WWV; Picking It Up Question

[BOBRR]

Hello,

Have a Winradio Excalibur SDR receiver.
Sure is different, and going to take a lot of getting used to.

Tuned it to 15 MHz, 20 MHz for the WWV broadcasts.

Interesting "something" I don't understand.

The spectrum display, and the other displays, show a nice, well defined, peak at these frequencies.

If I move and adjust the width of the selectable passband to "just" encompass the peak at the 15, 20 MHz frequencies, I receive nothing. (mode set to AM)

If I open the passband up to encompass some amount of the spectrum on each side, it then comes in clear as a bell.

There does appear to be a small peak on each side of the main one.

a. Are these likely Sidebands of the main freq. ?

b. Shouldn't a very narrow passband on Only the main peak, without encompassing these "sidebands"
be enough to pick it up clearly ?

c. Why must I encompass the sidebands (if that's what they are ?) also ?

Sure am missing something here.

Thanks, Bob
BTW: here's from the WWV site:

The signals broadcast by WWV use double sideband amplitude modu*lation. The modulation level is 50 percent for the steady tones, 50 percent for the BCD time code, 100 percent for the second pulses and the minute and hour markers, and 75 percent for the voice announcements.

 

[Token]

Using anything that gives you a spectral or waterfall display can be a real eye-opener as to what is going on in the signal. It is the only way to fly, something I found out 30+ years ago and have enjoyed every since when possible.

Answers:
a. Yes, these are probably the sidebands, depending on exactly what you are describing, there could be several possible peaks on each side of the main. But it should be more than a small peak, there should be wide and varying intensities during the speech portions of the TX, and solid tones during the "beeps". If you are talking about a small peak on either side of the main carrier that are very close in, say less than 100 Hz from the main, then these are not the information.

b/c. A very narrow filter, passing only the carrier, will result in no data, or modulation, being heard by the user. If the data is a 1000 Hz tone then your filter must encompass that 1000 Hz offset from the carrier, or your filter will be filtering out the sound you want to hear. The sidebands contain the intelligence of the signal, the part you want to hear. The main "peak" is just the carrier, and should contain no useful data/sound/intelligence. With a simple AM signal your filter should be wide enough to include the data on each side of the AM transmission to be useful.

Now, there are ways to only use one sideband of an AM signal. You can simply select either upper or lower sideband (USB or LSB) and only use the selected sideband data, ignoring the other sideband and the carrier. And also, specifically with the Excalibur, you can select AMS (synchronous AM), you can then select the option of excluding one sideband in the detected audio. Both of these techniques are useful if another signal is setting near the desired signal and killing one sideband.

For honest AM signals a selected bandwidth of 5 to 9 kHz is best, I leave mine right at 6 kHz unless I am trying to kill an offending signal. For USB/LSB I typically use 2.8 kHz or so, narrowing as needed. These are the starting points, I adjust from there.

I find that using the DDC display (the upper left window) in waterfall mode instead of spectrum mode helps to determine how wide a filter needs to be set. I also like to run the DDC display fairly narrow, 20 or 40 kHz, unless I am doing something that really requires a wider display. I zoom in on the full spectrum display (bottom window) to search around and click on signals. I also set this display to waterfall and speed 1 (slowest speed).

T!
Mohave Desert, California, USA

 

[K9WG]

Token is correct. The center carrier will not have any audio component so you need a passband wide enough to select the carrier and one of the sidebands in AM mode. You can also hear them in SSB mode as this selects the sideband to demodulate. Also you will notice some AM stations only use one sideband. (ex. CHU Canada). While you can hear these stations in AM mode you would need to select the correct sideband to hear them in SSB mode.

 

[jackj]

Token is correct, here is a little more info that may help you understand what's going on:

The AM sidebands are products of the modulation. That is where the information is located. If you filter out the sidebands, you remove the modulation and you're left with just the un-modulated carrier. The sideband offset from the carrier is determined by the frequency of the modulation. So a carrier of 1,000,000 Hz modulated with 1,000 Hz will have a upper sideband of 1,001,000 Hz, the carrier at 1,000,000 Hz and a lower sideband of 999,000 Hz. Both the upper and lower sideband contain the same information so you only need one of them to recover the modulation. In fact, you only need to transmit just one of the sidebands. This is why Single Sideband Suppressed Carrier works so well, all the transmit power is concentrated in just one sideband.

 

[BOBRR]

From OP

Hi all,

Just a quick thanks; that explains things very well for my feeble mind.
Appreciate the time you all took to help me out.

