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How to make a traditional diode envelope detector radio receiver

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Fluffyboii

Active Member
Hello.
As some of you know I recently made a small AM transmitter. It made me really happy even though it has a range about 3 meters due to not optimal antenna and low power of it. I want to improve that transmitter by adding RF amplifier of some kind but I can't figure out a way to amplify a signal about 5-10V peak to peak with simple CE amplifier since high input voltage swing causes BJT to get out of active region and weak RF output of that circuit would probably drop from the input impedance of an RF amplifier and I am not qualified to calculate and compensate for all of that. My small research revealed me that RF transmitters and amplifiers are just an endless rabbit hole so unless I decide to torture myself by getting some RF classes I will not be able to fully understand and come up with stuff myself.
Anyway since I made the transmitter but only radio I have here is one that plugs into mains and hard to move I can not show it to anyone in action. I want to make a simple envelope detector radio receiver to go with it. I actually made something like that at a lab session with op amps but it was very bad since we used silicon diodes for detector and overall gain was very low so it required direct feeding of AM signal into the circuit and didn't had the capability of getting the small signal in the air. I wasn't satisfied at that time:
1670458387657.png

Book explanation of envelope detector:
1670458184710.png

I don't know my transmitter bandwidth but from its sound quality I am sure it is not 20Khz but something like 5-10Khz at best. My carrier freq is about 1.2Mhz which would mean for 10K detector resistor I would need a capacitor at least few pF. I don't have 5-10pF capacitors around and I don't think having a value that low can be reliable so maybe I need a lower resistance about 1K without passing to much current over the diode. Anyway I have Germanium diodes with about 0.3V drop and Schottky diodes that have around 0.2V drop for the job.

Input filter can be made with the 30pF-60pF variable capacitor I have and a 50uH inductor but range will be 1.3Mhz-900Khz according to my calculation. I would like to have the whole MW range in there but honestly there are no radio stations here to listen that broadcast MW so it doesn't matter. I can always 3D print a larger variable capacitor housing and cut some metal discs or make a large adjustable inductor if I get a reliable circuit going and want to improve on it.

My real problem is the gain required and the finding antenna specs that are optimal. I saw many old, cheap MW radios that had 9 Transistors branding on them which is impressing to be honest. I honestly don't know how much gain I need but I assume at least 1000 since 100 gain doesn't even get close. I had this BJT amplifier with bootstraping at input to have a high input impedance here that has about 100 gain.
1670459608216.png

But since this is not a homework why wouldn't I cheat by using a mosfet at the input to have technically infinite input impedance. My only problem is that if I bias the mosfet with an voltage divider I would need to use very large resistance values about few megaohms to not load the weak input signal. I think I need something that biases the base with some kind of feedback network to avoid having a resistor to ground but I am not sure what is optimal here. Than after the mosfet buffer at the input I can have my 100 gain to feed my envelope detector than apply some more gain if needed. I will try to simulate it in LTspice. And at the last output buffer I can have a small toy speaker to hear the results. When I think about it the gate to emitter capacitance of the mosfet perhaps can be an issue. Cascode configuration was able to get rid of that miller capacitor and gave good gain perhaps it is a good idea to use it here.

For some reason whatever I tried I wasn't able to get and gain out of 2n7002 in LTSpice. I tried normal CS amplifier, I tried cascode with different configs, included perfect current sources for max gain but nope nothing got me gain more than 1. Maybe I am unable to see something in front of me since it is 5AM now and I used all of my mental resources. Added the LTSpice file. Unfortunately the one in the image with bootstrapping is lost.

I really can not get any transistor amplifier in LTSpice running now. Check Draft2 for example, there is no gain whatsoever. I don't know why maybe my LTSpice is broken.
I need to choose bias voltages more carefully. I totally forgot everything again.
 

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A sim of your first stage seems biasing is correct and recovered audio is there.

1 Mhz carrier, 1 Khz modulation.

1670592682840.png



Regfards, Dana.
 
I set the probe to 10x and checked the voltages. All bias voltages showed up very low when input was grounded. Around 35mV

When I directly feed the AC signal I can see demodulation happening. There is still attenuation at the pnp transistor as you can see, it is clipped because I am directly feeding it with AC but overall voltage swing is lower. Seems like it is root of the problem even though I connected it right this time.
 

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Base voltages are, 300mV, 600mV, 2.15V, 1.15V from left to right. My multimeter died, no matter new batts it reads voltage way lower than what it is. Rip.

Oscilloscope is reading my lead acid battery voltage as 8.4V which is actually about 11V, idk.
 
