On 7/1/20 4:59 PM, Bob kb8tq wrote:
Hi
Ummmmm …. errrr …..
If the phase spur is at 1.5 Hz at 10 MHz, it still will be at 1.5 Hz at 1296.
The multiplication process
does not change the offset frequency.
What you *do* get is a change in level. If the spur is 40 db down at 10 MHz,
then it goes up by 10 log (1296 / 10).
Net result is that your 40 db down spur is now at about 20 db down.
Bob
This is phenomenon that curses us in the deep space transponder world.
We like to use a DDS to generate the transmit frequency for the
downlink, but we really, really don't want spurs close in (say, within
30 kHz) because that's where the data subcarrier is.
At low, low SNR and low, low data rate, we modulate the data at, say, 10
bps, on a subcarrier using BPSK, then we phase modulate the subcarrier
on the main carrier with a max deviation of say, 70 degrees, so there's
a fair amount of residual carrier for the DSN station to lock onto.
On Jul 1, 2020, at 7:54 PM, glenlist <glenl...@cortexrf.com.au> wrote:
RRR
Stability over about a 2 minute period, preferably within a Hz at 1296 MHz , IE
about 1e-09 is all that is required. I'll make it and figure out what I missed
:-). The unwanted sideband (and some of the original will of course leak
through depending on the DBM balance) will generate spurs on the output. And
the few Hz spurs will multiply up 100 times ot being 150Hz instead of 1.5Hz
spurs at a gig ..... I also thought of downconverting (with any XO) by
aliasing and then using a 455kc or 10.7 or 4.5 5.5 MHz etc IF filter (and then
chopping and aliasing back up) . Hmm or a crystal. al la cheap crystal lattice
filter single xtal.. But now its getting a bit more complex than a 74HC4316
pair (as 2 x DBM) and a microcontroller.... although HC4316s etc start to
leak through a bit at 30 MHz. remember the SSB difficulty was as much as the
(wideband) polyphase quadrature filter (the mass of R and C) performance, and
design of the day limiting attainable long term carrier suppression. in this
case, it is just single freq quadrature at 30 Mhz I need to generate. We will
see !
cheers
On 02/07/2020 09:40, Bob kb8tq wrote:
Hi
The same “small fraction of a degree” and “small fraction of a db” issues that
plagued the analog SSB
generation process still get into this approach. For “good” ADEV you need spurs
down below the
-130 dbc range (and likely much lower). This only gets you to 60 db or so ….
Bob
On Jul 1, 2020, at 5:38 PM, glen english LIST <glenl...@cortexrf.com.au>
<mailto:glenl...@cortexrf.com.au> wrote:
Hi Bob
I imagine in physics there are times when you want an oscillator to move a few
Hz for offset, and the oscillator is fixed due to some physical / atomic
property.
yes, the whole thing will be phase locked, so no issue with freq error. For a
fixed frequency operation , +45 and -45 deg networks for the HF will be
accurate enough. I'll actually look at generating it out of the micro that
is already on the board (as it loads the PLL and also indicates to the user
presence of 10 MHz and input level) . see what the jitter calculates to be.
Anyway, thats one way to do it I guess if you have a source that is not quite
where you want it but otherwise good.
hi hi , an SSB generator ha ha who'd thought I would be back in analog SSB
generators after my years in SDR...
g
On 2/07/2020 12:14 am, Bob kb8tq wrote:
Hi
There is a NIST paper (somewhere) that has an example of doing this. Like any
image
reject mixer approach, it only does just so well. It’s no different than
generating SSB
the same way. You get a spur that is 40 to 60 db down at the “image” frequency.
You can
tweak this or that to get it to the 60 db point, how long it will stay there ….
that depends … :)
Since you are summing a very low frequency signal with a very high frequency
one, the
accuracy of the low frequency signal does not need to be very good. A 1%
accurate
5 Hz will be off by 0.05 Hz. Your combo also will only be off by 0.05Hz. For a
lot of
Ham sort of stuff, that’s plenty good enough.
Indeed, the noise of the 5Hz and the mixer setup does get into the act. That may
rule out a simple R/C oscillator as the source of the 5 Hz “tone”…..
Bob
On Jun 30, 2020, at 9:36 PM, glen english LIST <glenl...@cortexrf.com.au>
<mailto:glenl...@cortexrf.com.au> wrote:
Hello group
I have an idea that might work, and I wanted to discuss with likeminded that
might already have experience with the problem.
Shifting a fixed oscillator a few Hz using a image reject mixer.
background : From time to time I (and others) make lock boards for ham gear,
pulling the internal VCXO (vary from 11 to 55 MHz ish - which are out by a few
Hz ) in against a 10 MHz input. Frequency accuracy is required for narrow band
modes, and low phase noise 10kHz-200kHz is required as not to desense your ham
neighbours.
I use fast LVDS diff receivers to square stuff up and ADF4157 high res fract
and about 10Hz BW. That's all fine. ....That aside, there are a bunch of radio
that have only XOs, no control facility. Varying the supply voltage as a means
of control is one way, but that doesnt work for the ovenized nor internal
regulator types.
Other people to solve this problem by applying an external oscillator that has
been disclipined - usually like crappy SiLabs spury synthesiser chips that can
produce the oddball reference frequencies like 31.28234MHz ! Actually they are
pretty good for what they are, but they are certainly not as clean close in ,
and particularly poor in spurs far away. they are what they are.
Of course the great way is a DDS , and run something like a 400-700 MHz
VCO/SAW/BAW clock. One needs to go that high to get decent oscillator Q , and
of course the DDS needs the high clock. The clock is of course pulled to the
10 MHz with something like a ADF4002 etc integer synth running high BW like 200
kHz to kill to close in VCO noise. But that's alot of stuff
*** I thought in the shower this morning of inserted a block, and shifting the
internal radio oscillator (running at say 31.28234MHz the few Hertz either side
it needs to move.)
- by using an image reject (full complex) mixer with a +/- 5 Hz oscillator
applied.
- by using a analog or digital variable delay line to remove or insert delay as
to strecth or contract the period . almost like a phase modulator, but I think
it is going to wrap and cause trouble. Hmm if I play with the high and low
period separately, I might be able to fix it when it wraps. But that technique
will likely insert noise for any soet of small easy implementation.
- alias to close to baseband using another oscillator (fixed) and then alias
back up. Aliasing technique are very cheap and useful in DSP. Hmm that might
be soemthing I do in DSP for other signal processing tricks, but not on a small
board .
The cpx mixer is the 1st thought :
Perhaps a commutating HC-CMOS switch quadrature DBM (like HC4316) with the
complex LO +/- 5 Hz coming from something I can dream up.
For a single frequency, I would be able to get the quadrature matching at least
-60 over temperature in my experience with something like this..
That would be the unwanted sideband down that far. Of course with square wave
drive, the mixer will be sensitive to the harmonic series, but the input is
squeaky clean, so that's no issue. ALTHOUGH hmm the close in noise say + /- 10
Hz would get a say as it would be aliased in, but the 10Hz noise on those XOs
is usually prety good, and for this purpose, it is the noise at offsets of
10kHz up to 300kHz that are the most important. Control bandwidth only has to
track thermal drift in the radio, have be fractions of a Hz, so the system
could spend some time calculating and generating the LO.
Anyone tried this (IE shifting the frequency of the source) ? Comments ?
Glen
(VK1XX, AI6UM)
_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com
<mailto:time-nuts@lists.febo.com>
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
<http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com>
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
