something like this before.Making up a bunch of receivers & looking at their output would have been trivial for such firms for years.You want to use an SDR because you are poor----they can do it "brute-force" if they want to!(3)You need to really be "dead sure" your device identifies the VOR's correctly,as Tropospheric ducting in Summer months can present you with a very strong signal from a Station hundreds,or even thousands of km away.

author: rfeecs
author_rank: Frequent Contributor
timestamp: September 07, 2015, 05:30:59 am
content: Quote from: djacobow on September 07, 2015, 04:40:49 amI kind of wonder if the author actually built one of these, or just designed it on paper. He is using an I/Q downconverter. I assume it is a direct down converter, mixing down to baseband.It may be simpler for the RF portion to do as mentioned by other folks here and mix down to an intermediate frequency. In this case you only need one mixer and one local oscillator signal. The local oscillator could be a synthesizer, like ADF4150 which would be around $5.The mixer has a noise figure of about 6dB. If you need better, you can add an RF amplifier to the front end.

author: djacobow
author_rank: Super Contributor
timestamp: September 07, 2015, 05:31:30 am
content: Quote from: vk6zgo on September 07, 2015, 05:08:03 amIn aviation, area nav with VORs is more or less something that has come and gone. It was big in the 80's, but almost all such systems actually used a rho/theta approach, where they worked with a single VOR, but used a secondary system called DME to provude the distance to the station. With that, the pilot selected a VOR in reception range with good geometry, and then could track his absolutely position with distance and azimuth.A system using only angles can work with only two stations, depending on your geometry. Consider that the azimuth you measure has +/- a few degrees of error in it (VOR signals do, for various reasons). When you are at 90 degree angles from the stations you've chosen, you have the lowest position error, but when the stations are far and you are more or less in between them, your position solution becomes much less certain. In the worst case, you are directly between them. Then you can only determine you are somewhere on that line.Adding more stations to the mix gives your more choices for triangulation with hopefully better geometry, and if you combine them all mathematically, you can bound your position more accurately.Quote from: vk6zgo on September 07, 2015, 05:08:03 amIn airliner type flight management systems, they combine ALL the sensor data available and VOR info is definitely in the mix. I am not sure if any FMS can automatically determine a position from VORs alone, but I suspect the answer is that they can. So they do do it, but adding a few hundred bucks worth of complexity to a $100k's system is not a big deal.On the other hand, this approach has definitely not appeared in light general aviation aircraft. I think, if it was every very attractive, it was obviated by GPS. But most pilots were still trained to use the VOR system. However, all such systems are totally manually: the pilot selects the station to use, ID's it himself, and then navigates according to a radio of his selection. That's fine but it a bit of workload that the modern GPS-only pilot doesn't understand, or if they understand it, they're rusty.My idea has always been that fundamentally, there is no reason a computer with a database cannot do most of this work for you, ultimately putting a little airplane icon on a map just as the GPS unit does, except using ground-based sources. This could be a nice backup to GPS.All that said, if this system cost $1000's, it's not interesting. If it can be put together for $10's, it becomes interesting. It could even be something that is included in future GPS receivers as a backup or an aid to getting an initial fix.Quote from: vk6zgo on September 07, 2015, 05:08:03 amVOR stations transmit a 3 morse letter identifier, and there are no stations with the same identifier within some large distance, though there are stations that share the same identifier around the world. I'd have to look in datbases to see if any with the same identifier are also on the same frequency.I have never heard of VOR frequencies (VHF) being subjected to skip from far-off stations, but perhaps it is possible. However, the system has been in place since the 1950's and pilots have been ID'ing stations the olde fashioned way (by listening to the morse) since the beginning.

