Re: [TenTec] Orion and phase noise

[Ken Brown <ken.d.brown@verizon.net>]

John Rippey wrote:

> But what does this get you when the airwaves are full of solar and 
> man-made junk? Looking at past ARRL Lab reports, all but the ICOM PROs 
> show quite a lot of transmitted phase noise. So in a contest, such as 
> the upcoming 160 meter deal, there are lots of signals transmitting a 
> lot of junk. Nirvana would be if all the contestants were using 
> Orions, or ICOM Pros, but they're not. So the wonderful low phase 
> noise of the Orion's receiver sits there underneath all the signal 
> traffic that is being received and processed.

Hi John,

Phase noise in a transmitter results in RF output from the transmitter 
that occupies bandwidth outside the bandwidth that is actually being use 
for the communications. Various combinations of gain/loss factors such 
as using an amplifier, a high gain antenna, or just being close by, or 
really good propagation, can make the undesired RF output sufficiently 
strong that it raises the noise level noticeably at other peoples 
stations, making it more difficult for them to hear the signals they are 
listening for at frequencies other than what your transmitter is 
intentionally occupying.

Phase noise in a receiver's local oscillator makes signals outside the 
intended receive bandwidth also appear in the receiver IF and detector, 
raising the noise floor of the receiver. So a bunch of the signals and 
noise that are outside of the frequency your receiver is tuned to also 
add to the noise that the detector stage of your receiver converts to 
audio. The signals outside of your receiver's intended receiving 
bandwidth do not even need to be so strong that they cause distortion or 
intermodulation products in your receiver stages in order to increase 
noise due to local oscillator phase noise. The phase noise of the local 
oscillator effectively makes it not a single pure CW carrier, but rather 
a wide spread out signal. So more than the desired signals get mixed 
into the IF.

Measuring phase noise from a transmitter is pretty easy with a good 
spectrum analyzer. You just have to use the right attenuators to keep 
from generating distortion products in the front end of the spectrum 
analyzer. Still you are limited by the spectrum analyzer's dynamic 
range.  Measuring the effect of local oscilator phase noise in a 
receiver in the lab could be a lot more difficult. If you don't know the 
character of the local oscillator's phase noise (having not looked at it 
with a spectrum analyzer) then you don't know exactly how far from the 
intended receive frequency you need to generate another signal to check 
for worst case undesired response to signals outside the intended 
receive bandpass. Even if you know the character of the LO phase noise 
and put the signal on a frequency to get a worst case single signal 
measurement, the effect of LO phase noise in the real world is a 
cumulative effect of all the signals outside the intended bandwidth (and 
even inside), not just a single CW carrier from one generator. To fully 
characterize the effect of LO phase noise could take a long time and/or 
a lot of fancy equipment.

Yes it would be much better if everybody's transmitted signals and 
receiver LOs were as good as the Orion's. Still lacking that "nirvana" , 
having low phase noise in your rig reduces it's response to signals 
outside it's intended receive bandwidth, and is an advantage.

Ken N6KB


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