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F5OEO / rpitx

RF transmitter for Raspberry Pi
GNU General Public License v3.0
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What output Impedance could I expect from Raspberry 2B or 3B? #54

Open antonjan opened 7 years ago

antonjan commented 7 years ago

What output Impedance could I expect from Raspberry when I am transmitting with FM on VHF frequency 145Mhz (Ohm)?

F5OEO commented 7 years ago

That's a difficult question. It seems that output is around 300 ohm, but depend on frequency and dynamic in amplitude (not used in FM). Help required to better know impedance !

felixzero commented 6 years ago

Hello everyone, I did a few very simple experiments to try to estimate the output impedance of the Raspberry Pi using Rpitx. I build a simple RF probe using a BAT85S Schottky diode, a 22 nF capacitor, and a set of load resistors (1M, 10k, 1k, 100). I connected it to the GPIO4 pin and the nearby GND pin (pins 7 and 9). Here is a picture of the setup:

img_20180506_161540

The probe is then connected to a regular voltmeter set in DC mode. I used the "chirp" tool from librpitx to generate a signal at various frequencies (which bandwidth=0 so it is a pure carrier signal). Here is a table of the voltages I measured:

capture du 2018-05-06 17-13-22

Note: I stopped at 100 Ohm because I didn't want to fry (or bake :) ) my Pi as the current reached 14 mA already. Also, we can see a huge voltage drop above 200 MHz, but I don't think this is due to a lower output power of the Pi, but rather a combination of stray inductance and capacitance from my dirty wiring and probably from the response of the diode itself. From this, I could extract the voltage at zero load (assuming 1M is infinite...) and an estimation of the output impedance of the Pi:

capture du 2018-05-06 17-18-573

capture du 2018-05-06 17-18-572

From what I could measure, the zero-load voltage is about 2V (note: Vrms = Vmax for square waves and I assume the output is more or less square) and the source impedance should be close to 50 ohms. However, actually plugging a 50 ohm load at the output would draw about 20 mA, which I think exceed the rating of these GPIO pins.

-- felixzero F4VQG

F5OEO commented 6 years ago

Thanks for this very nice contribution to understand better GPIO impedance when it is used in RF. Maybe could be reproduced using a VNA ?

felixzero commented 6 years ago

Unfortunately I don't own such equipment. These can cost as much as a car...

kj7rrv commented 3 years ago

@felixzero have you heard of NanoVNA? They seem to be a bit more expensive in Europe than in the US, but it looks like you can get one at https://www.passion-radio.com/meter/nano-vna-918.html?search_query=nanoVNA&results=6#/221-screen_size-2_8_7_11cm for €65. 73 de @kj7rrv