regarding FM radiodesign

[audioguru]

It was the worlds first TV system, so obviously had a few problems - just like the USA adopted 525 lines and NTSC for colour, so has the worlds poorest colour TV system. Early adopters usually get an inferior system, that's just the way it is.

I know.
Britain (or Germany?) fixed the excessive pre-emphasis that America used for FM radio.
When America first broadcast FM radio, music sources and microphones didn'r have the extended high frequencies as they had later. America was stuck with excessive pre-emphasis but fancy compressor circuits and a "de-eeser" circuit tame it pretty well today.

 

[Nigel Goodwin]

I know.
Britain (or Germany?) fixed the excessive pre-emphasis that America used for FM radio.

I suspect it was probably Germany, they were VERY big on FM.

 

[RadioRon]

I didn't think about that, that you could actually do that by over driving an amplifier, but that is true. I was referring to the diode limiter. I know you know that...lol.

Anyway, how are you doing radioRon? I got into building a PLL just a single loop so far. Well, I was looking at the frequency and all and said to myself. Hmmm...I need a new radio. So here I go. I got the 1st IF strip and rf amp and home made crystal filter & mixer done and in a metal can. Great performance so far. I'm on the second IF with mixer. Audioguru gave me some tips on good stuff to use in the audio so I got a few chips on the way.

Hi SV. I'm impressed with how quickly you build things. I'm really slow by comparison.

 

[Space Varmint]

I know.
Britain (or Germany?) fixed the excessive pre-emphasis that America used for FM radio.
When America first broadcast FM radio, music sources and microphones didn'r have the extended high frequencies as they had later. America was stuck with excessive pre-emphasis but fancy compressor circuits and a "de-eeser" circuit tame it pretty well today.

Yeah and then Motorola came up with the ever famous compander chip.

 

[Space Varmint]

Hi SV. I'm impressed with how quickly you build things. I'm really slow by comparison.

Shoot, Me? I'm slow as molasses. I just got a little break from work and got bored. Always love putting a new home brew on the air. I hear the comments and it swells my head. Not good for me you know?

edit:

I knew this guy Duane...K4PDV. This guy would build a QRP transceiver over the weekend and then take it out and use it. This guy was amazing! You can't believe how much I learned from him. He knew everything. He even taught me how to make rf power amps that worked. You'd go to lunch with him and he would start drawing block diagrams on napkins. The guy was trip!

 

[audioguru]

Signetics designed the good compander IC then Philips bought them.

 

[theveekon]

Your block diagram is incomplete in my opinion because it doesn't show the filters that are critical to the function of a receiver. The most critical filter is the IF filter, found somewhere in the IF amplifier section. Many radios put this filter immediately following the first mixer while some include an additional amplifier between the mixer and the first IF filter. Can you guess why?

Other filters that are important include the de-emphasis filter which goes between the fm demodulator and any af amplifiers that follow it. It is also quite common to include a bandpass filter before the rf amplifier and between the rf amplifier and the mixer. Can you tell us why those are necessary?

I would like to know what frequencies you wish to receive with this receiver, can you tell us?

I am interested in the frequency range 87.5 to 108 mhz(the FM broadcast band which is widely used around the world).But anywhere between 90 to 100mhz is sufficient.I would like to receive three main stations(91.1MHZ,98.3MHZ and 93.5MHZ) in our location.

As u asked Mr. Ron, the purpose of the bandpass filter before the RF amplifier is to restrict the band to the desired FM broadcast band.At broadcasting station Pre-emphasis(boosting of high frequencies to eliminate high frequency noise)is done to the audio signal before modulation.In order to revert this we need a de-emphasis filter in the receier after demodulator circuitry.But to be honest i dont know the purpose of IF(intermediate frequency) amplfier and IF filter.Also I dont know where exactly to put the tuning circuit.Instead of simple RF amplifier can we use tuned Rf ampifier?

