Complete cheap SDR - Page 1
Forum section: electronics / projects

author: Noxer
author_rank: Newbie
timestamp: April 27, 2014, 01:17:13 pm
content: Hi everybody.I am relatively new to HAM radio and decided to make a SDR. I have ordered a DVB tuner stick and a LNA4ALL. I have also found a metal enclosure where I'll put the LNA and the SDR into. Now there are a loot of good sites out on the internet that I will use to configure the SDR stick but my problem is different. I need power for the SDR stick and power for the LNA. As much as I know the power output of the USB is around 4.8V and it has a loot of noise so I will add 12V DC external power to the whole project. Now the problem is the power filtering and regulation. The LNA4ALL accepts 6-12V DC because it has an on board AMS1117-5V or if you bypass it the LNA accepts also 5V but the SDR stick needs a 5V regulator ( like 7805). I could also put an external regulator for the LNA. The problem is I want a super low noise power supply for the LNA and the SDR stick. The 12V DC supply may have noise in it so I want to add a filter and regulation circuit into the metal box where the SDR and the LNA will be. I have a lot of questions about this and one of them is should I use two separate 7805 for each of the LNA and the SDR stick or could I use the same regulation circuit for both things and add just a few caps. I don't want the SDR stick to induce noise into the power rail and so I thought of using two separate regulators. The SDR stick also has some on board regulation but I can't identify it. Also I had an idea of using two regulators in series to reduce the noise if that is possible. I would put an 7808 regulator directly on the 12V power input and that I would feed that "clean" power directly to the LNA and also I would put an 7805 on that 8V rail to supply the SDR stick. Is it better to use the on board LNA regulator or an external one or maybe two in series? Now here are some links and some photos that will:http://lna4all.blogspot.com/http://www.ebay.com/itm/191073852799?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649Here is the metal case I will put everything in.Now the circuit ideas.Here I am using the on board regulator of the LNA but I have a 5V regulator for the SDR:This is the option using just one regulator for both devices(here the LNA internal regulator is not used):And here I have two separate regulators for each device(here the LNA internal regulator is not used):And here is the wierdest idea the regulators in series:On every schematic the USB power is not been used. and the DATA+ and DATA- are directly connected to the USB ( I could somehow use a optocupler there)Please tell me what should I do? What have I done wrong and please give me some advice. It would be nice to see some other ideas then just mine.Thanks Nox.

author: Noxer
author_rank: Newbie
timestamp: April 28, 2014, 07:30:27 pm
content: Can anybody please help me?

author: janoc
author_rank: Super Contributor
timestamp: April 28, 2014, 09:34:01 pm
content: OK, I am not an expert, but I can see a few problems.First, if you need 5V for both devices, why are you powering it from 12V? Depending on how much current the hardware needs, the 7805 regulator will be wasting quite a bit of power and getting hot. Some 9V would be likely a better idea, or even better, use an LDO and even lower input voltage.Also, careful about the 100uF caps on the output - if you use something like a barrel jack connector (which shorts its pins) for the input voltage, you will blow the regulator up unless using a protection diode, whenever the output cap is charged and the input is shorted (e.g. by pulling out the connector). That can destroy the rest of the circuit when you apply power next time if the regulator has failed short. These regulators don't like reverse current flow. The cap could be made much smaller. See here:http://www.learnabout-electronics.org/PSU/psu23.phpRe noise: if you hook your circuit up to the PC by USB cable, you will still get all the same nasty PC noise floating around, unless you use an optoisolator and heavy shielding (nice case, BTW!). External power won't help with that, really. External power is good for two things - when the devices require more than 0.5A that the USB port can deliver and stable voltage. If you wanted to power this from USB and the devices require 5V, you would need to use a boost converter - from this point of view an external supply is a much simpler solution.Circuit 1: That 7805 and the built-in regulator in the LNA are likely to get hot. Also the cap issue above.Circuit 2: Same issues as 1, I am not sure whether you actually need those large 100uF output caps, because both devices likely have their own reservoir caps on-board already, including all bypassing. However, I would keep one 0.1uF cap on the output for the sake of the stability of the regulator.Circuit 3: Same as above, just the wasted power will be divided between the two regulators (they will get less hot)Circuit 4: May not work - 7805 has typically 2-2.5V dropout voltage (i.e. it stops working bellow 7.5V input voltage). If you are powering that from an 8V pre-regulator, you are pushing your luck. In order to use this scheme to reduce the thermal load on the regulators (they divide the wasted power), I would use a 7809 there. However, then the 12V input could be marginal.And the same issue with the capacitors as above.

author: tonyarkles
author_rank: Regular Contributor
timestamp: April 28, 2014, 09:41:04 pm
content: janoc, you posted a lot of what I was planning on saying!If you do still want to go for the 7805, I'd probably try using a single 7805 for both, some capacitance on the input side (1uF, 10uF, whatever the datasheet says). On the output side, I'd have two circuits, each one isolated from the 7805 output through an RF choke and a pair of caps (one small one like 10-100nF, one larger one like 1-10uF). That will probably get you a pretty quiet supply and you have a lot less to worry about as far as one board passing noise into the other.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: April 28, 2014, 10:36:20 pm
content: Quote from: janoc on April 28, 2014, 09:34:01 pmYou mean a TRS? I've never seen or used a barrel jack connector that shorted out when unplugged.

author: Unknown
author_rank: Unknown
timestamp: April 28, 2014, 10:42:32 pm
content: Quote from: Noxer on April 27, 2014, 01:17:13 pmIf you're worried about noise on the 12v line, then you reallydon'twant to add your filter and regulation circuit into the same metal box where your LNA & USB stick are.You want to keep the noise out, not in...

