Help understadning why narrow signal causes interference to wide band signal - Page 1
Forum section: electronics / rf-microwave

author: 2N2222A
author_rank: Regular Contributor
timestamp: December 27, 2022, 04:18:39 am
content: Something I don't understand is how the transmitter power is measured with a wide band signal. Regular FM UHF communication is less than 15kHz wide. 802.11 is 20MHz wide. That's like 1333 15kHz signals all together. So if each 15kHz signal is 10mW, then 1333 of them together would be 133 Watts? So the wide band signal is actually very weak when only a small 15kHz section is considered?With a 2.4GHz analog TV transmitter the signal is clear 10M away on the TV. But a 2.4GHz cordless phone wipes out the TV signal completely, even with the phone 5M away from the TV receiver. But if I put the phone up to the TV transmitter it has to be less than 10cm away for the cordless phone to receive any interference, even though the phone is over 5M away from the base.Why is this? It is similar with 802.11 wifi and the cordless phone. The phone has to be close like 2M away from the WiFi device to be affected, but the WiFi is affected worse by the phone.

author: radiolistener
author_rank: Super Contributor
timestamp: December 27, 2022, 08:40:26 pm
content: Quote from: 2N2222A on December 27, 2022, 04:18:39 amBecause there is a limited dynamic range of receiver. If you put transmitter too close to the receiver it can saturate receiver input and receiver will be unable to receive frequencies even if they are outside of transmitter bandwidth. This is because receiver input saturation leads to clipping and other non-linear distortions, so saturated amplifier will multiply different signals and produce a lot of parasitic noise around transmitter frequency.If your receiver has high dynamic range which is enough to avoid input saturation from transmitter, it will be able to receive weak signals even if you place transmitter too close to receiver. This is because receiver input is not saturated with high power transmitter signal.

author: 2N2222A
author_rank: Regular Contributor
timestamp: December 27, 2022, 11:07:13 pm
content: This happens when the 2.4GHz cordless phone and the video transmitter are on interfering channels. I can change the channel on the phone or the TV transmitter and the TV transmitter no longer has a problem with interference.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: December 28, 2022, 01:15:11 am
content: Could be receiver desensitization. A strong nearby signal temporarily overloads the receiver and wipes out the signal.Refer to the discussion about this in the Extra Ham License Manual.

author: TheMG
author_rank: Frequent Contributor
timestamp: January 01, 2023, 08:58:47 pm
content: Another reason is that digital data transmission is more robust, the way the signal is modulated as well as forward error correction and other aspects of the data transmission protocol make it much more immune to interference than an analog signal, which will very easily be affected by any in-channel interference.

author: w2aew
author_rank: Super Contributor
timestamp: January 05, 2023, 03:18:13 pm
content: Quote from: 2N2222A on December 27, 2022, 11:07:13 pmThe "wideband" 802.11 signal is an OFDM signal, composed of several dozen evenly spaced sub-carriers - very similar to an "RF parallel bus". Most of the sub-carriers carry data, but some of them are "pilot" sub-carriers. The pilots provide an amplitude and phase reference for the receiver to help them lock/track on the data sub-carriers. If your narrowband signal interferes with one of the pilot sub-carriers, then the receiver won't be able to demodulate the data sub-carriers. Of course, it is possible that your narrowband signal could be taking out a few data-subcarriers instead, which could still interrupt communications

author: 2N2222A
author_rank: Regular Contributor
timestamp: January 07, 2023, 01:24:29 am
content: Quote from: w2aew on January 05, 2023, 03:18:13 pm802.11a/g is OFDM with 52 subcarriers. 48 data and 4 pilot. They are 312.5kHz apart. If a 100mW 11g signal had one 312.5kHz band pass filtered through, what would be the power of the remaining 312.5 kHz signal?

author: w2aew
author_rank: Super Contributor
timestamp: January 10, 2023, 06:58:08 pm
content: Quote from: 2N2222A on January 07, 2023, 01:24:29 amAll subcarriers are (ideally) at the same power level, so each contributes about 2% of the total channel power.

author: 2N2222A
author_rank: Regular Contributor
timestamp: January 12, 2023, 06:08:51 pm
content: Quote from: w2aew on January 10, 2023, 06:58:08 pmThis is exactly what I want to know!!!So the OFDM signal in a sense is actually only 2mW, as far as interference from a narrow band signal is concerned.Let us say that the digital cordless phone has a single data stream that is 312.5kHz wide and transmits at 50mW. So what is happening is a 50mW device is causing interference to a 2mW device. Even though the remaining 51 subcarriers of the OFDM signals are unaffected, it knocks out the 802.11 signal. With the phone's signal being 25 times more powerful, it now makes sense why the phone has to be 1/5 the distance to the 802.11 device to be affected by interference, compared to the other way around where the phone interferes with the 802.11 signal.