author: djacobow
author_rank: Super Contributor
timestamp: September 07, 2015, 05:57:32 am
content: That's an interesting idea, and I was definitely thinking that an initial scan just to see if there was power at a channel, and then using a subset of channels for a "closer look." Makes sense.Depending on where you are and your altitude, there are only a few stations typically that are worth receiver. Probably less than 6 in even the densest places.The morse is quite slow; about 4 seconds I'd say for the three characters. It is broadcast once a minute, I believe for a plain VOR and twice a minute for a VORTAC.The modulation is simple but interesting. The signal is simultaneously modulated 3 or 4 ways:1. the carrier is AM modulated at 1020 for the morse boops.2. there can be (but isn't usuallly) AM modulation of voice for weather or warnings.4. There is a 9960 Hz signal that is FM modulated at 30Hz. This resulting FM modulated signal is then AM modulated onto the carrier.3. finally, the carrier receives a 30 Hz AM modulation. The strength of this modulation varies based on your angle around the transmitterA receiver recovers the 30Hz signal from the 9960 FM subcarrier and that is the "reference" navigation signal -- it is the same in every direction. The 30 Hz signal that was directly modulated is also recovered. The phase difference between the two is the azimuth to the station.In the oldest transmitters, I believe the AM modulation of 30 Hz was actually accomplished by spinning the antenna at 30Hz. Modern transmitters doing it using a phased antenna array.Quote from: evb149 on September 07, 2015, 05:33:59 am

author: djacobow
author_rank: Super Contributor
timestamp: September 07, 2015, 06:04:33 am
content: By the way, from the description below it should be clear that this is basically a plain vanilla AM receiver. Everything odd about VOR decoding can be done with the AM decoded "audio." That's why, being able to find something off-the-shelf or close to it would be nice. In fact, even if I end up designing a radio, it would be nice to find something off the shelf now so I could just test out (and demo) the concept.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 07, 2015, 08:09:45 am
content: Quote from: djacobow on September 07, 2015, 05:31:30 am"It is possible"-------& a normal part of day to day VHF use.It isn't "skip" which is slang for Ionospheric Propagation,which is the mechanism by which long distance propagation on the HF bands is achieved."Tropospheric Ducting" is a different thing altogether,where the propagation is through lower levels of the Atmosphere---the part you mostly fly through in light aircraft.https://en.wikipedia.org/wiki/Tropospheric_propagationAmateur Radio Operators regularly make long distance contacts using this mechanism during the Summer,& indeed,at other times of the year.If your device misidentifies the Morse code,you could be led astray,or is there anuniqueidentifier in the signal other than the Morse & the frequency allocation?Under normal circumstances,the Pilot listens to,& identifies the VOR by frequency & callsign,but if you are rapidly switching between many,how do you do that?

author: tggzzz
author_rank: Super Contributor
timestamp: September 07, 2015, 08:27:47 am
content: Have you contacted any local amateur radio (ham) groups?They will typically have two types of members: ones that like to use radios and ones that like to build radios.

author: djacobow
author_rank: Super Contributor
timestamp: September 07, 2015, 03:11:55 pm
content: Quote from: vk6zgo on September 07, 2015, 08:09:45 amThanks for explaining it. Still, I've been flying for awhile and I wonder why I've never heard of this phenomenon affecting nav signals.Regarding ID'ing the station, yes, getting it wrong is a danger. It's also a danger to humans who sometimes misidentify, or much more often, just fail to bother (they tune the receiver and follow the needle without actually turning on the audio to do the ID). The modern VOR receivers (typically built into integrated GPS+COM+NAV units do typically display the decoded morse).Getting a wrong signal in the mix is definitely a danger. Hopefully, with a multi-station solution, this sort of thing would stick out and be detectable. Also, the decoding could throw out noisy and low power signals with ambiguous coding.The problem with rapid round-robin and IDs seems real enough, though. Back when I started this, I imagined having two radios. One to fast-scan, and another to slow scan and park on each of the channels providing a strong enough signal long enough to decode the ID. Then the computer could only use data from the fast scan from sources that have been ID'd in the last few minutes.This is not much different from the traditional use of the system. The pilot would ID the station when first tuning it, then he would fly with its guidance for potentially a long time. Back in the day, we only have one VOR receiver in the aircraft, so the pilot would be tuning the radio back and forth constantly if he needed to determine his position exactly, or follow a course that was not directly between VORs.Also, as stations came in and out, the system would not necessarily jerk the airplane around on the map, but would draw a different blob of uncertainty around the position.

