What is this component and purpose of the circuit? - Page 1
Forum section: electronics / beginners

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: November 05, 2017, 02:08:15 pm
content: Can anyone tell me what this component is likely to be please?It is marked J3 and is included in the antenna circuit on the main board of my RTLSDR radio. I have attached some images to clarify. I did start a thread in connection with the RTLSDR device here:https://www.eevblog.com/forum/rf-microwave/new-sdr-radio-does-not-seem-to-work/However, here I am specifically seeking to understand the antenna circuit on the board, which for whatever reason has been left disconnected at the points marked with an 'X'. It has been suggested to me that the J3 device may be a package containing two TVS diodes. Certainly it makes sense to provide ESR supression on an antenna, but usually these are connected across the source. In this circuit, they have small capacitors on both sides and the 'diodes' would be in series with the RTL2832 IC.I had wondered whether the 3.3v supply might be to provide a DC supply via bias tee when connected, but the 1meg resistor and series capacitor (R2 + C4) would seem to argue against that. The DC voltage would need to be fed in on the other side of the capacitor (C4). I am thinking about constructing a satellite antenna which has a requirement for a 5v supply via a bias tee, so I would need to take that directly from the incoming USB supply.So, I am trying to understand what the designer was trying to accomplish with the circuit around that J3 device as well as identify what that component marked J3 is. I can only add that when connected to the RTL2832 IC, it performs very poorly. Any further views would be appreciated.

author: orolo
author_rank: Frequent Contributor
timestamp: November 05, 2017, 02:35:05 pm
content: Looks like a bjt in common emitter configuration, crudely biased through the 1meg resistor. A quick look for j3 smd code gives 2sc3142, an vhf npn bjt.Link:http://www.semicon-data.com/transistor/tc/2sc/2SC3142.html#2SC3142.Pinout seems to match.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: November 05, 2017, 07:03:31 pm
content: Thanks for that. I always struggle to identify SMD markings as they can mean many things. I did find J3 as at least 3 different types of diode, but only in a two pin package. I did wonder whether it might be a BJT and the pinout would seem to match and it can handle up to 750MHz so more than enough for a HF application. The resistor is marked 105 so as you quite rightly point out is 1meg not 10meg, but even so with just a 3.3v supply does not seem to provide much of a bias current?Is there a definitive refertence for SMD markings anywhere or is it always a case of referring to different vendors?

