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Re: LF: Bluetooth instead?

To: <[email protected]>
Subject: Re: LF: Bluetooth instead?
From: "Alan Melia" <[email protected]>
Date: Tue, 18 Mar 2014 23:05:33 -0000
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Hi Stefan was there any problem with AGC on the MIC socket of the PC?? I doesnt seem to bea problem but I wondered if it explains your "strange effect".
 
Thanks for sharing that was a very interesting evaluation.
 
Alan G3NYK
----- Original Message -----
Sent: Tuesday, March 18, 2014 9:47 PM
Subject: Re: LF: Bluetooth instead?

Hi all,

I'm continuing with my report about experiments with a wireless AF bluetooth bridge that will (hopefully) allow to transfer a VLF or LF/MF spectrum between antenna and receiver.

Meanwhile i bought 2 RTX1 units (http://www.amazon.de/B-Speech-Stereo-Bluetooth-Receiver-Transmitter/dp/B00CB3KGYE/ref=sr_1_1?ie=UTF8&qid=1394458092&sr=8-1&keywords=B-Speech+RTX1+Stereo+Bluetooth+Receiver+und+Transmitter)

One is used as the TX and the other one for the RX. It is a stand alone system, actually the PC does not notice anything from the bluetooth stuff. That means, no software or driver or firmware and all that stuff is needed, just these 2 small devices.

Now, measurements:
The measurements were done with SpecLab and the RX connected to the MIC input of the PC where SpecLab is running.

Noisefloor: -100 dBHz, sometimes popping up to -90 dBHz.
Dynamic range (tested with a single sine tone at 1500 Hz, between Intermod and "O-copy"): 66 dB
Dynamic range (tested with a single sine tone at 10000 Hz, between Intermod and "O-copy"): 45 dB (distortion acceptable)
High cutoff frequency: ~ 20 kHz
Low cutoff frequency: ~ 20 Hz
Power consumption of the transmitter at 5 V DC (mini-USB) (including the red LEDs indicating a connection to the receiver): 59 mA (~0.3 W)

Spectrum distortion / frequency response: I've replayed a VLF recording for that test, including sferics, MSK stations, the Alphas... Left is original, right is passed though the 2x RTX1 system
https://dl.dropboxusercontent.com/u/19882028/VLF/L%3Doriginal_R%3DpassedthroughRTX1.png

White noise frequency response:
https://dl.dropboxusercontent.com/u/19882028/VLF/white%20noise%20test%20RTX1%20left%20in%20right%20out.png

Strange effects:
When suddenly removing the noise input, the output just slowly return to the background noise of the system. This can't be good for QRN!!
https://dl.dropboxusercontent.com/u/19882028/VLF/Rauschsprung.png


There can more QRM traces when the USB power supply is used on the RX. A traditional transformer power supply or a battery improves the situation. However this is not critical above 3 kHz.

Conclusion:
NOT suitable for our purposes! :-( I'll send them back...

...and i'll spend more time to learn what i have to choose and how to configure, maybe even homemade SMD ICs allowing to transfer much more bandwith without A2DC codecs and such a stuff.

73, Stefan/DK7FC


Am 10.03.2014 14:48, schrieb Stefan Schäfer:
Hi all,

I want to share my recent results of experiments trying to arrange a wireless AF connection, which can be used for a VLF antenna or any other RF spectrum such as 630m and 2200m bands. The goal is always to decouple local QRM sources from a RX antenna installed in a quiet(er) location, to improve the RX capability of the amateur radio station in noisy environments.

It looks like there are some cheap and useful devices on the market, using bluetooth technique. Last night i did some first tests!

First i simply wanted to transfer music from my smartphone to my netbook, using the bluetooth function (never used that before). This worked very well in the first attempt! There was no audible sound quality loss. The sound was played on the speaker of the netbook. But, more precise, it is not the speaker but the standard audio device! :-) That means, you can define virtual audio cable 1 as the standard output device! And here we are, we can feed the audio signal to SpecLab, without additional software except VAC which runs on many LF/MF/VLF PCs anyway. This allowed me to watch the spectrum and see if the signal is distorted or drifting or anything else. Again there was no audible and visible indication of a significant quality loss.

Next i wanted to get an impression about the possible dynamic range of this wireless bridge. I uploaded a VLF recording (the original one from 4X1RF, including my 8970 Hz signal :-) ) as a wav file to the smartphone and played it via the bluetooth link. The signal level in this recording is quite low and depends on the volume setting of the smartphone. Here i saw that the background noise of the system, i.e. the dynamic range, is 120 dB, with some noise lines/peaks peaking to -100 dB. This looked very fine! Also the full 0...24 kHz spectrum was transferred via the bluetooth link and it was possible to lock SpecLab to DHO38 which was included in the recording! Next, this allowed me to watch the frequency drift of the system. Looks quite normal for a crystal, i.e. useful for our purposes.

Now i want to try bluetooth for VLF and LF reception and i want to do some distance tests. 2 km distance, using a directional antenna, this would be very fine.

At amazon, there are relatively cheap bluetooth transmitters with an external antenna, e.g. http://www.amazon.de/B-Speech-Stereo-Bluetooth-Receiver-Transmitter/dp/B00CB3KGYE/ref=sr_1_1?ie=UTF8&qid=1394458092&sr=8-1&keywords=B-Speech+RTX1+Stereo+Bluetooth+Receiver+und+Transmitter

These do even have an external antenna and will allow to use a high gain antenna for higher distances and help to avoid collisions with other signal sources. Since my netbook has an internal bluetooth adapter and since i now know that the received signal can be fed to SpecLab, it is worth to buy the transmitter and to do further tests. This one seems to be one of the stronger versions with 100 mW RF power. Hopefully the power consumption is acceptable...

I will report about the progress...

73, Stefan/DK7FC



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