The reason there are two coils (and the same reason why tuning capacitors are usually a ganged pair) is because there are two LC circuits that need to be tuned together.

The first LC circuit is the main RF resonant filter.

The second LC circuit is the local oscillator which is usually set to the AM RF frequency plus 455kHz. When the RF signal is mixed with the local oscillator the result is two signals at the sum and difference frequencies. The difference frequency at 455kHz is called the intermediate frequency (IF) and this is amplified at the IF stage tuned to 455kHz.

That's a quick explanation of how a superheterodyne AM receiver works.

 

Yes, which sounds like a pain in the butt if you wanted to independently match the antenna
you need another trimmer anyway!

What's happening when you tune the RF resonant filter so far out of range that a carrier is detected again? (but no radio station).

 

Hello,

The reason there are two coils (and the same reason why tuning capacitors are usually a ganged pair) is because there are two LC circuits that need to be tuned together.

The first LC circuit is the main RF resonant filter.

The second LC circuit is the local oscillator which is usually set to the AM RF frequency plus 455kHz. When the RF signal is mixed with the local oscillator the result is two signals at the sum and difference frequencies. The difference frequency at 455kHz is called the intermediate frequency (IF) and this is amplified at the IF stage tuned to 455kHz.

That's a quick explanation of how a superheterodyne AM receiver works.

(Super)Heterodyne: I have seen the word before, of all things while reading about the development of radar years ago. I forget the context, or even the definition. The concise description is excellent, to the extent that I actually grasped a preliminary understanding of the method upon reading it. Thank you!

...
What's happening when you tune the RF resonant filter so far out of range that a carrier is detected again? (but no radio station).

I was thinking something along those lines while reading the description. As time permits I will try to find some more to read up on regarding.

However, back to the capacitor / tuner in Art's picture. It got my curiosity going to the extent that I made a quick and dirty experiment with iron dust, epoxy and a section of plastic biro. I have no idea how I should go about calculating the inductance of a coil with the inclusion of an "iron dust" core, but as I read that it is supposed to increase inductance "tremendously", I wound the coil for a considerably lower inductance than an air coil.

Pictures attached. It receives the AM band very well (surprisingly strong signal, for such a tiny coil). Yet again like another, earlier coil - and this is strange - it works with or without the ground, though obviously better with.

PS Edit:

...(surprisingly strong signal, for such a tiny coil)...

I keep getting caught out by that, as it still confuses me from time to time. I think I understand this correctly - the coil has little to do with the strength of the received signal, that is the antenna's job. All the coil does is "capture" the desired station by resonating at its frequency, if it is tuned correctly, either by varying capacitance or - as I now understand from this last experiment - inductance. I have written it for myself to consolidate it for once and all. Little or big coils have nothing to do with strength of reception; I hope I am right.

 

You can have that as well if you think it would be useful, and a commercial manufactured ferrite rod.
You probably already understand the mechanical one is not very good at also being an antenna like the one you made.

 

Yes, which sounds like a pain in the butt if you wanted to independently match the antenna
you need another trimmer anyway!

What's happening when you tune the RF resonant filter so far out of range that a carrier is detected again? (but no radio station).

I'm a bit lost trying to work out what you mean!


You could connect a series variable cap & "tweak" it for best results,but that was not normally done.
Most domestic MF AM radios had small untuned Primary windings on their antenna coils,which only lightly loaded them,so they were relatively immune to the effects of random length antennas.

Remember,the only time you get any output from the IF stages is when the difference between the Local Oscillator & the received signal is within the passband of the IF--in this case,centred around 455kHz.

Very early Superhets ( & some modern "Homebrew HF ones) used separate tuning controls for the LO & the aerial(antenna) coil.

There were two problems with this:

(1) The General Public had become used to one tuning control with Tuned Radio Frequency (TRF) radios,& weren't impressed,despite the better performance of Superhets.

(2) It was possible,for a given aerial tuning position,to tune the same station at two LO settings &,by adjusting the aerial tuning,different stations for a given LO setting.

Both these problems were solved by "ganging" the two tuned circuits & operating them from one control knob.

This introduced the problem of trying to tune two separate tuned circuits covering a different range of frequencies,maintaining a difference of 455kHz across the tuning range.
This quite profound problem was solved in millions of commercially made radios over tens of years.

The problem in (2) remained to some extent,but the aerial tuned circuit would always only have to have enough selectivity to reject a station 455kHz away,so "Image Interference "& "double-spotting"in most normal circumstances were no longer a problem.

Some Mantel sets had one,or several,Shortwave bands,& it is here where image Interference again reared its ugly head,as 455kHz at say,12MHz ,is a much smaller percentage of the desired frequency,so the required rejection is more difficult to obtain..

.

 

I thought I might add a bit of background on why Superhets are used:-

Why not just have several Tuned Radio Frequency stages,one after the other?
It seems a lot simpler!