author: djacobow
author_rank: Super Contributor
timestamp: September 07, 2015, 03:13:09 pm
content: Quote from: tggzzz on September 07, 2015, 08:27:47 amThat's a good idea. I think I, too, would enjoy building a radio, but I would also enjoy not falling into the most common first-timer pitfalls (board layout, component choice, etc, etc).

author: djacobow
author_rank: Super Contributor
timestamp: September 07, 2015, 03:17:52 pm
content: Quote from: djacobow on September 07, 2015, 03:11:55 pm

author: C
author_rank: Super Contributor
timestamp: September 07, 2015, 05:14:53 pm
content: Fast Channel change:If you want to change channels fast, you should use a digital AGC.AGC is just to control the amplitude of signal to your 12b signal dac. You could use the signal dac's output range to control AGC level. Store the AGC level on a channel basis so that you can change channels fast.Strong adjacent channel:This is an AM signal so if you have a strong signal on adjacent channel you could shift your signals so that in place of BW being centered on channel it is offset to one side making strong signal weaker in pass band.RF tunning:Many old radios had a separate knob to tune the RF filters. If you are using varactors, this could just be an additional DAC output when changing channels.C

author: tggzzz
author_rank: Super Contributor
timestamp: September 07, 2015, 05:18:17 pm
content: Quote from: djacobow on September 07, 2015, 03:13:09 pmMy strategy is to build some simple circuits, e.g. filters, and the test equipment to test them. I'm not too interested in building a complete radio from individual components; my interests lie in using components and modules to do something "new" and "interesting". (Where the definition of new and interesting is entirely egocentric)Currently I'm experimenting with an SDR dongle to see how it can be used as a spectrum analyser, scalar network analyser, and time-domain reflectometer. One day I'll get back to making my 2GS/s 4GHz scope.

author: djacobow
author_rank: Super Contributor
timestamp: September 07, 2015, 06:03:32 pm
content: Quote from: tggzzz on September 07, 2015, 05:18:17 pmWhich one are you playing with?It is gradually dawning on me (because I can be a bit slow) that a good part of the problem with the RTL-SDR is not the lack of front-end tracking filter, but that the IF filter after the mixer is just not good enough. That could be because the dongle was designed from 2 Msps or whatever for DVB-T and, though you can run the ADC slower, I'm not certain the IF filter narrows automatically or or anything like that.However, I am also in possession of a FunCube Dongle Pro +. It is designed for 250ksps and I suspect has the IF filter to match. I haven't flown with it to collect signals. I have found fumbling with a laptop and dongle to be a bit of a challenge while flying a plane, particularly in the local airspace. I need to work out that minor problem.I am also interested in another radio, the Airspy, which seems to have made a point of having good IF filtering. However, it is a high-bandwidth device (10MHz) which actually makes my life harder.Good luck with the scope. Projects...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 07, 2015, 06:23:09 pm
content: Quote from: djacobow on September 07, 2015, 04:40:49 amSort of. A passive mixer always loses 6dB by its operation, which means your SNR is 6dB worse, straightaway. Your dynamic range is also reduced by as much, plus whatever the maximum signal level the mixer itself can run at.With a preamp, SNR rises, so that by magnifying the input (and adding only a little noise in the process), the mixer's 6dB loss looks like only, maybe, 1dB, so that after the amplifier's say 2dB noise figure, the SNR is now 3dB off ideal, a 3dB improvement. Dynamic range has been reduced even further, however, because not only does the amplifier experience IMD and compression, it also drives the mixer that much harder. If the RF amp is part of the AGC circuit, this can be somewhat mitigated, but only with respect to the station being received, not over the total band (i.e., a station at 110MHz and 0dBm might overdrive the front end, while trying to receive 112.5MHz at -60dBm).The same SNR-dynamic range concerns apply to ADCs (the ENOB being a very obvious limit to both!). If the sample rate is high, much filtering and averaging can be applied to recover more ENOB (effectively, filtering the quantization noise improves the noise floor), so it need not be exactly 72dB. It's all about the processing.If you do wish to explore a hetrodyne converter (it's not really superhet unless the RF band is also being tuned as a tracking filter), I would suggest something like an SA602 with a varactor tuned LO (driven from a DAC, assuming you have one handy) and a frequency counter (so you can keep adjusting the DAC output to keep the frequency offset correct). This should target a 10.7MHz IF, provided by a ceramic filter -- a standard component in any FM radio. The SDR can run at whatever sample rate is desired, but if you have the capability and processing, you might as well keep it at maximum, since quantization noise is independent of sample rate, which means you can do more filtering to recover more SNR.Tim