author: janoc
author_rank: Super Contributor
timestamp: April 28, 2014, 11:19:31 pm
content: Quote from: suicidaleggroll on April 28, 2014, 10:36:20 pmI guess you are right, I have confused the two. However, even a barrel jack can short if you manhandle it or if you try to insert a smaller plug than it is designed for - shorting the pin in the receptacle to the body by the sleeve of the plug being pushed in.

author: janoc
author_rank: Super Contributor
timestamp: April 28, 2014, 11:20:51 pm
content: Quote from: Tac Eht Xilef on April 28, 2014, 10:42:32 pmI would be a lot more worried about the various digital noises from the USB stick and the common mode noise on the USB cable than the noise of the linear regulator.

author: Noxer
author_rank: Newbie
timestamp: April 29, 2014, 08:30:17 am
content: OK Thanks everybody. Now i know what I have done wrong. I have searched for a LDO regulator and came across the LM2940 (TO-220). It is cheap but if you have some better idea just say so. Now the input voltage is from 6 - 9 V and the heat dissipation should be less. I have also used an RF choke and some capacitors for each circuit. The datasheet says that this LM2940 has an short circuit protection does that mean that I don't need the protection diode for the big capacitors? As you can see I am not really god in this kind of stuff and I got no idea how to decouple the USB data with an optocoupler so if anybody has some kind of schematic that would be very useful. And in the end I could put a small metal case around the power regulator and USB inside the big metal enclosure. But just over the part that is not regulated and not decoupled.L= 2 turns around ferrite coreC1 = 100 uF electrolyticC2 = 10 uF tantalumC3 = 0.1 uFI am not quite sure if the C1 is necessary, you tell me please!And where should I locate these circuits? Should there LC filter be as close to the device as possible?Because I work with analog signals most of the time I know that I don't want any noise in the system. And that is the reason I am struggling to "kill the noise".

author: janoc
author_rank: Super Contributor
timestamp: April 29, 2014, 09:46:07 am
content: Quote from: Noxer on April 29, 2014, 08:30:17 amBe very very careful about the caps. First, why do you need so many caps on each of the devices? That looks like a massive overkill, moreover each of the modules for sure has its own caps inside as well. Second, make sure to look in the datasheet of you regulator as to what value and which type (material) the output caps have to be - LDOs are notorious for oscillating if you are not careful - e.g. too low ESR of the paralleled capacitors could make it oscillate easily.Quote from: Noxer on April 29, 2014, 08:30:17 amNo, that is output protection - the regulator will shut off if you short the output pins. See the link in my last post - the problem happens if you short theINPUTof the regulator (e.g. by a poorly chosen connector) while the output capacitor is still charged. Then the large output cap will discharge backwards through the regulator and burn it out. That's why you need to diode to bypass the regulator should this happen. The alternative is to make the output caps smaller, then the energy stored in them won't be sufficient to destroy the regulator in case of a short on input.Quote from: Noxer on April 29, 2014, 08:30:17 amUSB isolators are quite expensive and slow, unfortunately. Try without at first, the USB stick likely has some filtering on the USB port built-in already.Quote from: Noxer on April 29, 2014, 08:30:17 amThat would most likely be an overkill - you have much larger noise sources in all the switching logic inside of that USB stick and common mode noise coming down the USB connection and external PSU line than what a linear regulator (basically opamp + power transistor!) will produce. I haven't seen anyone shielding a 780x regulators - the one on the LNA is not in a separate can neither and it is right next to the signal lines!Quote from: Noxer on April 29, 2014, 08:30:17 amMake sure that the noise is actually a problem before embarking on "killing" anything. You may waste a lot of effort and money fighting a non-existing problem. Just shield the LNA properly and put some chokes on the power lines as described before. The USB stick doesn't really need to be shielded - it could even cause worse problems when "shielded" inside your metal case, if you don't separate the LNA and the stick properly into separate, isolated compartments, because all that digital noise will be right next to the LNA.

author: Noxer
author_rank: Newbie
timestamp: May 19, 2014, 09:28:27 pm
content: HiFirst thank you VERY much for the help.Yesterday my LNA has arrived and I have done some tests. The SRD draws about 270 to 370 mA and the LNA 50 to 60 mA. And I will follow your advice and chande the cirsuit. i have found some tantalum capacitors of 10uF and 22 uF. I will put two capacitors onto each device. And I have looked trough the LM2940 datasheet and saw that the capacitance can be increased without limit. But the ESR needs to stay between 0,04 to 5 ohm ( for 450 mA max). Now my question why to put two capacitors and how to put an aluminium one ( or something else)and still keep the ESR low. And what capacitance and material should I pick the other capacitor because i already have some 10uF tantalum as I said.And do I need a protection diode? The datasheet says that the regulator has a reverse polarity protection.

author: janoc
author_rank: Super Contributor
timestamp: May 23, 2014, 01:46:10 pm
content: Well, you are probably better off not increasing the capacity of the caps too high - that would cause issues both with ESR (think two "resistors" in parallel!) and stability of the LDO.I wouldn't bother with tantalums, put normal aluminium electrolytic caps there - both cheaper and thanks to the higher ESR better for this application. Tantalums have a low ESR and can cause stability issues. You don't need a low ESR cap for a linear regulator - those are needed when there is a strong AC component passing through it, e.g. in a switching PSU.The protection diode may not be strictly required but probably wouldn't cause any harm. Furthermore it is cheaper to replace a fried diode than an LDO. The LDO built-in diode may not be rated for high currents from the discharging capacitors, especially if you put some really large caps there.

author: dannyf
author_rank: Super Contributor
timestamp: May 23, 2014, 01:57:12 pm
content: QuoteSound advice and absolutely agree.