# Tag Info

52

Actually, yes, a receiver can affect the transmitter. Passive RFID is based on this principle. However, RFID only works at very close distances, where the receiver is absorbing something on the order of 10-4 to 10-5 of the transmitter's signal. In other words, the transmitter is sending out hundreds of milliwatts, while the receiver is absorbing a few ...

36

You could potentially break some radio transmitters by operating them without the antenna connected. Several things combine to make this possible. First, it's difficult to make power at RF frequencies, so the power devices are often fairly fragile, and run near their limiting conditions. Secondly, radio signals passing down a transmission line behave in a ...

28

Most useful radio receiver designs utilize some form of the superheterodyne architecture, where one or more radio-frequency local oscillator signals are used to shift the frequency of the received signal to an intermediate frequency where it is more readily processed. Typically, the local oscillator signal will radiate from a receiver to some degree - how ...

26

Ordinary UARTs have to be pre-configured with the desired baud rate (as well as word length, stop bits, parity, etc) traditionally by a human. For several decades now though there have been implementations of "auto baud" detection found in some settings, which typically works by timing key features of the waveform to deduce the baud rate. Early versions ...

24

It's technically possible to detect radio receivers if they are Superheterodyne receivers that use RF mixing to downmix the received signal to a well known intermediate frequency. You can scan for this frequency using a directional antenna and count the receivers around you. Though this doesn't sound like what you're inferring since the transmitter can't ...

18

This answer is focussing on radio receivers such as AM and FM. If you are only interested in receiving a signal from one station you may not need to have or use an intermediate frequency. You can build your receiver to tune in to just that frequency - the tuning needs to be sharp - you need to reject all possible other sources that may pollute the signal ...

16

Just to complement the excellent answer of Neil_UK and stress the fact that at RF frequencies voltages and currents don't really behave as those nice entities you know from KCL and KVL. You must drop Kirchhoff's laws and get your hands dirty with transmission lines theory, where the same concepts of voltage and current become a lot weirder! In other words,...

16

The reciprocity theorem is about the gain of the antenna. Let's say we have an antenna with 50% power efficiency, so 3dB more loss than a perfect antenna. If used for reception, it will lose half the power it receives, so decrease the signal to noise ratio due to the receiver input noise by 3dB. Not ideal, but no biggy, it just means a reduction in range ...

11

IF makes the receiver both more economical and higher quality. RF parts are trickier to make and use, and the circuitry more beset with problems of stray capacitance, inductance, noise, ground loops and interference. More so the higher the frequency. But we must have an RF front end because the signal at the antenna connection is just too weak to do ...

11

You could think along these lines: - simulate this circuit – Schematic created using CircuitLab The data you send needs to be DC balanced i.e. something like Manchester encoded or scrambled. The amplitude of the data needs to be significantly smaller than 5V - maybe 1Vp-p. You'll need a comparator/data slicer on the data out signal to recover the ...

11

No. There is no way for an AM or FM transmitter to determine how many people are listening. They provide exactly the same power output at carrier whether there are a million receivers within 1 mile or zero. Digital transmissions that require a subscription can on the other hand possibly know how many receivers there are, if there is a two way verification ...

11

Sounds like complete and utter cr*p for all practical purposes. The actual energy extracted by a receiver is microscopic. Though there is a story of a farmer who built a big tuned loop in order to extract free power from a nearby radio transmitter. Sufficient to distort the field pattern and be detected.

11

Ordinary LEDs can also be used to detect incident light, because they work as photodiodes too. You will need to amplify the incoming signal. Take a look at this projects (Jeff Han is the first one that used LEDs as sensors): https://cs.nyu.edu/~jhan/ https://hackaday.com/2016/06/17/ask-hackaday-whatever-happened-to-led-light-sensors/ Or google "LEDs as ...

10

Basically it's to allow the demodulation circuit to be made very sensitive with a narrow bandwidth. If the demodulation circuit had to be wideband (say, able to work for any frequency from 88-108 MHz for FM), keeping a flat response across the entire frequency range would be difficult. Instead, the tuner is wideband and then beat (heterodyned) to a single ...

10

At the point just before the first amp, the radio waves have been filtered to a particular frequency by L1, C1 & C2. That first amplifier is not an op-amp, it is an RF gain amplifier. It amplifies the incoming signal by a number of dB. The signal before and after will be an AC signal, equally biased around ground. The diode or detector gets rid of one ...