author: tggzzz
author_rank: Super Contributor
timestamp: September 07, 2015, 09:06:28 pm
content: Quote from: djacobow on September 07, 2015, 06:03:32 pmOne of the generic R820T2+R2832U devices. The front-end (partial) datasheet can be downloaded fromhttp://rtlsdrblog.rtlsdrblog.netdna-cdn.com/wp-content/uploads/2013/04/R820T_datasheet-Non_R-20111130_unlocked.pdfQuoteThat would be far too sophisticated! But the "SDR philosophy" is heavily geared towards minimal RF components and do all the interesting/clever filtering and demodulation using DSP after digitisation. They only back away from that where necessary.

author: rfeecs
author_rank: Frequent Contributor
timestamp: September 07, 2015, 09:17:25 pm
content: Quote from: T3sl4co1l on September 07, 2015, 06:23:09 pmJust had to comment on this. I have seen this statement often and it is not true. A passive mixer can have as low as 3dB conversion loss theoretically. The ADE-6+ has about 4.5dB conversion loss at 100MHz for example:http://www.minicircuits.com/pdfs/ADE-6+.pdfQuote from: T3sl4co1l on September 07, 2015, 06:23:09 pmNot sure what you mean be that. Superheterodyne just means supersonic heterodyne, meaning mixing radio frequencies instead of audio frequencies.

author: rfeecs
author_rank: Frequent Contributor
timestamp: September 07, 2015, 09:54:37 pm
content: Quote from: djacobow on September 07, 2015, 06:03:32 pmI have zero experience with the RTL-SDR and I'm a bit confused that you are not able to select 50kHz channels. I see people using them to receive FM broadcast channels which are 200kHz apart, so why can't it receive AM channels that are 50kHz apart?Apparently many of these units have an R820T receiver front end, which is a superheterodyne receiver incorporating an image reject mixer with 65dB image rejection. So you don't have to worry much about image frequencies.I would assume the RTL device properly filters the IF signal before sampling, so where is the problem coming in?Is it because there are very weak and very strong channels present at the same time?

author: djacobow
author_rank: Super Contributor
timestamp: September 07, 2015, 09:57:51 pm
content: Quote from: tggzzz on September 07, 2015, 09:06:28 pmI'm such a n00b here, I don't quite get it, but it seems to me that if your IF filter cannot block out stuff above the nyquist limit, then you will get images in your digital data, and all the DSP in world can't fix that.If I understand the datasheet, this particular dongle was designed for 6 MHz channels, and so has an IF appropriate for that, running the sampling at 6-8 MHz, too. As such, that works for TV, but running the sampling slower is an automatic invitation for trouble. Also, the adjacent channel spec in the datasheet is 47dB which seems not great, and that's assuming the 6 MHz channels. I know a lot of hobbyists are running the sampling slower because they want to use Raspberry Pi or USB1.1 and want it to be easier for the computer to keep up, but it seems like in the general case this shouldn't work very well at all.But now I am wonder, is it worth a quick-and-dirty attempt to hack one of these dongles to hack in low pass filter on the IF between the R820T2 and the 2832U, like at 15 or 20kHz, then operating the 820T2 in Zero-IF, so that the signal sits right at baseband. Now I get my narrow band without the reflections from other channels. Would this work? It seems like building a filter that would block the IF all the way down to audio frequencies would be easy.Could that work at all?