This kind of radio was made,called a TRF,with up to three such stages.

They had several problems:-

(1) The circuit tuning had to "track".
Although this is easier than making a Superhet track,it was still a fairly arduous task.

(2) As triode tubes were used in the original designs,there was the possibility of oscillation of these stages at some part of their tuning range.
This was less of a problem with a Superhet IF,as its fixed frequency made it easy to "neutralise" each stage to prevent oscillation.
Neutralisation of a stage which has to tune over a wide range of frequencies is quite a lot harder.
Ultimately,such problems were solved by tetrode & pentode tubes,but by that time Superhets had taken most of the market.

(4) Here's the biggie!

TRFs had to tune over a 3:1 frequency range in MF service,from around 500 KHz to just over 1600 kHz.

The selectivity of these TRF stages varied over this range---an interfering signal 15kHz away might be easily rejected at 500 kHz,& not at all well at 1600kHz.

If only we could get the same degree of selectivity across the tuning range!

Enter the Superhet!
The IF stages can be optimised for best selectivity at 455 kHz,& the RF stages now have the far less onerous task of rejecting an image frequency 455 kHz away from the station they are tuned to.

 

Having said that, it is interesting to note what happens if you shift the local oscillator such that the IF frequency is shifted from 455kHz down to 0Hz.

You have baseband detection or direct-conversion receiver (DCR).

If you phase lock the local oscillator on to the incoming RF carrier you end up with the output of the mixer at baseband, i.e. at audio frequencies. With today's high speed digital signal processors (DSP) it is possible to implement this in software.

There's a future project for my STM32F407.

 

This is varying capacitance across the RF half of the tuning gang,
and watching the AGC that is applied to it while the LO is fixed the whole time:
https://www.youtube.com/watch?v=GdmBbb_ulrc&list=UUTcutL5_iEQ-LRACW8CRh-w

 

This is varying capacitance across the RF half of the tuning gang,
and watching the AGC that is applied to it while the LO is fixed the whole time:
https://www.youtube.com/watch?v=GdmBbb_ulrc&list=UUTcutL5_iEQ-LRACW8CRh-w

As you tune the aerial tuning away from the frequency selected by the LO,the signal at that frequency is reduced due to the selectivity of the aerial tuning.---makes sense!

If you can tune that circuit to the image frequency (provided there is a signal there),you will either hear interference between the image & the desired station,or just the image (probably the latter),depending upon the relative signal strengths.

Of course,that's why the caps are ganged.

 

Well, a few weeks ago I thought I knew a little bit about electro-magnetic waves and propagation. Indeed, I did know a little. Very little, that is! Though it is not of much practical use, yet, I have been reading up about the subject. This is an enthralling hobby for my (slightly) more mature self now.

I wish I could comprehend some of those posts about superheterodyne radios, but as yet it is all still a bit beyond me. Indeed, though the lure has been taken and I am thinking and researching about how I might concoct a receiver of this type, I feel it might be a while before I take it on.

Other news is in regard to antennas. Though I have nothing tangible yet, I have outlined a few plans to test, and also researched antennas a bit more (in itself a whole subject, wow!). Additionally, there is at least one other simultaneous thread on the forum, started by PRS about AM antennas, that I am also following closely.

With reference to what I said in an earlier post about wanting to experiment with DX, the burning news of this particular post is that I have heard my first confirmed distant station on a home made set this evening.

The set is simple enough, and utilizes my "perceived understanding" of the Mystery Radio circuits. It consists of 12 turns of 21 gauge on a 2" form (for approximately 10 uH), this being the antenna circuit. This was then covered with a layer of masking tape, and a second coil was wound onto that, central on the first coil, EDIT: consisting of 29 turns of 26 gauge for approximately 62 uH. (quick correction there, a bit of confusion with what I had originally planned and eventually settled for, my apologies). I used the small hobby variable capacitor, as I would be needing finer control than for AM, for the purposes of going after (roughly) the 25 meter band.

It was immediately apparent that I was getting some reception, but was not sure if they were interfering AM or not. After some time, I suddenly got a very clear reception of a voice in English announcing WRMI, Florida. I have looked it up; it is on 9,955 kHz, confirming that I am indeed receiving a SW station from another continent, location 1,792 statute miles away. Quite exciting, really.

Yay!

 

Hello,

I might be tempted to say "bingo" at this point in time;

A thread right here on the forum. I knew there would be something about my own query already posted.

And a neat book on SW superhet design, "How to build Advanced SW Receivers", of which I have already read the first chapter (please note, the link is a direct PDF download).

I am very vague about this local oscillator component as yet, and I limit my present question to that subject only. Is the output of a 555 timer IC a good enough source for creating this oscillator? I have no way of determining what frequency of oscillation I might be getting (sorry, but I just know I am going to be unable to count how many times a LED blinks per second!), but this MK105 Assembly PDF (another direct download) explains how to make 4 different oscillators out of a 555. Is it of any use for the application?

