How to make a traditional diode envelope detector radio receiver

[Fluffyboii]

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:

Book explanation of envelope detector:

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.

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.

 

[Nigel Goodwin]

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.

It doesn't matter anyway, as 10KHz is more than high enough for the low quality of AM radio. Bear in mind as well, it's only a simulation, not real life, and doesn't appear to include the reflex action either?.

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.

Stick to the germanium diode, that's what has always been used - the modern suggestion of using Schottky rectifiers is simply because of the lack of availability of Germanium diodes. The OA91 was one of the classic diodes.

There are numerous different demodulators, but a simple germanium diode is hard to beat.

 

[Fluffyboii]

It doesn't matter anyway, as 10KHz is more than high enough for the low quality of AM radio. Bear in mind as well, it's only a simulation, not real life, and doesn't appear to include the reflex action either?.



Stick to the germanium diode, that's what has always been used - the modern suggestion of using Schottky rectifiers is simply because of the lack of availability of Germanium diodes. The OA91 was one of the classic diodes.

There are numerous different demodulators, but a simple germanium diode is hard to beat.

I see. I thought 10K was radio station frequency. If it is audio frequency as you said it doesn't matter since bandwidth is low anyway.

 

[Nigel Goodwin]

I see. I thought 10K was radio station frequency. If it is audio frequency as you said it doesn't matter since bandwidth is low anyway.

He's only simulating the audio part, he doesn't have the tuned circuit or the RF choke in place.

 

[Nigel Goodwin]

Something you might find helpful

https://ee.old.no/library/EE20-colour-en.pdf

Particularly pages 25, 66 and 67.

This is the kit I had in about 1967/68 for Christmas.

The radio, similar to the reflex design you're playing with, worked excellently, as did all the projects. Notice all Germanium devices, it was pretty well pre-silicon.

 

[Fluffyboii]

Something you might find helpful

https://ee.old.no/library/EE20-colour-en.pdf

Particularly pages 25, 66 and 67.

This is the kit I had in about 1967/68 for Christmas.

The radio, similar to the reflex design you're playing with, worked excellently, as did all the projects. Notice all Germanium devices, it was pretty well pre-silicon.

Yes it looks quite similar. This radio still works much better if I touch the end of the coil, adjustable capacitor is a must I guess. Maybe I should try this antenna on the orher "all band receiver" circuit and see how it responds. Would it be better if I used a large loop antenna. Ferrite sticks seems very directional.
Is there a ground connection requirement on these.

 

[danadak]

Sometimes ground makes a difference, sometimes not. And quality of ground
makes a difference, soil conditions, etc... The amatuer radio community dealing
with this since Moses on the mount. No "right" answer, but experimentation everything.

All in all as a SWL since early 50's longwires with an antennae tuner always
an excellent approach. Loops (LF and HF loops), not ferrite for this discussion)
best. Beverage antenna outstanding, if you have the real estate.

Internet Archive Search: antenna handbook

Regards, Dana.

 

[Nigel Goodwin]

Yes it looks quite similar. This radio still works much better if I touch the end of the coil, adjustable capacitor is a must I guess. Maybe I should try this antenna on the orher "all band receiver" circuit and see how it responds. Would it be better if I used a large loop antenna. Ferrite sticks seems very directional.

Ferrite rods are directional, but so are loops - both are commonly used for direction finding. In fact most aerials are directional, and even a vertical dipole (which is omnidirectional) is still directional vertically.

Is there a ground connection requirement on these.

As Danadek says - it's in the lap of the gods, yes, no, maybe? -perhaps only if there's an 'r' in the month?. It 'can' help considerably, it can make it worse, or it can have no effect.

As kids we used to listen to the Top 20 Countdown on the radio, a 'gang' of about half a dozen of us - 6:00pm Sunday evening. We found that if you placed the radio next to a telegraph pole it made it a LOT louder, so we always did that. I can't say I've ever really 'understood' the reason? - I've always presumed it was wirelessly coupling to the telephone wires and they acted as an external aerial?.

