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"broadband noise"

Oct 18th 2012, 21:03

W3ULS

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
I'm scratching my head about the following.

There was a discussion on another reflector in which the participants alluded to "broadband noise" as being an inherent problem for some transceivers (but not theirs). For instance, in a Field Day setup, certain transceivers operating QRQ CW were reported as obliterating reception for nearby transceivers on particular bands, making it impossible to operate the other transceivers.

What is "broadband noise" and what makes a "noisy" transmitter, particularly CW >40 wpm? Is it the same as the transmit phase noise graph QST has published for years?

John, W3ULS
Oct 18th 2012, 23:22

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

"broadband noise" can be caused by a number of different factors. The composite noise plot shows amplitude and phase noise. This can be caused by a poorly designed oscillator or synthesizer. It can also be caused by excessive transmit gain. Both of these are visible on the composite noise plot.

You can also get broadband noise from switching spikes, which can be missed by the composite noise plot, which is typically made while measuring a carrier. The sharper the rise/fall time of the spikes, the more broadband the noise. High speed CW may involve rapid switching between transmit and receive--making deficiencies in the transmit/receive circuitry more obvious. Some operators do operate more efficiently when they can hear while sending, but this has always been a technical challenge, especially at higher power levels

Zack Lau W1VT
ARRL Senior Lab Engineer
Oct 19th 2012, 13:55

WB1GCM

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Make sure to use a band pass filter on each transceiver; that will eliminate most of the noise.

Bob Allison, WB1GCM
ARRL Test Engineer
Oct 19th 2012, 20:28

W3ULS

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Thanks, men. I love it when experts give expert responses. It validates my ARRL dues!
Oct 20th 2012, 04:27

W0BTU

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Quote by WB1GCM
Make sure to use a band pass filter on each transceiver; that will eliminate most of the noise.

Bob Allison, WB1GCM
ARRL Test Engineer

Maybe I'm missing something obvious, but exactly how will a bandpass filter eliminate noise?

What kind of a bandpass filter are we talking about?

73, Mike
www.w0btu.com
Oct 20th 2012, 10:07

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

We are typically talking about a filter wide enough to cover an amateur band, such as 20 meters, but narrow enough to significantly attenuate noise in adjacent contest bands, such as 15 and 40 meters.

http://www.arrl.org/files/file/Field-Day/2012/2012-FD-Rules.pdf
Current Field Day rules exclude the use of 60, 30, 17, and 12 meters.

Ideally, the filter would handle full transmit power--typically, we are limited to 100 to 200 watts.

Rejection of noise on the same band, but different operating segments, is typically obtained by careful orientation of antenna nulls or cross polarization.

Zack Lau W1VT
ARRL Senior Lab Engineer

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