Cheap multimeters have crappy selector switches. So take apart and use
alcohol or contact cleaner on main selector switch. Usually fixes the problem.


Regards, Dana.
 
Cheap multimeters have crappy selector switches. So take apart and use
alcohol or contact cleaner on main selector switch. Usually fixes the problem.


Regards, Dana.
I took it apart and it was super clean. It is not caused by the contacts. It straight gone berserk, beeping randomly at modes that are not supposed to beep and lighting screen up for no reason etc. Maybe small chip inside of it got corrupted. I have a beautiful Simpson 260 multimeter that has a corroded, problematic middle selector that slowly stopped working even though I used corrosion removers and ISO multiple times on problem area. This is not like that. I have also another multimeter that likes draining 9V batteries while not being in use. I am never lucky with this stuff I guess. Maybe it is time to shill out some serious money for a fancy fluke with auto scaling.
 
Just saw this in an ad :




Regards, Dana.
 
I bought the high beta transistor MPSA18. I bought the ferrite stick and wound until I got 250uH. With two parallel 33pF caps it should be about 1.2Mhz. It works but it is very silent, probably needs fine tuning on capacitor values. But when I touch it or totally randomly it starts oscillating about 50Hz, very very loudly. Load inductor writing is 103 which is 10 millihenry. I removed those from a CRT TV, both have same inductance but big one has less resistance so I used it. Should I make like a faraday cage for it or is there another way of stopping it from raising hell. Tried changing feedback turns there since the guy wrote 8 then 3 on another schematic of the same thing but decreasing turns only decreases output volume.
 

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I think you are just coupling 50 Hz into it from ambient house/lab AC wiring.

Odd design where transistor not biased "properly".


Regards, Dana.
 
I think you are just coupling 50 Hz into it from ambient house/lab AC wiring.

Odd design where transistor not biased "properly".


Regards, Dana.
Yes, I immediately recognized the frequency. But weird thing is it latches randomly to that frequency. It stays until I unplug it. When I touch the feedback coil it radio sound gets louder and clearer until noise kicks in. I think capacitance is adding up from my body or I am acting as an antenna. Transistor biasing seemed off to me too but I assumed it was getting the bias voltage over the diode and voltage divider that biases it. Still very disappointing results but I guess it will do for a demonstration.
 
I got it working after messing with the LM386 connections. Seems like pot wasn't securely connected. Added 1uf film cap to power rail for better filtering. Used 220u for speaker but I think I should increase it to something like 470u or 1000u but tbh speaker is too cheap to hear a difference between 100u and 220u anyway. Since it is better than the clock radio in terms of sound quality I am thinking on printing a small box for it. I still need an adjustable capacitor though. Would this work for a while on a 9V battery or should I deploy 6x1.5 AA batt case? I will measure current.
I wish I didn't waste my time with the fist one I build and used LM386 instead. I was planning on winding a 500 ohm to 8 ohm transformer for a class A amp but seems like that is unnecessary. Would this also work for LW if I added more turns with a tap to the coil or use a large capacitor in paralel. Maybe I should make a foil capacitor like I did for my tesla coil instead of hunting for a 365pF radio cap that no one sells...
 
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Just saw this in an ad :




Regards, Dana.
Today while I was shopping for parts for my radio I bought the cheapest multimeter I found because it was way to cheap :/ I had one cheap one before but I lost it at some point. Quality is garbage but it works good enough. I will get something better when I need it or just fix the Simpson 260 with some strong corrosion remover. I wish I bought something like you send before extra tax and shipping costs for items outside country were added with new law. Now it will cost 3x to buy one from Aliexpress and god knows how much more because the lost value of money.
 
Overall performance :

View attachment 139610


Regards, Dana.
Why does it lose so much gain after 10KHz even though impedance is still high. Is it something with the diode being inefficient at high frequencies. I used a Germanium diode with Vd of 350mV maybe I should replace it with a rectifier diode with Vd of about 250mV. Are there any better demodulator circuits for short wave for example? Not that will keep making radios but curious.
These sinulations you do are very nice btw. Is it possible to do similar in LTSpice. I doubt I would be able to find MPSA18 model for example.
 
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I use Simetrix. It was Analog Devices sim until they bought LTC and decided to
standardize on that, I think because of all the LTC aps and sims they did on their
product lines.

Simetrix is vastly easier to do sophisticated probing and setup in my opinion.

The MPSA18 you are running at a light Ic, ~ 1 mA, and its GBW at that is low.
You also have significant - fdbk via R1 so that trades off G for BW.

You can do the sim in LTC, import the MPSA18 spice file.


Regards, Dana.
 

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