Can't see signal with scope even though I know it's present (solved, bad probe) - Page 1
Forum section: electronics / rf-microwave

author: psyon
author_rank: Contributor
timestamp: January 07, 2024, 08:37:44 pm
content: I have been working on a four channel SDR for a wildlife telemetry project that just uses CW pulses. The board works for the most part, but I am trying to track down where some attenuation is happening on one of the channels. When I try to probe various parts of the board, I can't read the signal though in most places. If I disconnect the coax from the signal generator, and touch the probe to the center pin, I can see the 150Mhz signal fine, so I know the scope can read the 150Mhz range fine. I can also read the 150.001Mhz signal coming out of the SI5351 going to the mixers as the LO. I can also see the resulting 1Khz signal between the op amp and the ADC pin on the STM32 board. But, if I touch the probe to the center pin of the SMA jack with the coax hooked up, I don't see anything on my scope. If I touch anywhere within the BPF circuit, I don't see anything. If I probe after the LNA, I can see the 1.65V bias, but no signal with it, and there is no signal after the coupling capacitor before the mixer. I also can't read the 1Khz signal coming out of the mixer, anywhere prior to the op amp. Like I said, I know the signals are there, because I can read them at the very end fine, and also read them with the ADC just fine. A few people I have talked to suggested that the signal was so weak that the scope just couldn't see it, but if I can read it on the coax, I should be able to read it at the SMA pin right? Also, the op amp does about 20db of gain (assuming I have that right with the 1k and 100k resistor), and if I put the signal generator to -20dbm for power, I saturate the op amp and get a full 3.3Vpp, so I assume that means I should have 0.03Vpp prior to the op amp, which my scope should be able to read just fine, and in fact, has read just fine when testing other stuff. I thought at first that the inductor shunt to ground for the BPF may affect things, but that wouldn't explain why I can't see the 1Khz signal before the op amp. So then I assumed since the mixer has transfomers connected to ground, that maybe that was the issue, but I can see the LO signal just fine. I am out of ideas for why I can't see the signal at various parts of the board. Scope is a Siglent SDS1204x-E, and I have attached schematic, PCB layout, and pic of completed board.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: January 08, 2024, 01:35:28 am
content: What type of probe are you using? If it's a 1:1 probe, then the probe cable impedance and the scope's input impedance (typically 10-20 pF) would load the signal enough to keep you from seeing it. A clue would be if the output from the audio amps goes away when you probe the RF signals. Using a 10:1 probe would decrease the loading by a factor of 10.

author: psyon
author_rank: Contributor
timestamp: January 08, 2024, 02:35:49 am
content: I am going crazy or something. I went to double check that the signal wasn't dropping at the output when I probe, and now I can find all the signals fine. I was messing around with this for a week trying to find why I couldn't read it, and now when I set it all back up, I can.

author: psyon
author_rank: Contributor
timestamp: January 08, 2024, 02:51:06 am
content: Ok, I am not crazy. One of my probes is bad. I had put the scope away, so when I pulled it back out, I grabbed a different probe, and it worked. I went through all 4 that I have, and one of them will show the stronger signals, but not the weaker ones. The bad one showed them, but very noisy, I just assumed my board was that noisy, so didn't even think probe may have an issue.

author: tautech
author_rank: Super Contributor
timestamp: January 08, 2024, 07:39:31 am
content: Quote from: psyon on January 08, 2024, 02:51:06 amIf the scope is under 1 yr old claim warranty for the probe.The scope offers 3 years but probes just 1.

author: Randy222
author_rank: Frequent Contributor
timestamp: January 08, 2024, 02:21:31 pm
content: Quote from: psyon on January 08, 2024, 02:51:06 amGeneral practice (hobby land) is to verify the test equip when things appear to be in the "bewildering" realm. I typically allow scope to warm and then I put probe on the internal 1kHz generator and cycle the x1 x10 probe switch just to make sure the probe appears ok. I see this like a pilot person who walks around airplane prior to flight looking for any obvious issue.Glad to hear you found bad probe as culprit.

author: radiolistener
author_rank: Super Contributor
timestamp: January 09, 2024, 12:28:11 pm
content: Quote from: psyon on January 07, 2024, 08:37:44 pmNo, you can't see weak signals on oscilloscope, the amplitude of input signals from antenna is about 10000 times lower than max oscilloscope sensitivity.For example, the input signal can have amplitude 5 uV (which is enough for receiver), but max oscilloscope gain is 500 uV/div and it has about 500-1000 uV noise in that mode. So you will be unable to see 5 uV on the background of 1000 uV oscilloscope noise.But even if you put RF amplifier between receiver input and oscilloscope it still can't help you to see the signal, because antenna output has a lot of different signals from all transmitters and noise sources around you. So, your weak signal of interest will be flooded in the noise. It requires the good filter to cut-off unwanted frequencies and RF amplifier to amplify it for about 10000 times in order to see the signal on your oscilloscope.You can see local oscillator signal because it has much-much higher amplitude than input signals. si5351 has output amplitude more than 1000000 uV and this is why you can see it on oscilloscope.PS: and Siglent SDS1204X-E has some attenuation for 150 MHz, so it shows it with reduced amplitude, which makes monitoring of that signal on your oscilloscope even more difficult.You can try to setup 500 uV/div mode and switch oscilloscope into FFT mode. There is a chance that in FFT mode you can see a peak at 150 MHz, which will indicate presence of 150 MHz signal. But it will be hard to distinguish it from LO interference (due to low FFT resolution), which frequency is too close and signal level is much higher... So, you're needs to power off LO source (si5351) to detect 150 MHz signal from antenna on your oscilloscope.

author: psyon
author_rank: Contributor
timestamp: February 05, 2024, 03:20:24 am
content: Quote from: radiolistener on January 09, 2024, 12:28:11 pmFor testing I was feeding a signal in from a signal generator, which is putting out a stronger signal, but before I ever started this project, I hooked an antenna directly up to my scope and was able to see a signal from a transmitter on it.

author: radiolistener
author_rank: Super Contributor
timestamp: February 05, 2024, 09:58:37 pm
content: Quote from: psyon on February 05, 2024, 03:20:24 amthis can happens when transmitter is strong and located too close to antenna. In such case it induce high power signal in antenna and you can see it on oscilloscope. But you can't see signals from remote transmitters in such way, just because their power will be reduced for about 60-100 dB and more. The oscilloscope sensitivity is not enough to detect even presence of so low power signal.For example the signal amplitude from antenna which receive remote transmitter can be 5-10 uV (microvolts) = 0.000005 V. This is enough signal level for a good receiver. But you cannot detect even presence of so weak signal with oscilloscope, just because noise floor of 100 MHz oscilloscope is about 500 uV which is 100 times higher than the signal amplitude.

author: tggzzz
author_rank: Super Contributor
timestamp: February 05, 2024, 10:33:13 pm
content: Quote from: psyon on January 08, 2024, 02:51:06 amWell done for noticing the problem, finding the cause - and reporting backTraditionally it happens with RF cables. Anybody caught putting such a faulty cable back on the rack is taken out and shot at dawn. The correct action is to widlarise the cable/probe with diagonal cutters.

author: psyon
author_rank: Contributor
timestamp: February 07, 2024, 04:22:37 am
content: Quote from: radiolistener on February 05, 2024, 09:58:37 pmYeah, the transmitter is sitting on my desk next to everything else I am working on. I wasn't diagnosing a problem by trying to pick up signals that were far away.