Bruce Griffiths wrote:
Esa Heikkinen wrote:
Hi Bruce...
Try increasing C2 and C8 in the white emitter follower circuit schematic
to 100nF.
Doesn't seems to change anything:
http://www.amigazone.fi/files/gpsdo/544-15.png
http://www.amigazone.fi/files/gpsdo/544-16.png
When doing this measurement I noticed that the 3rd and higher harmonics
level are changing! First I thought that the capacitor change was some
effect on harmonics but then those peaks come back...
Time domain analysis about 3rd harmonic level gave some explanation:
http://www.amigazone.fi/files/gpsdo/544-17.png
The EF circuit was turned on at start of sweep. Something is heating and
raising the harmonic level. The bump and the end of sweep is a test
where I momentary switch off the 12V feed to the EF circuit to see how
it reacts.
Temperature sensitivity was also verified with cold spray:
http://www.amigazone.fi/files/gpsdo/544-18.png
Well, that's not an issue but makes measurements harder because even
without any changes the results can differ. However the temperature
effect on the 2nd harmonic frequency was very small.
The switcher sidebands will still be there, they are just buried in
the spectrum analyser noise floor.
Yes you're right. Just changed some settings and there they are again:
http://www.amigazone.fi/files/gpsdo/544-19.png
About 90 dB below carrier.. I would say that it's good enough! The LPRO
for example, gives higher spuriouses, but far away from fundamental.
Does the board use the recommended LC filters and regulator for the
oscillator supply as depicted in Figure 3 on the 10544A data sheet?
I haven't analyzed it fully but it seems to be just the datasheet
circuit having LM723, 10 uH coil etc. But it's layout is totally wrong
because oven switcher current runs via wrong trace. It would also been
possible to have other side as grouding copper but this was not done.
The PCB is manufactured by Cubic western data.
The required parts shouldn't be too expensive, however you may need
to wind your own inductors for the series tuned LC circuits.
Air core or powdered iron core inductors should be OK as long as you
use shields between filter sections etc.
Sounds like hard. How it's possible that lower grade ocxo's (like in
thunderbolt) output so much better spectrum? Is it all about ocxo output
driver circuit? Would it be easier to modify the 10544A itself than
trying to clean the distortion? Has anyone tried that kind of modification?
Esa
The circuit that is used to extract and buffer the signal from the
crystal oscillator can have a significant effect on the distortion.
In the case of the 10544A the cascode buffer contributes most of the
distortion.
The emitter follower contribution to distortion with a 1k load is much
smaller, however it is sensitive to slew rate distortion with
significant capacitive loading.
Using a common base stage as in the 10811A can have lower distortion, as
the emitter current is filtered by the crystal.
If one uses RF transformers between stages then one can cascade common
base stages and increase the output signal current.
An output stage with a 50 ohm output impedance is useful in that the
connection to external circuitry can then be a 50 ohm transmission line.
Some of the early OCXOs even used a crystal filter to clean up the
oscillator output.
Modifying the 10544A circuit would require a new circuit board and the
mechanical space available would restrict what you can fit in unless you
you use some surface mount components.
You would need to reuse the trimmer capacitor.
Bruce
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Esa
Attached circuit schematic for the While emitter follower optimises the
bias circuit frequency response.
The bias circuit has a bandwidth of around 40kHz so that will reduce the
effect of flicker noise should RF transistors be substituted for the 3
transistors comprising the white emitter follower.
The modifications also remove peaking in the bias circuit frequency
response.
Bruce
Bruce Griffiths wrote:
Attached circuit schematic illustrates how you could filter the harmonic
content of the 10544A below -50dBc.
Ok that's something what I can't build from shelf parts anomore... What
kind of coils should be used here? The transformer may be tricky, I
think only way is to build it but first it's needed to find right kind
of core.
--
73s!
Esa
OH4KJU
Esa Heikkinen wrote:
Bruce Griffiths wrote:
Attached circuit schematic illustrates how you could filter the harmonic
content of the 10544A below -50dBc.
Ok that's something what I can't build from shelf parts anomore... What
kind of coils should be used here? The transformer may be tricky, I
think only way is to build it but first it's needed to find right kind
of core.
Esa
The inductors need to have a self resonance frequency (SRF) above 10MHz.
Try Farnell's EPCOS range (at least they specify the SRF).
The transformer is easy to do.
I used air core inductors but powdered iron toroids would suffice.
Just place a couple of windings on one core and adjust spacing turns,
etc until you get the required mutual inductance and then add an
additional inductor to the primary and secondary to pad out to the
required primary and secondary inductances. This is easier than trying
to wind on one air cored former and adjusting the spacings to achieve
critical coupling.
For 10MHz a #6 or #7 powdered iron core is appropriate.
Just choose cores with an appropriate A: with sufficient room for the
required number of windings.
I dont know how easy it is for you to get these.
From memory my 3 air cored formers were about 15mm diameter and about
10cmm long.
Bruce