another cool book

He also seems to be a bit fuzzy on the actual inventors of radio! I can't see any mention of Marconi, Hertz, Henry, Lodge et al. :-/

Marconi is prominent in the book. Marconi's love life is prominent, too. Maxwell and Hertz are mentioned as background, but are not big players in the development of radio as such.

David Sarnoff comes off better than I'd expected. He was a real visionary. Armstrong was a good guy. De Forest was a creepy lunatic.

..and Cy Elwell?

Marconi's greatest love was Preece. And it was mutual.

Hertz not a big player in the development of radio?? A burger and fries says you're wrong!

Ferdinand Braun is sometimes celebrated, he brought us tuned circuits and selectivity and multiple wireless "channels". He shared the Nobel Prize with Marconi.

piglet

The book is about radio, stations broadcasting talk and music to people in their homes. I don't think Hertz envisioned that.

Armstrong invented regeneration, tube oscillators, the superregen, the superhet, and wideband FM. Not bad. The FM thing was especially impressive, as it had been mathetically proven, and generally accepted, that FM was inferior to AM.

I am delighted that Marconi's name is not mentioned. To set the record straight, Marconi plagiarized ALL the work of the famous Indian physicist and bio-physicist Sir Jagadish Chandra Bose. Sir Jagadish Chandra Bose designed and built transmitters and receivers for microwaves. Most importantly he was the first to identify and exploit the properties of semiconductors in his radio physics work.

But AHH.. the QUALITY of the sound! (not to mention the noise rejection)

The noise rejection was key. AM had lots of static, and restricting the bandwidth helped some, but trashed fidelity. Carson had proved mathematically that FM was worse, so everybody but Armstrong accepted that. But he assumed narrowband FM and especially assumed a linear system. Armstrong intuited that wideband modulation with limiting receive amplifiers would work, and it did. Gain-bandwidth was expensive in those days, so the FM signal chain was a very progressive idea. GBW is dirt cheap now.

Anyway, the book is about the people and the history, not the details of the technology.

Marconi is a major character in the book.

To set the record straight,

Marconi liked the coherer detector and didn't much like crystals or tubes.

The counterintuitive thing is that you get less noise for the same TX power by making the bandwidth _wider_. (Only in the high-SNR limit, but still.)

Cheers

Phil Hobbs

Do you know of someplace that it's explained. (There are links to Armstrongs IEEE article in the Wiki link of JL's ... but behind the pay wall.)

Never mind.. here it is..

George H.

Yeah, the "FM capture effect" of the limiting amp is magical. Armstrong may have suspected it, but he did hundreds of breadboards until it really worked.

Limiting gets rid of the AM component of the noise but not the FM component, so by itself it improves the SNR by only 3 dB in a narrowband system.

Besides suppressing noise, capture prevents distant stations or adjacent channels from changing the instantaneous average frequency of the carrier. When you add two phasors of different lengths and similar frequencies, the average frequency with which it loops round the origin is just that of the larger one. The smaller one can make it bounce back and forth, but can't make it loop faster or slower.

Normally, the detected noise density goes up as you increase the IF bandwidth, because all the noise components intermodulate with each other in the detector, and some of that creates baseband products.

Interestingly, because of the large carrier amplitude, the noise doesn't intermodulate with itself much, but instead is linearly downconverted at the frequency discriminator's output. What this does in the frequency domain is to confine the relevant noise bandwidth to +-15 kHz of the instantaneous carrier frequency. Thus given a fixed baseband lowpass filter, the detected noise doesn't go up as you widen the IF, and the detected signal gets bigger as you increase the deviation, all assuming that you stay in the high-SNR limit. That's a cool thing that I just learned today, from Armstrong's paper (thanks, George!). (I had thought that the noise voltage went up like sqrt(BW), but it doesn't.)

Floyd Gardner's PLL book has a good section on loops with limiters. The math gets fairly hairy.

Cheers

Phil Hobbs