Ferrite rod antenna oscillation - Page 1
Forum section: electronics / rf-microwave

author: AntonYuzhakov
author_rank: Newbie
timestamp: November 23, 2019, 05:17:42 am
content: Hello, i'm not sure if this is the right forum section to ask this so i apologize if it isn't.So, i needed to receive and demodulate on-off keying modulated signal with carrier frequency of 114 kHz. I decided to use ferrite rod antenna (parallel LC circuit) because it was easiest to build (at least i thought it was) and an amplifier made from reasonably fast Op Amp in non-inverting configuration.Problem is that the signal i got isn't what i expected and i don't know if this should've been expected result or i should change something (what?) about my antenna to get what i wanted.Yellow trace is a signal received by placing transmitter in a loop made by shorted oscilloscope probe (what i was expecting to get, more or less), blue trace is a signal from my antenna after amplification. As you can see it oscillates too long after transmitter turns off, which makes it almost impossible to demodulate.My question is: can this be fixed and how?

author: vk6zgo
author_rank: Super Contributor
timestamp: November 23, 2019, 05:48:09 am
content: As far as I can see, it looks like your ferrite rod antenna may have too high a Q at your carrier signal, hence it wants to continue oscillating.Try damping it with a parallel resistor.Another possibility is that your Op Amp is unstable, & is on the verge of oscillation, but cannot maintain it.The incoming bursts are enough to allow it to oscillate, then taper off back to a more stable state.Some Op Amps are a bit "touchy" about reactive circuits in their input or output.

author: radiolistener
author_rank: Super Contributor
timestamp: November 23, 2019, 09:25:02 am
content: This is normal behavior of resonant antenna.As you know, high Q antenna has very narrow bandwidth. This is because it keeps oscillating.But at the same time, high Q antenna has amplitude gain for Q times.The relation between Q and bandwidth is very simple:BW = 1 / Qwhere BW is fractional bandwidth:BW = [bandwidth Hz] / [center frequency Hz]

author: profdc9
author_rank: Frequent Contributor
timestamp: November 23, 2019, 09:48:55 pm
content: This is exactly the principle of a regenerative receiver. A regenerative receiver has feedback, just enough to prevent oscillation. A received signal is amplified, with the amplification being greater the closer the circuit is to achieving positive feedback and oscillating. It was used for radio receivers when valves were expensive and so a single valve could both perform the function of filtering and amplification. However, it is unstable as you see, and requires the amplifier to be just on the verge of oscillation. This is a major reason other receivers such as superheterodyne were developed. But it still instructive to build one as you have.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 23, 2019, 09:54:37 pm
content: Not sure what you mean. The bandwidth appears to be almost perfectly matched to the bitrate. If you are having problems, look at the ADC. Implement or improve ALC, hysteresis, clock rate detection, etc.If you wish to receive higher bitrates, you will need a lower Q (wider bandwidth). This should all be part of the system design; the antenna response isn't something that just happens, it is completely controllable.Tim

author: AntonYuzhakov
author_rank: Newbie
timestamp: November 23, 2019, 11:41:11 pm
content: Quote from: T3sl4co1l on November 23, 2019, 09:54:37 pmI was hoping to do that without digital processing using envelope detector and comparator but if it all fails i guess i should do it that way. Can you point me where to look for info about that? I've never done that before, so any tips would be greatly appreciated. I tried to search for info on that topic but i guess my google-fu is weak.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 24, 2019, 04:24:14 am
content: e.g.https://www.google.com/search?ei=JQTaXbSBAsqisAXuhpC4BQ&q=ask+keying+front+endAnything about ancient code transmission -- Morse code -- will do. There's a century of radio design and build history to read. Amateur/HAM radio, simple receivers, famous receivers; their building blocks; texts discussing them, like the ARRL Radio Amateur's Handbooks; etc.Tim

author: radiolistener
author_rank: Super Contributor
timestamp: November 24, 2019, 10:24:10 am
content: Quote from: AntonYuzhakov on November 23, 2019, 11:41:11 pmYou can start from article about Q-factor in the "Радио 1948-3", see page 22.And "Радио 1948-4", see page 19.http://archive.radio.ru/web/1948/03/http://archive.radio.ru/web/1948/04/

author: AntonYuzhakov
author_rank: Newbie
timestamp: November 24, 2019, 10:53:07 am
content: I'm more interested in digital demodulation algorithms after digitizing the signal asT3sl4co1lmentioned.

