Radio and Aliasing

Here's one of your posts:

"Okay. Let me make a modification. Lets make the the the bandwidth of the receiver 150 Ghz [notice that 'G'] but keep the carrier frequency of 150 Khz. Now, what is the maximum frequency the the modulation can be? I guess its 750 Mhz. Do I guess correctly?"

No you did not, and only a little homework on your part would show you why.

Here is an example: You have been advised that the carrier frequency must be at least two times the highest (or only) modulation frequency. Let's assume 2x is good enough, so obviously in a multiple guess test, an answer of 75 kHz for a maximum modulation frequency is the right answer. The process of modulating 75 kHz with a 150 kHz carrier will produce a sideband signal at 75 kHz (lower sideband), and 225 kHz (upper sideband). The carrier will also be present in the output as will other products. The bandwidth required is nominally 150 kHz, in the range of 75 to 225 kHz.

Another example: You cannot modulate a high frequency on a 1 Hz carrier. But now you know better, right? Anyone wishing to argue about this should start a new thread in alt.sci.electronics.dumbideas

Google is your friend.

If the the carrier's frequency must be at least 2x the modulation frequency, then how can a 150 KHz station carry a 225 Khz tone?

If it's SSB, and not AM, you can modulate any amount of signal bandwidth onto a 1 Hz carrier.

John

(snip)

Now you are wasting bandwidth and oxygen. Since you understand nothing you are told and refuse to do any study on your own, I declare you a Troll.

This is the wrong thread for this topic, however.....

You really should read what you wrote. It is totally off the wall.

Don

The highest "audio" frequency is about 20kHz, so my opinion is that anything above 40kHz will do it really nicely and >=20kHz will 'just' do it. (I'm more familiar with digital and the Nyquist theorem, but surely similar rules apply.)

... Steve

OK, take the carrier sinewave and express it as a sequence of impulses at sufficient density to trace the sine shape pretty well. Number those, say, 64 impulses and apply the sampling math to each. Now sum the amplitude and phase of the resulting signal samples, all 64 of them, and correct for the artifacts.

Or stay continuous use a trig identity or two.

John

Whose wall? It's correct. All you do to make a USB signal is shift the spectrum up by some "carrier" frequency, and 1 Hz is as good as any other value. All a SSB receiver does is shift it back down.

John

Along the same lines, but not quite on topic, there may be an advantage in using a square wave carrier in a low-level (ring, etc) modulator, rather than a sine wave. A single "AM modulation" sample amplitude is time variant; it includes a span of time of almost a half cycle of the carrier frequency. It seems to me that a square wave carrier would provide a longer sample than would a sine wave. Filtering the output of carrier harmonics, etc, would be simple. I think there should be a measurable difference in distortion.

What do you think?

Don

It's nonsense.

Tell me how you modulate a 300 to 3000 Hz voice signal using a 1 Hz carrier. Where is the 2:1 carrier to highest modulation frequency in this concept of yours?

Don

The 2:1 limit is for AM, where you obviously get into trouble if the lower sideband has frequency components that get below zero frequency and fold back. SSB/Upper sideband doesn't have that problem, because all the modulation products are *above* the carrier frequency. As I said, USB is merely an up-shift of the signal spectrum, by any amount.

The two common SSB generation techniques are the phasing method (which works fine for your example) and the balanced mixer followed by a sideband filter, which doesn't work with these numbers. But a double-conversion mixer+filter method will work: modulate the signal with a carrier, say 100 KHz; filter out the lower sideband, so that the spectrum is now 100300 to 103000 Hz;; mix that with 99,999 Hz to shift the spectrum back down, now 301 to 3001 Hz. Voila, a 300-3000 Hz signal SSB modulated onto a 1 Hz carrier.

You can purchase an audio-band frequency shifter. Set the shift to +1 Hz and the result will be exactly the same as audio modulated SSB onto a 1 Hz carrier. If it lets you, set the shift to 28.2 MHz and you'll be generating legal USB signals for the 10-meter ham band. It's all the same.

John

No, all you did was shift the spectrum down. I certainly accept SSB as a form of AM, but you did not modulate a 300 - 3000 Hz signal, with a 1 Hz carrier.

Don

John, we're not in disagreement at all here, really. All I've been trying to say all along is that you CAN make an analogy to sampling, if that happens to be a more comfortable place for a given person to be coming from to consider what's going on. No, AM radio is NOT sampling, and the analogy (like any) breaks down if you look to closely at it. But analogies like this are often useful for getting the basics across to people. OK?

Bob M.

Well, a classic AM modulator is based on a Class C output stage, which I guess is sort-of what you're thinking of here. In short, it doesn't matter if the modulator is fed with an actual "sinusoidal" carrier, as long as you can control the amplitude of the output via the modulating signal, and you can be assured of throwing out any undesirable components that might result. A Google search for "modulation transformer" will no doubt turn up more information on this approach, but that basic idea is that you have a high-powered audio amp which is plate-modulating a Class C RF amp via a transformer. The advantage, of course, is that you have a much more efficient RF stage than would be the case if you had to modulate the carrier and THEN amplifier the result linearly. This is a very common approach in AM (but not for SSBSC, for obvious reasons).

Bob M.

Rather pointless, though, since the result is equivalent to the modulating (baseband) signal shifted up by

1 Hz....:-)

OK, but not if they mis-state quantitative reality by 2:1.

John

And that's all SSB is! The old analog long-distance telephone carrier stuff wasn't as extreme, but they stacked SSB voiceband signals every

4 KHz, with the first one at baseband. So the second signal was 300-3400 Hz modulated onto a 4 KHz carrier. They did all this with tubes, and saved a huge amount of copper and telephone poles. One FDM version (Western Electric type "O") placed the first carrier at 6 KHz, but put two voice signals on each carrier, one USB and the other LSB; that put 6 KHz of signal onto a 6 KHz carrier, but it was two different signals!

John

Scrapped O Carrier filters were great for homebrew sideband equipment.

There was one telco carrier system arrangement that was "extreme;" that was the use of the EB bank with K and L Carrier. The Emergency Bank put two 1.8 kHz channels in the 4 kHz spectrum of the K or L Carrier's channel spacing. As I recall, the A channel of the two had normal orientation (USB), and the B channel had LSB orientation. The (suppressed) carriers were 4 kHz apart. The B channel of the L chan 1 shared the carrier of the A channel of L chan

1. The whole stack gave us 24 crappy channels in the space of 12 good channels.

Don

No, I specified a low-level modulator such as a diode ring, which can be easily filtered. I don't think I would try it with plate modulation