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I got a bunch of AM stations, but NO FM!

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mstechca

New Member
When I connect the antenna to the base of the 1st transistor amplifier, I get a loud AM radio receiving the strongest station. When I change the resistance value from the base of the transistor to the positive of the 5V power supply, the station changes. If I turn it too far, I hear hums. I read on the internet that a bandpass filter passes frequencies I want. I am looking for a circuit that uses any of the following to make the simplest bandpass filter. Capacitors, Inductors, Resistors, and transistors. If a bandpass filter doesn't work, then how do I remove the interference, hum, and the other junk that comes to my receiver?

To prove that the sound amplification is decent, when my finger touches the base terminal of the first transistor, I hear a hum.

By the way, I am making this receiver from scratch.
 

kinjalgp

Active Member
You can use "Squelch" Circuit which mutes the power amplifier in the absence of carrier signal. So you won't hear any hum when tuning in-between stations.
 

mstechca

New Member
how do I make that?
 

Ricky

New Member
If you want to get FM station better to use a suitable antenna tuned for that general band and a high pass filter might be helpful to filter the AM
low freq.
 

stevez

Active Member
I will try to break down the process of radio reception to the extent that I understand it.

The radio frequency energy must be captured so that it can serve as an input to the radio. That's the job of the antenna. A wire, pipe, coil, other conductor or even a wet string can serve to capture and conduct RF energy to the radio. Most radios have an internal antenna for AM and a telescoping antenna for FM broadcast and other bands. Antennas can be very general in terms of frequency or made to be somewhat selective - they work well within a narrow range of frequencies.

There is a lot of radio frequency energy out there - zillions of stations spread from below 100 kHz to several gHz and beyond. Unless you have the means to select the frequency of interest you will not get where you want to go. The traditional crystal radio is a good example - near populated areas with many broadcast stations an untuned crystal radio will pick up many stations at once.

In order to get closer to the goal of receiving one station at a time the selectivity of the radio must be increased to a point where the station of interest is amplified to the greatest degree and the others are not amplified or attenuated sufficiently. How much selectivity is needed depends on many things - what frequency, signal strength of desired station compared to the others, bandwidth of the desired station, etc. You might like to think of selectivity as a way of describing a window or doorway to the rest of the radio. An amplifier with any amount of gain at all can function as an AM radio with a wide open window or doorway to any frequency that appears.

Sensitivity speaks to the ability of the radio to make use of stronger or weaker RF energy. All other things being equal, increasing the sensitivity of a radio or the stage of a radio means that more stations will be there to possibly compete with the frequency of interest.

Some of the stages in a radio address selectivity (the window) and selectivity together. Imagine an antenna conducting the energy from 3 radio stations - at 950 kHz, 1,000 kHz and 1,050 kHz all at similar strengths. In order to select the one we want we need to filter two of them out. Filters can be inductor/capacitor (tuned circuits), crystal or other but they all are far from perfect. Their window or selectivity allows for some of the undesired RF to pass but that's an improvement. If we tuned a filter for 1,000 kHz and attached it to an amplifier we might see the signal level of the 1,000 kHz signal to be 10X what it was at the input and the 950 and 1,050 kHz signals still at the same level as the input. The stuff we don't want is still there but what we want is 10X stronger. If we do this several times in a row we have what we want at 1000X stronger than it started and 1000X stronger than anything else.

Now that we have this very strong signal (station) that we desire it is still radio frequency - if AM it's a carrier with sidebands, if FM it's a carrier that varies wildly in frequency, if FM it's only sidebands, if CW it's just a carrier with a little noise. In all cases the RF began at the transmitter as just pure RF with no intelligence, data or audio. At the transmitter we mixed in the intelligence so what we have in the antenna, the amplifier is precisely that - RF plus intelligence. We need to separate the intelligence out of the mess. That process is called detection. The detector for AM is different than FM. Detecting sidebands only is a different challenge. Once you separated the intelligence from the mess (detected it) all you need to do then is put it thru an audio amplifier.

