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LNA4ALL on the VHF close to antenna effect

LNA4ALL on the VHF close to antenna effect submitted by /u/Adam-9A4QV
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LNA4ALL on the VHF close to antenna effect [00:04:44]

Comment on TETRA Decoder Plugin for SDR# Now Available by Eddy

Not working for me same error as Xanax too…WIn7-64/win8.1-64 and have tried all ways on this page but no lucky.

RTL-SDR Discussion • Direct sampling dongle V3 modification; input impedance of RTL2832

Reading these posts I decided to remove the original transformer in my dongle (recent production; version V3) and to measure the input impedance. Not with an impedance meter, but by adding and removing outside series and parallel impedances; complex and real. And this is what I measured: Ri differential is 6000 Ohms with a parallel capacitance of 12 pF.

Then I wanted to modify the lower frequency sensitivity loss. The 10 uH for antenna supply current, the diplexer components and the BGA2867 input and output changed the response according expectations and did not give a new signal path problem. But then I found that the BGA2867 is used far out of the specified frequency range and easily can go into a DC latch-up situation. Far too easy. So I removed the BGA2876 also. Then I made a new transformer 77 uH primary and 288 + 288 uH secondary (for my ferrite transformer 6 on 12 + 12 windings). And now the frequency response is (1 MHz is reference 0 dB) -1 dB at 180 kHz, -3 dB at 95 kHz, -6 dB at 52 kHz, -10 dB at 32 kHz and -20 dB at around 12 kHz.

The input is now on a separate SMA-cable direct to the transformer. No amplifier in this line. The sensitivity is 18 dB less than the original dongle without this modification. The S-meter reading minus 13.6 dB corresponds with the signal generator dBm value. The minimum discernible signal is now -90 dBm. So the amplification lacks about 20 dB; that equals the amplification of the removed amplifier.

There is, after this modification, no input bandwidth limitation. I added (externally) a 14 MHz low pass ( two inductors of 1 uH, input and output capacitance 150 or 180 pF and in the middle 300 or 330 pF to ground). And 10 pF across the first inductor and 33 pF across the second inductor. See attachement for diagram and simulator response.

The input impedance is NOT 50 ohm any more. This modified dongle is INTENDED to use with 50 ohm source. The measured sensitivity (over frequency) and frequency response are almost perfect when measured with a 50 ohm source impedance. The actual input impedance is higher, 600 ohm parallel 77 uH parallel 150 pF.

When using this filter with the MODIFIED dongle (higher input impedance) then it is advised to match the output by insertion of a 50 ohm attenuator of a few dB, or just a 100 Ohm resistor across the output. And then connect it to the dongle input: direct to the modified transformer.

Advise for followers of the modification: replace the BGA amplifier (NEC has identical amplifier, but an inductor must be added from output pin to plus: no latch-up) and replace the transformer. Be prepared for extreme miniature work: first check that you cam produce such a transformer; otherwise buy it. Check if it fits in the elegant housing. The original V3 dongle design has no further shortcomings.


Statistics: Posted by Brightnoise — Tue Jan 30, 2018 11:13 pm

Audio Jammer using a SDR

Hi All, A little bit on the problem statement : I was just thinking if there was a way to detect voice recording devices in say a meeting room. Because especially when it's a confidential conversation it almost becomes impossible to assure that the VoIP phone in the room or someone could be recording all of it on their phones. Existing solutions: Existing solutions use Audio jammers which emit White noise that makes it impossible for the passive recording device to capture the conversation. I was wondering if this could be implemented using a SDR with it's TX capabilities.

Any help or advice is appreciated.


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RTL-SDR Discussion • Re: LNA and bias tee understanding

rtlsdrblog wrote:
Tue Jan 30, 2018 10:05 pm
Remember that an LNA will only really help at 420 MHz if you have a long lossy run of coax cable. Using a low NF LNA may help by a few dBs as well. But too much gain could make things much worse by overloading the dongle.

The NooElec doesn't have a bias tee, but yes you can wire power directy to the LNA instead. At 420 MHz an eBay LNA should be fine.

But do remember what I mentioned earlier, that too much gain can make things worse. So unless there's a lot of loss in your coax 32 dB might be too much. I'd recommend adding a 420 MHz filter as well to block those strong out of band frequencies which could cause issues with high gain.
Thank you for the reply. The antenna I have is connected to the stock coaxial cable that the noelec came with so I don't think it's very long. I will measure the length tomorrow and post back. By NF do you mean noise filter? The LNA I am looking at currently suggests that the gain can be altered depending on supply voltage so that might be one way of fixing a high gain issue. I am not very knowledgable in sdr and rf stuff as I am learning but going by your suggestion for a 420MHz filter I looked up something called a rf preselector bird, is this what you mean?

Statistics: Posted by usernamehere — Tue Jan 30, 2018 10:40 pm

Comment on TETRA Decoder Plugin for SDR# Now Available by Xanax

Sure. I tried several diffrent versions : 1583, 1631, 1637

Всем доброго времени суток, может кто то собирал вот такой девайс.. есть…

DIY Kit 100 кГц до 1.7 ГГц УФ ВЧ RTL sdr приемник USB тюнер R820T 8232 CW FM купить на AliExpress
Всем доброго времени суток, может кто то собирал вот такой девайс..https://ru.aliexpress.com/item/DIY-KIT-100KHz-to-1-7GHz-UV-HF-RTL-SDR-USB-Tuner-Receiver-R820T-8232-CW/32812455991.html?spm=a2g0v.search0104.3.127.90960633xF1xD&ws_ab_test=searchweb0_0,searchweb201602_4_10065_10344_10068_10342_10343_10340_10341_10543_10084_10083_10618_10307_10615_10301_10313_10059_10534_100031_10103_441_10607_10624_442_10606_10623_10622_10621_10620_10142_10125,searchweb201603_1,ppcSwitch_5&algo_expid=18f233f3-efd2-4eb0-a8ab-0221a143ecc6-18&algo_pvid=18f233f3-efd2-4eb0-a8ab-0221a143ecc6&transAbTest=ae803_5&priceBeautifyAB=3 есть вопрос... по номиналу одного элемента , полагаю что это резистор , пришлось выпаять для удобства припайки проводов, но неловкое движение занесло его в список пропавших безвести, буду очень признателен за помощь..

Troubleshooting Help • Re: NooElec NESDR SMArt — Error: Cannot access RTL device

Drivers look okay. If it's happening on two separate PCs then it might be a hardware fault.

One thing to try first might be to disable Windows automatic driver updates. Sometimes this can break things.

Also double check the rtlsdr.dll is inside the SDR# folder.

Statistics: Posted by rtlsdrblog — Tue Jan 30, 2018 10:15 pm

RTL-SDR Discussion • Re: Satellite Tracking

You won't find an ADC that works that high. You'll need a tuner on the front end to convert the frequency down.

I still don't understand the project though. You won't be able to design an ADC from scratch - that's high level design work requiring a silicon chip factory. My guess is that they're asking you to build an SDR from scratch using an off the shelf ADC component.

I would recommend Googling for "homebrew SDR". There are so many well documented projects out there that you could reference.

Statistics: Posted by rtlsdrblog — Tue Jan 30, 2018 10:10 pm

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