1GHz signal on 100MHz scope - Page 1
Forum section: electronics / projects

author: dobsonr741
author_rank: Frequent Contributor
timestamp: May 25, 2025, 06:42:20 pm
content: My daily oscilloscope is a Tektronix TDS1012, which has a 1 GSa/s sampling rate and 100 MHz bandwidth. I tried to check whether a ~1 GHz signal source was working.Even though this frequency is well beyond the scope’s bandwidth, I expected to at least see some indication—like a noisy or scattered waveform due to aliasing or strong attenuation. I was using AC coupling on the input.However, what I observed was different: when I connected the 1 GHz signal, the baseline noise level shifted up by a few hundred millivolts compared to when no signal was connected. But there was no distinct waveform, no aliasing, no visible sign of the 1 GHz signal—just the same noise, but now showing up at a different DC offset.What could explain this behavior?

author: coppercone2
author_rank: Super Contributor
timestamp: May 25, 2025, 06:48:50 pm
content: rectifier capacitoryou can measure it through a AC block to be sure there is no DC offset. You can't measure for precise DC offsets on high frequency equipment with a DMM accurately unless you have a AC block because it makes them lieLike the same circuit you use to feed power to a GHz rail, but used for measuring.I would love to hear about a 'proper' AC block ("measurement" bias T, not for Bias) for using to test voltages on RF power. A simple DC feeder block just has like a inductor in there and maybe a feedthrough cap (LC circuit). Do they have something like this meant for more... isolation then just blocking some amplifier power supply (which might have its own filter that is good already). Like for hooking up a DMM to microwaves.RLC? LCL? higher order?Like how accurate is a regular bias T for measuring voltage to make sure the meter is not being screwed with. There could be some kind of bias T not suitable for putting current into it but can measure voltage.

author: 2N3055
author_rank: Super Contributor
timestamp: May 25, 2025, 06:56:57 pm
content: Quote from: dobsonr741 on May 25, 2025, 06:42:20 pmScope's 100 MHz BW explains it.Aditionally, if you applied large signal, some parts of signal path might have rectified some signal so change in DC..

author: ejeffrey
author_rank: Super Contributor
timestamp: May 25, 2025, 07:00:41 pm
content: Quote from: dobsonr741 on May 25, 2025, 06:42:20 pmThat is the expected aliasing. If you have a 1 GHz signal and a 1 GS/s sample rate, you will see the same voltage for every sample.If you set the generator to 900 MHz you will probably see what looks like a heavily attenuated 100 MHz signal. Alternately, try adjusting the sample rate if you can.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: May 25, 2025, 07:59:34 pm
content: QuoteThe frequency is just approximately 1GHz, so I would not expect a straight line, hitting the waveform at identical phase.I agree with the suggestion of a non-linearity, acting like a diode and the capacitance after can create the DC offset in the input or the input protection stage, after the DC block capacitor of the scope.

author: TimFox
author_rank: Super Contributor
timestamp: May 25, 2025, 11:00:44 pm
content: When faced with a similar problem, where I feared an amplifier was oscillating at maybe 3 GHz, but I had no oscilloscope nor spectrum analyzer with appropriate bandwidth:The amplifier was mounted on a test fixture with a female N connector for the input (for gain and noise measurements).I used a reasonably-priced "detector" module with a male N connector, and measured the DC voltage on the "video" output (BNC connector).Presence or absence of oscillation corresponded with a shift of the DC quiescent voltages from expected values.

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 27, 2025, 10:17:59 am
content: There is nothing to expect 1 GHz from digital 100MHz oscilloscope because of aliasing. But some old good analog 250-500 MHz scope can show a signal, though more or less attenuated.I've seen 900 MHz signal with my 350 MHz Tektronix 2465A.But it's much easier and possibly cheaper now to buy 1.5-6 GHz DIY spectrum analyser (like $150 TinySA or TinySA Ultra etc) then a good shape analogue scope like those mentioned Tektronix.

author: CaptDon
author_rank: Super Contributor
timestamp: May 27, 2025, 01:07:20 pm
content: Don't forget that many of the TDS model Tek scopes were available in different bandwidths depending on price. The TDS 2004/2014/2024 series comes to mind. They used an identical motherboard but for the lower priced models they placed 'rolloff capacitors' in the signal chain to limit the upper frequency response and may have included jumper solder pads to set a lower sample rate. Looking at your unit the rolloff at 1GHZ would be huge. Also, as the frequency goes up the allowable input voltage goes down so you may not be able to apply enough input voltage to overcome the rolloff loss without damage to the scope. Another thing to look at in the specs is that the specified bandwidth of the scope may only be true at the 1V/DIV and higher or possibly .1V/DIV and higher. When you crank up the vertical gain to things like 10mV/DIV or 1mV/DIV don't be surprised if the vertical bandwidth drops by 50%.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 27, 2025, 01:36:58 pm
content: RF voltmeter? Sampling scope?

author: Terry Bites
author_rank: Super Contributor
timestamp: May 27, 2025, 04:17:46 pm
content: You might like thishttps://hackaday.io/project/167292-8-ghz-sampling-oscilloscope

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 27, 2025, 04:50:59 pm
content: Can you make a simple diode probe?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 27, 2025, 08:45:07 pm
content: How about a SDR dongle?

author: David Hess
author_rank: Super Contributor
timestamp: May 28, 2025, 01:34:17 am
content: It is not properly triggering. A countdown trigger is needed, which means using a divided signal for the trigger.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 28, 2025, 08:55:23 am
content: Quote from: dobsonr741 on May 25, 2025, 06:42:20 pmMy guess is what you see is caused by the 1GHz signal being rectified by the analog front-endnon-linearities.Any non-linear component with an asymmetric I/V response (asymetric relative to the origin of the I/V axis) will also act as a rectifier, similar with how a diode works. The I/V curve doesn't have to be "diode-like". As long as it has an asymmetric response, any device will also produce some DC at its output, similar with a diode detector.