General > General Technical Chat
Any Shortwave Radio Tips for a Noob Considering the Hobby?
<< < (7/20) > >>
edy:
I'm continuing to read through some basic radio electronics books because I'd like to understand what is going on. I know it is probably more technical than someone needs to know just to get licensed and involved in amateur radio, but for my own education I would like to really be able to start building some basic stuff to understand how it works. Right now I'm working through a series on the website http://www.learnabout-electronics.org/Oscillators/osc10.php on oscillators. I am looking at the LC tank circuit and the equation for frequency looked very familiar. It looks like that of a simple pendulum:



Compare to....



I find this fascinating. Probably no surprise to most of you. Is there some analogy one could make with L/g and LC to help learn out things from one to the other? Note that this is the small-angle approximation for pendulum. I wonder if the "angle" limitation also is analogous to some parameter that we need to keep small for an LC tank, otherwise the approximation formula no longer works? I feel that the relationship is like kinetic and potential energy being transferred back and forth. The capacitor stores potential energy, and the inductor is the kinetic.

I spoke to someone who said an outboard BFO will not work on a radio like the PL-310ET because of DSP (or probably most modern radios). Since the output of a CW being received by a radio sounds silent (since there is no modulation on top of the CW) and because of DSP handling all the audio before it even gets to the speaker, the BFO would have to be wired into the board internal to the radio where you can somehow patch it in after the receiver most likely converts the signal to the intermediate frequency the radio uses, but before the 2nd detector (demodulator) kicks in to pull out the audio from the CW (https://en.wikipedia.org/wiki/Beat_frequency_oscillator). In this case the beat is analogous to when you are tuning a guitar and you use 2 adjacent strings... you can hear a "beating" if the strings are not same frequencies which gets less and less until they are in tune to each other?

I'm also trying to understand SSB.... there is USB/LSB. My understanding is that there is a frequency "X" which is being "tuned" to. However, the LSB is being transmitted about 3 kHz below and USB is at 3 kHz above. So are we really just tuning to a frequency X-3 or X+3? If your radio has sharp enough resolution to tune to minute frequency steps, could you just tune into the LSB or USB frequency on your way through the spectrum? Or are you actually tuning to "X" and using some kind of method to pull out stuff that is slightly below or above that is not exactly tuning? For example if you had a trim somewhere in your radio for calibrating frequencies and it was off by 3 kHz in say the down direction, would you be tuning in the LSB every time you jump to certain frequencies you thought were in the middle (e.g. "X")? My confusion is in that frequencies have a "bandwidth" and not exact, and how it is that some signals can be pushed lower or higher yet we still say we are tuned to a frequency. Oh so much to learn!

bob91343:
USB and LSB are opposite.  If you analyze the spectrum of an amplitude modulated signal you find a carrier wave with two sidebands, one on either frequency side of the carrier.

For a low frequency tone modulation, there will be two side frequencies close to the carrier.  For a slightly higher modulation frequency, the side frequencies move farther from the carrier, spaced from it by the modulating wave's frequency.

It was obvious that the carrier wave carried no information so circuits were devised to avoid transmitting it to allow more power to be put into the sidebands.  But since both sidebands are identical, circuits were devised to eliminate one or the other.  This is called SSB, single sideband.  In order to detect the signal, one must reinsert the carrier signal locally in the receiver.  If you are receiving upper sideband, USB, you carefully reinsert a local carrier signal at a frequency just below the signal, where the carrier signal would have been.  For LSB you reinsert the carrier on the high frequency side.

The frequency of the local signal is very critical.  To get intellligible speech, its frequency must be within perhaps 50 or 100 Hz from where it should be.  If too far from the sideband, the signal sounds high pitch and unnatural.  If too close, it really gets unintelligible due to the 'folding' of the low frequencies.

If you don't reinsert carrier, you get garble.  It takes a practiced hand to tune the local oscillator satisfactorily.  It's kind of fun, too.

Most good receivers have 'brick wall' filters in the IF so you can tune just one sideband's worth of signal.  Another benefit of this system is that it occupies half the spectrum that an AM signal occupies.  As a net result, for the same power you get about 9 dB more received signal.  And because, for speech at least, the average power is much lower than the peak power, you can gain even more by taxing the amplifier a bit more, since it won't overheat as soon.

That means that today's radio amateur can produce a signal that is perhaps 10 or more times the power than back in the old AM days, and with more compact equipment.
fourfathom:

--- Quote from: edy on December 12, 2019, 03:19:32 am ---I spoke to someone who said an outboard BFO will not work on a radio like the PL-310ET because of DSP (or probably most modern radios). Since the output of a CW being received by a radio sounds silent (since there is no modulation on top of the CW) and because of DSP handling all the audio before it even gets to the speaker, the BFO would have to be wired into the board internal to the radio where you can somehow patch it in after the receiver most likely converts the signal to the intermediate frequency the radio uses, but before the 2nd detector (demodulator) kicks in to pull out the audio from the CW (https://en.wikipedia.org/wiki/Beat_frequency_oscillator). In this case the beat is analogous to when you are tuning a guitar and you use 2 adjacent strings... you can hear a "beating" if the strings are not same frequencies which gets less and less until they are in tune to each other?
--- End quote ---

Probably correct about not being able to use an external BFO with the PL-310ET.  It is remotely possible to inject a strong signal at the radio input, offset the right amount from the desired input, and through overload the two signals will mix and generate the desired beat frequency.  But that's not really practical, and that BFO you were looking at is designed to be injected at an internal point in the (fixed-frequency) IF (intermediate frequency) signal chain.  DSP radios such as the PL-310 don't have such an IF chain, even in the digital domain.  Instead, the input signal is directly mixed down to DC (well the audio range anyway) and the traditional filtering is done via quadrature mixing and signal processing.  It's really quite elegant, and now practical because of cheap digital technology.


