I know.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.
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.
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.
Hi SV. I'm impressed with how quickly you build things. I'm really slow by comparison.
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?
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.
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?
He also explained how the vacuum tube actually worked to it's inventor de Forest.
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?.
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?.
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.
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?