 

[Fluffyboii]

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.

I see, that biasing setup is oftenly used because they want to keep input resistance quite high to avoid loading down the input signal, I see it with mic preamp circuits as well. Low current causes output capacitor time constant high because high resistance, right?
I kind of want to make a regenerative receiver with moving feedback coils now :/It just looks too cool.

 

[Fluffyboii]

It turns out the 10K pot I used was defective. After replacing it it gets loud enough to drive that 5W speaker into distortion region. I assume that is not good since this LM386 is rated for about 0.7W But it will not get that loud when I try to listen to stations far away. I also noticed that those 2 copper pieces make an ideal adjustable capacitor with 1.2Mhz resonance value at the middle of them. I may make a smaller adjustable capacitor if I can cut some aluminum with laser cutter at collab space but one way or another I will make a box for it and try to listen another stations at night.

 

[Nigel Goodwin]

The speaker probably isn't been driven in to distortion, the amplifier is - it will be clipping heavily. The speaker also looks to be rather optimistic if it claims to be 5W

You're going to a lot of effort to listen to low quality AM radio It makes me rather ashamed to be sat 2 metres away from a Hacker RP35B, probably the best (and best sounding, and loudest) AM portable radio ever made - which hasn't been used this century, or in the latter part of the last century.

 

[Fluffyboii]

The speaker probably isn't been driven in to distortion, the amplifier is - it will be clipping heavily. The speaker also looks to be rather optimistic if it claims to be 5W

You're going to a lot of effort to listen to low quality AM radio It makes me rather ashamed to be sat 2 metres away from a Hacker RP35B, probably the best (and best sounding, and loudest) AM portable radio ever made - which hasn't been used this century, or in the latter part of the last century.

Haha, true. That speaker is the cheapest speaker I got from the speaker shop after saying I wanted something bit "meaty" but as cheap as possible. I do have other speakers I harvested from other stuff at home and they are all the same. I do have a very sensitive Hitachi ferrite antenna radio as well but making one myself was very alluring for some reason. I was able to get some Long wave signals with it about 4-5AM and lots of MW radio channels. I am sure if I used it in Istanbul instead of Ankara it would get even more reception. Its analog signal meter doesn't work though and it drives me insane.

 

[audioguru]

A speaker needs a proper enclosure to produce low frequencies. Without an enclosure then it sounds like a shrieker.

 

[Fluffyboii]

A speaker needs a proper enclosure to produce low frequencies. Without an enclosure then it sounds like a shrieker.

Does it need to be something specific. Most of the time in cheap radios, speaker is just screwed behind a simple grill and that is all. This speaker doesn't even have screw holes so idk how I will hotglue mount it.
Today I was able to get few radio stations with the reflex radio at 3-4AM. Not as good as my Hitachi, that one probably has a way larger ferrite antenna but It surprised me with it's performance. I will post a video when it uploads.

 

[Nigel Goodwin]

Does it need to be something specific. Most of the time in cheap radios, speaker is just screwed behind a simple grill and that is all. This speaker doesn't even have screw holes so idk how I will hotglue mount it.
Today I was able to get few radio stations with the reflex radio at 3-4AM. Not as good as my Hitachi, that one probably has a way larger ferrite antenna but It surprised me with it's performance. I will post a video when it uploads.

They are usually held in place by a couple of small half-circular clamps, but hot melt is fine.

On the Philips kit radio I built back in the late 60's I used to listen to Radio Luxembourg on 208 - which was a fairly serious distance away.

 

[audioguru]

All the radios I have seen have the speaker mounted on the grill of the important enclosure.
The enclosure prevents sounds from the rear of the speaker (that have the opposite phase of the sounds from the front) from coming around and cancelling mid and low frequencies.