So, I am now proceeding to be quiet for a while.

 

the 555, having very squared off output, will have a lot of harmonics, causing lots of spurious signals in your reciever.

 

A 555 will battle to make any output at the frequencies needed for a MF Local Oscillator.
Apart from that,a 555 produces a non-sinusoidal waveform which will need a lot of filtering.

You are probably better off trying to build an LC oscillator.

The OP in that other thread got bogged down with the idea that he had to "design" everything,something which was beyond his capabilities.
I would strongly warn against going down the same path.

 

if you look into the SA602 chip, it has all you would need for the front end of a simple or complex reciever, low noise amp, mixer, oscilator.

 

Thank you for the advice regarding the 555. It was just an idea, as that PDF showed a way of obtaining a sine wave. I do have a couple of oscillator modules, but they are fixed frequency, as far as I know as yet, and the wrong frequency for the intended band. That said, I am wondering if, before going superhet in my projects, I might try a regenerative circuit with transistors for a SW radio first? I am still very much in the paddling pool and learning about more advanced designs.

I have loads of questions regarding CW and SSB, but I am still reading up...

The OP in that other thread got bogged down with the idea that he had to "design" everything,something which was beyond his capabilities.
I would strongly warn against going down the same path.

Thank you for the warning. Noted. I was, indeed, thinking along those lines.

EDIT:
@alfacliff: Sorry, we posted simultaneously and I did not see your post until after I had submitted mine. I will look into that; sounds promising. Thank you.

 

Moving on;

With my aforementioned idea of making a regenerative SW radio as a "natural" beginner's progression from straightforward crystal radio, I noticed that the couple of designs I looked up employ transistors - obviously, I suppose. I have never used one of these components before. Without wanting to "reinvent" the concept, as per vk6zgo's very valid warning, but certainly with the desire to become a bit more familiar with the component, I salvaged one from a defunct PCB.

After a read of Chapter 4 of the AAC Manual III: SemiConductors, I proceeded to "test" this transistor. First, to identify it as an NPN, then which one was the emitter and collector (the lower resistance from the base, I understand, is most likely to be the emitter, right?). As the transistor came off a PCB with a 120 VAC to 12 VAC transformer + rectifier on board, I assumed that using a 9 VDC source would be a reasonably safe source to do my tests, as I have no idea what transistor it is. For the radio, of course, I will be getting a known component.

Please look at the attachment. I am about 70% sure I have grasped the function of the component. However, I do just want to be sure that I have got this right. I have used it as an amplifier and a switch by simply swapping the LED and +9VDC line on the emitter and collector (as I understand them to be). I took some measurements, while it was functioning.

Am I on the right track, please?

 

Hello,

A bit of an off-topic, though related question. In my "office" there is a digital HF radio, covering 2.8 to 24 Mhz. I have never been interested in it before and there has never been a need to use it. My current interest in SW caused me to have a brief listen to it today, as time allowed. I picked up several stations, plus some people talking to each other - "hams", no doubt. But what interested me was a certain pair of "tones" I found at several points across the range; I was scanning around mostly between 8 to 10 Mhz.

They are continuous tones, one low and the other higher pitch, on frequencies immediately adjacent to one another. My first though was that they were carrier waves, or perhaps a channel of some sort, as they occur in pairs right next to each other. I have no idea what they are, and neglected to jot down the frequencies.

Any ideas, please?

 

Morse and RTTY are transmitted as a carrier, the decoding comes at the receiving end, one method by BFO (beat frequency oscillator).
Reason, all you need is to receive the carrier, not any modulated carrier prone to interference or corruption of the modulation.
Max.

 

+1

FSK or AFSK

I don't remember what it sounded like on AM.

When I operated a Ham RTTY station, I tuned receiver to the FSK position, and it was beadle beadle beadle sound.

That was mostly 2M band. FM

Gave it up when they started using them new fangled computer machines.

 

When I operated a Ham RTTY station, I tuned receiver to the FSK position, and it was beadle beadle beadle sound.

Now I cannot say where or when, but I have heard that sound before. My current experiments have never produced it, though. The tones I was getting were continuous, much like the old TV test screen tone. It really appears to be a channel, to my limited appreciation. It is not a DRM test, or something?

Next time I get a chance I will try and record them and attach an audio file, as well as noting the frequencies.

Morse and RTTY are transmitted as a carrier, the decoding comes at the receiving end, one method by BFO (beat frequency oscillator).
Reason, all you need is to receive the carrier, not any modulated carrier prone to interference or corruption of the modulation.
Max.

It could be that. Are amateur radio Morse CW carriers actually heard as a continuous tone on two frequencies, 1 Khz apart?