--- Quote ---I'm also trying to understand SSB.... there is USB/LSB. My understanding is that there is a frequency "X" which is being "tuned" to. However, the LSB is being transmitted about 3 kHz below and USB is at 3 kHz above. So are we really just tuning to a frequency X-3 or X+3? If your radio has sharp enough resolution to tune to minute frequency steps, could you just tune into the LSB or USB frequency on your way through the spectrum? Or are you actually tuning to "X" and using some kind of method to pull out stuff that is slightly below or above that is not exactly tuning? For example if you had a trim somewhere in your radio for calibrating frequencies and it was off by 3 kHz in say the down direction, would you be tuning in the LSB every time you jump to certain frequencies you thought were in the middle (e.g. "X")? My confusion is in that frequencies have a "bandwidth" and not exact, and how it is that some signals can be pushed lower or higher yet we still say we are tuned to a frequency. Oh so much to learn!

--- End quote ---

First you need to understand AM modulation.  Here we take a carrier signal at "frequency X" and use an audio (or digital) signal to increase or decrease the amplitude of the carrier signal. Wikipedia has a good article on this, and here's one with less math: https://www.electronics-notes.com/articles/radio/modulation/amplitude-modulation-am.php.  This modulation process generates sidebands.  With a 1 KHz modulation signal, sum and difference products (sidebands) will be generated at "X+1KHz" and "X-1KHz".  The carrier is also transmitted in AM.  A basic AM receiver can demodulate this AM signal -- this can be as simple as a diode and headphones (you need a strong signal for this to work).  With Single Sideband the carrier and one of the sidebands are eliminated, and only one sideband is transmitted.  This was traditionally done with filters, and occasionally with analog phasing techniques.  These days this can be done with DSP.  Wikipedia and the link above also have discussions of SSB.  The BFO is used to essentially re-insert the carrier signal at the detector, modulating the SSB signal to produce the original audio.  Mixing (modulating) "X" and "X+1KHz" gives you sum and difference products: "2X+1KHz" (which we discard) and "1KHz" (your original audio modulating signal).
eti:

--- Quote from: VK3DRB on December 08, 2019, 12:28:29 pm ---
--- Quote from: eti on December 08, 2019, 04:27:53 am ---I'll give you a guess which rock-solid, simple communication system based on physics and not on endless chains of servers and switches, I'd pick to save the life of my family, come a nuclear war? It's pretty simple, and here's a hint; it AIN'T the internet. We think we're SO SO "clever" and we are, but then radio preceded the "ever so pleased with itself" internet by MANY, MANY decades, and they don't use Skype or FaceTime to pilot submarines, so there's a clue as to which is better, merely by virtue of its' simplicity.

--- End quote ---

It is true that if the Internet were to go down many of us in countries whose infrastructure is addicted to the Internet will suffer a catastrophe. You wont be able to buy food, gasoline, medicines, no nothing. You water supply may well run out and you won't have electricity from the power grid. It just needs any of the lunatic countries that hoard nuclear weapons like Russia and the USA to push the button either on purpose for by accident. The doomsday clock is only TWO MINUTES TO MIDNIGHT. Nuclear attack might not be a matter of if, but when: https://en.wikipedia.org/wiki/List_of_nuclear_close_calls

But if you have a ham radio you will be able to communicate, assuming your infrastructure is destroyed and you were not vaporised. If this sounds far fetched, look what happened in Cyclone Tracey in 1974 which wiped out an entire city of Darwin. Ham radio was the ONLY form of communication to the outside world for several days. The main link was between a ham with a generator in Darwin and a ham in Melbourne.
 
If the doomsday happens, ham radio could also be used to coordinate emergency relief. And be used as an aid to hunt down the leaders of the countries that fired the nukes.

A film worth seeing is the 1964 film "Fail Safe". Such a scenario it is very possible. It almost happened in 1983.

--- End quote ---

Tell this to captain cynical in the post above you ^
edy:
Ok thanks, now I got it. I read a few articles on electronics-notes.com which explained it. So in a mix of pure frequence X (carrier) plus Y (modulator) you get X+/-Y. In cases where Y is also a simple wave at a specific frequency you get a peak at each side of the X. But since audio has a range of frequencies that are modulating the peaks flatten out into bands so these 2 bands (LSB and USB) are representing the audio and carry the actual information. Part of my confusion stems from the visualization of frequency spectrum and wrapping my head around the simple AM wave graph showing a simple carrier wave with the amplitude of it being "enveloped" by the modulating wave (audio input). When you look at that diagram you can't really imagine other frequencies... all you see is the CW with a modulation, you don't see the side and frequency visually. Although peaks and valleys combine in such a way that you can end up producing signals that can be Fourier transformed to represent those other frequencies. That also explains channel spacing and other stuff.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod