Chinese 433 MHz receiver noise - Page 1
Forum section: electronics / rf-microwave

author: mikro
author_rank: Contributor
timestamp: September 04, 2016, 03:04:50 pm
content: Hey,I have one of those chinese 433 MHz receiver and transmitter pairs that are used in many of the hobby projects, they look like thishttps://dmohankumar.files.wordpress.com/2015/03/433-mhz-transmitter-recei.jpg?w=549. Now I'm trying to decode a temperature transmitter that transmits on the same frequency. I have the protocol quite figured out but the problem now is that I dont get a clean signal from the receiver that I can sync to. The temperature transmitter sends a signal which starts with 3 short pulses and a ~9 ms low state pulse (preamble), and I'm trying to sync to that low pulse. But the receiver has so much noise on the data pin, that I miss the sync pulse a lot of the times. I noticed that when I put a 1 Kohm resistor from the data to VCC, the noise disappears. I am just wondering is the resistor right thing to do?, because I have never seen in any schematic that would have had a resistor connected like that.

author: JimRemington
author_rank: Regular Contributor
timestamp: September 04, 2016, 03:27:16 pm
content: If you don't have a decent antenna on the receiver, use 17 cm of straight wire on ANT to improve reception. A balanced dipole is better.

author: wraper
author_rank: Unknown
timestamp: September 04, 2016, 03:30:47 pm
content: Quote from: mikro on September 04, 2016, 03:04:50 pmProbably it has open drain/ open collector output.

author: JimRemington
author_rank: Regular Contributor
timestamp: September 04, 2016, 04:01:23 pm
content: The module linked has an op amp output, so I don't know why a resistor would work.A typical schematic for those modules is shown here:http://electronics.stackexchange.com/questions/42923/what-do-i-need-for-a-basic-rf-circuit

author: JimRemington
author_rank: Regular Contributor
timestamp: September 04, 2016, 04:42:00 pm
content: The identical-looking 433 MHz receiver in my junkbox has a clearly labeled LM358 dual op amp as IC1 in the linked schematic, so it is not OC/OD.

author: mikro
author_rank: Contributor
timestamp: September 04, 2016, 05:46:25 pm
content: Thank you all for the answers and helping me out. The IC on the board is LM358 as JimRemington said. If the output would have been OC/OD, that would probably solve the mystery. I took some photos of the data pin activity with the scope. The scope is triggered on the thermometer's data pin (blue channel) going to the transmitter.This is the DATA-pin without the said resistor.This is the DATA-pin with the added 1 Kohm resistor on between DATA and VCCAs you can see from the photos, the noise before of the burst is gone, and also the glitches from the bits are gone.If zoomed out, the DATA-pin would look like full of noise without the resistor, but with the resistor it has some weird signal with period of 200 ms (picture).

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 04, 2016, 07:01:01 pm
content: your communication seems too slow i suspect the receiver will go high gain mode after few ms or so. try communicating at 10Kbps (100us per pulse), use longer preamble to stabilize receiver into low gain mode, and use 5V or greater power supply.

author: mikro
author_rank: Contributor
timestamp: September 04, 2016, 07:09:32 pm
content: Quote from: Mechatrommer on September 04, 2016, 07:01:01 pmI dont think that the autogain has that fast attack time. I am not the maker of this communication protocol, it is from a commercial thermometer using simple ASK-OOK. I am using 5 V regulator. I just noticed that the ground level is lifted for about 1 volt when the resistor is in place.

author: wraper
author_rank: Unknown
timestamp: September 04, 2016, 07:39:33 pm
content: Probably opamp is oscillating then.

author: JimRemington
author_rank: Regular Contributor
timestamp: September 05, 2016, 12:10:10 am
content: The attached schematic is almost entirely correct for the receiver that I have, and the output op amp is a Schmidt trigger stage. It may be that the added resistor on the output is affecting the bias or otherwise reducing the sensitivity of the stage.

author: mikro
author_rank: Contributor
timestamp: September 05, 2016, 02:15:06 pm
content: Quote from: JimRemington on September 05, 2016, 12:10:10 amThis schematic looks quite the same as my receiver with some differences like the R12 in the schematic is 220 Kohm, and mine has 200 Kohm. Could it be that the added resistor somehow changes the schmitt triggers trigger point higher, so it wont trigger to the low amplitude noise? There's not much going on on the pin 3 of the op-amp, but the DATA-pin is still full of noise.