 

[RadioRon]

As u asked Mr. Ron, the purpose of the bandpass filter before the RF amplifier is to restrict the band to the desired FM broadcast band.At broadcasting station Pre-emphasis(boosting of high frequencies to eliminate high frequency noise)is done to the audio signal before modulation.In order to revert this we need a de-emphasis filter in the receier after demodulator circuitry.But to be honest i dont know the purpose of IF(intermediate frequency) amplfier and IF filter.Also I dont know where exactly to put the tuning circuit.Instead of simple RF amplifier can we use tuned Rf ampifier?

Good answers. You are correct about the function of the bandpass filter, however you do not explain why we want to restrict the band. The reason is that in all receivers there is a finite range of voltages over which the amplifiers do their work without distortion. Once an amplifier runs out of voltage swing and begins to clip or limit its AC output, it becomes a mixer, and a bad one at that, and mixes all the incoming signals together creating an awful mess of output products. These output products interfere with the signal we want to hear. Another problem that happens when an amplifier output is limiting is that the amplifier's gain starts to go down and so it does not amplify as well anymore. Again, we lose the signal we wanted to hear. So the first job of bandpass filters in the RF section is to exclude interference.

Often you will see a bandpass filter before the first amplifier and then another one between this amplifier and the following mixer. This second bandpass filter has an additional job of reducing the noise coming from the first amplifier at the mixer image frequency. If we did not do this, the image noise would reduce the sensitivity of the receiver by a few dB.

The IF filter has a similar job to the RF bandpass filter except that it excludes interference from all channels except the one we want to hear. It is the only filter narrow enough in frequency span to pass only the narrow channel we want to hear and reject all others. It is this filter that determines our selectivity.

Yes, a tuned RF amplifier is exactly the right way to integrate filter and amplifier together. You should include a tuned input as well as a tuned output.

 

[theveekon]

Good answers. You are correct about the function of the bandpass filter, however you do not explain why we want to restrict the band. The reason is that in all receivers there is a finite range of voltages over which the amplifiers do their work without distortion. Once an amplifier runs out of voltage swing and begins to clip or limit its AC output, it becomes a mixer, and a bad one at that, and mixes all the incoming signals together creating an awful mess of output products. These output products interfere with the signal we want to hear. Another problem that happens when an amplifier output is limiting is that the amplifier's gain starts to go down and so it does not amplify as well anymore. Again, we lose the signal we wanted to hear. So the first job of bandpass filters in the RF section is to exclude interference.

Often you will see a bandpass filter before the first amplifier and then another one between this amplifier and the following mixer. This second bandpass filter has an additional job of reducing the noise coming from the first amplifier at the mixer image frequency. If we did not do this, the image noise would reduce the sensitivity of the receiver by a few dB.

The IF filter has a similar job to the RF bandpass filter except that it excludes interference from all channels except the one we want to hear. It is the only filter narrow enough in frequency span to pass only the narrow channel we want to hear and reject all others. It is this filter that determines our selectivity.

Yes, a tuned RF amplifier is exactly the right way to integrate filter and amplifier together. You should include a tuned input as well as a tuned output.

but Mr. Ron is that we can we use RF tuned amplifier instead of RF amplifier so that we don't need a bandpass filter. Because by using this, we will be able to tune our desired stations at the RF stage itself. Anyway we are going to attenuate all other frequencies in the band other(than the required station) at some point before demodulation.So why maintaining undesired frequencies till the IF stage.Can u get what i am trying to say? and also don't know the exact purpose of the IF amplifier which is after the mixer section. Can u/anybody clear my doubts?

 

[RadioRon]

but Mr. Ron is that we can we use RF tuned amplifier instead of RF amplifier so that we don't need a bandpass filter. Because by using this, we will be able to tune our desired stations at the RF stage itself. Anyway we are going to attenuate all other frequencies in the band other(than the required station) at some point before demodulation.So why maintaining undesired frequencies till the IF stage.Can u get what i am trying to say? and also don't know the exact purpose of the IF amplifier which is after the mixer section. Can u/anybody clear my doubts?

