AM radio

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" the processing is quite easy in a DSP."

Darn computers keep spoiling all the fun.

George H.

You'll have zero image rejection, which is educational in itself.

John

On a sunny day (Wed, 16 Nov 2011 12:59:05 -0800 (PST)) it happened George Herold wrote in :

Just look for ferrite rod. Buy a long one. Wind about 70 turns 0.3 mm diamter wire on it. Use a 15-500 pF old fashion tuning cap in parallel.

To drive a transistor, ad some 20 turns of coupling to match the input impedance, or use a FET on the main coil.

If you do not have the tuning cap: Wind the wire on a paper tube, add a 500 pF capacitor in parallel, and slide the ferrite rod in and out to tune. Should give a huge signal. It is called permeability tuning.

Yes, I bought an internet radio, now I get 15,000 stations without hiss or fading. Also, no need to wait till sunset to receive long distance stations. It takes a toll on my interest in AMBCB antenna receiving systems. Yes, "Darn computers keep spoiling all the fun." Mikek :-)

P.S. Maybe computers just present different fun.

Sorry to say, you're right. The Net has effectively killed ham radio in the younger generations.

--

Tauno Voipio, OH2UG for 50 years

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Umm, can you tell me what that means? (Or some wiki page that talks about it.)

I was thinking the students could first try my 'bone headed' mixing right down to zero frequency, and then use the 'fix' suggested by Tim.

George H.

Hi Mike, I forgot the smiley face at the end of that statement :^)

Without all the digital stuff I wouldn't have a few hundred dollar LO that I can set to a hertz out of a mega hertz.

George H.

It means that there are two LO frequencies that will receive any given AM station. So in some cases, stations will overlap at one LO setting, but you can separate out one of them at some other LO frequency.

Not wanting to be too critical, but if you want to teach about modulation and demodulation and filters and sidebands and stuff, you should try to understand it yourself first. Get an undergrad EE text on "Signals and Systems" or some such. It's actually pretty cool stuff.

John

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OK that's easy.. (It's the nomenclature that can cause confusion.)

Grin, Well I guess that=92s part of what I=92m doing right now. In some ways I=92m the perfect foil, for developing new labs. I=92m not smart enough to know everything already, and I=92ll try all sorts of different things. Not knowing anything about a field can sometimes be an advantage. I=92m not stuck following the same rut as everyone else. (though most of the time the rut=92s there for a good reason.) Every new project is a new learning experience. (which is perhaps the best part of my job.)

I=92m not a complete novice when it comes to modulation and all, Way back in gradual school I did my research on electron spin resonance. There was a superheterodyne Ku band spectrometer (mostly built by my advisor) that could see signals from something like 10^9 electron spins. Two klystrons one locked 60 MHz. above the other... balanced mixers, circulator=92s, magic Tee,s Microwave plumbing was some weird bit=92s.

But using something and really understanding the nuts and bolts can be very different.

George H.

I'm not going to have time to play with AM today there's a pile of diode laser's outside my door waiting to be tested.

the

An ARRL Handbook may be better. In grad school I was kind of embarrassed to be designing radios with the Handbook open on my bench for guidance.

Then I interviewed at an RF place, and nearly every engineer (and most of the technicians) in the place had one in their bookshelf, generally front and center, and generally looking pretty dog-eared.

It's lighter on the theory than the school stuff, but the theory it presents is correct as far as it goes, and the thing is just _stuffed_ with practical information.

--
www.wescottdesign.com

Another vote for the ARRL Handbook.

Mathematics is exact science. Physics is a science of how to approximate math in nature. Technology is how to approximate physics in practice.

The above is not as weird as it seems on the first sight: Most of the technical information is where in the vast area of physics to find the practical solution. Plenty of rules of thumb.

The Handbook is mostly of the last group.