author: djacobow
author_rank: Super Contributor
timestamp: September 07, 2015, 10:05:31 pm
content: Quote from: rfeecs on September 07, 2015, 09:54:37 pmWell, I don't know. I would not rule out user error. But I think one constraint I maybe failed to mention is that running the unit at its full sampling rate of 6MHz puts a load on the computer and USB that introduces other problems. And because the signal I was interested in is not nearly that wide, I thought I'd want to run slower. But I think everyone has made it very clear to me now that if the IF is designed for 6 MHz, you really have to run the sampling that way.Now the issue of strong and weak station could be another issue. I can imagine that upping the gain to get the weak station you want could cause the strong station to overdrive the input LNA and the mixer and introduce all kinds of distortion. Could that cause images even if the IF filter is correctly matched to the sampling rate? It seems if that is the case, then front-end filtering is the solution. But why don't scanners and normal receivers have this problem?I'm starting to feel guilty about asking so many questions. Really, if there is reading that is the right place to start, I'm open to it. I feel like I have read plenty of basic DSP and several RF intro things, but I have't found the practical "Art of RF Electronics" I'm imagining is out there.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 07, 2015, 11:10:23 pm
content: Quote from: rfeecs on September 07, 2015, 09:17:25 pmI thought that felt wrong... 3dB should be it. A single balanced mixer for example drops half the signal (it's alternately being connected to the output, or not, by the LO drive), so loses half the voltage; half of which, in turn, is at DC (baseband + harmonics) (the average reads half, or -6dB) and the other at LO + harmonics. A double balanced mixer alternates between on and on (+/-), so only loses 3dB, which is equivalent to splitting fo into fo + LO and fo - LO.Although, 6dB is at least a typical ballpark (your 4.5dB example not being terribly far off, and truly terrible mixers like single diodes being even worse still).QuoteIt was always my intuition that "super" was because it's a hetrodyne set (i.e., converts RF to IF via LO + mixer), but "super-" because of the addition of tuned-RF tracking (e.g., a standard AA5 set).Wikipedia says Armstrong called it "super" to begin with, so I guess that is indeed the historically correct term, and there's no such thing as a "hetrodyne" set!Tim

author: rfeecs
author_rank: Frequent Contributor
timestamp: September 07, 2015, 11:11:54 pm
content: Quote from: djacobow on September 07, 2015, 10:05:31 pmYou may need to find someone that's experienced with the RTL-SDR. But it looks like there is an additional programmable anti-aliasing filter inside the RTL chip. It's not clear to me. But I don't know how it could work as an SDR without one.Yes, if you overdrive the front end it could screw everything up. You will get intermodulation distortion causing channels to mix together. Large signals in two channels could cause IM products in other channels and swamp out smaller signals for example.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 08, 2015, 12:51:00 am
content: Quote from: T3sl4co1l on September 07, 2015, 11:10:23 pmWell,a Direct Conversion radio or a Regenerative Receiver could probably be called "heterodyne sets".Both were designs which existed,& were used at the time.A DC set or a "Regen" in the oscillating mode produce an audible beat note or heterodyne at the Rx output.Frequencies above the Audible range were commonly called "Supersonic" frequencies,with little differentiation on whether they were referring to an Acoustic or Electronic signal.A Superheterodyne was so called because the resultant heterodyne was above the range of Audible frequencies.The other use of "Supersonic" was to refer to the speed of an object,such as an aircraft,projectile,etc.Since that time "Supersonic" has come to only have the latter meaning,being replaced in the other sense by "Ultrasonic".

author: djacobow
author_rank: Super Contributor
timestamp: September 13, 2015, 04:16:30 am
content: So, I flew the other day with the Funcube Dongle Pro+ with a simple rubber ducky and a 108-118 MHz bandpass filter in front. The FCDPP has a 192kHz sample rate and appropriate filtering after the mixer to go with.Results were much better than I ever got with the RTL-SDR. With some fiddling I was able to get usable signals from most VORs that I should have been able to receiver. Of course, the dongle was quite deaf compared to the ship's radios, but they are working from properly tuned antennas on the OUTSIDE rather than a ducky inside.However, still not smooth sailing. With some fiddling of AGC and gain at various points, I was able to get a nice signal, but there didn't seem to be a setting that got good results in all conditions of received power. That's not good for what should be an autonomous system.Overall, I'm encouraged, but this project just gets harder and harder. It is one thing to fiddle with SDR# or even gnuradio-companion and casually decode signals, but making a system work is seeming much harder.