On 7/10/19 8:45 PM, Steve Wilson wrote:
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
On 7/9/19 6:26 AM, Steve Wilson wrote:
I built a 60-MHz amplitude/phase digitizer as part of my
interferometric confocal microscope back in my mid-twenties.ĂâĂ The
calibrator was a lot more complicated than the digitizer--it had two
sources whose relative phase could be walked in 1-degree steps via a
pulse-swallowing counter, and the aforementioned crystal ring-down
amplitude calibrator.ĂâĂ The digitizers ran at 50 kS/s, so 1-dB per
millisecond was a very convenient number for calibrating.
On re-reading the instruments paper, I discover that I
misremembered--this gizmo didn't use the ring-down amplitude
calibration scheme. I used that in another project about ten years
later, which was another laser interferometer (a handheld 3-D
scanner).
Cheers
Phil Hobbs
Yes, I was starting to get a bit confused. But what I was reading was
still very interesting.
Any chance to get a copy of the later ring-down amplitude calibration
scheme?
Something vaguely like this, but with a much longer ring-down.  IIRC my
actual one used a PIN diode rather than a pHEMT, but I just picked up a
thousand ATF38143s on eBay for $300, so I have plenty for protos.
(Also for small production runs.)
To oscillator mavens:Â There are lots of sins in this oscillator design,
but one needn't care because the oscillator isn't even in the circuit
when the ring-down is occurring. So don't give me agita.
Cheers
Phil Hobbs
Thanks for the lovely ASC file. That shows how to generate a damped sine
wave. I especially appreciate your labels on the critical nodes. I wish
more people did that.
I was hoping to find out how you sampled the ringdown signal to get 12
digits of accuracy, especially with the devices that were available at
the
time. That is a big mystery to me.
Thanks for your help.
In my thesis project, the Plessey DLVA building blocks produced a
logarithmically compressed AC waveform, which I full-wave rectified with
a Mini Circuits transformer and a couple of Schottky diodes. The
resulting DC went into an op amp and then into a 12-bit digitizer (iirc
it was an AD ADC12something, but I no longer have the datasheet).
snip
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
On 7/9/19 6:26 AM, Steve Wilson wrote:
I built a 60-MHz amplitude/phase digitizer as part of my
interferometric confocal microscope back in my mid-twenties.ĂâĂ The
calibrator was a lot more complicated than the digitizer--it had two
sources whose relative phase could be walked in 1-degree steps via a
pulse-swallowing counter, and the aforementioned crystal ring-down
amplitude calibrator.ĂâĂ The digitizers ran at 50 kS/s, so 1-dB per
millisecond was a very convenient number for calibrating.
On re-reading the instruments paper, I discover that I
misremembered--this gizmo didn't use the ring-down amplitude
calibration scheme. I used that in another project about ten years
later, which was another laser interferometer (a handheld 3-D
scanner).
Cheers
Phil Hobbs
Yes, I was starting to get a bit confused. But what I was reading was
still very interesting.
Any chance to get a copy of the later ring-down amplitude calibration
scheme?
Something vaguely like this, but with a much longer ring-down. IIRC my
actual one used a PIN diode rather than a pHEMT, but I just picked up a
thousand ATF38143s on eBay for $300, so I have plenty for protos.
(Also for small production runs.)
To oscillator mavens: There are lots of sins in this oscillator design,
but one needn't care because the oscillator isn't even in the circuit
when the ring-down is occurring. So don't give me agita.
Cheers
Phil Hobbs
Thanks for the lovely ASC file. That shows how to generate a damped sine
wave. I especially appreciate your labels on the critical nodes. I wish
more people did that.
I was hoping to find out how you sampled the ringdown signal to get 12
digits of accuracy, especially with the devices that were available at the
time. That is a big mystery to me.
Thanks for your help.
On 7/9/19 6:26 AM, Steve Wilson wrote:
I built a 60-MHz amplitude/phase digitizer as part of my
interferometric confocal microscope back in my mid-twenties.ĂÂ The
calibrator was a lot more complicated than the digitizer--it had two
sources whose relative phase could be walked in 1-degree steps via a
pulse-swallowing counter, and the aforementioned crystal ring-down
amplitude calibrator.ĂÂ The digitizers ran at 50 kS/s, so 1-dB per
millisecond was a very convenient number for calibrating.
On re-reading the instruments paper, I discover that I
misremembered--this gizmo didn't use the ring-down amplitude
calibration scheme. I used that in another project about ten years
later, which was another laser interferometer (a handheld 3-D
scanner).
Cheers
Phil Hobbs
Yes, I was starting to get a bit confused. But what I was reading was
still very interesting.
Any chance to get a copy of the later ring-down amplitude calibration
scheme?
Something vaguely like this, but with a much longer ring-down. IIRC my
actual one used a PIN diode rather than a pHEMT, but I just picked up a
thousand ATF38143s on eBay for $300, so I have plenty for protos.
(Also for small production runs.)
To oscillator mavens: There are lots of sins in this oscillator design,
but one needn't care because the oscillator isn't even in the circuit
when the ring-down is occurring. So don't give me agita.
Cheers
Phil Hobbs
Sorry, another misremembered fact. I think it was actually the MC13055.
(I had to leave my lab notebooks behind when I left IBM,
unfortunately. They probably threw them out.)
The RSSI (Received Signal Strength Indicator) output is logarithmic
within about a decibel over a pretty wide range, if you get the
inter-stage attenuation right. If you cascade too many stages the noise
starts to dominate, so the log conformance suffers.
You can see how it works from Fig 15 of the MC13055 datasheet. There
are several cascaded differential pairs, driven by current sources, and
each having an extra transistor hung off the common emitters that
generates a current proportional to the positive-going excursion of the
CE node. Those currents are summed and mirrored before going out the
RSSI pin.
While a given pair is in its linear range, the emitters hardly move at
all. When it starts to reach cutoff on the negative swings, the
transistor on the positive-going half cycle drags the emitter node along
with it, generating more RSSI output current but no more output voltage
because the current source doesn't provide any more current. When the
previous stage saturates, the RSSI contribution maxes out, so the
overall gain drops by the gain of that stage. Thus for 10-dB stage
gains, your gain drops by 10 dB for every 10 dB signal increase, so
dVout/dVin ~ 1/Vin
which is the differential equation for a logarithm. Pretty slick
actually.
Cheers
Phil Hobbs
Sorry, another misremembered fact. I think it was actually the MC13055.
(I had to leave my lab notebooks behind when I left IBM,
unfortunately. They probably threw them out.)