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Designing a simple AM radio receiver

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bugmenot

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I'd like to design a simple AM radio receiver. It should be able to drive a small speaker, and the antenna shouldn't have to be longer than a meter.

I plan to use 2 transistors as common-emitter amplifiers, to get a gain around 15000 (and perhaps another transistor or two as emitter followers to increase the current), an LC tuned filter, and a simple diode + low pass filter for detection.

A few questions:
1. Is this enough to drive the speaker? What type of signal do I need to drive a small, inexpensive, stock speaker?
2. What kind of voltage signal can I expect off the antenna? What type of output impedance / current?
3. Should I amplify, then filter; or filter, then amplify; or amplify some, then filter, then amplify more? What are the tradeoffs?
 

RadioRon

Well-Known Member
It sounds like you plan to build a class of receiver that we call TRF or tuned radio frequency. This is a simple structure indeed, but suffers from some difficulties that you may want to consider. The most typical problem is that you plan to put a lot of gain, you say 15000, and I presume you mean voltage gain, in your receiver. This is a gain of 83 dB. You will find that most likely the amplifier chain will oscillate since it is very difficult to control positive feedback due to unintended and parasitic paths. To be successful, you would likely have to build using surface mount parts, full groundplane pcb layout and separate shields over each stage. You would also have to be liberal in your use of bypass capacitors and ferrite beads for isolation.

I suggest that you start with a single amplifier stage and see how good it works first.

To drive a speaker, you need current and lots of it. The typical speaker has an impedance of 8 ohms and you need to push as much as 100 mW (roughly) into it to get decent volume. Even a tiny speaker, if it is a conventional type, needs a few tens of milliwatts. So you must include an amplifier to drive this low load impedance and this amplifier must follow your low pass filter. Usually we put a volume control between the two sections.

AM radios don't use rod or whip type vertical antennas for a good reason. AM broadcast frequencies are so low that such an antenna has very very poor performance. You absolutely must use a ferrite bar or ferrite rod antenna such as you find in any off-the-shelf AM radio. The voltage that you get off of this type of antenna will depend a lot on how close the stations are, but a very very rough guess might be 100 uV to 1000 uV. The impedance of your first amplifier should be fairly high for easiest voltage coupling from the antenna. Plan for about 2K ohms input Z.

Filters are put in various places in a receiver for a few reasons. The most important reason is to exclude signals away from your frequency from getting to the detector. The characteristic that describes how well your filter does this is "selectivity". The other reason is to keep other signals on other frequencies from driving any of your active stages into limiting because when this happens the signal you are receiving will not be amplified and it will be heard as a drop in your signal. This is interference that can be excluded by your filter. For these reasons, it is usually best to put some filtering before the first active stage, but if you use a ferrite rod antenna, the antenna itself provides part of this function. So put a filter between the antenna and the first amplifier if you like, or follow the first amp with a filter. Maybe experiment with various places to see which works best for you.
 

audioguru

Well-Known Member
Most Helpful Member
Radios have automatic-gain-control so that they are sensitive to weak distant stations but are not overloaded by strong local stations.

I think your simple "radio" will be overloaded by all the stations in your city at the same time.
 

bugmenot

Banned
Thanks for the great points. It seems that there's a lot more to even beginner's radio than just applying what I'm learning from Horowitz & Hill. Can you recommend a good book or two - in the spirit of Horowitz & Hill - which will guide me to basic radio?

I'm looking for a solid understanding of radio circuit basics - enough to design and build my own simple, practical devices. (That is, I don't want the level of detail and sophisitication that a professional EE would use for commercial devices.)
 

bugmenot

Banned
Yeah, I've read many of the web pages, but I'm looking for something more structured and complete. An good book is what I'm looking for - in the spirit of Horowitz & Hill, but on radio.
 

audioguru

Well-Known Member
Most Helpful Member
An aM radio is pretty simple. The circuit has not changed for about 80 years. A couple of pages describe its circuit.
 

ericgibbs

Well-Known Member
Most Helpful Member
I'd like to design a simple AM radio receiver. It should be able to drive a small speaker, and the antenna shouldn't have to be longer than a meter.

I plan to use 2 transistors as common-emitter amplifiers, to get a gain around 15000 (and perhaps another transistor or two as emitter followers to increase the current), an LC tuned filter, and a simple diode + low pass filter for detection.

A few questions:
1. Is this enough to drive the speaker? What type of signal do I need to drive a small, inexpensive, stock speaker?
2. What kind of voltage signal can I expect off the antenna? What type of output impedance / current?
3. Should I amplify, then filter; or filter, then amplify; or amplify some, then filter, then amplify more? What are the tradeoffs?

hi,
If you can still get one of these they make a simple radio.
Some coil and ferrite winding data in the pdf.

Whats your location.?
 

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bugmenot

Banned
Great! But I'd be interested in getting a fuller background, so I really understand the basic fundamentals, and not just following someone's circuit. Is there any good, intuitive book on radio?
 

stevez

Active Member
Radio amateurs include experts and people who barely understand radio. Publications by and for amateur radio enthusiasts (hams) are often prepared in a way that helps someone who might not be an RF engineer understand the concepts. If all else fails take a look at those publications.

Years ago I found a chapter in a radio repair manual that might have what you need. I can tell you that it helped me a lot. For an AM radio, the author explained what was going on at each stage including various signal levels, gain and so on. What you are searching for does exist in some form.
 

ericgibbs

Well-Known Member
Most Helpful Member
hi,
Taking Steves point I just Googled for:

fundamentals of radio reception

There is a lot of basic info on line.:)
 
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