Perhaps I was not clear before. Yes, if you are receiving a relatively narrow bandwidth signal (and broadcast FM is indeed relatively narrow), then a tuned RF amplifier is the right way to go so that a separate filter is not necessary.

You are incorrect when you say that you will be able to tune the RF stations at the RF stage, and this goes to the heart of why we have an IF at all and why we use a superheterodyne configuration. The problem is that it is not possible to make LC (inductor-capacitor) filters narrow enough at 100MHz to allow only one FM station to pass. In addition, it is very very difficult to make a narrow filter that tunes evenly across the entire band. The station bandwidth is only about 180 Khz. This is a percentage bandwidth of 0.18% and such a narrow filter cannot be practically made using simple coils and capacitors. It is possible to make a narrow filter with other technologies such as crystals or SAWs, but such filters are made at fixed frequencies and are not tunable over a range of 88 to 108 Mhz.

This is an old old problem that was faced by early receiver inventors many years ago. They discovered that the most effective kind of receiver is the superheterodyne structure. In this kind of receiver, a special filter is manufactured with the exact bandwidth of one channel (eg 180 Khz for FM) but at some specific fixed frequency, usually a lower one than the band you are receiving. One common frequency used over the years has been 10.7 MHz and the channel filter that I mentioned is often made out of ceramic materials for this frequency. To make this filter useful, we use a "frequency converter" to change the frequency of the signal from what we receive on the antenna (eg 99 MHz) to the frequency of the channel filter. The frequency converter is designed to be tunable over the range 88 to 108 MHz and at any frequency it is tuned to it converts that frequency to 10.7 Mhz.

This idea has the additional advantage that it is much easier to make high gain amplifiers that remain stable at a lower frequency than at a higher frequency, and at a fixed frequency these amplifiers can be optimized very carefully without having to worry about frequency variation. It is also much easier to make a good demodulator at one fixed frequency than over a variable range of frequencies. So it is common to put most of the amplification, all of the channel filtering, and the demodulator at a lower, fixed, frequency and use a frequency converter in front of this. We call this fixed frequency section the "Intermediate Frequency" or IF section of the radio.

So, the short answer to your question is that we have an IF section so that we can practically have good selectivity. Without an IF filter, you will not get good selectivity.

 

[Space Varmint]

Man I like this guy ^^^^^ Your in good hands over here. Hey RadioRon, let's add a few important factors to this concept.

The inventor of this process is known as Edwin H. Armstrong. This guy is the same guy who invented FM radio because he didn't like static. He also explained how the vacuum tube actually worked to it's inventor de Forest. Inventor of superhetrodyne etc. He had 42 patents!

This guy hardly thought inside the box. He was way out there! The poor guy got screwed every which way by the corporate giants and ended up jumping out of the 13 floor window of his NYC apartment.

Here's a Wiki link but there is so much more to this guy. Well worth looking into.:https://en.wikipedia.org/wiki/Edwin_Howard_Armstrong

 

[Nigel Goodwin]

He also explained how the vacuum tube actually worked to it's inventor de Forest.

I suggest you try google to find out who actually invented the valve (it's original and correct name), de Forest merely expanded on Flemings original invention to create the triode.

 

[radiomate]

Armstrong didn't explain to De Forest how the tube (valve) worked, these guys were in court over who invented the regenerative circuit. Armstrong invented the superheterodyne and FM circuits. The superhet caught on fast, but FM kept dragging behind because David Sarnoff of RCA wanted AM to stay champ. Best Armstrong could do was... jump :-(

 

[theveekon]

hei folks i apologize for the long gap and to start to the thread again.

Perhaps I was not clear before. Yes, if you are receiving a relatively narrow bandwidth signal (and broadcast FM is indeed relatively narrow), then a tuned RF amplifier is the right way to go so that a separate filter is not necessary.