author: mikro
author_rank: Contributor
timestamp: September 05, 2016, 03:43:12 pm
content: Ok, so this is weird. I was testing the module with lab power supply (was using reg before), and when I got over the 3,7 V limit, the DATA-pin would start to have that noise again. Under the 3,7 V limit, the DATA-pin is looking like I think it is suppose to look with only random pulses now and then. And this test was without the 1 Kohm resistor. The module says RF-5V and with all the info I found, I think this is suppose to work at least with 5 V.

author: JimRemington
author_rank: Regular Contributor
timestamp: September 05, 2016, 09:33:18 pm
content: Yep, 5V only. I recall that the module I have won't work below 4.0 V.I think the superregenerative detector circuit is voltage-critical.

author: mikro
author_rank: Contributor
timestamp: September 05, 2016, 09:47:02 pm
content: Quote from: JimRemington on September 05, 2016, 09:33:18 pmCould you fire one up and check with a scope how the DATA-pin looks on your module? I really cant get my C-program to work with the module with 5 V or without the resistor in place, there is simply too much edges to trigger on.

author: JimRemington
author_rank: Regular Contributor
timestamp: September 06, 2016, 01:16:59 am
content: The receiver output is a complete mess until a decent signal arrives, then it "cleans up" when the AGC kicks in and there is no problem decoding the output.Here is a typical trace. I use Audacity on a laptop for analyzing these transmissions, because you can record for basically ever, and easily zoom in on regions of interest. (The spikes on the leading/trailing edges are artifacts from the sound card).

author: mikro
author_rank: Contributor
timestamp: September 06, 2016, 09:58:04 am
content: Yes, this is same how mine looks. Before the AGC goes in "low gain"-mode there is just high frequency noise, which I dont understand that why does it have to be there why wont the schmitt trigger remove it.

author: JimRemington
author_rank: Regular Contributor
timestamp: September 06, 2016, 05:08:25 pm
content: I don't see it as a problem. Micros are cheap and with this sort of receiver, you can dedicate one to the task of waiting for a transmission and decoding. The more expensive receivers all have a micro built in.

author: mikro
author_rank: Contributor
timestamp: September 06, 2016, 05:57:58 pm
content: Quote from: JimRemington on September 06, 2016, 05:08:25 pmAt the moment I am writing a code to sync to the low state after the preamble. I have implemented a simple glitch filter in the code, but still when I use an unmodified receiver, I dont get any good packets. I modified the receiver by replacing the resistor R12 (in schematic you posted ) to 500 Kohm resistor. Now I can run the receiver with 5 Volts, and get packets in about 60 % success rate.

author: cool53
author_rank: Newbie
timestamp: June 15, 2020, 05:41:18 pm
content: HiThe topic is 4 years old, however the problem still remains.My receivers all had the noise problem, no matter what voltage, pullup or power condensor I used. Even a speed change did not provide any improvement. For some reason, I took the power supply away from the arduino and connected it to a separate source (DC/DC converter). That resulted in a perfect pulse shape response at pulse speeds between 500 and 2 mSec.MY conclusion is, that the the arduino produces noise on the 5v supply confusing the RF receiver, and a separated supply (de-coupling) gives the best result.cool

author: walter_wpg
author_rank: Newbie
timestamp: October 12, 2021, 08:45:47 pm
content: What would be a really useful mod for these receivers is a "squelch", or else some way to limit how high the AGC cranks up. We'd really want the output to stay low when the signal level is very low. In my playing with these with Arduinos, I have used a "preamble" on the transmitter, to set the receiver AGC. Then, turn off transmitter, which causes a period of "output is zero" at the receiver. THEN you can start your actual data transmission, while the receiver AGC is at a reasonable level, and is looking at a real signal from the transmitter.

author: radiolistener
author_rank: Super Contributor
timestamp: October 12, 2021, 11:07:52 pm
content: Probably your sensor transmitter working at a little different frequency, so receiver needs a little time for auto-tune frequency.