"Could you elaborate on this a little if time permits? I'm more a
'digital person' but it sounds interesting. Thanks in advance, Herbert "
Yes Herbert
here is;
first divide 24MHz by two you get a very good quality absolute 50% duty
cycle 12MHz, than you feed that 12MHx into mixer [which could be a
transitional gate like HC4066 ] the other --LO input of the mixer you
need to drive with a 10MHz oscillator, the output of the mixer will be
2MHz, which you filter amplify and using to drive a freq multiplier, you
have to multiply by 5 to get 10MHz that is your 10MHz input of the
mixer's LO port,
That was a very old style but very reliable way to do and since you have
the LC filters you will not have to much jitter issue.
Also you could go a more modern way; divide the 24MHz by 6, but make it,
that you have 50% duty-cycle [you could make it with a CMOS device, by
loading during the counting a hex counter ] use one 10MHz better quality
crystal oscillator, of its output has to be divided by 5 also with 50%
duty-cycle output! than use one EX-or style phase detector and close the
loop with proper filter. Depend on the quality of your crystal
oscillator you will have a very low phase noise 10MHz source.
And you could make a few other variant by mixing the idea of the two
previous and others .
It is important to use the components at the frequency at which they
well perform, but keep the phase comparation at as high frequency as you
could [side bands are easier to filer out if they are fare away from the
carrier ], also use proper shielding and power-supply filtering
73
KJ6UHN
Alex
On 4/8/2016 3:13 PM, Bob Camp wrote:
Hi
If you start from a 24 MHz TCXO (different modules use different TCXO’s):
On an 8 MHz output, most of the time you divide by three.
On a 10 MHz output, you need to divide by 2.4. The net result is that you
divide by 2 sometimes and 3 other times.
In the 10 MHz case, there is a *lot* of energy at 12 MHz and 8 MHz, along with
the 10 MHz output.
In the 8 MHz case, most of the RF energy is at 8 MHz.
====
To correct the output by 1 ppm on the 8 MHz output, you need to either drop or
add one pulse out of every million pulses. Effectively you divide the 24 MHz by
2 or by 4 when you do that. You get a bit of 12 MHz or a bit of 6 MHz as a
result.
That can be filtered out with a RF filter. The same is true with a (somewhat
more
complex) filter on the 10 MHz output.
In addition to the “big” RF spurs, you get a low frequency component to the
output
modulation. You are “phase hitting” the output eight times a second. That gives
you
an 8 Hz sideband along with the further removed stuff. Since it’s not simple /
clean
phase modulation, there are more sidebands than just the few mentioned above.
What messes things up even more is that you never are quite doing one ppm. You
are doing
corrections like 0.12356 ppm this second and 0.120201 ppm the next second.
The pattern of pulse drop and add is not as simple as you might hope. The low
frequency part of the jitter (and it will be there) is no different than the
noise on
a 1 pps output. You still need to do very long time constant (or very narrow
band)
filtering to take it out.
Bob
On Apr 8, 2016, at 7:06 AM, Herbert Poetzl <[email protected]> wrote:
On Mon, Apr 04, 2016 at 06:07:54PM -0700, Alexander Pummer wrote:
and it is relative easy to make 10MHz from 8MHz with analog
frequency manipulation, which generates less jitter
Could you elaborate on this a little if time permits?
I'm more a 'digital person' but it sounds interesting.
Thanks in advance,
Herbert
73
On 4/4/2016 4:27 PM, Attila Kinali wrote:
On Mon, 4 Apr 2016 17:56:29 -0400
Bob Camp <[email protected]> wrote:
The variable frequency output on the uBlox (and other) GPS
receivers has come up many times in the past.
If you dig into the archives you can find quite a bit of
data on the (lack of) performance of the high(er) frequency
outputs from the various GPS modules. They all depend on
cycle add / drop at the frequency of their free running TCXO.
Regardless of the output frequency, that will put a *lot* of
jitter into the output.
That's why you should put the output frequency of the ublox modules
to an integer divisor of 24MHz. Ie 8MHz works but not 10MHz.
Attila Kinali
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