Elenco AM/FM 108ck, dig scope and AWG - Page 1
Forum section: electronics / beginners

author: ttx450
author_rank: Regular Contributor
timestamp: July 05, 2021, 07:50:15 pm
content: Hi all, looking for some guidance on using a Siglent sds1104-u and a juntek AWG psg9080. This is my first dig scope and AWG. I have limited knowledge on using a analog scopes. I purchased the Elenco kit and thought it would be a good learning process along with the new equipment. Went through the AM side of the radio and now on the FM. Manual is herehttps://www.elenco.com/wp-content/uploads/2017/10/AMFM-108CK_low-res-2.pdfI been stumbling with the first step on the FM alignment of the ratio detector on page 45. Using method #2 RF generator and a scope, seems like I cannot not get my amplitude low enough to get a 1kHz on the scope. I am always greater then 1kHz.Attached is a picture of the scope with the AWG @ 0Vpp and the unit on. My scope reads over 1kHz with no amplitude from the AWG? What am I doing wrong. Thks..

author: Manul
author_rank: Super Contributor
timestamp: July 05, 2021, 09:53:19 pm
content: First, what you see on the scope is "noise". That means unwanted signals coming from inside of device under test, scope itself or surounding environment. It is so visible, because the scope is set to 5mV per vertical division, which is quite low, so it is very sensitive to small signals (and noise). Such noise, like seen in the picture likely comes from SMPS (switching mode power supply). The scope's frequency counters interpretation of the frequency could be a bit randon with noisy signals, but most likely it shows frequency of highest amplitute peaks.Now, what the book tells you to do is to increase the amplitude until you will see 1khz frequency apearing. It does not mean, that the frequency will be less or more and then become 1khz. It means, that 1khz will start to apear, it will rise in amplitute and become a visible wave. After it apears, you adjust coils T4 and T5.To be able to observe 1khz, you should adjust your horizontal timebase. In the picture I see 1 microsecond. That is much too short. 1khz wave will almost look flat when "time zoomed" like that. Choose around 500us - 1ms.

author: ttx450
author_rank: Regular Contributor
timestamp: July 05, 2021, 11:02:20 pm
content: Quote from: Manul on July 05, 2021, 09:53:19 pmHi, thanks for the response and the time reading the manual. I was wondering if all that was noise, but was not sure. Been working on this several weeksI had to turn the horizontal to 200ns with 'modulation on' to get somewhat of a clear sinewave. Why dose my scope show a frequency much greater then the 1kHz? Stats are running around 10.7MHz, is that reading based on my RF generator that is set to 10.7MHz, with MOD at 1kHz and 22.5kHz deviation? On the scope the f number in the upper right corner is around 6.46MHz. What reading is this? So to achieve a 1kHz sinewave the scopes stats don't show that?Thks..

author: Manul
author_rank: Super Contributor
timestamp: July 06, 2021, 12:22:47 am
content: I will also note, that as I see you have probe set at 1X. Don't forget, that at 1X probe will have much lower bandwidth, like 5-10Mhz. It means that higher frequencies start to look lower amplitude (attenuated) then they really are. If you want to use full bandwidth of your scope (100Mhz) you need probe set at 10X. Measuring 1khz is fine at 1X.In general we want noise to be as low as possible, because if it is high, it masks the signals we actually want to see. If noise is high, small signals will be buried in it. Sometimes it is worth the effort to indentify noise source and reduce/eliminate it. Then you see more of what is useful.Now to your subject. Based on your question, you seem to have some misconception, but I'm not sure what exactly. Frequency of generator 10.7Mhz, modulated by 1khz. 10.7Mhz is a carrier. 1khz is a carried signal. I haven't read the manual, but as I understand the goal is to measure signal after demodulation (extraction of signal from carrier). So if circuit is working correct, it will output 1khz. There might be some carrier left on the output, but it is not important. You should look for 1khz signal to apearvisually. Ignore the frequency counter. With a correct time base, you will see a nice sine wave rising from noise. If the amplitude will be high enough maybe frequency counter will lock on it, but again, just ignore it. It is all about amplitude of demodulated signal, you already know that it is 1khz, because you set it on your AWG, so there is no need to look what value scope shows.Top right corner is harware frequency counter. It counts the cycles in the signal, independent on selected timebase. It could be wrong in some cases. It works best with clean sine waves, square waves, pulses and so on.In the stats there is software frequency counter which calculates the frequency based on acquired signal samples, usually what you see on the screen at the moment. It might show different results then harware counter. Imagine that your input is low frequency, lets say 1khz sine wave and you see it on screen. Hardware counter will count it easy and show it correct. Stats will also show it correct. But if you zoom a lot by selecting much shorter timebase until only part of wave period is on screen, hardware counter will still show correct, but stats will not. Because stats count what is on screen and there is no periodic wave on screen anymore. Hope all this makes sense.

author: ttx450
author_rank: Regular Contributor
timestamp: July 06, 2021, 01:24:48 am
content: Quote from: Manul on July 06, 2021, 12:22:47 amyes all this makes much sense. You dont know how much time I spent on this and your couple post has taught me a lot. Thank you much for that! I was doing a lot of nothing for several weeks, now I know I am not that crazy, maybe somewhat. I been running the radio with a switchable Hanmatek power supply, maybe at fault for the noise. Can that be filtered out? Your explanation on how to read the frequency on the digital scope. I over looked the bandwidth from setting the probes 1x-10x, always was only thinking voltages. Now I can move along with this project. Big help, learned a lot. Thks!

author: Manul
author_rank: Super Contributor
timestamp: July 06, 2021, 09:16:17 pm
content: Quote from: ttx450 on July 06, 2021, 01:24:48 amWell, switching power supply will always be somewhat noisy, that is why linear supply is highly preferable for sensitive applications. Some noise may be removed by winding power supply wires on a ferrite ring. Also, because mostly it is common mode noise, shorter ground wire on probe will help. If you would use ground spring which is likely included with your probe - you would see much less noise.

author: ttx450
author_rank: Regular Contributor
timestamp: July 07, 2021, 12:25:11 am
content: Quote from: Manul on July 06, 2021, 09:16:17 pmthank for all the tips, very helpful. yes I got all the extra stuff with the scope. I will have to try the ground spring, which I have not yet. I been wanting to get a linear power supply, I did not realize how much it could affect the circuit. thks