author: radiolistener
author_rank: Super Contributor
timestamp: November 24, 2019, 11:02:13 am
content: Quote from: AntonYuzhakov on November 24, 2019, 10:53:07 amthen you can start from article that I mentioned above, digital signal processing is the same as analog. If you don't understand how works analog receiver, it will be hard to understand how works DSP and SDR radio...This article explains your question about ferrite rod antenna. You're needs to know that in order to process the signal.All these analog RF things even more important when you deal with high speed ADC and digital electronics. And signal processing is the same, the only difference is that operations are performed in digital domain.For example, when you understand how works analog superheterodyne receiver, you will understand how to implement it in digital domain with ADC connected to antenna. Because signal processing in digital domain will consists of the same blocks as analog one (LO, mixers, filters, detector, etc).You're needs to starts from basic things like how works LC circuit, it's properties, etc. Without it you will not be able to build receiver. If you open any book about digital filters, you will see the same things as in book about analog filters. The same properties. And these basic things are explained in article about Q-factor of LC circuit, that I suggested you above.Regarding to "digital demodulation algorithms after digitizing the signal", in digital domain it works exactly the same as it works in analog domain.

author: iMo
author_rank: Super Contributor
timestamp: November 24, 2019, 11:34:54 am
content: ..And always show your schematics.. Schematics is the way people in electronics communicate

author: iMo
author_rank: Super Contributor
timestamp: November 24, 2019, 12:39:42 pm
content: You have to couple your opamp such the antenna's L2C1 couples in an low impedance, it seems (ie the L3).Anyhow, the L2C1 has to be damped a bit.

author: radiolistener
author_rank: Super Contributor
timestamp: November 24, 2019, 01:38:22 pm
content: There is a nice book, which explains modern receiver architectures:"MODERN RECEIVER FRONT-ENDS Systems, Circuits, and Integration" JOY LASKAR, BABAK MATINPOUR, SUDIPTO CHAKRABORTY, Wiley, 2004:https://www.wiley.com/en-us/Modern+Receiver+Front+Ends%3A+Systems%2C+Circuits%2C+and+Integration-p-9780471474869

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 24, 2019, 08:34:43 pm
content: Digital, you can do it at whatever level you like.You can directly digitize the signal (at perhaps a few 100 kSps to a few meg, as the oscilloscope does), and analyze it there. But note that, the oscilloscope has an adjustable scale -- there is an analog front end (AFE) in there providing gain, impedance matching (typically an oscilloscope has a 1Mohm input resistance, and its ADC needs a low impedance like 100 ohms, and differential) and filtering (optionally).Then you can do everything in DSP, probably filtering to ensure you have the desired carrier, taking the RMS of the wave to detect it, divide by average amplitude to normalize it (ALC), then apply a threshold to reduce the number down to single bits. That's a lot of hardware, but nothing a modest FPGA or MCU can't do.(Though if you aren't familiar with any of these terms, the amount of work involved is probably quite daunting. The subject is literally worth several undergraduate-level courses!)But, your ADC might have 8 to 14 bits of resolution, and for signals weaker than a few LSBs, you can't get anything reasonable. That's not much sensitivity (a typical analog receiver might have 20-22 bits equivalent dynamic range). You have to perform ALC (automatic level control) in analog, by changing the gain of the input stage, to improve this.You can detect the carrier wave, using analog tuning, ALC and detection, then digitize the detected wave, at perhaps a few kSps. You get a pretty stable level this way, so a hysteresis comparator should suffice for detection.Heck, you might as well put the comparator in the analog domain as well, and then you can simply sample the digital output directly.Note that, most generally, an ADC is a converter from analog to digital, period. Obviously enough. But, this means comparators are 1-bit, continuous-time (not sampled) ADCs. The familiar multi-bit, discrete-time (sampled) ADC is a special case -- it just happens to be a very popular one.Tim

author: radiolistener
author_rank: Super Contributor
timestamp: November 24, 2019, 10:24:15 pm
content: Note, if you don't want to get aliased signal, you're needs to use ADC with sample rate at least twice higher than highest frequency component in your signal. It means that you're needs to put anti-aliasing analog filter before ADC. The goal of such filter is to cut-off all frequencies which are outside of first Nyquist zone. If you don't do that, all these frequency components will be folded into first Nyquist zone and it will leads to aliases.You can use oversampling with filtering in digital domain in order to improve dynamic range. Since ADC output has quantization-noise, it's dynamic range is limited by maximum value and noise floor. But you can apply filter to reduce bandwidth and it will reduce quantization-noise power, it means that dynamic range will be increased. This is known as process gain.As you can see, in digital domain wide bandwidth leads to low amplitude gain and narrow bandwidth leads to high amplitude gain. The same as with analog LC circuit (see Q-factor equation above)