What is CCW?
Amateur Radio Coherent CW was invented by Ray Petit, W7GHM. He is also the
inventor of Clover now manufactured by HAL Communications. The first
amateur QSO was by Andy McCaskey, WA7ZVC using a Ten-Tec PM-1. CCW was
promoted by Chas. Woodson (Woody), W6NEY a professor at Stanford University.
Woody published a newsletter in the early 1970's. Ade Weiss, W0RSP wrote
some articles in CQ and Woody, W6NEY publish a series of articles in QST in
1979 - 1981 period. In February 1994 VE2IQ published his circuit for CCW
using a PC and DSP techniques. Peter Lamb, G3IRM wrote a newsletter on CCW
techniques in the early 1990's.
CCW moved on to BPSK techniques and is presently being used on 80 meters. A
lot of this work, software, etc. is available on the web. The ARRL had
information in the 1980's handbooks and still has some material in the
current issues.
Amateur CCW was developed before we had nice microprocessors, DSP and other
current technology. It's been around for 25 years, is only as complex as an
SSB transmitter, and certainly within the building ability of all most all
amateurs. One does not need power ... it is a QRPp mode.
CCW is slow ... 12 wpm CW. You need a good freq standard, but today we can
use GPS timing (see TAPR web site). It works in noise and under poor
conditions and has been proven to work on the ham bands.
See all that here.
https://midnightdesignsolutions.com/ccw/
73
JC
N4IS
-----Original Message-----
From: Topband <topband-bounces@contesting.com> On Behalf Of JC
Sent: Monday, December 24, 2018 6:46 PM
To: 'K4SAV' <RadioXX@charter.net>; topband@contesting.com
Subject: Re: Topband: FT8 - How it really works
Jerry
The new mode FT8 is not all that new, actually, there are several aspects to
consider, like detect the signal, decode the signal detected, make a
decision to accept the decoded signal. The improvement on signal to noise
ratio concept is very old, just the internet made it possible with time
synchronization. The decode uses new algorithms and some very intelligent
way to guest the decoded signal.
Check this out. 1975 Sept QST; Coherent cw test!
Experiments show 20 db
Signal Boost over QRM,
http://www.arrl.org/files/file/Technology/tis/info/pdf/7509026.pdf
The improvement on signal to noise ratio is just because a narrow bandwidth.
The gates opens at the right millisecond window. On FSK the secret salvage
is time synchronization. You can record the audio and play it back, the
decode will happen only if you synchronize the time of the recording with
the time in ms of the PC clock.
I did that, and it worked, I have a SDR QS1R and using HDSDR software to
record the I/Q file, RF file. I used to record rare DX expedition signal and
the bandwidth is 50 KHz, I can see the FT8 guys on 1840, My question was ,
can I decode them from the digital file recorded several month ago?
I started plaining the file at the top of the second count, and voalah!!!,
The WSJT-X decoded several station, weak as -21 db. The weak signals are
there, buried in the noise on my old digital recorded file.
Then I decided to test my HWF, the practical result measuring cw signal is
that the signal to noise increase around 20 db, 10 db due the directivity
RDF 11.5 and another 10 db from the polarization filter. The Horizontal WF
attenuation on vertical signals is over -90 db. The manmade noise vertical
polarized is reduced below the MDS of the receiver and cannot be amplified
by the receiver.
The IC-7800 has two identical receivers. I connected my HWF on receiver MAIN
and the TX antenna on the receiver SUB, I installed two instances of the
WSJTX program, one for each receiver. After 15 minutes the number of decodes
on the HWF was 20 or times more than the vertical full size vertical, my TX
antenna 120 Ft high.
Signals decoded around -21 db on the vertical was decoded on the HWF 0 to +1
db. Signals less the -5db decoded on the HWF was not decoded using the
vertical, The HWF was decoding hundreds of signals that would be -40 db on
the decode using the vertical.
I think the s/n reported by the program as ball part is actually very good
and close to the real s/n improvement of 2 Hz BW, depending on the mode.
