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How to make a traditional diode envelope detector radio receiver

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Fluffyboii

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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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Use the 1N4001 :D

It might be a bit of a 'black hole' - but the excellent old UK Magazine 'Radio Constructor' published endless radio designs, as well as loads of other stuff - you can find most of it here:

 
Use of varicaps and considerations, some excellent notes here :


C range everything if you are doing a receiver :

The 1N400x series have been used but they have a limited range of C for reasonable Vrevbias
control voltages :

1671198655851.png


As you can see the Rev bias range of 1 to 10V (a reasonable control V range)
only gives a ratio of ~3:1. The hyper-abrupt varicaps usually highest ratio.

If you wanted a 10 : 1 range in freq you would need

1671200004795.png


Looking at Digikey I can see a max of 25:1 (not sure of V control range to get that). I saw a part
yesterday that was 56:1, now cant find it.

There are alternative ways, like using a variable R in series with cap, to produce an effective change in
its parallel equiv C. But that de-Q's circuit so demands more G in oscillator loop and a whole bunch
of other considerations.

The series to parallel transofrmation (useful in impedance matching work) to find the equivalent parallel
C of a series RC circuit :


1671201079680.png


Manufacturers of varicaps (check actual availability) show on this page -


Not sure how many of those vendors listed still making varicaps.....



Regards, Dana.
 
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Yes, the Cmax/Cmin ratio is rather limited for diodes like the 1N4001.
This will give a range of Fmin to Fmax which is significantly less than the range of the medium wave broadcast band.

Varicap diodes have been made for this tuning application, the BB212 is a typical example with a wide Cmax/Cmin ratio.

I am sure that other diodes suitable for this application are available, but whether they are available in Fluffys location I do not know.

JimB
 
I tried few different diodes and noticed I can tune to my 1.2Mhz radio transmitter with 1n5819 diode as varactor but range was very limited as expected.
Did you try your 1N4001?, as the one you mention is a Schottky rectifier, and I've no idea who well they work as varactors?.

I'd 'imagine' that they would be poor, as presumably they have low capacitance, in order to increase their speed?.

Also, what voltage did you have feed it?, as you need quite a voltage range to get the most change.
 
1671220196108.png

I first tried making a stable 555 circuit with very high charge resistor like 40Mhoms which was blinking led slow enough to see with very low capacitor values and connect the ghetto varactor to that to change the blinking speed, which did not work for some reason. Either it capacitance was too low and it was blinking too fast to see or idk. I then modified my radio circuit with the one above and replaced the MV1405 with different diodes. I unfortunately did not had 1n4001 but 4007, 4004, small voltage silicon diodes, leds and such so I tried what I had. Sınce range was bad with all of them I did not spend too many hours manically testing every possible combination but at some point I was able to tune to my radio.
1671220523886.png

There is the graph from the site I quoted earlier.
 
Use of varicaps and considerations, some excellent notes here :


C range everything if you are doing a receiver :

The 1N400x series have been used but they have a limited range of C for reasonable Vrevbias
control voltages :

View attachment 139625

As you can see the Rev bias range of 1 to 10V (a reasonable control V range)
only gives a ratio of ~3:1. The hyper-abrupt varicaps usually highest ratio.

If you wanted a 10 : 1 range in freq you would need

View attachment 139626

Looking at Digikey I can see a max of 25:1 (not sure of V control range to get that). I saw a part
yesterday that was 56:1, now cant find it.

There are alternative ways, like using a variable R in series with cap, to produce an effective change in
its parallel equiv C. But that de-Q's circuit so demands more G in oscillator loop and a whole bunch
of other considerations.

The series to parallel transofrmation (useful in impedance matching work) to find the equivalent parallel
C of a series RC circuit :


View attachment 139627

Manufacturers of varicaps (check actual availability) show on this page -


Not sure how many of those vendors listed still making varicaps.....



Regards, Dana.

I guess one way of getting high capacitance change would be simply making a simple boost converter to boost the voltage to 100V then feed it with a large resistor to the diode so that it's capacitance can get much lower. But all the AC garbage that booster would produce... And simply having 100V chilling in a radio wouldn't be the nicest thing I guess.
 