You are incorrect when you say that you will be able to tune the RF stations at the RF stage, and this goes to the heart of why we have an IF at all and why we use a superheterodyne configuration. The problem is that it is not possible to make LC (inductor-capacitor) filters narrow enough at 100MHz to allow only one FM station to pass. In addition, it is very very difficult to make a narrow filter that tunes evenly across the entire band. The station bandwidth is only about 180 Khz. This is a percentage bandwidth of 0.18% and such a narrow filter cannot be practically made using simple coils and capacitors. It is possible to make a narrow filter with other technologies such as crystals or SAWs, but such filters are made at fixed frequencies and are not tunable over a range of 88 to 108 Mhz.

As you said Mr. RadioRon, we cannot tune desired station at the rf stage itself. But i dont know exactly the design of the bandpass filters before the RF amplifier stage( i.e. between rf amplifier and antenna) and after rf amplifier (i.e between mixer and rf stage). All my desired stations are within
90-100MHZ band. Then is it enough that i design bandpass filter with the passband for above mentioned band?.

 

[Nigel Goodwin]

As you said Mr. RadioRon, we cannot tune desired station at the rf stage itself. But i dont know exactly the design of the bandpass filters before the RF amplifier stage( i.e. between rf amplifier and antenna) and after rf amplifier (i.e between mixer and rf stage). All my desired stations are within
90-100MHZ band. Then is it enough that i design bandpass filter with the passband for above mentioned band?.

You can, and it's done that way in lower quality radios - but it gives far better performance if you can tune the RF stages as well.

 

[audioguru]

Cheap radios do not have a tuned circuit between the antenna and the RF amplifier. Then their RF amplifier is overloaded by strong local stations if it is sensitive and the result is that weak or distant stations cannot be received because a few strongest stations cover the entire dial.

My cheap Sony Walkman FM-stereo radio has a local-distant switch that attenuates the antenna so that local stations can be received without overload interference and distant stations can be received if you are on Saturn or Mars.

My cheap clock radios don't have enough sensitivity to receive weak distant stations or be overloaded by strong local stations.

 

[RadioRon]

hei folks i apologize for the long gap and to start to the thread again.




As you said Mr. RadioRon, we cannot tune desired station at the rf stage itself. But i dont know exactly the design of the bandpass filters before the RF amplifier stage( i.e. between rf amplifier and antenna) and after rf amplifier (i.e between mixer and rf stage). All my desired stations are within
90-100MHZ band. Then is it enough that i design bandpass filter with the passband for above mentioned band?.

Yes, that would be OK. Design a bandpass filter for 90 to 100MHz. Often, we integrate such filters into our amplifiers, by putting one at the input and another one as the output load of the transistor. This makes a Tuned RF Amplifier.

Don't make your filter too complicated. It would be fine if you could attenuate the signals outside your band by 30 dB or maybe 40 dB, and in this case, you might consider using a 4 pole configuration. Such a filter would typically include two inductors and two capacitors. This is as complicated as you want to get for your filter, anything bigger is just too much. Simpler is OK of course. One efficient approach would be to have a single LC (two pole) tank circuit at the input of your RF amp and another LC at the output. You can tune the center frequency of each of these parallel resonant circuits at slightly different frequencies so as to get a flat response from 90 to 100MHz. Try googling on Tuned RF Amplifier for some typical schematics.

 

[rmn_tech]

It was the worlds first TV system, so obviously had a few problems - just like the USA adopted 525 lines and NTSC for colour, so has the worlds poorest colour TV system. Early adopters usually get an inferior system, that's just the way it is.

NTSC (No Two Similar Colours)

 

[audioguru]

NTSC has worked fine in North America (maybe not in Mexico) for the last 20 years on cable TV. It worked poorly when ghosting was caused by signal reflections from buildings when antennas were used.

 

[Hero999]

It's still inferior and lower resolution to PAL but what do you expect?

America was the first to adopt colour TV and the earlier pioneers always seem to get stuck with an inferior system, the same is true here in the UK with DAB.