Monitoring HF ssb output via DSO? - Page 1
Forum section: electronics / rf-microwave

author: vinlove
author_rank: Frequent Contributor
timestamp: September 28, 2019, 08:39:24 am
content: Are there ways to monitor HF ssb output via DSO for frequency, bandwidth and PEP power output?Or do I need a spectrum analysis machine for that?

author: vk6zgo
author_rank: Super Contributor
timestamp: September 29, 2019, 03:20:39 am
content: Quote from: vinlove on September 28, 2019, 08:39:24 amNot sure how accurate the frequency readout is on your DSO, so that may be a limitation.The FFT should work OK for bandwidth.For PEP you would normally feed two tones of unrelated frequencies into your radio, at a level corresponding with your microphone audio level.The 'scope will then display an envelope pattern.Determine the peak to peak voltage of that envelope, then from that, calculate the RMS voltage (don't rely on your 'scope's RMS readout, as it might calculate over the whole envelope).Knowing the resistance of your test load, you can then calculate the power level present at your monitoring point.If it is a built in test point, or a tap on your load, you then have to determine how much lower the power level at that point is compared to that dissipated in the load, & make the correction.

author: vinlove
author_rank: Frequent Contributor
timestamp: October 03, 2019, 06:58:02 am
content: My DSO is a Rigol 1052E, and I ran FY6800 connected to it. The freq reading of the 1052E was accurate.I have been monitoring any stray RFs from my antenna feeder when transmitting using one of the kit DSO from JYTech, and was quite effective in the monitor.It gave voltage readings in mV, and usually 100 - 200mV and sine wave, when stray RF is strong to none depending on the band and freq. of the transmission.

author: radiolistener
author_rank: Super Contributor
timestamp: October 03, 2019, 07:31:30 pm
content: if you're planning to use standard 1:10 probe, you can burn out your probe and there is even risk to damage your oscilloscope.High voltage 1:100 probe may work ok up to 50 W, but above that it is also may be unsafe.The more safe way is to use 40 dB attenuator on the TX output and then connect it to oscilloscope with coax cable with 50 Ohm pass-through dummy load on the oscilloscope connector.

author: bd139
author_rank: Super Contributor
timestamp: October 03, 2019, 07:38:54 pm
content: This.Also make sure it's a 40dB POWER attenuator and it is the correct way round!

author: vinlove
author_rank: Frequent Contributor
timestamp: October 04, 2019, 12:47:29 pm
content: Thank you for the info. So, maybe it is better to use just PEP power meter and spectrum analyser.I don't want to risk blowing off my Rigol DSO by high voltage RF.

author: radiolistener
author_rank: Super Contributor
timestamp: October 04, 2019, 05:13:53 pm
content: you can use something like this:https://www.aliexpress.com/item/32780784273.html

author: fourfathom
author_rank: Super Contributor
timestamp: October 04, 2019, 07:31:35 pm
content: Do test your attenuator before you use it with high power, especially if it isn't a "brand name" one. I bought a 50 dB / 100W attenuator via Amazon (made by Shenzhen BECEN Communication Technology Co.,Ltd), and the actual attenuation was 39dB. It was flat enough from DC to 2 GHz, and the return loss was good, so I suspect this was a mis-labeled 40dB atten. It's still useful to me so I kept it and put a new label on it, but I do wish it was actually 50dB. So be careful before you burn out a piece of test equipment!

author: w2aew
author_rank: Super Contributor
timestamp: October 04, 2019, 08:46:22 pm
content: I did a bunch of videos talking about how to couple HF RF signals to a scope. Attenuators are one way to go. Directional couplers are another. But, more often, a simple RF tap or sampler is easiest and safest.Here's one on RF Taps and Samplers:Here's one that shows a simple RF pickup:Even a piece of wire wrapped several times around your coax will pick up enough RF to be useful. Not calibrated for power, but good to look at the envelope.This very old video shows a simple resistive tap:Here's one on how to properly interpret RF envelopes:Keep in mind that for SSB signals, you usually want to view the envelope of your voice/audio modulation. This typically means that you'll have the horizontal scale setting around 1ms/div. At this setting, the waveform sample rate of most DSOs may be fairly low and will alias the RF signal, especially if the waveform memory is small.Since there is no carrier for SSB signals, there is no constant frequency to measure (unless you whistle and add/subtract [for LSB/USB] your whistle's frequency from your measurement to get the suppressed carrier frequency).Depending on your scope, an FFT may work OK for looking at the spectrum of the signal.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 05, 2019, 02:20:05 am
content: Quote from: radiolistener on October 04, 2019, 05:13:53 pmOr, some test loads have a low level monitoring point (either a built in coupler, or as in my case, a low resistance tap on the earthy end of the load itself.)

author: vinlove
author_rank: Frequent Contributor
timestamp: October 06, 2019, 08:50:05 am
content: Thank you W2AEW for your videos. I watched some of other videos of yours on youtube before, and they were great.But these videos are real gem for the topic.I will watch them over a few times to understand the theories and logics behind it fully, as they have a lot to take in.