Erik,
On 7/9/22 22:06, Erik Kaashoek via time-nuts wrote:
Getting the simple PNA to lock was a bit difficult due to the overly
simplistic translation of the mixer output to the Vtune of the OCXO
To get some more flexibility I added a summing opamp that summed the mixer
output with the output of the coarse tuning potmeter. As the summing causes
inversion one extra inverting opamp was added. This made the loop gain
constant
To ensure the mixer is in quadrature another opamp was added that amplified
the mixer output into two LEDs. One LED on when below zero ouput from
mixer, the other on when above zero and both dim when zero output. This
made tuning the coarse frequency simple. Turn till the blinking stops and
both LED's light up dim. The fine frequency potmeter was no longer needed
and the frequency counter is also no longer needed to get into lock
With the summing opamp it is also possible to add an integrator but this
has not been done yet.
So, this is where you should attempt the PI loop.
In theory, you have one proportional path P and one integrating path I
that sums to form the EFC. You can imagine this as two op-amps having
inverted gain and then a summing amp to sum these two up. Thus, you have
for the P path a resistor in the negative feedback path and for the I
path a capacitor in the negative feedback path.
Such a setup is nice for testing, but a bit excessive as one progresses.
One can actually reduce this to a single op-amp with the resistor and
capacitor of the negative feedback to be in series, having a common
input resistor.
The integrator part will hold the state that ends up being the DC part
of EFC. The proportional path will provide the AC path and set the
damping factor for the PLL, you want it well damped.
This would replace your normal loop filter. You would still want a
filter to reject the sum-frequency out of the mixer.
The P gain is proportional to the PLL bandwidth time damping factor.
The I gain is proportional to the PLL bandwidth squared.
The capture range is for all practical purposes infiinte (it's wide
enough). The capture time depends to the cube on the PLL bandwidth, so
altering the PLL bandwidth between unlocked and locked conditions have
proven very useful approach to speed things up if one has a need for
larger lock-in frequencies. Rough-tuning with a trimmer can reduce it
significantly. The lock-detection is very simple detection of the
presence of beat-notes or not, that AC component dies away as it locks.
Anyway, the benefit of the PI loop filter is that you can be rather
brutal with parameters, it will lock. So, it can be worth experimenting
with it. I've found that one can ball-park things fairly quickly knowing
how to change the P and I for wished PLL bandwidth and damping. Very
experimentally friendly.
I should advice you that any PLL will provide a low-pass filter of the
reference input, and a high-pass filter on the noise inside the loop,
which includes that of the oscillator. This can help you identify likely
sources of disturbances as per their frequency in relation to the PLL
loop bandwidth.
Cheers,
Magnus
Shielding is now the biggest problem as any nearby coax connected to a
10MHz source will cause a huge amount of spurs when not at exactly the same
10MHz
Ultra low noise opamps have been ordered to hopefully reduce the internal
noise of the PNA but the reference OCXO may already be the limiting factor.
The REF voltage output of the OCXO turned out to be rather clean. Much
cleaner than a 8705 voltage regulator
Erik
_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-le...@lists.febo.com
_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-le...@lists.febo.com
