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Radio Receiver Systems - "Seek" / Auto tune function

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Dirka21

New Member
Hello everyone

As you can see I'm still a very new guy here. I was hoping that someone could shed some light on my problem.

I need to find out how the "Seek" button or the auto tune function in a radio receiver works. If anyone could refer me to some websites that would be great.

I've tried to find it on Google but my attempts seem to be in vain. My personal theory is that it must have to do with the adjacent channel rejection that has to be greater than 80db and occurs in the IF stages. Unfortunately I don't know how this is done.

So if anyone could refer me to a website or four that would give me some good information than that would be great. Thank you very much.
 

Dirka21

New Member
Yes it has to do with the signal strength, that is where the 80db ACR comes from.

It is for an assignment that I have to do, and my lecturer said that we're not allowed to use wikipedia as it is not always correct and no one moderates it. So if there are any other websites that anyone could refer me to that would be great. Thank you once again.
 

Mikebits

Well-Known Member
Well I would have to do the same thing that you would and Google, since it is your grade, guess who should do the search?
 

Dirka21

New Member
For sure, I know that and besides posting here I am actually running searches for the info that I will need so, but still any references are welcome. Also as the old saying goes the more the merrier, the more info I can get the better.
 

MikeMl

Well-Known Member
Most Helpful Member
FM receivers have an FM detector called a "discriminator" or "ratio detector". As well as outputting the detected base-band audio, these detectors also put out a DC voltage level which varies as the signal is tuned through the pass-band of the receiver's IF. When the received signal is "centered" in the pass-band, the detector level is "zeroed". This was used as the "seek" detection in older analog receiver (also for AutomaticFrequencyControl AFC). With the advent of synthesized receivers, I suspect that "lock" happens when the limiters see a minimum signal level AND the discriminator is centered. This prevents the receiver from stopping on noise only.
 

Dirka21

New Member
Hello Mike

yes I did some research and found that it is done via a phase lock circuit, and when this is incorperated into the receiver, the receiver is called a Digital symthesized receiver.

I don't agree with all your terms though, or maybe I'm just misunderstanding them. When your talking about the "discriminator" or "Radio detector" do you meen the demodulator, halfwave rectifier? (AM signals)

Also when talking about AFC, the signal that is fed into it, is it the same signal used for AGC that is fed into the phase lock circuit that picks up when a signal is "centered"?
 

MikeMl

Well-Known Member
Most Helpful Member
Hello Mike

yes I did some research and found that it is done via a phase lock circuit, and when this is incorperated into the receiver, the receiver is called a Digital symthesized receiver.

I don't agree with all your terms though, or maybe I'm just misunderstanding them. When your talking about the "discriminator" or "Radio detector" do you meen the demodulator, halfwave rectifier? (AM signals)

Also when talking about AFC, the signal that is fed into it, is it the same signal used for AGC that is fed into the phase lock circuit that picks up when a signal is "centered"?
Most modern receivers utilize a digitally phase-locked local oscillator for tuning ("synthesized"). All FM receivers, synthesized or not, must have an FM demodulator, which is either a discriminator or ratio detector. Both of these detectors by their very nature are insensitive to AM.

AM receivers do not use discriminators or ratio detectors.

AFC is a ±voltage proportional to how far the receiver is tuned off frequency (how far off-center the tuned signal is with respect to the IF passband). It has nothing to do with AGC, which is used to reduce gain of IF stages with strong signals.
 
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andrebc

New Member
Need help building an AM radio receiver

Hi all!

I would like to work on a project this summer, I need to build an AM radio receiver using discrete components, no IC's. I kindda know about all the sub-systems of the project, except the local oscillator and the mixer. I know what they do and why I need them in my circuit, but I am still not sure how to start designing and building them. I am assuming I can build a local oscillaor using BJt transistors, not sure if I would be able to get a sine wave as my output though. On the other hand, I suppose the mixer is a muliplier that will multiply the signal, coming from my antenna after it was amplified, with the sine wave generated by my local oscillator. Now how I would implement this multiplier circuit, any hints or clues on this. Please relpy with any little info. It would all help, and thanks all in advance.
 

hassam

New Member
hey...
i have assigned a project of making a FM modulator and demodulator working at 100MHz. would somebody help me making the circuit diagram of the project.
 
I am assuming I can build a local oscillaor using BJt transistors, not sure if I would be able to get a sine wave as my output though.
Here's a question that gets asked a lot. Here's a design I did for a longwave xcvr. Since an AM BCB xcvr isn't high frequency, the same idea could be applied to your project. The series tuned Colpitts makes a fine low to medium frequency oscillator. However, it doesn't necessarily produce sine waves. That was the very problem I had with this: making the coupling capacitors larger made sine waves, but the feedback fell off too rapidly, and it would drop out at too low a frequency. Increasing the coupling by making these capacitors smaller caused the output to look more like exponential type pulses. So include the voltage amp to sinewave shape the output, and fix tune that stage to 190KHz with a Q= 8 to help level the output.

Since this drives a passive ring DBM, the RF power amp is needed to get approximately the 40mW of RF it needs to operate correctly. Its output pi-L network further cleans up the output. The output from BJTs is going to be much dirtier than what you'd get from a hollow state oscillator, and so will always require some sort of BPF before the mixer. Otherwise, you'll inject too much extra noise into the mixer, which is noisy enough as it is.

On the other hand, I suppose the mixer is a multiplier that will multiply the signal, coming from my antenna after it was amplified, with the sine wave generated by my local oscillator. Now how I would implement this multiplier circuit, any hints or clues on this. Please relpy with any little info. It would all help, and thanks all in advance.
You have just ESSSSSSSSS- loads of design choices for that. Since this is an AM BCB, you can eliminate the DBM from consideration since this type rejects the LO and the RF carrier, so it'll sound like receiving SSB, and I suppose you won't want that. You could do a passive singly balanced mixer or an unbalanced mixer (could be as simple as just a single diode) or the active mixers. Will need more details concerning what you're trying to accomplish here.
 

sarax0908

New Member
hello guys..
i'm a new user here..and i have a problem..
how to make a fm radio receiver tune by itself??
it's mean that, we not need to tune to get the frequency that we want,,it will tune by itself..
for example..im from location A and i want to go to lacation B..and my first frequency at location A is 90.00Mhz..and when i received at location B, i want the frequency still remain with the same frequency which is 90.00Mhz..anybody can help me??plzz...i need some idea to this project..
 
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