Decoding a PSK Stream from A DRA818v Module  - Page 1
Forum section: electronics / rf-microwave

author: kml183
author_rank: Contributor
timestamp: March 24, 2023, 06:23:42 pm
content: Hello! I am trying to decode data send from a DRA818v (1W) module at 144.39Mhz by connecting it through an 80dB attenuator (SWR is about 1.2) to an RTL based SDR. When I look at the data the SDR is receiving, I noticed the channel with of the signal is much larger then the 2.5Khz channel I configured. I don't have a LPF on the end of the module so I am getting a harmonic components. I am receiving the data with the SDR on a RPI. The RPI is hosting a rtl_sdr server which I am connecting to with GQRX on my main computer.Why is the bandwidth of the signal I am receiving so large? is there some other harmonic component here that I am missing? Attached is my settings, the signal I am receiving and my setup.Thanks

author: eb4fbz
author_rank: Regular Contributor
timestamp: March 24, 2023, 08:38:52 pm
content: You may be overloading the RTL SDR

author: geggi1
author_rank: Frequent Contributor
timestamp: March 24, 2023, 08:45:46 pm
content: you can try to set it in 0,5W mode by putting a jumper on the lowpower link and see it it becomes better.You should also look in to how hard you are driving the transmitter start by lowering the drive

author: kml183
author_rank: Contributor
timestamp: March 24, 2023, 09:24:44 pm
content: Quote from: eb4fbz on March 24, 2023, 08:38:52 pmQuote from: geggi1 on March 24, 2023, 08:45:46 pmHmm I tried dropping the input power but I just get the same spectral components. Just checking the specifications the max input power is 10dBm. My input power would be -50dBm. I just double checked my rtl_tcp setup and I am not enabling the built in LNA or the bias T.Quote from: geggi1 on March 24, 2023, 08:45:46 pmAre you talking about the frequency that I am transmitting information at? I send the command to transmit about 1Hz

author: Marsupilami
author_rank: Frequent Contributor
timestamp: March 26, 2023, 05:18:24 pm
content: I have no idea about either of the devices you're using, but I'm curious.I think geggi1 meant to check the audio input level to the DRA818 (if there's any)From your spectrum shot it looks like the group spacing is about 45kHz. I wonder where that is coming from.I might try playing around with the available frequency settings on both the transmitter and the receiver to see which one if any affect the spacing of the peaks and groups. If your transmitter is compressing, the settings of the receiver should cause no change whatsoever in the spectral composition other than a shift. Maybe try to adjust the ratio of the overall receiver frequency and the receiver hardware frequency setting.

author: kml183
author_rank: Contributor
timestamp: March 27, 2023, 03:42:30 am
content: Quote from: Marsupilami on March 26, 2023, 05:18:24 pmFor the test I was doing I just wanted to see the carrier. So there wasn't any audio input. When I look at the output on my spectrum analyzer I just see the carrier frequency. Its only the SDR that I see the 45khz group spacing.I will try and adjust the ratio of the over reciever frequency and the reciever hardware frequency. Maybe there is some distortion from the IF section in the SDR. Thanks!

author: Marsupilami
author_rank: Frequent Contributor
timestamp: March 27, 2023, 03:56:24 am
content: You could’ve started with that you have a spectrum analyzerIs there an LNA in the SDR? Or whatever kind of programmable gain? Maybe that’s too high.

author: geggi1
author_rank: Frequent Contributor
timestamp: March 27, 2023, 04:03:42 am
content: Quote from: geggi1 on March 24, 2023, 08:45:46 pm1. Pull pin 7 low and you will have 0,5W power output.2. Maximum drive i 700mV if the drive is over this you will get splatter on the signal. You should start very low for instance at 100mV or lower and adjust the drive up until the signal is is readable on your SDR.