86. Diode Detector and Automatic Gain Control
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- čas přidán 29. 07. 2024
- A continuation of the Elenco radio build. This video analyzes ho the AGC and envelope detector work and then test the system.
I had some triggering problems in this video so bear with me. The Sync out on my FG decided to stop working. - Věda a technologie
I just wanted to thank you so much. I've always loved building electronic kits, Heathkit and I go back a long ways :) I picked up my first copy of this radio kit about 3 years ago and started building it. At the time I really didn't know much but this kit really piqued my interest and I found your videos. Wow, so much to learn... I didn't even have the basics down very well. So, I decided to learn more about radios and electronics. Over that time period I worked on the radio, took a break to study electronics in more detail, came back to build more of the kit and watched your videos again. Then I took a break, studied for my ham licenses and finally ended up with my Amateur Extra license. Now I'm back to my studies on electronics and have started anew on my kit. Each time I've watched your videos I know more from my studies which makes your videos make even more sense to me. I've got 3 of these radios, in different states of construction and I love making deep dives off how all this works while I build and study some more.
In short, your videos are like candy to me. They scratch all my itches: electronics in general and the magical world of RF! Thank you for the time you've spent. You have done a service for many including myself and I'm deeply appreciative.
thank you for the great videos.
I prefer the capacitive reactance description of the detector output. the 2.2k resistor and 0.01uF cap form a voltage divider that is frequency dependent. where the voltage at the output is higher at the lower frequency signal.
Fantastic video! Clear, simple, effective. Priceless informations for self-taught guys like me.
Excellent video. I learned several things, particularly the real cause of audio distortion in an envelope detector. One detail worth mentioning: the audio frequency response of an AM signal is not limited to 5 kHz. Most US stations transmit up to 10 kHz, following the standard NRSC audio curve. Even though the channel bandwidth is only 10 kHz, stations are occupying up to 20 kHz. The true limitation of the high frequency response is in the bandwidth of the receiver’s IF section.
Thank you so much. Very well explained theory and clear practical testing demonstration. I've learnt a huge amount. Much more interesting and useful than just soldering the kit together. 😃
Thank you very much. I have always tried to explain theory and prove it in practice. Thanks again for watching and for the Super Thanks. Yours is the first I have ever received :)
@@TheOffsetVolt That's very sad. Unappreciated genius. You clearly have a very great depth of knowledge and super talent for presenting it. I'm still working my way through the AM amplifier, so I've got lots of watching and learning to go. Thank you so much once again...
HI, I love the clear explanations in these videos, the close ups of the test equipment certainly helps to understand how to test the circuits. I have however hit a problem with my kit at this AGC testing, at TP5 I only get 0.133vDC, the R38 is getting hot, I have tested R35, R36 and R38 (off the board) and are fine. Diode D4 is correctly orientated (also tried a replacement has there are 2 in the kit). So at this point I'm lost as what to do, I have contacted Elenco but as yet no reply. Any help would be greatly appreciated. Keep up the good work.
(EDIT) C32 Faulty, I'll leave this post here if OK just encase anyone else comes across the same issue. ( I did test the capacitor before putting the board!)
I have just started watching you videos and wanted to let you know how much I like them. I have this kit and already built the AM side, but after watching I am realizing how much I just skimmed the surface. Your content has opened up the circuit for me. Thanks!
Wow, thank you!
Excellent explanation on AGC, exactly what I was searching for - thanks!
This is much better than the explanation in the manual
Thank you. The manual does a fair job of explaining the operation but I always like to go beyond that.
@@TheOffsetVolt I can't wait till you talk about the 2nd IF amp. I think that's the part I blew out.
Many thanks for your video. Trying to service the AGC of a Sony CRF-320 and now I have a better idea of how to test it.
I have this kit, I had to wait for your videos to really learn something. TY!
Glad it was helpful!
Awesome !! thanks for sharing your knowledge. I am trying to TS a radio which I suspect has an AGC issue, I have the tools now!
Glad it helped Victor - it's like G.I. Joe, "knowing is half the battle."
awesome explanation, thank you
The gain is changed because the control voltage on Q8 Base shifts the AC signal on Q8 Base from the steeper part of the IC vs VBE curve to the flatter part of the curve.
Hello, thank you very much for your videos. Why can't I see the signal detected at the cathode of the detector diode on my oscilloscope??? At the anode I see the modulated signal from my generator but not the detected one. Thank you so much
Thx. And... ...you saved my marriage! :D
how can we turn off the sound at an empty (noise) frequency on the fm radio, what should I use for this?
Please what's the name of the textbook you use? Thanks.
Why is C36 bypassing the second transistors base to ground? What is the value of C36, is that maybe some picofarad to elimate oscillation?
I found the manual online and it says that stage is a common emitter amplifier and that C36 bypasses 455kjz to ground, i dont get at all why they do that on the base. The emitter makes semse, to increase the gain. Wny would you ever put a capacitor on the base to ground on a common emitter
It is a common base amplifier. Common base amplifiers are mainly for impedance matching.
A little confused, the LTC is 22 micro-seconds and the STC is 200 micro-seconds. 22 usec is less than 200 usec. Why is it called the long time constant?
That square wave with the clipping and the ripple wasn’t a good representation of what can happen with an AM signal, since it contains harmonics far higher than the maximum 5kHz. The maximum dV/dt for a 5kHz sine is Vpeak*2πf = Vpeak*10,000π, which is significantly easier for that filter to handle than a square wave. On the topic of filters, I wonder about using an LC filter instead of an RC filter, for the steeper cutoff. It’s not like there aren’t already inductors or transformers already on the PCB. I think an active filter would be an option too.
Go ahead and calculate the inductor needed for a LC filter with a reasonably sized capacitor. Remember that for a filter, you'll want components with high quality factor. The inductor you'd need would be the size of your fist.
Hi James. I appreciate the feedback and I agree up to a point. The modulation signal I used was just 200Hz so the harmonics were a lot lower and the example did a fair job of showing what different RC combinations do to the demodulated signal. An active filter would be nice but this was part of the Elenco radio kit so that was out.
Can this be used to control sound volume?
Yes, it can. It will probably need mods to meet your specific requirements but the principle is the same.
What is the purpose of the diode? Why is it called a *_detector_* ?
The diode removes one half of the IF signal. A low pass filter is used to eliminate the higher frequencies of the IF signal to just pass the audio through, but putting a low-pass directly on the IF signal would average the whole thing to 0; there aren’t actually any audio frequencies there. By clipping half the signal off with the diode, we are left with an asymmetric signal that a low-pass filter will average to the signal’s envelope. This can be done directly to a tuned RF signal like in a crystal radio circuit, or to the tuned IF signal like in this superheterodyne circuit. It’s called a “detector” because it’s the critical component in turning an imperceivably high frequency radio signal into an audio signal that can be listened to.
See the wikipedia pages for amplitude modulation, envelope detector, crystal radio, and superheterodyne receiver for more information.
what is the minimum input signal that is detectable?can I get your email ID to share my observation with you and get help from you too?