10

But I wonder how does this handshaking takes place between a Master and Slave when the Slave is sender and the Master is receiver and only the Slave (sender) knows when there is no more data to send to the receiver? This isn't supposed to happen. i2c is a very defined protocol and each slave device should be known to each master. Typically, the master ...

9

It intentionally DOES oscillate. It goes beyond a regenerative receiver and is called a super-regenerative receiver, invented by Edwin Armstrong in 1922. The values of R3 and C3 are arranged so that after oscillation starts the voltage builds up on the emitter due to rectification of the oscillations. After some period (10's of microseconds typically) the ...

9

First of all is it possible to create 10 GHz wave on a PCB? Any clue on how to do something like, regarding the circuitry? If by "creating" you mean propagating and/or radiating, the answer is yes. However, you need to use specific board substrates like Rogers 4000 series, with low losses and controlled dielectric constant up to that frequency. Your ...

9

There is a bigger picture to consider... For the reciprocity theorem, if an antenna has a low efficiency in transmission, it has the same efficiency when receiving. Just to be absolutely clear about this: you can have a very good and fairly efficient receiving "antenna" that makes a very poor transmitting antenna. Reciprocity theorum doesn't always ...

8

Depending on the distances involved, sensing / timing precision available at master transceiver, response speed at slave transceiver, and computational power available on the master device, Time of Flight mechanisms may be usable for distance estimation. ToF measurement involves transmitting an identifiable, unique bitstream from the master transceiver, ...

8

I think that there is a very simple answer to this question that has nothing to do with electronics. A number of airlines prohibit the use of GPS receivers in flight. Their rationale does not actually matter. In order to comply with their rules, the watch must be able to turn off the GPS receiver somehow or it would be prohibited from being on the plane ...

8

Two UARTS "agree" on baud rate by means of documentation and by operator/user setting the baud rate by hands, including handshake protocol, stop bit size, etc.

8

Yes, certainly is possible. For further info, I suggest finding a copy of a MERL reseach report by Dietz et al from 2003 how they did it. It does not in fact produce enough voltage to detect it with the MCU directly. It usually is done in a way that enables the generated photocurrent to discharge the parasitic junction capacitance of the LED which is first ...

7

First you have to decide how represent your signal. Sound is easy: a microphone gives you a voltage proportionate to sound pressure. If we send this signal (with amplification) to a speaker, you get a reproduction of the sound. But how do we represent this signal as a radio signal? The answer is modulation. Perhaps the easiest modulation to understand ...

7

Assuming the the field in question is the electromagnetic field, and all interactions are in the 'far field', then the question is 100% no, no you cannot sense increased load. RF is just the production of light, albeit a frequency much lower than visible (WiFi runs at 2.4 GHz. Red light is ~400 THz). Does a star experience more 'drain' because its light is ...

7

In this particular circuit application, the demodulation of amplitude modulated RF, no, the polarity of the diode does not matter. If you reverse the diode, you simply track the positive part of the envelope rather than the negative part. Either will give you the demodulated signal.

7

First, I do not understand how the LC filter selects the frequency. I know that it's impedance is 'infinity' at the resonant frequency, but I don't understand how it rejects the other frequencies: no matter the impedance of the LC filter for a given frequency, the voltage is applied at the 'X' node and hence every frequency is applied at the input of the ...

6

Neither of your circuits make any sense. That is simply not how transistors, capacitors, and crystals work. Unfortunately, they are so far off that trying to explain why they won't work is too large a task for a reasonable answer here. Also, you'd need to understand more electronics to understand the answer, and trying to teach all that electronics first ...

6

Here's how it works but a couple of factoids first: - According to this source, the most common frequency of transmission is 437 MHz Antenna gain$^1$ for a 1m dish is about $0.5\cdot\dfrac{(\pi D)^2}{\lambda^2}$ and at 437 MHz $\lambda$ is about 0.69 metres and gain will be about 10 and in dBs this also is 10dB Link loss from satellite to earth is: - ...

6

This is explained here in detail: So, in the optimum case half of the power absorbed by the antenna is immediately re-radiated. Clearly, an antenna which is receiving electromagnetic radiation is also emitting it. This is how the BBC catch people who do not pay their television license fee in England. They have vans which can detect the radiation emitted ...

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