author: orolo
author_rank: Frequent Contributor
timestamp: November 05, 2017, 08:57:39 pm
content: If the transistor is active, using KVL: 1k·Ic + 1meg·Ib + 0.7 + 510·Ic = 3.3, let's assume beta=100, so Ib = Ic/100. We have: 1000·Ic + 10000·Ic + 0.7 + 510·Ic = 3.3. So: 11510·Ic = 2.7, meaning Ic = 2.7/11510 = 0.235mA, good enough for this kind of transistor. Spice predicts about 0.32mA for a 2N3904.I think the idea is a matching network for the antenna, then a 47 ohm load, and then a capacitive coupling to this amplifier that, with Ic=0.25ma (approx), and assuming that C5 is a short to ground at signal frequency, has an input impedance of about 1K, much higher than the 47 ohm load. A transistor this fast in this configuration could give reasonable gain, 20dB or so, well into the tens of MHz. This, assuming C6 and C7 don't load the amplifier: if this is a typical CE amp, C7 should be high impedance, but I don't know what C6 is coupling to, if C5 is a short.Try connecting only the output of C7 to the rest of the circuit, leaving C6 alone. Maybe when you connect C6 you are spoiling the amplifier.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: November 05, 2017, 09:35:41 pm
content: Yes, I get the first bit with the network, the 47ohm load and the capacitive coupling. So if it is indeed a transistor and everything is connected then we are presumably dealing with something like in the attached.Quote from: orolo on November 05, 2017, 08:57:39 pmWell its certainly worth a try. I initially connected the two pins on the RTL2832 IC. This worked and some HF was received, but sensitivity was extremely poor. I could pick up only the strongest of signals. I then connected the 3.3v line thinking that this might make an improvement, but disappointingly it seemed to make no difference. I was still getting the same very poor sensitivity until I modified the circuit by removing the series capacitor and the 47ohm resistor and adding a 1:4 transformer, and also a pair of diodes on the antenna input for ESR protection. The difference was incomparable. Clearly if this is intended to be an amplifier, it does not seem to work very well.It is difficult to know what the designer intended. There appear to be two variants of this RTLSDR device. The more expensive (about 40GBP) 'UK upgraded' version has an additional switch. Does this come with HF connected and a switchable bias tee? Then there is the 25GBP version which I have, and which has the HF antenna circuit built in but disconnected. This is why I was evidently getting absolutely nothing on HF until I connected up the two pins on the RTL2832 IC. Since there were no instructions of circuit diagram that came with it, I had to reverse engineer the antenna circuit in order to provide it here. I am working rather in the dark and have no way of knowing the capacitor values or how the designer intended for the HF antenna circuit to be connected to the RTL2832 IC. Does it require an impedance matching transformer be placed between the two decoupling capacitors (C6/C7) and the RTL2832 connection points or just links? On the other hand is the very poor performance caused by a design flaw? For example, does R1 adversely impact to reduce the gain? (It is not present in the older BA5RA version of the RTLSDR - on which my mod is based). Presumably if I want to supply a 5v feed to the antenna then it will almost certainly have to be removed anyway along with the two protection diodes that I added, so then how do I then protect against ESD? Perhaps some of these questions will only be answered by further experimentation.

author: orolo
author_rank: Frequent Contributor
timestamp: November 06, 2017, 12:15:38 am
content: Well, I didn't know anything about the RTL2832 chip, so I looked around. I'm a bit lost, but it seems that some time ago people discovered that directly injecting a signal (bypassing the tuner chip) into either of pins 1 or 2 (or both, with a balun) managed to catch HF transmissions, with some modifications to the drivers, if I understood well. They also warn that this bypasses the chip's ESD protection, so doing this mod the chip could be fried, use it at your own risk, etc. Here is a youtube video about this mod:So in your case the circuit could be similar: the initial section is a low pass filter to keep higher frequencies away. Then, an amplifier. And finally a balun to inject the signal into pins 1 and 2. Still, if I understood your circuit, you are lacking the balun, unless the transistor is being used as some kind of crude phase splitter.Maybe this part of the circuit was intended for direct signal injection into pins 1-2, with a switch somewhere to turn off the tuner chip. Dunno.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: November 06, 2017, 11:19:49 am
content: Quote from: orolo on November 06, 2017, 12:15:38 amThanks for that. I have seen others similar and am aware that either pins 1&2 or 4&5 can be used. Pins 4&5 are usually n/c and in this case have pads provided on the SDR board so I used these. I notice that the HG7AN mod has a 'Monolithic Amplifier' includedbetween the network and the balun and he has used the same transformer by Mini Cicruits that has been recommended elsewhere.Quote from: orolo on November 06, 2017, 12:15:38 amWell if pins 1&2 are being used in some versions then it might make sense to provide a switch because the R820 is also connected to those pins. The switch would toggle between HF and VHF/UHF. If the transistor is being used as a crude amplifier/phase splitter then all maybe only the balun needs to be added.

author: orolo
author_rank: Frequent Contributor
timestamp: November 06, 2017, 04:48:21 pm
content: I had read that the quadrature pins could be used, but I wasn't sure. Inthis page(needs translation unless you speak Japanese,this is the datasheetfor the balun), they do just that, with a small balun. Then, there isthis other post, where a common base amp + balun is used; the transistor works in a different mode, but it's an interesting example of transistor amplifier + balun. So, this does not strike me as very clean, but I wonder if they connected the primary of the balun to the emitter and collector of J3, and then the secondary of the balun to Pins 4/5.