Not sure how effective this would be but use a C multiplier :


I have never used one, so started fooling around with SIM. The multiplier
does multiply C1 by ratio of R1/R2 but also adds a series R with the equivalent
C. But in 1 Mhz area, with a 10 Mhz or better opamp might be interesting to
play with.

1671238031317.png



Update, hold off on this, I am getting results that need more investigation.


Regards, Dana.
 
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Not sure how effective this would be but use a C multiplier :


I have never used one, so started fooling around with SIM. The multiplier
does multiply C1 by ratio of R1/R2 but also adds a series R with the equivalent
C. But in 1 Mhz area, with a 10 Mhz or better opamp might be interesting to
play with.

View attachment 139637



Regards, Dana.
That is an interesting circuit I may try making it on breadboard if I can find few high bandwidth op amps. Tbh using two metal copper plates is still unreasonably effective. If I make an envelope to reliable slide the pieces I may just stuck that under the radio box I am designing. I could also leave few holes on the box so if I made something like this work or get a varactor I could convert it to that. Still I will look into this.
 
I made a box for it. Unfortunately maybe because Laser cutter was bit missaligned it didn't fit well and it was impossible to screw bottom capacitor holder.
 

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The wires to and from the volume control are not shielded audio cables then the long ordinary wires are antennas for mains hum and other interference.
 
The wires to and from the volume control are not shielded audio cables then the long ordinary wires are antennas for mains hum and other interference.
I see. I thought that wiring could be an problem as well but I was more concerned about the antenna and tuning capacitor. I assume some aluminum tape around wires to ground would suffice but I don't have any around. Can I put multiple turns of wire that is grounded around those as a means of blocking random AC. I really need a 365pF capacitor though. I tried adding more turns to the loopsitck and wired it as a tap on a switch but it seems like I have more than enough inductance since my small transmitter at 1.2Mhz barely gets in tune with the long tap when capacitors are barely overlapping each other. I should instead remove some turns it seems or put some capacitance series with the capacitor. Still it is hard to pick stations that are too close to each other and feels like it is not going low enough even though it certainly does.
 
I tried the radio with signal generator and saw it can do around 500-1200 with larger middle tap and 650-1700 with smaller one I build on later but it wasn't really consistent. These were with capacitor having paper tape as insulator. When I use other side of the capacitor it decreases in capacitance since there is also plastic between it's sides and it can then capture high frequencies about 1900Khz with small coil but then low end is missing. Instead of opening it up and messing with inductor or capacitor I decided to leave the inductors as they are connected to second switch to be selected and soldered an alligator cable head on the capacitor cable. This way I can select which side of the capacitor I am using. BTW I cut a alligator cable for this but I chose one that went open. They always go open for some reason. When I checked the connections I noticed it wasn't soldered and connection was made by bare copper touching the clip. And one side was oxidized. I can not believe that was the reason, I always thought they went open because copper cable in them was too thin and it was breaking at some point. Other than that I made a loop antenna. I originally put the feedback coil of the radio to screws outside the box so I can connect external antennas (It can be seen from my previous message) if I wanted to experiment. But coupling alone makes it much better so I guess that is not needed. I was gonna go with the 60cm loop with 17 turns as it was mentioned in Antenna book that was posted here, short antenna section, but it requires like 43M of copper wire and when I asked teacher about getting some he got very surprised at the amount needed and "kindly" reject my offer XD So I went with a smaller one I saw from a youtube video and used regular wire instead of 1mm enamel covered copper. Works good enough.
 

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I accidentally used an impostor jack with thick pin and no female connector I have fits it. It is glued too securely in there so I guess I am stuck with 9V battery. Is it another standard.

With coupling I accidentally got a radio signal in my dorm from nothingness while I was trying to listen my own transmitter. I never got signal in my room before, there are just too much interference and it isn't even 3am yet so it means it is working stupidly good.
 

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I accidentally used an impostor jack with thick pin and no female connector I have fits it. It is glued too securely in there so I guess I am stuck with 9V battery. Is it another standard.

There are a number of different standard sizes, you have to ensure you use the correct matching ones - 2.1mm is probably the most common pin size, and 2.5mm comes second. 'Roughly' the thicker the pin, the higher the current rating - and the 2.5mm ones were often used on small portable colour TV's (like the Nikkai Baby 10).
 
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