Regards,
Bob

 

[BOBRR]

From OP: A Bit More Clarification On Tuning, Please

Hi again,

Just a bit more in the way of clarification, please.

Re the normal local AM broadcasts:

For the local AM broadcasts, is it also that there is no info on the carrier, but only on the sidebands ? When you tune exactly to a frequency as given in the listings, are you tuning to the carrier, or one of the sidebands ?

Forgetting my previous WWV question for the moment, when you tune to a SW AM station, as listed, is it that you arealways just tuning to the carrier freq. (which has no audio info.) but you better have a wide enough bandwidth to encompass a sideband to actually be able to hear anything ?

Suggested BW settinh, if so ?

Thanks again,
Bob

 

[Token]

For the local AM broadcasts, is it also that there is no info on the carrier, but only on the sidebands ? When you tune exactly to a frequency as given in the listings, are you tuning to the carrier, or one of the sidebands ?

Forgetting my previous WWV question for the moment, when you tune to a SW AM station, as listed, is it that you arealways just tuning to the carrier freq. (which has no audio info.) but you better have a wide enough bandwidth to encompass a sideband to actually be able to hear anything ?

For any AM broadcast or signal (be it in the AM BCB, on SW, or at aviation frequencies in VHF) there is no information on the carrier itself. All of the information is in the sidebands.

You tune to the carrier frequency, the carrier frequency is what is listed in "listings". Yes, you need to have a filter width that is wide enough to receive the signal. You can pinch down on the signal so that the filter is more narrow than the actual signal to a certain extent, but when you do so you start leaving part of the signal outside the band pass, cutting off those parts. What is cut off you will not hear.

So, for example, you have a music broadcast on 7285 kHz. The audio signal is 5 kHz wide, that is to say it includes audio frequencies up to 5 kHz. That means the RF will be 10 kHz wide on the display, each sideband will have the full 5 kHz audio in them. In order to fully receive all of the possible audio your filter needs to be 10 kHz, or more, wide assuming you have AM mode selected on the radio. If you reduce your filter below 10 kHz in width, say to 6 kHz width, you will still receive the signal, but not including the higher frequencies. In this case you will receive the audio frequencies from close to 0 Hz to about 3000 Hz, because your filter is allowing + and - 3 kHz (if you add up the + and - 3 kHz you get 6 kHz) of signal through. This reduced bandwidth works fine for speech, because most speech has little data above 3 kHz, but music often has audio frequencies above 3 kHz, so music might sound a little pinched, or off.

Suggested BW settinh, if so ?

As I said before, for most short wave AM use I start with the bandwidth set at 6 kHz. In the AM mode it is one of the selection buttons above the spectral display in the upper right area of the WinRadio window. I adjust as needed form that 6 kHz point after I have tuned to a station, but really the only reason I ever adjust it is to reduce interference from another station close in frequency to my target station. 90+ % of the time it just sets on 6 kHz.

T!

 

[kb2vxa]

"Forgetting my previous WWV question for the moment..."

Never forget it, not even for a moment. The answer to all your questions may be found in the answer to that one question.

 

[majoco]

For best reception of a strong AM (broadcast band or shortwave band) station, your bandwidth should be twice as much as the highest audio frequency - usually 5kHz is the highest so a 10kHz bandwidth will be good.

Hoever for shortwave stations if you get selective sideband fading, just selecting the carrier and one sideband may result in a clearer signal, so carrier + sideband = 5kHz. You may also try switching to SSB or AMS.

If there is another station on the next frequency (most SW stations are 5kHz apart) you may get a whistle from the adjacent carrier - then reduce your bandwidth to 9 or 4.5 kHz.

On your spectrum you should be able to see the next channel along and adjust your bandwidth to suit.

Cheers - Martin ZL2MC

 

[BOBRR]

From OP

Hi,

Thanks.
Believe I understand it now.

You guys really give good, clear, explanations.

Thanks again,
Bob

 

[jackj]

Token made one slight mistake in his answer to your last question. The carrier DOES contain the modulation and is used to demodulate or recover the information. I am not familiar with your radio or the ability of the spectrum analyzer function of the software but if you can spread out the display enough to show the sidebands as well as the carrier then you will see the level of the carrier changing with the modulation. The modulation and the sidebands can be thought of as different aspects of the same thing. If you remove the modulation then you also remove the sidebands. If you remove the sidebands then you also remove the modulation.