 

[Fluffyboii]

All the radios I have seen have the speaker mounted on the grill of the important enclosure.
The enclosure prevents sounds from the rear of the speaker (that have the opposite phase of the sounds from the front) from coming around and cancelling mid and low frequencies.

I see, so as long as it is in a enclosure behind a grill it is fine. Will make a box and cut it with laser out of some acrylic. Should I keep these copper piece capacitors. I am not sure they cover the whole range. Loopstick is 250uH seems like people usually go for bit lower like 180uH which explains why I get some channels when copper pieces are barely overlapping but seems like I am missing on the low frequencies a little. I am unable to cut conductive metal with laser to make myself a better tuning capacitor. I need something simpler to build like a roll of aluminum foil. I made such capacitors for my tesla coil which passed 10nF when fully rolled but those were fixed.

They are usually held in place by a couple of small half-circular clamps, but hot melt is fine.

On the Philips kit radio I built back in the late 60's I used to listen to Radio Luxembourg on 208 - which was a fairly serious distance away.

I think that radio still operates. I was barely able to get a signal at that frequency with my radio and it was written on the scale. I doubt there are any LW stations that use the same frequency but it was hard to believe since it was over 2000km away from my location.

NTE618 is bit pricey but it looks like it can be used with this radio if I can find someone selling it I may buy one.

 

[Nigel Goodwin]

I see, so as long as it is in a enclosure behind a grill it is fine. Will make a box and cut it with laser out of some acrylic. Should I keep these copper piece capacitors. I am not sure they cover the whole range. Loopstick is 250uH seems like people usually go for bit lower like 180uH which explains why I get some channels when copper pieces are barely overlapping but seems like I am missing on the low frequencies a little. I am unable to cut conductive metal with laser to make myself a better tuning capacitor. I need something simpler to build like a roll of aluminum foil. I made such capacitors for my tesla coil which passed 10nF when fully rolled but those were fixed.

I think that radio still operates. I was barely able to get a signal at that frequency with my radio and it was written on the scale. I doubt there are any LW stations that use the same frequency but it was hard to believe since it was over 2000km away from my location.

It's MW, not LW - and it's 208 METRES (not KHz) - MW/LW stations were normally listed by wavelength, not frequency.

NTE618 is bit pricey but it looks like it can be used with this radio if I can find someone selling it I may buy one.

Use a simple silicon rectifier (like a 1N4001 etc.), these make pretty good varactor diodes - don't use asmall diode, it needs to be a decent sized rectifier, in order to give a decent capacitance range.

 

[JimB]

It's MW, not LW - and it's 208 METRES (not KHz) - MW/LW stations were normally listed by wavelength, not frequency.

Correct.

The frequency was 1440 kHz in the medium wave band. Although...
in its earlier days 1920s, 30s and 40s, Radio Luxembourg did transmit in the long wave band.

JimB

 

[Nigel Goodwin]

Correct.

The frequency was 1440 kHz in the medium wave band. Although...

I can't say I was aware it was 1440 kHz, as I've never been interested enough to work it out

in its earlier days 1920s, 30s and 40s, Radio Luxembourg did transmit in the long wave band.

JimB

Interesting, I never knew that - but it was before my time

 

[Fluffyboii]

Use a simple silicon rectifier (like a 1N4001 etc.), these make pretty good varactor diodes - don't use asmall diode, it needs to be a decent sized rectifier, in order to give a decent capacitance range.

What about full bridge rectifiers. I do have 1n4001 I think but I also have some full bridge rectifiers with high rating. Since they are just very chonky 4 diodes in specific configuration it may work. Ok it will probably not work since I can not isolate a single diode from it. I guess a Large transistor would work like 2n3055.

I have 10A10 diodes which are very large maybe that would work.

Someone actually test several diodes as varicaps

Rectifier diodes as varicaps (varactors)

I am a simple enthusiast attempting to convey my love and enjoyment of radio, electronics, and computing and perhaps to inspire YOU to build something too.

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