Load shedding by Smart Meters

[winston1]

I seem to remember there was an international agreement to align all frequencies on some standard channel spacing. Probably 6kHz.

The standard spacing was 9kHz, but more important all channels were multiples of 9. Two nearby transmitters were both allocated 180kHz so they broke the rules and quickly moved to 177 and 183kHz respectively. The 177 in former E Germany is now gone but Saarbrucken is still active on 183kHz.

 

[JohnW2]

The standard spacing was 9kHz, but more important all channels were multiples of 9.

That sounds reasonable. I did have my doubts when Detlef suggested that it might have been 6 kHz, since that would be pretty close for AM signals.

Kind Regards, John

 

[JohnW2]

I liked to be able to check my test gear was accurate, I have three commercial grade frequency counters, so I needed to check they were accurate.

Back in the days I'm talking about, 'we' didn't have frequency counters, commercial grade or otherwise

Although it was often useful to be able to 'measure' a frequency fairly accurately, the only crucial thing was the legal requirement to ensure that our transmissions were within the allocated amateur radio bands. Since the boundaries of those bands were all multiples of 100 kHz (!00 kc/s in those days ), the harmonics of a 100 kHz crystal oscillator was adequate to identify the edges of any of the bands. As I said, in those days we 'fine tuned' the oscillator simply by comparing its second harmonic with the Droitwich 200 kHz signal - but, as I also said, had I needed to do it more recently, I would have compared the 198th harmonic of one-hundreth of the oscillator frequency (divided down) with the 198 kHz Droitwich signal.

Kind Regards, John

 

[JohnW2]

With Phase Modulation there is no overall affect on the carrier frequency there will be 198,000 cycles in any given second. The last zero crossing point of the carrier waveform may be slightly displaced timewise. The carrier can be used as a reference frequency

Indeed.

With Frequency Modulation the number of cycles per second will be more or less 198,000 per second. With a NRZ data format the frequency can be 199,000 kHz or 197,000 kHz for long periods of time. The actual deviation from 198,000 depends on the depth of modulation and permitted band width. The carrier cannot be used as frequency reference.

Fair enough - I hadn't considered the possibility of that sort of 'data format'. My only real experience of FM (at least, 'back then') was modulating an RF carrier with an audio signal (with an average amplitude of essentially zero). In that situation, the average carrier frequency over any appreciable period of time (like 1 second) would be essentially constant.

Kind Regards, John

 

[JohnW2]

On reflection ....

With Phase Modulation there is no overall affect on the carrier frequency there will be 198,000 cycles in any given second....

and

...With Frequency Modulation the number of cycles per second will be more or less 198,000 per second. With a NRZ data format the frequency can be 199,000 kHz or 197,000 kHz for long periods of time.

... there's something I'm not getting here. With such a data format, if one were using phase modulation, if every cycle of the carrier were 'phase shifted' (although one has to ask 'shifted relative to what?') "for long periods of time", why would that not manifest as an "overall effect on carrier frequency"?

Kind Regards, John

 

[bernardgreen]

It the data is NRZ then Phase Modulation cannot normally be used.

Recovering the modulation from a Phase Modulated carrier requires the receiver to have a stable oscillator running at the same frequency as the carrier. The phase of the incoming carrier is compared to the phase of this oscillator and the difference is the modulation signal.

 

[DetlefSchmitz]

P

The standard spacing was 9kHz, but more important all channels were multiples of 9.

Yes thanks Winston. It was a long time ago. 6kHz would certainly have been challenging in those days. 9 makes a lot more sense. (I would have researched it better but the flight for Sicily was just starting to board).

 

[JohnW2]

It the data is NRZ then Phase Modulation cannot normally be used.

For the reason I mentioned? If so, then the apparent contrast between PM and FM that you tried to present is surely not really valid, is it - since neither type of modulation will result in an constant carrier frequency if the data is of that format, but both will have an essentially constant carrier frequency if the modulating signal is 'symettrical'.

