Dan:
Thanks for the information.
The AD9958 seems to promise a great improvement over AD9854 which Gerald
uses in the SDR-1000. If Gerald were not trying to do an all shortwave
receiver and ham band transceiver, he could easily do the PLL spur
removal circuit but it is a bit hard to do one that tracks 50 Mhz of
reference. For dedicated ham band units, such as the NC2030 and the
Elecraft K2, the problem is solved by having no DDS or having a DDS as
the reference to a PLL circuit and then restricting the tuning range to
something reasonable. It appears clear to me that this problem of
spurious emissions from DDS is well on its way to being resolved and any
future designs for Flex Radio SDR's should take into account that almost
surely, you will want to replace the DDS section. The AD engineers are
really pushing hard. They are also urging great caution about using
the 9951/9954 in quadrature for multiple reasons. The large spurs are
at submultiples of the clock and having done the BDR measurements (which
I consider to be altogether basically unimportant in comparison to IP3,
IP2, and IMD-DR), it appears they are good as seen in the ARRL lab
results. Their results are consistent with my measurements.
However, as I said, the proof is in the eating of the pudding. I have
eval boards on the way.
Bob
Tayloe Dan-P26412 wrote:
Let me say that for the future AD9954 and AD9958 just look spectacular.
I have ordered the development boards for both of them. Especially on
the low bands, either of these would give spur performance where the
spurs are below the noise floor at all but sub multiples of the clock.
It is my understanding that the 9958 has made spectacular strides in
this "keeping the clock submultiples" off the output.
The very first thing you need to do when evaluating a new part is to read the
spec sheet. Once again, the spec sheet for the new AD9958 device can be found
at:
http://www.analog.com/UploadedFiles/Data_Sheets/383477232AD9958_prd.pdf
Read it. See what it says. Spec sheets are normally optimistic, not
pessimistic. The real world results are rarely much better than what is in the
spec sheet.
The close-in spur results specs are given on page 42. The specs for 1.1 MHz
are not significantly different than those for 15.1 MHz. The close in spurs at
1.1 MHz are 90 db down +/- 10 KHz, 88 db down +/- 50 KHz, very similar to the
numbers that are shown for 15.1 MHz and the numbers at 40.1 MHz.
These spurs would not be as much a problem if they were sparsely distributed.
Again, the information is contained in the data sheet for us to get a handle on
this. Page 12 shows the spur distribution for both 1.1 MHz (Figure 12) and
15.1 MHz (figure 15). In both cases, there are numerous spurs in the 80 to 90
db down region.
What does this mean to the receiver?
In the case of my NC2030, a very low power, high performance "hardware" SDR, the blocking dynamic range is 130 db at 10 KHz, and over 140 db at 20 KHz. On 20m, the sensitivity in a 500 Hz BW is -135 dbm. This means that a signal has to be over +5 dbm 20 KHz away to cause the receiver front end to go into compression, and -5 db when only 10 KHz away. If the LO has numerous spurs in the -80 to -90 db region and the sensitivity were -135 dbm, then signals 80 to 90 db higher than the receiver noise floor will get mixed on frequency. Thus, even though the receiver front end might be capable of rejecting signals up to +5 dbm 20 KHz away, a LO spur at 80 db down will cause a signal at only -55 dbm to appear on frequency as crud.
This is not good. The LO limitations have now caused the receiver to give up
60 db of blocking dynamic range when this happens. It might not be so bad if
we were only talking one close in spur, but from the sheets above, the spurs
are numerous. In a contest weekend, we can have many signals mixed on
frequency due to this effect, artificially raising the apparent noise floor of
the band, and potentially masking the signals we want to hear.
If you have a very clean signal generator such as an old HP8640B, you can go
looking for these spurs yourself. Set the generator to a level 100 db above
the noise floor of the signal and sweep it across a range 2 to 100 KHz away
from the receiver center frequency. You should actually see spurs pop up and
move around as you sweep the generator across this region. Try a couple of
different bands and a couple of different center frequencies and see what kind
of variation you get.
You need to understand what DDS chips were designed for in the first place to
understand their performance limitations. These chips were designed originally
for cellular telephone base stations. These base stations have a performance
requirement that is quite different than what hams need. A cellular telephone
base station has a group of mobiles that are in the range of its coverage. A
closed loop power control mechanism is used between the base station and the
mobile phone to reduce the phones power such that its signal is just sufficient
for good communications with the base station. In CDMA phones (Verizon,
Sprint, Alltel), the speech data is sent in 20msec data packets, and it is
typical to set the mobile received power at the base station such that there is
a 2% packet erasure rate.
The bottom line is that to a cellular base station receiver, ***** all mobiles
arrive at about the same power level *****. The base station does not have to
typically worry about very strong signals adjacent to weak signals. In such an
environment, spurs only 80 db down is perfectly fine. Likewise on transmit,
the base stations need spectral purity of (I think) 70 db down. This is much
more stringent than ham transmitters, but again, spurs 80 db down or more are
ok.
Thus DDSs work great for the intended application, cellular base stations.
However, ham receivers do not face signals that are all uniform in strength.
We have very weak signals right next to very strong ones. Thus ideally we
would like to have high sensitivity in order to hear the weak ones combined
with high adjacent signal rejection, thus blocking and IP3 specs.
In my case, IP3 is more important than blocking specs. A receiver will distort
20+ db before it overloads. However, I do not have any big gun stations in my
area and this is not the case for other folks. For anyone that has a big gun
station near by, blocking performance is also very important. All it takes is
one very large signal to cause a receiver a problem.
If all you want is 80 to 90 db of dynamic range under crowded band conditions,
a DDS is fine. If you look at QST receiver reviews, there are a lot of good
receivers that have this kind of IP3 performance, although it would be kind of
poor blocking performance. This level of performance would definitely limit a
great rig such as a K2, an Orion, or in my case the NC2030. If you want to see
more information on the NC2030, go to www.norcalqrp.org/nc2030.htm and read
some of the presentations there.
Any body can throw stones at a receiver design. It is a very complex subject with lots of tradeoffs. Thus, let me offer a few suggestions on how to improve the LO chain performance. First, I would suggest going to a PLL VCO design that tunes in something like 10 KHz steps. With the SDR approach, sub Hz tuning is simply not necessary since the SDR software can tune to all the frequencies in between. Since the PLL reference will be a clean crystal oscillator, the VCO should inherent some of the reference oscillators clean phase noise characteristics. It will have spurs at the reference frequency harmonics, but these will be much fewer than in DDS and can be greatly minimized by known, good PPL design. The PLL approach is common in high performance ham rigs.
Another possibility that came to mind is coming up with a set of "magic" DDS
frequencies. If the rig really only needs to have 10 KHz LO tuning steps, you could take
advantage of the fact that not all DDS tuning steps have the same spur problems. It
seems to me that there is a subset of DDS tuning setting that may have very few (if any)
close in spurs. If some how these tuning setting could be identified, you could get the
best of both worlds, great phase noise, and minimal close in spurs. It may be that
optimum settings could be determined mathematically. I am not sure this approach is
really possible, but it seems worth investigating.
It is also very neat that something as simple as a "soft rock" has the
potential of zero problems with spurs or phase noise because of the use of a crystal
oscillator.
Fun stuff!
- Dan, N7VE
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