So, to sum up the process - you need to capture the RF via some means- an antenna. You need to amplify the signals and while you are doing that you need to separate them from everything else thats out there - senstivity and selectivity. Once that's done you need to separate the stuff you want out - detect it. Lastly, you need to amplify it to a level that is useful.

I took the time to write this because I thought it might help. I'm also going to print it out for myself because I've had to explain much the same thing to others. You certainly don't need a PhD to construct radios but it does help reduce frustration to know some of the basics.
 

mstechca

New Member
thank you for that wonderful explanation. Can you show me the explanation as a circuit (up to the amplifier stage).

Ricky, I can't access that website for some reason. I'll try again later.

Here is what I tried before that wondeful explanation came up. Using my homemade AM radio, I took all connections that led to the BASE of the first transistor amplifier. (filters, antennas, everything like that), except for the resistor from +ve to the base of the transistor.

Next I connected the cathode of a diode to the base of the transistor. the anode is connected to one end of a capacitor and inductor. The other end of the capacitor and inductor was connected to ground. I also connected the anode of the diode to the antenna. The results were worse. I got no stations in. A guy who lives across the street from me used a machine outside and I can hear the exact same sound through the receiver.

Do you think I need to give the amplifier the boost, or do I need to change the filter circuit?
 

Gene

New Member
Since you say you can receive an AM station, it seems you have a working basic radio. The reason for all the noise and the machine across the street probably is the result of no selectivity (the ability to tune to a specific frequency and block others). You can overcome this (to some degree) by working on the tuning section. I did a search for "AM tuner circuit" on the web and found this article you might find helpful.

http://www.ensc.sfu.ca/~ljilja/ENSC220/Fall98/Labs/lab_5.html

Now, with a simple tuner you can expect a bit more rejection of noise and the ability to tune more than one station - if there are several strong station in your area. But, with a single stage tuner, don't expect a great radio. As to the machine across the street - it probably messes up commercially made AM radios - especially those plugged into the mains on the same side of the power company's transformer.

Your subject line implies that you are looking for FM. If you are trying to build an AM radio and tune it to an FM frequency, you should know that FM is not just the result of a different frequency. If you want FM, try

http://users.auth.gr/~mixos/projects/pcb/rf/007/

this is a neat little one IC radio that is easy to build and could feed into your audio section.
 

Pilot

New Member
mstechca

Could you post a schematic of what you have so far and include the component types & values.

It sounds to me - and please forgive me if i'm wrong - that you are trying to amplify the output of a crystal set.
:lol:
 

Gene

New Member
Pilot - I agree. He said that the antenna goes thru an inductor and capacitor - then thru a diode - and then to an amp. I am confused. With all the talk about filters and FM, I think a circuit diagram and a statement as to what is to be accomplished would help.
 

mstechca

New Member
Here is what I have so far. (see picture). As I change the input, I hear a local AM station. What I am trying to achieve is an FM radio. I am currently trying to pick up 102.9Mhz on my receiver. 102.9Mhz belongs to a local station that is about 2 to 4 km away.
 

Attachments

mstechca

New Member
I got another question. Can someone tell me how to only upload one image? Every time I add an attachment and then choose submit, two copies appear.
 

Sebi

Active Member
This is a cristal-detector receiver for AM stations, with AF amplifier (and what is a transistor B-E serial with piezo-speaker?) You can build an FM-demodulator with another circuit, called discriminator.
 

Pilot

New Member
In this circuit the coil/cap pairing is only tuning the antenna input so you will only get the strongest signal.

I would suggest thet you change the 5.1pF capacitor for a variable, 0 - 10pF, to enable some very broad tuning.

A better solution for building an cheap FM receiver is to use the MC3362 or the more modern variant MC13135.

Have a look at the example in the datasheet. Also other design with this chip are available on the web

http://www.oselectronics.com/downloads/mc3362.pdf

I've used this in a lot of home built transceivers and found it to be more than adequate at an affordable price.

:lol:
 
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