--

Tauno Voipio

--
Almost. ;)

Here's what Tim's talking about:

If you take a 1MHz sinusoidal carrier and modulate (mix) it with, say,
a 10kHz sine wave, you'll generate sidebands at the sum of the
frequencies of the carrier and the modulating signal,(1.01MHz) and at
the difference between the frequencies  between the carrier and the
modulating signal (9.99kHz), like this:

                         1MHz
                  990kHz   |  1.01MHz
                     |     |     |
---------------------+-----+-----+----------------------

If you then connect the output of the mixer to an antenna, you'll
broadcast all three signals.

When these signals enter a superheterodyne AM receiver, they're
generally mixed with a local oscillator which is 455kHz above the
incoming carrier, and will produce sets of sidebands which are located
455kHz above, and 455kHz below the LO.

If the LO is chosen to be 1455kHz, then the spectrum - from a first
order point of view - into and out of the mixer will look like this: 


                                       1455kHz
        455kHz                1MHz        |       2455kHz
  445kHz  |   465kHz   990kHz  |  1010kHz | 2445kHz  |  2465kHz
    |     |     |        |     |     |    |    |     |     |
----+-----+-----+--------+-----+-----+----+----+-----+-----+---
    
    |           |        |           |    |    |           |
    +--IFOUTLO--+        +---RFIN----+   LOIN  +--IFOUTHI--+
  

Then a modest transformer with an input tuned to 455 kHz (which will
then reject RFIN, LOIN, and IFOUTHI) can be centered about IFOUTLO,
with the output of its secondary rectified and low pass filtered to
yield the modulation envelope.

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Thanks Tim, I've got two copies of the ARRL handbook. 1959 and

1992. It's a bit too practical for me at times. Regenerative amplifiers are discussed in terms of a specific circuit rather than in general. (To give an example from the 1959 volume I've been paging through.) ('59 was easier to bring home) I'd like a text with a little more theory if anyone has a suggestion.

George H.

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Nah, perfect sense. Technology provides the 'imperfect' tools to do physics with. It's what you can do with the tools at hand, or maybe make a better tool.

George H.

The problem is that, if the LO is 1455 KHz, there are two stations that the radio will receive: 1 MHz and 1910 KHz. That's the "image" problem. The fix is to put a tunable, tracking bandpass filter between the antenna and the mixer, to preselect 1 MHz but reject most of the

1910 KHz.

My mom used to listen to WSMB in New Orleans. It's at 1350 KHz. She had a really terrible radio with a very unbalanced mixer. It apparently mixed the third harmonic of the IF with the carrier and produced an audible whine at the difference frequency. It was very annoying. The station programming was annoying, too.

John

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=A0 =A01455kHz

=A0| =A0 =A0 =A0 2455kHz

465kHz

=A0| =A0 =A0| =A0 =A0 | =A0 =A0 |

=A0 =A0| =A0 =A0| =A0 =A0 =A0 =A0 =A0 |

--
>
> Then a modest transformer with an input tuned to 455 kHz (which will
> then reject RFIN, LOIN, and IFOUTHI) can be centered about IFOUTLO,
> with the output of its secondary rectified and low pass filtered to
> yield the modulation envelope.
>
> --
> JF- Hide quoted text -
>
> - Show quoted text -

Thanks John,  I think that pretty much describes my understanding.
But I was going to try mixing it down to 100kHz, instead of 455kHz.
For a station at 1 MHz, I'll dail the LO to 900kHz, (or 1100kHz).  Of
course I won't know til I try it.

George H.

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...

There's just about nothing good left on AM anymore. Or FM for that matter. I can't even find a good rock station around here, every point on the dial has morning shows. The clock radio's BRAZZ-BRAZZ-BRAZZ is more entertaining.

I'm often a little amused when we're testing our high frequency induction heater. The guys in the shop usually have a radio tuned for 620. (I know, AM in an induction heating business? They must not have great expectations for reception to begin with.) Just as the frequency readout sweeps past 210kHz, you hear the squigglies. :) It's actually pretty useful for tuning PIDs and PLLs, since an audible indication of tuning is more qualitatively useful than a scope readout, or heaven forbid, a numerical readout (like you'd get from the FPGA debugger).

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Take a look at "Introduction to Radio Frequency Design" by Wes Hayward.

John S

I forgot to mention that you might find it at your local library where you can check it out before buying.