Ok , so then I am a little confused are standard AM radio stations broadcasting SSB or fully orignal AM (DSB with carrier)?

I read that DSB with carrier is just wasting alot of power and bandwidth since all the info is in one sideband and the carrier is the largest amount of power need of the 3 elements. So I thought AM radio stations where just broadcasting one sideband to be more efficient. Thought this was some FCC regulation thing ... maybe I am wrong but with TV thru the air NTSC and ATSC use 8VSB or some variation which is basically using SSB supressed carrier with half of the other side band... But of course TV broadcasting maybe be different since standard TV may have TV tuners geared to do product detection as their standards,...etc

If the professional stations and amatuer radio stations broadcast as SSB without carrier? I would think the standard AM receivers would have to have product detectors as opposed to the simple enevlope/diode detectors?

Curious a transmitter that broadcasts orignal AM modulation (i.e DSB with carrier ) if it can be recieved by SSB carrier supressed AM recevier?
I think this is true

Basically what I am trying to figure out is will product detectors detect both DSB (carrier or not carrier ) and SSB ( carrier or not carrier )
4 different conditions.

Where as enevlope/diode detectors could only detect 2 of the 4 SSB and DSB with carriers broadcasts.

Correct me if I am wrong!

 

Questions

Ok , so let me get this straight the diode/enevlope detector will work with
SSB and DSB with carrier but won't work if the carrier is taken out in either one of these. Yes/No?

Yes

If this is true then for detecting SSB without removed carrier would you have to offset the tuner by the sideband frequency shift or keep it the same as orginal DSB not removed carrier orignal AM diode detector?

No, the tuner would still need to be tuned to the carrier. The carrier is the reference.

If the carrier is completly taken out i.e SSB removed carrier then is the only way to detect/demodulate it by product detectors (i.e where you use a local ossilation and a mixer/filter ,...etc etc)???

Yes

And are standard AM receiver that you buy at stores normal built for orginal AM (DSB with carrier) , or (SSB with carrier) , or SSB with out carrier or DSB without carrier?

Thanks

A standard AM radio uses a variation of the diode detector, since it is the simplest way to do things. BTW, a transistor BE makes a good diode detector with built in amplification.

Almost all (an extremely high percentage) use super heterodyne, to simplify the RF filtering/amplifier section.

Most AM stations (including shortwave) use the standard AM modulation. If you go through the advantages I listed the other modulation techniques will be used, but they are unusual.

If you're interested I'll go through some areas where they are used, but at this point I'll refrain until you are comfortable with the concepts and have digested what has been said.

I assume product detectors are where the carrier is synthesized, it will work with standard AM. There can be a problem with minor offsets, if the frequency is off there will be a beat between the two carriers.

 

Ok, all set so if I make a enevlope detector and tune into any AM stations frequency then demodulation will work and I should hear this fine after I amplify it and send it to the speakers?

Cool , At first I thought the professional stations where broadcasting only SSB suppressed carrier (to conserve on power etc etc ) but no that I know that the carrier is almost always broadcasted them I am ok.

If you're interested I'll go through some areas where they are used, but at this point I'll refrain until you are comfortable with the concepts and have digested what has been said.

Please do!

Also I am trying to learn how fm can be demodulated.
The simplest method is slope detection where the frequency variations are converted into amplitude variations and then enveloped detected I believe using the AM method.

But I have looked at the circuit for it and I just cann't see how it is converting the fm to am ??? (slope detection circuits look as easy to make as the envelope detection for AM)

Main questions for FM slope detection

How do you know how much to offset the tuning circuit from the carrier frequency?

Why does this offset produce the equivalent fluctations in the Amplitude as the frequency is fluctation at???

Hear is the link I am looking at http://www.tpub.com/content/neets/14184/css/14184_163.htm

Thanks for any help

 

The biggest use of SSB suppressed carrier is/was telephone. Truth, I don't know if they do it this way anymore, it is analog and a lot of telephone equipment has gone digital.

Basically ever 4 Khz is an AM SSB suppressed carrier channel, stacked on on top of each other. The technique was developed during WWI, where lives were risked (and lost) laying a simple pair of wires. They wanted to get the maximum use out of them, so they came up with this method of adding channels up to 64 Khz (I believe). Since voice in phone is 300Hz to 3.4Khz it was relatively easy to put them in the 4Khz slots.

The base band telephone system I worked on had DC to 8.5Mhz, slice that up in 4Khz chunks and that is around 2,125 channels. This signal was called a base band signal, and they FM modulated on a microwave carrier, either satellite or land line.

I worked on modems for these racks that synthesized the local frequency using phase locked loops and a common time base. They were fun to work on, interesting stuff.