The only real way to increase signal to noise ratio is increasing the
directivity of the RX antenna, more real RDF means real signal to noise
ratio improvement. I used real because it is very easy to destroy the
directivity with integration, leaking, intermodulation, low noise figure
etc.
One bad concept, bidirectional unterminated beverage with two lobes one in
the back and one front, it just does not work because the RDF is 6 db down a
terminated beverage. Same for BOG's the RDF is bad, a K9AY works better
because has more RDF. A simple Flag can deliver 9 db RDF is tis easy to hide
too. Two Flags in phase 11.5 db and four Flags 14 db RDF, and a very clean
pattern besides real broadband from 1 MHz to 10 MHz
As you can see on the ARRL 1975 article, there is nothing new about
improvement of signal to noise ratio reducing the bandwidth. On the article,
the test was CW at 12 wpm and 9 Hz filter BW , no ring using WWV as time
source for the synchronization.
That was state of the art back in the early 70's, almost 50 years ago.
73's
JC
N4IS
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of K4SAV
Sent: Monday, December 24, 2018 3:10 PM
To: topband@contesting.com
Subject: Re: Topband: FT8 - How it really works
Although I have finished my FT8 testing, there is one final thought I would
like to leave with you, and also to correct one statement I made earlier.
Someone thought FT8 measured the noise in the interval when the FT8 signals
were off, and I replied that would result in a real S/N number. That is not
true as you will see in the info below. You would get a real S/N number if
the RF was sampled, but not if the audio is sampled.
I spent many years designing electronic circuits professionally, so I still
think that way. So for a few minutes lets think about a circuit that can
decode something below the noise floor .If you think about FT8 or anything
similar, from a designers point of view, you suddenly realize that making a
statement of "the circuit can decode down to X dBs below the noise floor" is
almost an impossible task, that is, if you are talking RF noise floor as
most people will be assuming.
Since you will be dealing with audio, not RF, the receiver will convert the
RF into audio and compress it into something that has a lot less dynamic
range. How much less? Say the volume is set to a level such that the
strongest signals do not clip, then how far down is the noise?
You can expect that to vary on each band too.
Now comes a real complication. If you were taking samples in the RF world,
you could see the noise level on your S meter and estimate it relative to
the strongest signals. However your circuit will be dealing with audio.
Surprisingly, when the signals disappear, the receiver AGC voltage drops and
the receiver gain increases. That produces a lot more audio signal. The
audio noise in the case of no signals becomes higher than the audio level
for strong signals if you are using USB bandwidth and receiving something
similar to FT8. That condition is not nearly as pronounced when using a
narrow CW bandwidth. Even if you put the receiver into AGC slow mode it
won't hold for the 3 seconds when FT8 is off, so you still get the increased
audio in the off period. Then there will be a sudden increase in audio when
the first signal reappears, until the ACG kicks in and lowers it. This
happens even with fast AGC selected. It's fast enough that you don't notice
it when listening, but if you put a scope on it you can see it. Yeah, all
that surprised me too when first thinking about it.
Take a close listen and
see if you agree. If you can't hear it, put it on a scope or anything that
displays an audio waveform and it will become very obvious.
If you made a statement that this circuit can decode X dBs below the noise
floor, most people will be thinking RF noise floor. So what is it in the
audio world that represents the noise floor in the RF world, and what would
your statement mean?
Of course you could turn off the AGC and decrease the receiver RF gain and
that would make the audio very low when the signals disappear. That would
also severely limit the dynamic range for your circuit since you would no
longer have the compression supplied by the receiver.. Your circuit would
have to cover a much wider dynamic range, similar to what a receiver does.
So your circuit would need what? maybe 100 dB dynamic range to cover the
strongest signals to the weakest noise floor, forgetting about decoding
below the noise floor. Actually that wouldn't really happen because
receivers can't produce a dynamic range of 100 dB in the audio. They may do
it in the RF world, but not in audio.
Receivers have no need to do that.
Jerry
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