Topband: Digital mode spurious issues

[Jim Brown]

On 12/30/2013 7:55 PM, Tom W8JI wrote:
> IMD **requires** two or more signals at once, and does not appear 
> anything like sideband leakage. 

Actually, IMD can be produced by ANY keying waveform -- for example, the 
envelope of a CW  or RTTY signal. To understand this, think of the 
keying envelope of a signal as the amplitude modulation of a carrier by 
a square wave, which, from a spectral point of view, consists of many 
harmonics, the relative strength of which vary with the rise time. 
Indeed, clicks are the IM products of that envelope.

During contests, I often hear clicks from RTTY rigs that have high 
levels of IMD somewhere in the system. Elecraft recently released new 
firmware that significantly reduces the TX bandwidth of FSK RTTY by 
carefully shaping the keying waveform in the same manner that they do 
for the CW waveform.

There's also the mechanism of incidental AM as the result of non-flat 
response of the filter through which the signal is being transmitted and 
received. This is clearly visible as flicker of a wattmeter on the 
output. I noticed a significant reduction in this when I replaced the 
2.7 kHz 5-pole filter in my K3 with a 2.8 kHz 8-pole filter.

On 12/31/2013 7:13 AM, Tom W8JI wrote:
> The entire thing for digital modes was poorly planned. 

I think it's long past time to stop wringing our hands about a very 
minor issue and start worrying about more consequential ones. Nothing 
short of official allocation restrictions by national bodies (FCC and 
equivalent around the world)  is going to change the frequencies used by 
various digital modes within CW sub-bands. Besides -- the vast majority 
of those using WSJT and PSK on the HF bands are running flea power -- 
20W is QRO for this crowd.

Not only that, this group of operators is, as a whole, more or less 
self-policing. All are using some sort of waterfall display, and it's 
common for a dozen or more QSOs to be taking place in the 2kHz or so 
bandwidth of a SSB signal, so anyone generating audio distortion is 
going to be quickly noted and they will let each other know on email 
reflectors devoted to JT65. Granted they won't notice the inferior 
sideband suppression that W8JI has observed, but FWIW, I hear a ton of 
that during most SSB contests.

I think we ought to be paying a lot more attention to the issues 
highlighted in this report by SM5BSZ, which is quite illuminating with 
respect to the RF trash produced by a selection of popular rigs. 
http://www.sm5bsz.com/dynrange/dubus313.pdf

.It shows, for example, that the phase noise from a K3 is at least 17 dB 
lower in amplitude than most other rigs, including the IC-7600. 17dB 
means that there is 50 times the power (more than three real S-units) in 
the trash, and some rigs are a lot worse than the 7600.

In any contest, these rigs are all running at least 100W, the trash 
produced is broadband and cumulative, most of them are driving power 
amps of varying quality that boost the level of the trash by 10-15 dB, 
and there are a LOT more of them than the handful of digital operators. 
Given these realities, it's just plain silly to obsess over a few QRP 
digital signals. It would be far more productive to work on reducing 
that trash (by applying peer pressure to get these guys to clean up 
their act), which is raising the overall noise level on all of our bands.

As to the post about WSPR -- although it was developed by K1JT, it is an 
entirely different system designed to observe worldwide propagation in 
real time, uses an entirely different modulation method, and 
transmitters typically operate at 1W or less. The WSPR website reports 
only on the reception of WSPR transmissions. It does not report JT65 
transmissions.

Most operators using JT65 on the HF bands use software called JT65HF, 
written by W6CQZ using K1JT's protocols. It's a multi-decoder system 
with a waterfall display. It can optionally be set to report all 
received signals to W6CQZ's website. I've used it to test the 
effectiveness of my 160M antennas, by statistically averaging MANY 
reports of my signal from stations east of the Mississippi as I switch 
from one antenna to another.

http://jt65.w6cqz.org/receptions.html

A fair number of excellent operators use JT65. I've worked KH6LC and 
NO3M on JT65 on 160M.

73, Jim K9YC