I'm not sure what television NTSC used to use, I think it was something similar. The picture was SSB AM with a carrier, and the sound was a FM signal inside the AM signal (demodulate the AM, then send the video to have the hidden FM signal demodulated).

It is a common theme, you can have a second signal modulated within the main signal.

 

You guys lost me when you started throwing in all the math, but being one that tries to understand things in simple terms, here are my thoughts on the slope detection for FM. Tell me how close I am.
The key to successful slope detection is in the amount of variation from center frequency. The frequency deviation of a standard broadcast FM signal is much wider than the frequency deviation that is the result of amplitude modulation. The Q of the tuned circuit is what actually provides the slope and would ideally be tuned to a frequency equal to the center freq either + or - 1/4 the total deviation. Rectified and filtered output of that tuned circuit would then track the modulation frequency. Like I said, let me know how close I am with my simple understanding.

 

Don't forget how much more information was intended to be included in an FM broadcast signal. Beside the greater signal bandwidth, the left and right channels are multiplexed using a 19 KHz pilot reference. There is also (or was) a 38 KHz pilot used to decode subcarrier channels, plus the space for that information. Some of that was the horribly boring background music in stores.

 

I'm not sure what television NTSC used to use, I think it was something similar. The picture was SSB AM with a carrier, and the sound was a FM signal inside the AM signal (demodulate the AM, then send the video to have the hidden FM signal demodulated).

They use either a version of 8VSB (which is amplitude modulation with
1 1/2 SSB ,...etc ) or a version of QAM probably QAM64 ( which is a form of amplitude and phase modulation a together ).

Eitherway I am still wondering how the slope detection converts FM to the equavlant AM by offseting the carrier. Main question is why does this offseting produce the equivalent amplitude variations.

Could somebody give/design an easy slope detector circuit so I can see how it works the one on that site I gave you doesn't really make me see why the wave is converting into frequency into amplitude.

What would the typical basic theoritical circuit look like for this???

Thanks for any help on this

 

You are looking at frequency response here. With a tuned circuit, the further away you get from center the more the signal is attenuated (which you knew). The diode will create less voltage with less input.

While the AM detector will demodulate FM on its slope, a discriminator is the preferred method, though I suspect PPL is the preferred method nowdays.

http://www.electro-tech-online.com/...ews/87433-fm-foster-seeley-discriminator.html

 

When I first heard of SSB it took me a while to understand it.

It turns out that the end result is simply the modulating speech (or whatever) upshifted by the nominal transmitter trequency. Think of a voice pitch changer, shifting by megahertz.

Thats why demodulation by carrier injection works - the difference is the original audio, shifted down again (and why mis-tuning gives pitch-changed voice).

 

Yes , but I want to build this discriminator FM detector but every diagram I see doesn't give you the resistance , capacitance , inductance values to use ???

I am assume the LC tank is calculated by the standarded 1/2*pi*sqrt(LC)

but as for the rest of the cap , resistors , ...etc don't know what to use for values.... I am assuming their is specific values you should use ????

Also what type of diode would switch diodes work ,...???

Thanks for the help

 

That is a problem. My approach is to design two tank circuits 200Khz apart, since that is the specification for commercial FM stations (200Khz deviation). Figure the 3db rolloff around 100Khz, and use two diodes to add the signals together. Something like this...

The idea is one tank or the other is receiving the moving carrier wave.

Something that needs said, I'm not an expert. I've had a lot of experience, but that was repairing what other people designs. Something like this receiver requires superheterodyne, because it only works on one frequency.

Collins receivers would take a 70Mhz intermediate frequency and decode it to a base band that went from DC to 8.5Mhz.

 

Thanks but I think this is more a heterodyne , basically I want to not do a mixer/product detector yet I want to first learn how to make ratio and discriminator FM detector but they don't give the capacitor values in any of the schemetics how do I determine these?

Anyway in your detector is their any way to make it work for more then one station.... would all you have to do is put in a varible capacitor???

Thanks again.

 

Anyway in your detector is their any way to make it work for more then one station

The detector usually processes the signal after a couple of intervening stages. You might be interested in looking into tuning, heterodyning, and IF (intermediate frequency) stages.

 

Yes , but right now I am not trying to learn heterodyning or superheterodyning .

I just want to

how to make ratio and discriminator FM detector

what are the values ...??? Or how do you determine these

 

The point is many of these detectors only work on one frequency (especially FM, simple diode detectors not so much), they have to be tuned for best linearity at this frequency, so the concept of superheterodyne is integral to the design.

I've shown you what I know. Note the flat spot between the peaks of the discriminator, they are the resonance points for each tank circuit.

