Yesterday I scored a pocket sized Seiko "Atomic" alarm clock at a yard sale for $1 and it looks brand new and works great. It is fun to watch the minutes click over in exact synchrony with the WWV audio.
There is a very interesting description of how these work on the WWVB website and in one of the technical articles where they discuss interference to the clocks they mention that the signal used is transmitted from WWVB at a frequency of 60 kHz using 1 bit/second amplitude modulation so it takes a full minute for the complete data frame to be received. The receiver bandwidth is 10 Hz.
What caught my attention was a statement that if there is a nearby broadcast station operating on a frequency where 60 kHz is a subharmonic then this can cause reception problems. Dividing my 1680 kHz part 15 transmit frequency by 60 kHz yields 28 so there is, according to what I read, the potential for "subharmonic interference" yet the clock syncs just fine.
My question is what is this all about? Is there an explanation as to how a signal operating on a much higher frequency than the receive frequency can cause problems because of the (sub)harmonic relationship? I can understand problems resulting from receiver overload, desense, or IMD but not from "subharmonics". Am I missing something?
Any ideas?
Neil
Why would you be confused about sub-harmonics? Sub-harmonics are no different from any other produced harmonic, and cause just as much problems.
In fact sub-harmonics can cause more interference because it is a multiple or division of your main carrier, and whatever is on that carrier (modulation) be it in FM or AM or whatever, will be that much more in intensity (modulation percentage) than the main carrier.
Take your spec-an and scan that sub harmonic from your 1680 signal modulated and note the modulation intensity being greater on the sub-harmonic than on the main carrier.
RFB
Help me understand. If I transmit a signal on 1680 kHz with or without modulation what frequencies are produced where they would be considered "sub harmonics"?
For example, a 1 MHz carrier perfectly AM'd with a 5 kHz pure sine wave will produce signals at 5 kHz, 995 kHz, 1 MHz, and 1.005 MHz.
Would not a sub harmonic be these signals divided by integers and if so where are they?
Neil
Not so long ago I mentioned that while I was testing my loading coil with AMT3000, I noticed spurs popping down at 216kHz during modulation peaks. RFB said that meant I had a mismatch in the antenna system and was having reflected wave back into the transmitter that somehow put pops down in the LW.
I thought I had it fixed when I added a .001uF capacitor between the coil and antenna, but on review, it didn't help after all.
To stop the spurs I have lowered modulation.
Is that the same as sub-harmonics? Don't know.
"Would not a sub harmonic be these signals divided by integers and if so where are they?"
One would conclude that sub harmonics are exact multiples or divisions of a main carrier. However not always the case since our signal can "intermix" with another, as well as our signal's harmonics intermix with other harmonics. This kind of intermixing can cause the odd ball sub harmonics, which usually end up somewhere completely different from calculated multiples or divisions of a carrier.
Also we have to account for receiver IF and local oscillator. These also can produce sub harmonics internally, which can produce the odd ball results.
Normally these levels of sub harmonics are incredibly weak, but may be strong enough to cause a problem in a sensitive device, like the atomic referenced clock. 60Khz signal with a 1 bit 10 hz wide pulse would require very sensitive and very high-level notch filtering for a 10hz wide bandwidth window looking for a 1 bit pulse spread out over 60 seconds. Lot's of interval noise can take place within that window. I bet that clock has got a very interesting front end on it!
Modulation can also produce sub harmonics..ie side band sub harmonics with a greatly reduced carrier, almost like dual sideband with a slightly suppressed carrier. These are where you will see the larger intensity modulation compared to common multiple/division harmonics.
Sometimes I can see odd ball sub harmonics interacting between my AM and FM signals, most noticed on the FM band. These sub harmonic signals are very tiny, but noticeable on the spectrum analyzer, and sometimes tunable via the receiver. Certain days they are there, other days they are not there. I would conclude this is caused by a reaction between my station's signals and others.
My 1670 signal has an odd ball that appears on 770Khz. Now mathematically it should break down like this..1670, 835, 417.5, 208.75 and so on going in division. So where the heck does 770Khz come from?!!
RFB
I suspect that my problem grasping this is the use of the term "subharmonic". I am very familiar with how RF signals can mix an produce all sorts of chaos and this is why I referred to a "perfect" modulated signal with a "perfect" audio sine wave and no mention of receiver electronics. Now, in the real world of non-linearities mixing products come into play and I can understand and predict some of the effects.
It would be interesting to know more about the receiver used in this clock and a reading of the WWVB technical articles on the subject give some hints. From an article "We recommend that RCC products shouldbe sensitive enough to successfully synchronize to signals from WWVB with a field strength of 50 μV/m, if the signal to noise ratio exceeds 20dB. The RF bandwidth of the receiver should be narrow, typically ±10 Hz or less." They mention that the received signal is usually passed through a crystal tuned to 60 kHz but for 50 uV/m sensitivity considering the wavelength and very small antenna there has to be some high gain circuitry involved.
There is always something new to learn isn't there?
Neil