Electromagnetic compatibility and Goertzel Mode - what can be done safely?
Christof Eb.
Posts: 1,214
Hi,
after I got working Goertzel mode, I want to experiment with it but I am feeling uneasy about the radio effect.
I want to output a sine frequency signal Vpp=3.3V via a resistor, say R=1k5, to a coil or an electrode plate. Perhaps a LC combination.
Are there rules of thumb to select frequency and resistor in an acceptable way?
Thank you very much!
Christof
Comments
Allowing just the wrong RF out of your experiment can bring a heavy knock on your door (Eventually), but anything at logic levels and short wires is probably, possibly, maybe OK. For instance, The time standard is broadcast on 2.5,5,10,15,and 20 MHZ. messing that might annoy people with 'atomic' clocks.. Ham radio operators may hunt you down, introduce themselves and help you avoid this if it happens on a frequency of interest to them...
Running right at the resonant frequency of your external components might cause weird stuff, and maybe even damage to happen. check out the P1 objects starting with 'Inductor' for some discussion and hints on how to find and safely use resonant attachments with digital logic. (Which MAY be very helpful to your project)
Let us not get too paranoid about this.
At this power levels (single digit micro Watts at base frequency) and with short connections in a range of centimeters your experimenting should abolutely not get you in trouble. Sure there are numerous rules that aply to the RF frequency bands and emited power levels but these are country specific and most likely do not aply to such tiny amount of energy emitted. Your connection wires will act as antennas but mostly very poor ones as you are not going to match their imedance to the source impedance and even if you did, at these lenghts the frequeencies would need to really go much higher than even third or fifth harmonic of the base signal frequency the P2 can run at.
It is highly appreciated you thought of that EM compatibility issues but honestly, I do not think these types of experiments should get you in trouble or impact your/your neighbours' RF signal noticeably.
Thanks for the inputs!
At the moment, I have used a frequency 999kHz, that I could receive with an am radio. A 10cm wire at the 990Ohms output without additional resistor seems not to radiate much. (I can hear my wireless mouse and serial transmission (921600 baud) at 999kHz, if the receiver is on the same table.) Perhaps it is not a bad idea, to use a frequency, that I have a receiver for.
You've got the right idea using the AM radio on a non-used station frequency to test how efficient your radiating setup is.
I did some early tests with the P2 using some alternating test tones, and was able to hear it a few tens of metres down the street, but it faded off beyond that.
Agreed, I didn't want to discourage experimenting. But OP did mention LC combo, which can be surprisingly efficient at radiating under the right (Or Wrong.. depending on what you are trying to do..) conditions.
Here is an example of what you can do with some very low power and the right antenna:
https://www.mike-gualtieri.com/posts/rockmite_qrp_rig