Radio Intermediate Frequencies

I'm not sure that any general-purpose circuit simulator will be up to helping you much with superheterodyne radio design. AFAIK you pretty much have to use mathematics, red in tooth and claw, to analyze the performance if the thing. Fortunately the math doesn't have to be that hard.

I suggest you look up the Amateur Radio Relay League (ARRL). Their "Handbook" has many many basics including (probably) a superhet receiver if not two or three. Also look at "Experimental Methods in RF Design" by Hayward et all, and "Introduction to Radio Frequency Design" by Hayward. I have the latter, and I've seen good recommendations for the former.

On to your problem. Since it isn't clear about what you don't understand it's hard to recommend a circuit. Let me give you this thumbnail explanation, and you tell me where things are unclear:

The principal of the superheterodyne radio (i.e. one that uses an intermediate frequency) is this: it is relatively easy to build a good radio as long as all of the finely tuned circuits have fixed tuning, and it is relatively easy to build a gizmo that takes a radio frequency signal and shifts all of it's content by a fixed frequency offset. So you build a really good fixed frequency radio and a really good frequency shifter, stick them together, and voila! a good radio.

The fixed-frequency radio part is called the 'IF strip', and the frequency at which it operates is the 'intermediate frequency (IF)'.

The frequency-shifting part is called the 'front end*' or the 'mixer' or (in older material) the 'first detector'. It's frequency is set by the variable frequency oscillator (VFO) if the radio is tunable.

• Purists: Yes, the mixer is just part of the front end, at least in many radios. This is a basic, educational post. Go away.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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This stuff has been beaten to death lately, but here goes. ;-) Since all RF circuits are, by nature, sensitive to variations in frequency, radios tend to use an IF (intermediate frequency) for achieving good sensitivity, linearity and selectivity. The IF is usually chosen to be the "difference" between the desired incoming signal and the LO (local oscillator).

In most portable AM type receivers, the IF is chosen to be 455kHz. That means in order to receive 545kHz thru 1650kHz, the LO must be able to tune a range of 1000kHz thru 2105kHz. The LO is "mixed" (technically this is a multiplication process even though it doesn't seem that way) with the incoming signal. This process produces four output frequencies (the two originals, the sum and the difference).

For example if we're trying to receive a station on 1000kHz, the LO would be tuned to 1455kHz. This would produce a mixer output containing signals on 455kHz (the difference), 2455kHz (the sum),1000kHz and

1455kHz. At this point the signal is passed thru a low pass filter to remove everything above about 500kHz, a relatively simple process at these frequencies. Then the signal is fed to an amplifier stage(s) that are designed to work best at exactly 455kHz. Then this signal is passed thru a diode (detector) and another low pass stage to remove the RF and leave only the audio signal.

This is a simple description that only scratches the surface of the theory behind all of it. There are technical reasons to pick specific IF frequencies and to even have multiple IFs in a radio. Still other times you may want the ultimate in simplicity and choose not to have an IF at all. Google must surely be filled with pointers to information on this.

Geez, and you didn't even mention the preselector. (;-)

Building and simulating may not be the best way to learn about this. Google for articles on the Superhet principle.

Figure out why this equation is important..

sin(x) sin(y) = 0.5 cos(x ? y) ? 0.5 cos(x + y),.

The hell you say?

Jim

At this point the signal is passed thru a low pass filter to

I want to light a torch bulb from a battery source. The torch bulb specification is it is the smallest one found in the market.A MES is good enough.

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Thanks.

Masroor.

Tim Wescott wrote:

In sci.electronics.design James Douglas wrote: : I am looking for a schematic that I can build that will teach me about : intermediate frequencies and radio reception. I want to be able to : simulate the circuit in CircuitMaker then build it and test it.

: I am building/learning about radio reception and am not going to : continue building the radio without a through understanding of how : these intermediate frequencies work.

James,

Find a diagram of a superheterodyne receiver. I'm sure you might find one by just doing a google search for "superheterodyne receiver." It will help you follow along with what I am saying.

I don't have time to post a detailed explanation, but in a nutshell, the IF is a free parameter in a (super)heterodyne receiver that is chosen to trade off the first stage filter "Q" or bandwidth/sharpness with second stage filter/amplifier bandwidth/complexity.

The higher the IF, the the lower the Q of the first stage filter. Choosing a higher IF may allow for the use of less expensive first stage (RF) filters (Lower Q generally = Lower cost,) although these days, it's more common to see lower or even 0 IF (also called direct conversion) receivers.

The lower the IF, the lower the band of interest of the components of the second stage (filter, amplifier, mixer.) Choosing a low IF may allow for the integration of the IF filter in an IC receiver (which is what I am most familiar with) or using a cheaper external filter (crystal vs. SAW, etc)

So, in a general sense, you can think of the choice of IF as a way of trading off first vs. second stage complexity. For lots of types of receivers (FM, TV, etc) there exists a "customary" IF around which lots of ICs (which may implement an entire second stage of an FM receiver, for instance) have already been designed. Therefore, you don't see the IF being changed in those types of receivers, although it can be.

