On 06/08/2014 09:21 PM, Chris Johnson wrote:
Pardon my ignorance, but can I get some clarification on a few
things?
1) Is phase noise the measurement of the instability of an DDS?
Yes. To oversimplify a bit, if the frequency/phase of the oscillator is
varying back and forth at a 20 kHz rate then there will be phase noise
at a 20 kHz offset from the carrier.
If so, does this mean a cheap part is being used?
Not necessarily. Phase noise can creep in at many places in a frequency
synthesizer. You have to get the entire design exactly right to have a
clean signal that has low phase noise and low spurs at all frequency
offsets.
Does a GPSDO or a OCXO reduce phase noise?
Maybe. Normally it would only improve the phase noise within the loop
bandwidth of a PLL-type synthesizer. With a DDS-type oscillator it
would depend on whether the phase noise from the clock or the DDS itself
dominates (at any given offset).
2) How does this impact TX only, and why does it create such an
issue to nearby listeners?
If the same local oscillator is used for both the receiver and the
transmitter, then the phase noise will be the same. In both cases it
causes the oscillator spectrum to "spread out". In the transmit case,
that can cause interference on nearby frequencies. In a receiver, it
causes "reciprocal mixing" which makes it sound like nearby strong
transmitters have excessive phase noise even if they don't.
3) Does phase noise go down if you use a faster master clock? The
Flex 6700 uses a 983.04mhz vs a 122.99Mhz clock in the 6300.
Maybe. Every time you divide an oscillator's frequency you potentially
reduce the phase noise by up to 6 dB per octave (division by 2).
However, that only helps if the high-frequency oscillator has good phase
noise to begin with and the dividing process introduces no phase noise
of its own.
4) Do low phase noise radios allow in-band use, such as someone on CW
on 20M and someone up on voice on 20M?
That's the goal. Almost all the early synthesized transceivers of 30-40
years ago had horrible phase noise. Hams soon discovered that they were
useless in multi-transmitter environments like Field Day. It was a
problem both on receive and transmit. People resorted to using old
non-synthesized tube rigs like Drake and Collins.
> What is considered a low value?
It's a matter of opinion. The top receivers in Sherwood's chart are on
the order of 140-ish dBc/Hz at 10 kHz offset.
http://www.sherweng.com/table.html
Alan N1AL
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