Recovering the modulation from a Phase Modulated carrier requires the receiver to have a stable oscillator running at the same frequency as the carrier. The phase of the incoming carrier is compared to the phase of this oscillator and the difference is the modulation signal.

Indeed - and that is also essentially one of the ways of demodulating an FM signal.

Kind Regards, John

 

[JohnW2]

Yes thanks Winston. It was a long time ago. 6kHz would certainly have been challenging in those days. 9 makes a lot more sense.

Indeed. Probably fairly 'challenging in those days' in terms of receiver selectivity but, as I wrote in response to winston, also 'limiting' in relation to AM (by definition imposing an absolute maximum modulation frequency of 3 kHz).

Kind Regards, John

 

[DetlefSchmitz]

So does anyone know the specified transmitted bandwidth and filtering specification for the modulating signal for an a.m. station? When it was all relevant we didn't have good access to information, and I took it all for granted.

 

[bernardgreen]

The bandwidth is determined by the highest frequency of the modulating signal. Band width is twice the maximum modulation freqency,

With 9 kHz channel spacing this (in theory ) limits audio broadcasting to a maximum of 4.5 kHz but in practise the adjacent channels are not used ( in the local area ) so the audio band width can be greater than 4.5 kHz. Where the channel is speech only then adjacent channels can be used with the modulation limited to a maximum of 3 (or 3.5 ) kHz ( Germany use 4 kHz due the higher frequencies used in German speech )

 

[DetlefSchmitz]

Ok thanks. I was aware of the basic theory. I wondered more about the tx filter shape. As we couldn't suddenly cut off at a particular frequency, I assumed, say, 3dB at 3kHz and then so many dB per octave above that. I was wondering if that had been formally defined. Sounds like it was more pragmatic than that then?

 

[JohnW2]

Ok thanks. I was aware of the basic theory. I wondered more about the tx filter shape. As we couldn't suddenly cut off at a particular frequency, I assumed, say, 3dB at 3kHz and then so many dB per octave above that. I was wondering if that had been formally defined. Sounds like it was more pragmatic than that then?

Although I have no experience of commercial/broadcast transmitters, I suspect that it may well have been essentially 'pragmatic' (or, at least, 'implicit'). In terms of amateur AM transmitters, I certainly don't recall any explicit steps being taken to limit the 'local' bandwidth - we just relied on the fact that the audio modulating signal would never be more than 2-3 kHz. The only TX filtering we did was on a much wider (frequency) scale, to limit the extent to which harmonics etc. of the modulated RF signal got to the aerial.

Having said that, given that channels with 9 kHz spacing were formally defined/allocated, one imagines that there very probably was also a formal limitation on bandwidth (to ensure that no transmitted signals were more than 9 kHz wide - e.g. if someone were mad enough to try transmitting high frequency data etc. by AM!). However, again, I suspect that may well have usually been achieved essentially 'pragmatically' (on the basis of the maximum likely frequency of the modulating audio), rather than by explicit filtering.

If it were done, I presume that it would be the modulating signal (audio) that would have had a low-pass filter, since it would be infinitely easier to create a filter with a rapid fall-off after say, 3 or 4 Hz than it would to achieve the same with the modulated RF signal. Even 'back then', it was quite feasible to create (even with discrete components!) an audio frequency low-pass filter that had very rapid fall-off beyond a particular frequency.

AM is, of course, an inherently inefficient means of communication, both in terms of power usage and bandwidth required - much of the power is in the carrier, which carries no information, and one of the sidebands is redundant (hence making the bandwidth twice as wide as it needs to be), since it is merely a mirror image of the other sideband. That is why 'SSB' is preferable (strictly 'SSBSC' - "Single Sideband with Suppressed Carrier") - occupying half the bandwidth and wasting no power in the carrier. Most of the ways in which SSB is generated would also, as a 'side effect', limit the bandwidth of the one 'surviving' sideband (i.e. 'the signal').

Kind Regards, John