The simplest AM detector is a diode and a diode is used to rectify AC into DC current. The output of the diode detector is half of the carrier, either positive or negative pulses. The output is fed through a low-pass filter which removes the high frequency RF and leaves the low frequency audio. You can't recover the modulation without the carrier so when you receive a SSB signal, you inject a signal which takes the place of the carrier. The sideband will cause the injected signal's amplitude to vary and the diode will detect that variation.

 

[BOBRR]

From OP: For Jackj et al

Hi,

Talk about timing.

I was just about to ask this also.

Was wondering, therefore, why not just tune solely to a sideband, and have the bandwidth only encompass a single sideband

And also why not just post one of the sideband freq's as the "tune-to" frequency

Your explanation makes sense of all of this now.

Didn't appreciate that you had to have the Carrier in the pix also for de-modulating the signal.

Thanks again,
Bob

 

[k9rzz]

Flame shields up: So, it's not so much having trouble hearing WWV, the signal is there, it's more trouble learning how to use the SDR radio. Isn't there a separate section of this forum for that now?

 

[Token]

Token made one slight mistake in his answer to your last question. The carrier DOES contain the modulation and is used to demodulate or recover the information. I am not familiar with your radio or the ability of the spectrum analyzer function of the software but if you can spread out the display enough to show the sidebands as well as the carrier then you will see the level of the carrier changing with the modulation. The modulation and the sidebands can be thought of as different aspects of the same thing. If you remove the modulation then you also remove the sidebands. If you remove the sidebands then you also remove the modulation.

My error was one of over simplification. I was not going to go into how the amplitude of the carrier is modulated in power to produce the intelligence in its sidebands, or what the carrier envelope is doing. I did not want it to get complicated when we got to SSB-SC or DSB-SC and how that applied to the use of his filters.

For his application, in AM mode, the central peak he sees on the display is not the important part, and he was originally filtering out all but that. If he does this he will not hear the audio of the transmission. The sidebands are the important part, you can filter out the carrier (by filter out I mean put the carrier outside of the filter and use an SSB mode of the receiver) and still recover the intelligence, you cannot filter out both of the sidebands leaving only the carrier and do the same.

Was wondering, therefore, why not just tune solely to a sideband, and have the bandwidth only encompass a single sideband

And also why not just post one of the sideband freq's as the "tune-to" frequency

Your explanation makes sense of all of this now.

Didn't appreciate that you had to have the Carrier in the pix also for de-modulating the signal.

Bob, in my first response I wrote “Now, there are ways to only use one sideband of an AM signal. You can simply select either upper or lower sideband (USB or LSB) and only use the selected sideband data, ignoring the other sideband and the carrier. And also, specifically with the Excalibur, you can select AMS (synchronous AM), you can then select the option of excluding one sideband in the detected audio. Both of these techniques are useful if another signal is setting near the desired signal and killing one sideband.” But this only applies if you have a radio that can receive USB or LSB, and while the Excalibur does fit that description but not all radios do.

And you only have to have the carrier “in the pix” as you say to demodulate in AM mode on the receiver. When in SSB the receiver affectively applies its own carrier to the sideband information and so the transmitted carrier can be ignored (this is what Jackj described in teh part about the simple AM detector). In fact, you must either completely filter out the carrier or you must zero beat the receiver to it or you will have a heterodyne issue to contend with.

You have gotten turned around here a bit, and the discussion has become a bit circular.

An AM transmission is intended to be received by an AM receiver, not an SSB receiver, even though SSB receivers can be used to listen to AM broadcast. Many SW receivers are AM only. It would not do any good to reference the sidebands in that case. So the station is listed by its carrier frequency.

However, you are missing part of the puzzle. What do you call the upper sideband frequency of a 10000 kHz AM signal? I mean, how would you express it to someone to list it? 10000 kHz USB.

T!

 

[majoco]

jackj said

"The carrier DOES contain the modulation "

Sorry, it does not - the carrier is exactly that - it carries the sidebands which contain the information. If the carrier contained the modulation then SSB would not work.

If you look at a spectrum analyser display - such as those shown on Excalibur and the others - on a reasonably strong Broadcast band AM station that is not changing rapidly in signal strength, with a display about 10kHz wide you will see that the carrier does not change in amplitude at all - all the information is in the two sidebands.

Cheers - Martin ZL2MC

 

[jackj]

Martin ZL2MC, then the carrier is not needed to detect the signal and recover the modulation?

 

[majoco]

Correct.

