<whitequark>
so, my issue was, most stability plots in literature are done for a/q on axes
<whitequark>
which is relevant if you want to keep frequency constant
whitequark changed the topic of #tinyqma to: design of an open hardware DDS-based QMA with a low-voltage dc/rf stage :: http://irclog.whitequark.org/tinyqma
<whitequark>
but that doesn't really work for a fixed rf voltage, changing dc voltage and rf frequency
<whitequark>
so the foremost priority in the design would be to work out stability conditions for these two parameters
<whitequark>
bofh__: ^
<bofh__>
yeah, your a needs to be totally different for starters
<bofh__>
heck even at ~1nF the thought that I might be able to directly feed the output of the hackrf into this is sot of hilariously silly (max output power ~150mW)
<whitequark>
1nF at 13MHz is quite a bit
<whitequark>
that's... 65W
<whitequark>
also hackrf doesn't give you 100V
<bofh__>
yeah it also isn't close to frequency-stable
<whitequark>
so what I'd use here is a linear amp
<whitequark>
directly from DDS output
<bofh__>
exactly what I'd use as well
<whitequark>
100V... might be a stretch, but not by much
<bofh__>
I can chain the output of a nice solid-state amp into a 3CX800A7-based linamp
<bofh__>
that gets me 1kW @ ~200V if need be
<whitequark>
holy fuck that's an enormous one
<bofh__>
and that's just from crap I have lying around
<bofh__>
$7 / 1.8m is, again
<bofh__>
about an order of magnitude smaller than what I guessed it'd be
<whitequark>
actually, I'm not even sure what linear amp to use for this
<whitequark>
needs a tiny gain and large supply voltage
<whitequark>
actually, why even bother with supplying it directly with 100V
<whitequark>
1:10 trafo is trivial
<bofh__>
...good point
<whitequark>
air core
<whitequark>
power it with 12V
<bofh__>
(also if you need high voltage @ near unity gain, I believe I know the tube for that as well)
<whitequark>
nah, I want a tiny PCB that can be mounted mechanically onto the flange
<whitequark>
with the entirety of electronics
<whitequark>
100V is well within what can be just mounted on a PCB
<whitequark>
take advantage of this!
<bofh__>
yeah, 100V I'd not bother
<whitequark>
heck even 600
<bofh__>
600V...meh. 1kV is when solid-state starts to really start being a pain.
<whitequark>
have you heard of a cascode ladder?
<bofh__>
built a few :P
<bofh__>
(yes)
<bofh__>
also wow
<bofh__>
for Q/A ~ 2000 f is sub-MHz
<bofh__>
with 2mm rods
<whitequark>
yeah
<whitequark>
this is actually fairly convenient that the relationship is with 1/omega^2
<bofh__>
I am just surprised how small all these numbers are. Totally different from what I'm familiar with when I think QMA.
<whitequark>
instead of just omega^2
<whitequark>
because the resolution of DDS rises with m/z ratio
<bofh__>
yeah
<whitequark>
unlike regular QMA, where the resolution of the supply usually falls as you venture into HV
<whitequark>
this is even better of an idea than what I initially thought
<bofh__>
yeah wow
<whitequark>
the annoying part here is keeping U/V ratio, as usual
<whitequark>
the characteristics of the 1:10 air core trafo are not good
<whitequark>
or rather, they are not precisely known offhand
<whitequark>
in principle, it's possible to sample the rectified RF output
<whitequark>
which introduces loss on the diode, etc
<whitequark>
a better idea I think is just calibrate the instrument on a gas with known atomic weight
<whitequark>
we know r0 to +-75 micron, omega to sub-ppm, and U to at least 1%
<whitequark>
so we tune at, say, argon
<whitequark>
singly ionized
<whitequark>
or to hell with argon, just water, it's not like you can get rid of it even if you really want
<whitequark>
and then you raise U until you just get out of range
<whitequark>
that will be the vertex of the stability plot.
<whitequark>
yeah, this sounds better, especially since you sidestep any confounding variables
<whitequark>
such as crappiness of your ADC, losses in your diodes, parasitic inductance or capacitance
<whitequark>
a second problem is high leakage inductance of an air core transformer.
<whitequark>
so you'd need to also compensate for that to keep a constant U/V ratio among all the working frequencies
<whitequark>
0.06 Hz resolution. that's... four orders of magnitude better than what we need
<whitequark>
hrm, it's up to 8 MHz
<whitequark>
so we'll start from m/z=3. no big deal
<bofh__>
just about any of the DDS chips AD makes that go up into the MHz range *should* work. TI makes similarly absurdly overkill-specced ADCs so that's not going to be an issue.
<bofh__>
I'm slightly worried about the air-core transformer though.
<whitequark>
bofh__: how in particular?
<bofh__>
you mentioned leakage inductance, in my limited experience with them it's been very high
<bofh__>
like you can compensate as you mentioned above, but I'm just wondering if there's a better part to use here
<whitequark>
I haven't found RF transformers
<whitequark>
8MHz is ... way past what you'd use a ferrite for
<whitequark>
there are premade RF baluns
<whitequark>
but not 1:10
<whitequark>
I could use some microstrip insanity at 8MHz, probably, barely
<whitequark>
but not at 200kHz
<bofh__>
I feel like I've seen 1:9 premade RF baluns but I can't find them right now so okay yeah air-core is our best bet here