More help with my Elenco AM/FM108CK build - oscilloscope ?'s - Page 1
Forum section: electronics / beginners

author: Hank30721
author_rank: Contributor
timestamp: February 08, 2023, 08:51:11 pm
content: Hey folks...I started the FM section of the kit After assembly of the FM detector, there are several static and dynamic tests. All of the static tests are fine. The dynamic is fine to a point. I have to adjust a transformer by peaking the amplitude on an oscope - no problem. Once it is peaked you move the scope probe to the Base of the transistor in the circuit and try to get a Vpp reading. (pic of manual attached).Looking at the equation for AC Gain, my reading should be 6 to 7mVpp. I can't get the scope to stabilize enough to read any Vpp. I borrowed a Siglent 1202X-e and get the same results as my old Tektronix TDS 340. It's just a measurement, but I'd like to understand what I am doing or trying to do. I even used the auto setup feature on the Siglent but I can't get a reading that small it seems.Am I missing something on scope setup?

author: m k
author_rank: Super Contributor
timestamp: February 14, 2023, 02:14:03 pm
content: Your situation should be very basic, just moving a tip from a point to the other.Maybe it's a grounding issue.Have you tried different trigger settings?

author: Hank30721
author_rank: Contributor
timestamp: February 14, 2023, 04:09:27 pm
content: Thanks for the reply!!!I have not tried different trigger settings.Would the auto-setup feature on the Siglent 1202x-e not account for that? I am by no means even a novice when it comes to using a scope.I turned on amplitude measurements on both scopes, the numbers change so fast and they never stabilize.The scope is AC coupled - is that correct? And Hi-Z set. The probes are 10X for both scopes. The Tektronix auto-sets to 10X and the Siglent is manually set to 10x. I can change the Siglent probe to 1X on the probe and then change the scope to match - would that even make a difference?

author: Grandchuck
author_rank: Super Contributor
timestamp: February 14, 2023, 07:27:41 pm
content: Auto mode should work. Small signals with normal triggering often show nothing.

author: m k
author_rank: Super Contributor
timestamp: February 14, 2023, 07:30:24 pm
content: I guess it's some manual settings then.(and then manual the paper)

author: floobydust
author_rank: Super Contributor
timestamp: February 14, 2023, 08:41:36 pm
content: It's best to know what the noise source is. Are you powering the radio with a battery or power supply with noisy SMPS? How is the grounding done?You can direct trigger the scope from the signal generator to eliminate hassles over that. Just use a BNC to EXT TRIG and set scope to EXT TRIGGER.Although the signal is low amplitude, so you might want to use the signal generator second output with more than a few mV if it can be in-phase with the first channel.It looks like the FM ratio detector is assembled first so no signal comes in. That's why the noise might be from an electrically noisy bench or power supply.https://www.eevblog.com/forum/beginners/more-help-with-my-elenco-amfm108ck-build-oscilloscope-s/msg4690118/#msg4690118https://www.eevblog.com/forum/rf-microwave/elenco-amfm-radio-tuning-instructions/

author: Hank30721
author_rank: Contributor
timestamp: February 21, 2023, 06:38:18 pm
content: A battery vs. power supply makes no difference in the signal noise.I setup the scope to externally trigger from the signal generator. The 2nd channel can be in phase with the 1st. I was still getting noise so I started searching for switching power supplies etc. that may be the culprit. My old Weller soldering station was the culprit - turned it off and now the signal is not so trashy that I can read the oscope clearly. It's still not great but the measurement was close enough to verify proper assembly so far.Thanks for all of the help - I'm now down to the final alignment. My signal generator does not go above 30MHz, so the alignment has been more by trial and error. (I'm not sure I can do the 1/5 of the desired frequency using a square wave as suggested in this post -https://www.eevblog.com/forum/rf-microwave/elenco-amfm-radio-tuning-instructions/Would the modualtion even still work like that?? I do have a 50mW FM transmitter I could use for alignment - any reason that wouldn't be a good idea? I've got the FM dial close but the band edges are still not right and the dial is way off still.Here is a station tuned-in to 90.5MHz which is basically the bottom of the dial:https://youtu.be/0Pb14emYriMHere is my AM side:https://youtu.be/iRM3vXHntlc

author: TomKatt
author_rank: Frequent Contributor
timestamp: February 21, 2023, 07:00:06 pm
content: I can't imagine that 4pf really screws things up, but I do note the test procedure indicates to use scope probes <=12pf or it will detune the circuit and cause reading errors. The SDS1202X-E kit probes (PP215) are spec'd at 16pf in 10X range.Just an observation.Edit - 12pf probes seem like an uncommonly low value compared to general use probes in the 200Mhz and under range...

author: Hank30721
author_rank: Contributor
timestamp: February 21, 2023, 07:42:21 pm
content: Quote from: TomKatt on February 21, 2023, 07:00:06 pmThe baggy my Siglent probes are stored in say 13.5pf - which is still too high. My old Tektronix TDS 340 probes are also 13.5pf (stamped on them). Would changing the Siglent probes to 1X impact the capacitance?

author: TomKatt
author_rank: Frequent Contributor
timestamp: February 22, 2023, 12:04:00 pm
content: Quote from: Hank30721 on February 21, 2023, 07:42:21 pmX1 is basically straight cable and has substantially more capacitance. The PP215's are spec'd at 85pf X1 / 16pf X10.The probe adjustment only affects the X10 setting on switchable probes... X1 is pass-through and the frequency adjustments do not affect anything in X1.

author: Hank30721
author_rank: Contributor
timestamp: February 22, 2023, 06:55:24 pm
content: I've enjoyed the kit, but wish I had a scope and RF generator mentor!