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 <[email protected]> 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)
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