On Fri, 24 Jan 2014 13:04:48 -0500, Jerry Stuckle
wrote:

The easiest way of seeing it is looking at the output of both tube and
transistorized transmitters on a spectrum analyzer. You will see much
more hash on the transistorized transmitter.

Amazing. You might see more hash with a synthesized transistor
xmitter, but for crystal controlled, they noise is quite a bit less
with transistors.

Back in the 70's, I ran a CAP repeater from my home. Transmit and
receive antennas were separated by about 25' vertically. It was a
surplus Motorola tube rig, running 25W. I was able to run it without
any desense without duplexers. Yes, the channel spacing was 4.25Mhz,
but you can't do that with a transistorized rig.

Amazing. These days, 2 meter repeaters do 0.600 MHz spacing using all
transistor equipment, a single antenna, and a notch type duplexer.

Yes, nowadays, there are transistors with lower noise figures. But they
are relatively expensive, and you won't find them in the less expensive
receivers.

Rubbish. pHEMT devices are available with 0.75dB NF at 1GHz for under
$3/ea. For example:
http://www.minicircuits.com/pdfs/PSA4-5043+.pdf
Note that this isn't just a single device but a MMIC amplifier capable
of belching 100 mw (+20dBm) with 1dB gain compression.

I seem to remember that most tube sets of the old days were stating about .5
uv senstivity on ssb, and many of todays ham trasceivers are way less than
that.

That depends on the frequency. The atmospheric noise is so high on
the lower HF bands that improvments in receiver sensitivity simply
results in amplifying both the noise and the signal but the same
amount, resulting in no net improvement in signal to noise ratio
(SNR). Adding more gain also decreases the receiver dynamic range
because at high signal levels, the added gain will cause the receiver
to overload at a lower signal level. Hint: Use only as much gain as
necessary and no more.

Plus or minus, that is about right. But that wasn't because of the
tubes; they could have done better but it would have required more
amplification and higher cost. Plus with a decent antenna, the
atmospheric noise was higher than that, so there was no need for more
amplification. It would have just been lost in the AGC circuitry.

Mostly I agree except for the part about AGC. If your receiver is
already into AGC for weak signal conditions, you're effectively
reducing the receiver sensitivity at the same time. The way AGC is
suppose to work for a SSB receiver is that the AGC starts just above
the level where you can hear an intelligible signal. 12dB SINAD is
about right. If the AGC were perfect (i.e. no slope), then any
increase in signal level above that point will result in no
improvement in SNR because the AGC will do its best to keep the SNR
constant. With a real AGC (dual slope, controlled attack and release
time) the SNR improves somewhat as the input level increases until it
reaches some SNR, where it levels off. I think this is called
"ultimate SNR" or something similar, which is just the SNR of a very
strong receive signal.

Even back in the early 70's, commercial tube VHF radios could easily get
.15mv for 20db S+N/N ratio. Not much different than the transistorized
versions today.

I think you might mean 0.15 uv/12dB SINAD. A 0.15mv receiver would be
considere comatose.

In the 1960's thru about 1983, I was involved in various radio
service, radio manufacture, radio sales, and radio consulting
companies. I had plenty of experience with everything from wideband
GE Pre-Prog thru cellular radios including tubes. I never saw a tube
receiver with 0.15uv sensitivity. Photos of the shop and various
repeaters. Most were UHF.
http://802.11junk.com/jeffl/pics/Old...ers/index.html
The main site was on Santiago Pk:
http://www.trabucooutdoors.com/assets/images-1/odds_ends/santiagopk.jpg
The mess on the far right is the antenna farm.

9 GE Progress Line repeaters with Alpha tone panels (which I helped
design).
http://802.11junk.com/jeffl/pics/Old%20Repeaters/slides/Santiago-01.html
See anything missing? There are no duplexers. There was one receive
antenna and 9 transmit antennas. All tubes. With a typically
1uv/12dB SINAD receiver, the isolation was sufficient. Don't ask
about the tx intermod, which was horrible.

Typically, these tube type repeaters would start out with about
0.5uv/12dB SINAD with new tubes. That's measured directly into the
receiver input with no additional filters. After about 6 months, the
sensitivity would settle down to about 0.75 to 0.90uv and stay there
for about 6 to a year, when it needed retuning. This was using a 6AM4
triode. The only time I saw better sensitivity with tubes was when
someone tweaked the audio freq response, or excessively narrowed the
IF bandwidth.

Somewhat later, in the 1970's, I found myself designing marine radios.
Typical VHF sensitivity was about 0.25uv/12dB SINAD using a dual gate
MOSFET front end such as a 40673 or 3N212. We ocassionally used
JFET's such as a U310 but the sensitivity was about the same.

The problem is that 0.15uv is just too close to the receiver noise
floor to be realizeable. With a 25KHz receive bandwidth:
noise floor = -174dBm/Hz + 10*log(25KHz)
= -174 + 10*4.40 = -174 + 44
= -130 dBm or 0.071 uv
To obtain a 0.15 uv sensitivity, you would need a receiver noise
figure plus a detection SNR of less than:
10 log(0.15/0.071) = 3.3dB
With an analog FM demodulator, that's barely possible and usually
requires a perfect noise-free front end. However, with a 0.25uv/12dB
SINAD sensitivity, there's 5.5dB of margin, which is more than enough
for real receivers.

Mo
http://www.r-390a.net/Receiver-Specifications-Explaned.pdf



--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558