Doubt on using a SWR meter for SDR transmitter - Page 1
Forum section: electronics / rf-microwave

author: Alejos
author_rank: Newbie
timestamp: January 09, 2024, 03:55:36 pm
content: Hi all,I am a very amateur guy just starting working on HAM projects and I have a simple question that I cannot solve. I am using a transceiver based on a SDR transmitter (LimeSDR) connected to a 20W power amplifier and then the antenna.I want to measure the SWR at the output of the power amplifier and that is my doubt. If I am using a digital modulation (2FSK in my case), is it possible to use any SWR meter (I am using DG-503) in the correct band or should I use a specific SWR meter for digital modulations/signals?I guess I don't understand how the SWR measuring works with digital signals...In case I need a specific SWR meter for digital signals, could you recommend me any?Thank you very much in advance for your help.Alejos

author: radiolistener
author_rank: Super Contributor
timestamp: January 09, 2024, 04:08:50 pm
content: In order to properly measure SWR, you're needs to transmit continuous CW carrier. Any modulated signal is not good for VSWR measurement, because it is represented with variable signal, so your measurements will be variable.If your transmitter don't have CW, you can use FM modulation with silence on the mic. For SSB you're needs to transmit stable sine tone (for example 700 Hz or 1 kHz)

author: Alejos
author_rank: Newbie
timestamp: January 09, 2024, 04:18:01 pm
content: Hi Radiolistener,Thanks for your reply. The problem is that I try to do that just sending a continuous tone modulated in FSK, that is, I send all 1's so the carrier will be only one tone but the SWR meter does not work, it says that I am transmitting 90W although the SWR measure seams good (1.2), but the power is completely wrong. I would expect to measure around 10 Watss, no 90W.Can any power/SWR meter be used for measuring the power of digital signals? What could be wrong?Thanks again.Alejos

author: A.Z.
author_rank: Super Contributor
timestamp: January 09, 2024, 04:26:50 pm
content: Quote from: Alejos on January 09, 2024, 04:18:01 pmThe power meter will just measure the RF power, the point is... what frequency are you transmitting on ? See the DG-503 has two pairs of input/output connectors, one is for the 1-60 MHz range and the other pair for the 125-525MHz range, so if you are outside one of those ranges or if you are using the wrong pair of ports, you'll have wrong readings, also, the reading can easily be influenced by harmonics, in such a case your power amplifier may probably output 10W on the fundamental, but also emit a bunch of harmonics (not a good thing at all)

author: Alejos
author_rank: Newbie
timestamp: January 09, 2024, 04:31:55 pm
content: I am using the connections for UHF band since I am transmitting in 430MHz band.A.Z., by your response, should I understand that it is ok to use the power meter for digital modulations/signals?Using an SDR like Airspy, I can see that there are not too high harmonics, so the main power is the fundamental.

author: A.Z.
author_rank: Super Contributor
timestamp: January 09, 2024, 04:41:55 pm
content: Quote from: Alejos on January 09, 2024, 04:31:55 pmOkQuoteAs@radiolistenerwrote, to have a correct measurement you'll need to send out a carrier, be that in AM or FM, it shouldn't be so difficult to achieve through some code, then the power meter just measures the RF power, it has no "notion" of modulation, but the latter may affect the reading, for example if using SSB w/o any modulating signal the meter will read zero

author: A.Z.
author_rank: Super Contributor
timestamp: January 09, 2024, 04:56:09 pm
content: As a note, the LimeSuite implements a signal generatorhttps://github.com/myriadrf/LimeSuite/issues/281which can be used to test the output power

author: Randy222
author_rank: Frequent Contributor
timestamp: January 09, 2024, 06:12:16 pm
content: So, what does "output of the amp" mean?Just VSWR the antenna "system" using VNA. Yeah, a sine wave test.The amp should approx see that VSWR on it's output port.From there you can run the math to obtain real and reflected power.

author: RFDx
author_rank: Regular Contributor
timestamp: January 10, 2024, 02:09:22 am
content: Quote from: Alejos on January 09, 2024, 03:55:36 pmThe signal that goes to the antenna is in the analog domain, no matter what type of (digital) modulation you use. 2FSK is a constant envelope signal, simply put, a sinewave that shifts in frequency. If you are transmitting with 20W into a 50 Ohm load your SWR-meter would ideally show you 20W FWD, 0W REV and a SWR of 1.

author: fourfathom
author_rank: Super Contributor
timestamp: January 10, 2024, 02:36:39 am
content: Quote from: Alejos on January 09, 2024, 03:55:36 pm2FSK is typically a constant-amplitude modulation, and the small frequency deviations will not effect the SWR measurement. As long as the SWR meter is good for the transmit frequency it should work fine.FWIW, I regularly send 4FSK through my SWR meter, no problem getting accurate results.