Consider a single sideband signal. In the transmitter the audio signal is modulated onto the carrier and produces two sidebands plus the original carrier. If the modulating tone is 1kHz and the carrier is 1MHz then the two sidebands have frequencies of 1Mhz-1kHz = 999kHz, and 1MHz + 1kHz = 1001 kHz. Which is what you can see when you display WWV on your spectrum except the tones are 400Hz. Now the two sidebands contain equal signals and the carrier does nothing. Therefore in the transmitter the carrier and one sideband tone are filtered out, leaving the other sideband which is a radio frequency and can be transmitted.

In the receiver, to make the sideband back into audio again, you mix the sideband with a frequency the same as the missing carrier, then filter out the original sideband and inserted carrier, thus 1001kHz - 1Mhz = 1kHz.

Not quite as simple as that, but you get the principle.

Cheers - Martin ZL2MC

 

[jackj]

If the carrier isn't needed to recover the modulation then why do you have to mix the sideband with a locally produced signal that is the same as the carrier? Please read my post of 3:32 PM, yesterday. I think I explained it in that post.

 

[Token]

If the carrier isn't needed to recover the modulation then why do you have to mix the sideband with a locally produced signal that is the same as the carrier? Please read my post of 3:32 PM, yesterday. I think I explained it in that post.

The disconnect here could be one of terminology and the mixing in the discussion of SSB and AM basics and techniques. I strongly suspect we are all seeing essentially the same things in our heads and saying them differently in text.

I think the issue is not if a carrier, of some type, is required to demodulate or detect SSB, but rather the point about the carrier and intelligence. For example I said, about an AM signal and what started this portion of the thread, "there is no information on the carrier itself. All of the information is in the sidebands." Your response was "Token made one slight mistake in his answer to your last question. The carrier DOES contain the modulation and is used to demodulate or recover the information.", the first part of your response is incorrect, or possibly partially correct depending on how you view the process (the carrier does not contain the data, but the modulation envelope derived from the original carrier does), and the second part applies to demodulating an AM signal when using an AM only receiver with an envelope detector. Absolutely true that the carrier is needed to demodulate the signal in an envelope detector, but the carrier itself does not have to contain any data other than its basic oscillating frequency to achieve this task and that is not the only way to recover the data.

Later you said "The modulation and the sidebands can be thought of as different aspects of the same thing. If you remove the modulation then you also remove the sidebands. If you remove the sidebands then you also remove the modulation." and this sounds pretty much like what I was saying. The intelligence is in the sidebands, not in the carrier.

From there the conversation spiraled into why the carrier does, or does not contain data, and how it is possible to detect the data in only the sidebands of a signal without regard to the carrier.

However, I will use your illustration to attempt to clarify. You asked, "If the carrier isn't needed to recover the modulation then why do you have to mix the sideband with a locally produced signal that is the same as the carrier?"

_A_ carrier, or beat frequency, is indeed needed in the detector stage, but not the _ORIGINAL_ carrier to the signal. This is because the original carrier has, as I stated, no data germane to the intelligence of the signal, all of the data is in the sidebands, indeed possibly in only one of the sidebands. A simple, pure, data-less carrier can be locally generated in the product detector and beat against the down converted sideband, producing the intelligence. If the original carrier had any required data (other than its simple operating frequency, and the BFO is generally a completely different frequency from the original transmitted carrier) how would the product detector know to develop that data to apply to its BFO? After all, the original carrier is not present in an SSB-SC signal as received by a remote station, or rather; generally the original carrier is suppressed as much as the radio designers could possibly make it, ideally to the point of being below detection thresholds.

I stand by what I originally said, the carrier contains no intelligence with regards to the transferred data, the sidebands contain all of the data in the transmission. That does not mean that the carrier was not originally required to generate an SSB-SC signal or that an AM signal does not need the carrier to be demodulated in an envelope detector. But you could, if you wanted too and at the receive end, completely remove the original carrier from a received AM signal via filters, essentially producing DSB-SC, and insert your own pure, definately data-less carrier in its place, without loosing any of the data in the sidebands, indicating the carrier is not important to the data itself. And at the other extreme, if you filter out the sidebands so that all that is left is the carrier frequency you will have no data on the received transmission, indicating that there is no data in the carrier itself.

T!

 

[majoco]

I agree with all that Token has said.

SSB was initially developed as a efficient communication method, not necessarily for entertainment, hence the reduced 'fidelity'. It is efficient because it only transmits one sideband which is only 25% of the original total AM power at 100% modulation. Reduction in the required bandwidth at the receiver results in less noise and another improvement in 'talk power'.

Cheers - Martin ZL2MC