That's the general idea...

Joe

It generates double side band, local to receiver. By the way, why is it important mixing to get intermediate freq instead of detecting audio directly from RF?

Thanks for all the great information! I will be reviewing everything today as I'm sitting home with a sick kid. I was hoping that I could simulate a circuit with two input frequencies, for example 1Khz and 5Khz and somehow view/measure the output to be XKhz? I will continue to research the superhet type devices. I do have that AARL book around here somewhere.

Better yet, go for broke. Pretend that the whole world is made of nothing but complex numbers. In other words, pretend that complex numbers are not a special case of regular numbers that we learned in grade school, but the end-all in general of quantities, and that it is we, the humans, who have been operating in a mode of deficiency since the very first time we learned to count.

Then you can assert that all functions are complex, where every part that make them up is potentially complex. Then, for a wide variety of functions, it is true that those functions can be represented as sums of complex exponentials on t:

x(t) =Sum {-infinity, +infinity} (complex coefficient)*e-to-j-omega-t.

It would do you great benefit to take random "grade-school" functional patterns of t that you make up yourself (sines, cosines, ramps, boxes), and see if you an represent the functions as a sum of clumps where each clump is a complex co-factor applied to e raised to j omega t. Keep figeting with the per-omega clumps to get the signal to look right in the time domain. This is most likely what Fourier did before he arrived at his convictions.

If you view the world this way, as if all numbers were complex, including the number of pieces of fruit that you last bought at the supermarket, you will feel a lot better about all of this, because there will be no more special cases, as everything will be complex, and the vast majority of quantities that we experience each day, the complex part just happens to be zero.

Then take the two pure sinusoids that you plan to mix, use Euler's Theorem to treat them as two complex functions as above.

Multiplying them together (heterodyning) will quickly reveal, by definition of multiplication of *any* two exponential functions (add the exponents), that the frequencies will add in the resulting signal.

Then if you take a x1 to be sum of two sinusoids, and x2 to also be sum of two sinusoids, and multiply them, you can see the blobs that they make in the frequency domain (again by adding).

If you keep adding sinusoids to x1 and x2 so that they become "rich" in time (and therefore spectral pattern becomes less spike-like), you will see that the multiplication in time domain results in convolution in frequency domain.

-Le Chaud Lapin-

oops where did all the ? marks come from.

Well I guess you are all clever enough to know what I meant.

The normal coupling out of a mixer is with an RF transformer tuned to the IF frequency. Let's see why a lowpass filter isn't of much use using your example of a 1000kHz signal, a 1455kHz local oscillator, and 455kHz. IF frequency. Let's go still further and postulate a 2-pole LPF which can be made about the same size as that IF transformer. Put the cutoff right at

500 kHz. so that we don't lose a lot of the 455kHz. energy.

What's the attenuation at the signal frequency? Well, the slope of a

2-section RC filter is 12 dB per octave, and that is exactly an octave, so you reject 12 dB of the unwanted signal frequency. 1455 isn't much above that, so you lose perhaps another couple of dB. Big deal. When dealing in an environment that has to handle microvolts to millivolts, 15 dB or so is a drop in the bucket.

However, with an IF transformer, you will be down something on the order of

50 to 60 dB at the signal and about the same at the LO. Now we're talking some decent attenuation.

Jim

The basic process of detecting audio (or data) from RF involves filtering the RF, then detecting it as if the signal of interest were the only thing there. That filtering step gets extremely complicated if you try to make it tunable to any old RF frequency. By translating the signal to match a fixed frequency filter-and-detect strip (the IF strip) you simplify the radio design.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google?  See http://cfaj.freeshell.org/google/

You can: you will see an output consisting of 4 and 6 KHz tones.

Mix, say, 4 and 5 KHz and you will get 1 and 9. Filter that and you are left with 1 KHz. Then, vary one of the frequencies and see what happens. Or apply 4 and 4.5 on one input and5 on the other of the mixer, and check what happens.

Excellent books. You need to get a feel for how this works - once you graps it all kinds of things become possible.

Thomas

I believe a Direct Conversion design, which eliminates the need for an IF, makes separating the modulation from the RF signal quite simple. Direct Conversion uses demodulation rather than envelope detection, so the signal is baseband audio straight out of the mixer. A simple low pass filter (RC) will prevent RF from affecting the following audio stage(s). If one wishes, wideband and narrowband audio filters can be switched in/out for "music" and CW.

Don

Damm, you guy's are either really smart or good bullshitters, I am thinking smart. I appreciate all the help and now am armed with new information to continue my experimentation.

They are not necessarily mutually exclusive characteristics.

Best regards, Spehro Pefhany

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;-)

Hope This Helps! Rich