PPL (phase locked loops) is much simpler to work with due to it's repeatably. This translates into a much simplier tuning, usually a single resistor instead of two capacitors and two resistors. A simple PPL circuit is a FM discriminator as is.

Any time you tune anything like RF filters or discriminators, plan on being there a while. It is not a straight forward process. This I am an expert on.

How to calculate values? I'm not sure, I gave you my best guess of the process. Do you know how to calculate resonance and Q of a simple tank circuit?

 

How to calculate values? I'm not sure, I gave you my best guess of the process. Do you know how to calculate resonance and Q of a simple tank circuit?

Ofcourse


But suppose we where back before we had this superheterodyne and heterodyne detectors .

Their must have been some way to tune into FM stations.
Obviously AM is pretty straight forward just use enevlope/diode detection.
Which was the first think for AM detectors the cat whisker detector was the equivalent of a diode detector. Later we used product detection which is a form of heterodyne/mixing signals for AM.

But other then product detection and envelope detection I think those are the only methods to detect AM signals.

As for FM detection we have slope , ratio , discriminator , superheterodyne/heterodyne/mixing/product detector and that is all the ways to detect fm I think that we have.

Curious what the difference between the words superheterodyne ,heterodyne ,mixing , and product detection are ???
aren't they the same thing what makes them different???

Also is their any good references out their that explain how you calculate all the components and values for these fm detectors???

AM is pretty easy I have been using enevlope/diode detectors and varible capacitor for the LC tank and I have been able to tune into an AM station I want almost. Only thing is the quality is some what crappy but good enough for me.
And sometimes I have to recompute the RC constant after the diode to get the correct enevlope of the signal but other then that I am cool with AM.

Any time you tune anything like RF filters or discriminators, plan on being there a while. It is not a straight forward process. This I am an expert on.

Ok but I want to beable to make a FM detector that can be tuned to detect different carrier/station frequencies. I don't want to be confined in having to remake the circuit for every different station I want to tune into. Could I just use varable caps...?

Also in your diagram is the output of your detector next stage going to an amplfier and then to the speaker or straight to the speakers?

Same question but for the input is the stage before the input the antenna or tuner ???

 

FM was a later system, by the time it came around superhet was an established technology. The reason superhet exists is it dramatically increases sensitivity of a RF receiver, and allows a multistage RF amplifier section that is highly tuned to exact tolerances.

There are other systems for FM that work quite well, but I don't think you can separate the FM discriminator from superhet because of the reason you stated, you don't want to retune the circuit every time for a new station.

One I would dearly love to find again was an old Radio Shack kit. It was a regenerative FM receiver that used something like two transistors, and was reasonably sensitive. The only drawback (a major one) was that it killed the station for all other receivers in the area. Weird, but that was how it worked. Basically a regenerative receiver works by setting up transistor circuit where it will almost (but not quite) oscillate. When you pump a little energy from a resonant RF signal into it it does oscillate, but in rhythm with the RF signal. They were standard design before superheterodyne was invented due to the minimum parts required.

My admittedly lame schematic shows an audio out. You will probably need to amplify this signal, but it would be enough to drive a simple earphone, same as a crystal radio. I'm not familiar with any FM receiver equivalent to an AM crystal radio.

 

http://quazar31.home.comcast.net/~quazar31/FM/index.html
this site has some more 'complete' schematics with component values for radio circuits AM and FM.

 

Ok , so basically the only way other then using slope detection for FM demodulating is using a product detector/superhet?

curious is their any simple mixer/local ossilating FM detector circuit I could make to plug directly into the mic port or line in port of my computer?
I am think I can use the soundcard as the amplifier/speaker stage.
And then just have to make the tuner/detector circuit. This would be a good way to test if I am makeing good detector circuits.

I have done it for AM using enevlope detector directly to the mic port and I could tune into any AM station ,....

don't know is their some simple mixer using LM366 op-amp or something to do FM detection (a simple circuit)....?

Also since we have talked about AM and FM detection how is PM detection done differently then FM detection? I would think PM detector or FM detector could pretty much be interchange or at least you would be able to hear something... seems closely related the detectors for these.

If you had the same carrier wave and modulated it 3 different ways AM , FM , PM . Would the AM and FM not interfer but the FM and PM interfer. Seems to me you can us the same carrier to modulate it with FM and AM so that both AM and FM recievers (if they where on the same channel would beable to hear it)

But I am unsure if modulating the same carrier with FM and PM modulation could be done.
I think only AM and FM or AM and PM can be done in combination?
Curious though.

 

Hello,

For AM detection you can also use a synchon detector.
For FM you can also use a Phase Locked Loop detector.

Greetings,
Bertus