RF Modulation Technique  - Page 1
Forum section: electronics / rf-microwave

author: stern0m1
author_rank: Contributor
timestamp: February 18, 2018, 01:45:36 am
content: Hi,Does the attachment look like OOK Modulation?The yellow is digital output from an encoder blue is the RF.I think its obvious but i'm a beginner i'm just looking for reassurance.Thanks!

author: ataradov
author_rank: Super Contributor
timestamp: February 18, 2018, 01:54:34 am
content: RF typically does not have DC component. Where and how this measurement was performed?

author: stern0m1
author_rank: Contributor
timestamp: February 18, 2018, 02:12:14 am
content: Yes, its not the actual RF in the air.I attached the pcb with an arrow marking where I probed.Thanks

author: ataradov
author_rank: Super Contributor
timestamp: February 18, 2018, 02:13:38 am
content: If it is measured referenced to the ground, then yes, it does look like simple OOK.

author: stern0m1
author_rank: Contributor
timestamp: February 18, 2018, 05:27:30 am
content: I think I'm not probing the right spot.Are you able to identify where on the pcb I attached previous ly I can probe for a representation of the rf signal?Thanks

author: ataradov
author_rank: Super Contributor
timestamp: February 18, 2018, 05:31:24 am
content: I'd draw a schematic. Should not take too long.But it is 433 MHz, are you sure your scope has enough bandwidth for that?The pin you are probing is probably close enough. Even though is says XTAL, the device is actually a SAW filter. And the whole thing is a single transistor oscillator, so it would be OOK or ASK.Why do you need it?

author: stern0m1
author_rank: Contributor
timestamp: February 18, 2018, 05:48:46 am
content: I'm working on sort of duplicating this transmitter.My scope is only 100mhz but I thought i should be able to make out at least the modulation technike.

author: ataradov
author_rank: Super Contributor
timestamp: February 18, 2018, 05:51:53 am
content: If you want to duplicate it, draw a schematic.But it will be something like this:And you can just assume that it is OOK.

author: stern0m1
author_rank: Contributor
timestamp: February 18, 2018, 01:14:09 pm
content: The encoder is HCS300 which outputs PWM.I dont understand how PWM can become OOK or FSK which just simple circuitry.Is there a concept of PWM RF modulation?Thanks

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 18, 2018, 01:30:50 pm
content: its not PWM, its digital (binary) data. when output is high, the oscillator will output 433MHz signal to antenna indicating binary "1", when output is low, oscillator will be off, no signal (carrier) indicating binary "0". if you want to probe the RF transmission, it maybe on the lowest via (after the white capacitor) which is i think the antenna below (or you should solder in one), but a 100MHz scope will show not much amplitude and you have to be very good at triggering the scope to capture those carrier stable on the monitor, ymmv.

author: stern0m1
author_rank: Contributor
timestamp: February 18, 2018, 01:50:26 pm
content: The datasheet for hcs300 states PWM.

author: Andy Watson
author_rank: Super Contributor
timestamp: February 18, 2018, 02:11:59 pm
content: It looks like the PWM format given in section 4 of the data sheet - Short pulse is a 0, long pulse is a logic 1. The yellow trace looks like it is the PWM data that is to be modulated on to the RF carrier. Usually, these key-fob type transmitters use the simplest of RF modulation techniques possible - i.e. OOK, the RF carrier is turned on for the high portion of the PWM data signal.

author: stern0m1
author_rank: Contributor
timestamp: February 18, 2018, 07:45:54 pm
content: Is it possible that the PWM is being transmitted?Thanks

author: ataradov
author_rank: Super Contributor
timestamp: February 18, 2018, 07:47:15 pm
content: PWM is not a modulation (in RF sense). You can transmit PWM signal using OOK modulation. So yes, PWM can be transmitted this way.This simple transmitter is not capable of transmitting anything other than OOK. You can stop thinking about options right here. If you want to replicate it, focus on the output of the MCU.

author: stern0m1
author_rank: Contributor
timestamp: February 18, 2018, 07:54:29 pm
content: You cant have rf signals that the width would vary to indicate a 1 or a 0?Can you share an image showing PWM being transmitted as OOK?Thanks

author: ataradov
author_rank: Super Contributor
timestamp: February 18, 2018, 07:58:29 pm
content: Yes, you can vary the duration of ON and OFF states to transmit PWM signal using OOK modulation.Your original image is good example, given that 100 MHz scope filtered out 433 MHz carrier. Take your pictures and imagine that 433 MHz sine wave is multiplied by the yellow trace.But in this case it looks like the information is encoded in the pulse with completely discarding the pauses between the pulses. So short pulse is 0, and the long one is 1, or vice versa. Once again, this is not what is commonly understood by PWM.Correction. It looks like the duration of the interval is actually fixed, and the width of the pulse encodes information. But for the purpose of decoding, you can safely ignore the pauses.So duplication of this device is going to happen in two stages - get 433 MHz OOK transmitter (plenty on the market already) and duplicate the output of the MCU.

author: stern0m1
author_rank: Contributor
timestamp: February 18, 2018, 08:05:13 pm
content: I thought ook by definition means on for 1 and off for 0.I understand from you that changing the thickness for a 1 or 0 is also a form of ook.Thanks

author: ataradov
author_rank: Super Contributor
timestamp: February 18, 2018, 08:11:05 pm
content: Quote from: stern0m1 on February 18, 2018, 08:05:13 pmOOK is just a carrier that is turned on or off. There may be some academic definitions, but they have nothing to do with actual engineering, The way you turn on on and off the carrier transmits the information. And you are free to do it however you like. Most commonly seen hobby transmitters and receivers just connect raw UART output to the transmitter and input to the receiver and get wireless UART this way without doing any additional encoding.Quote from: stern0m1 on February 18, 2018, 08:05:13 pmHow you change control (modulation) signal is up to you. In this case someone decided to encode it this way.Encoding information in a pulse width is probably a bit more robust, since you can filter out pulses that are too long or too short (noise or interference).

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 18, 2018, 08:26:15 pm
content: Quote from: stern0m1 on February 18, 2018, 08:05:13 pmOOK is a concept or method in rf signal modulation transmission, somewhat similar to AM, except instead of analog values in AM (amplitude) OOK is either ON or OFF hence digital. the interpretation whether ON is 1 or 0 is defined by another protocol or encoding method, such as Manchester encoding, or you can make your own encoding protocol, or simply just take it plain raw i suspect like your PWM signal. the receiver end will duplicate this PWM signal as is as its receiving it. but as you dig deeper, there are shortcoming, such as when there is noise coming from outside you'll have nasty PWM coming out of the receiver end. more complex protocol will double or triple check its receiving signal for communication integrity this will include checksum, parity check, signatures ID or data redundancy, fwiw...