OK Bob,
it clears the topic - thanks.
We have to wait for an upgrade to some newer chip....
73 !
Jiri
Ok1RI
On Tue, 6 Dec 2005, Robert McGwier wrote:
There is a lot of confusion here. Let me attempt to straighten it out.
1) The extra noise occurs (as the original note said) when SPUR REDUCTION IS
<<<OFF>>>. This is because the hardware is tuned EVERY time the dial is
changed. 2) Spur reduction ON, does NOT move the spur out of band. It does
something quite different and clever. There are "good frequencies" where
the spur generation which is caused by truncation of the phase word and by
the number of bits that can be applied to the DAC are minimized. If you are
not on the good frequencies the phase accumulator has fractional parts that
are not exact values to give the DAC in the synthesizer. This leads to
"walking on and off" an exact DAC value in the DDS. This "walking on and
off" of the good points is a periodic process and and because the amplitude
and phase are just a little off when you are not exactly on a table phase
(DAC bits are nonzero but the rest are zero), this little bit of amplitude
and phase distortion generated spurs. Spur reduction recognizes this process
and limits the HARDWARE DDS oscillator settings to these good frequencies.
The remainder of the tuning is then done in the "perfect" software oscillator
inside the code. So when spur reduction is ON, and the frequency request
changes by 3051.7578125 Hz from one of these good frequencies, we move the
hardware frequency only then. This approach has pluses and minuses but it is
felt the pusses outweigh the minuses.
You can still hear spurs from the DDS but these are due to DAC nonlinearities
and clock leakage and mixing in the AD9854. These are almost gone in the
AD9954 and really gone in the AD9958.
Analog is learning along with the rest of us.
Bob
N4HY
Jiri Sanda wrote:
OK - so if I understand it well one should run it without the reduction
unless there is a problem - a spur audible - and than switching on might
help ?
73 !
Jiri
OK1RI
On Mon, 5 Dec 2005, Ahti Aintila wrote:
Jiri,
My understanding is that the "spur. reduction" moves the spurious signal
generated by the DDS away from your listening passband. There is a good
probability that another spurious signal does not hit the same frequency.
73, Ahti OH2RZ
----- Original Message ----- From: "Jiri Sanda" <[EMAIL PROTECTED]>
Cc: <flexRadio@flex-radio.biz>
Sent: Monday, December 05, 2005 7:04 PM
Subject: Re: [Flexradio] Interesting behavior when connected to a dummy
load
I do not understand ?
If the transmitted noise get so much worse when "spur. reduction" is on
why is it there at all ? What positive it does ?
73 !
Jiri
OK1RI
On Mon, 5 Dec 2005, Eric Wachsmann - FlexRadio wrote:
Jeff nailed this one on the head. The jumping around is because when
spur reduction is turned on, the radio hardware is only tuned every
~3.051kHz. We do the fine tuning using a software oscillator. Also
worthy of note is that we use an 11kHz IF. So what you are seeing is
the junk around DC on the left side of the spectrum.
Eric Wachsmann
FlexRadio Systems
_______________________________________________
FlexRadio mailing list
FlexRadio@flex-radio.biz
http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz
--
Laziness is the number one inspiration for ingenuity. Guilty as charged!
