#robotics Logs

Jul 03 2018

12:46 AM mrdata: what range
12:50 AM mrdata: 100pF is typical of variable caps these days
12:50 AM mrdata: tho old radios have larger ones
01:26 AM rue_: well I need an oscillator to do a 10x freq range
01:27 AM rue_: pref from 1Mhz to 10Mhz
01:27 AM rue_: so its capacitance would need a 10x difference
01:27 AM rue_: say 1pf to 10pf
01:27 AM rue_: I think
01:28 AM rue_bed: that said, that dos't make sense
01:29 AM rue_bed: the old radio one I had went from less than 1 to 160pf and I was getting 8 to 16Mhz
01:29 AM rue_bed: maybe too much stray circuit capacitance?
01:30 AM mrdata: your 100pF variable cap should do the 10x
01:30 AM mrdata: from 10pF to 100pF
01:30 AM mrdata: wait
01:30 AM rue_bed: so whats going wrong
01:30 AM rue_bed: pretty sure the formula is L*C somewhere
01:30 AM mrdata: square root of capacitance must be 10x
01:30 AM rue_bed: so a 10x in cap should be a 10x in freq
01:30 AM rue_bed: er... oops
01:30 AM mrdata: you want 100x cap
01:31 AM rue_bed: :/
01:31 AM mrdata: 1pF to 100pF is tough to accomplish
01:31 AM mrdata: because 1pF is in the parasitic range
01:31 AM rue_bed: I think the one I had was <1 to 160
01:31 AM mrdata: even 10pF is
01:31 AM rue_bed: hmm
01:31 AM rue_bed: thats prolly what happened
01:31 AM mrdata: so you want like 30pF to 3000pF then
01:31 AM mrdata: maybe
01:31 AM rue_bed: huh
01:32 AM mrdata: thats going to be big
01:32 AM rue_bed: cnc a lot of aluminim plates
01:32 AM mrdata: mhm
01:32 AM mrdata: what voltage
01:32 AM rue_bed: 5
01:32 AM mrdata: so they can be quite close together; good
01:33 AM mrdata: they can even be close enough that they would touch, but-for them having an insulative coating, say
01:34 AM rue_bed: small radio ones use plastic sheet
01:34 AM mrdata: ok
01:34 AM mrdata: i wondeer if digikey has something
01:34 AM rue_bed: or I find an RF osc I can drive up to 10Mhz
01:34 AM rue_bed: er, RC
01:35 AM rue_bed: 10Mhz is crazy, but not impossable
01:36 AM mrdata: some old radios used variable inductors
01:36 AM rue_bed: hmm, that would be easier to make
01:36 AM rue_bed: ferrite slug
01:37 AM mrdata: thats one way
01:41 AM mrdata: the biggest range for a variable cap on digikey is 24x, a 25 to 600 pF cap
01:42 AM mrdata: which would give frequency range of 4.899
01:43 AM mrdata: and theyre expensive, $25 to 50
01:43 AM rue_bed: hmm
01:43 AM mrdata: https://www.digikey.ca/product-detail/en/sprague-goodman/GZN60100/SG9058-ND/5278718
01:43 AM mrdata: ish
01:43 AM rue_bed: I dont think any of my browsers right now can open digikey
01:44 AM mrdata: check later
01:44 AM rue_bed: china is smart, you only need the securrity to buy, not browse
01:44 AM mrdata: it's got a 14 week lead time anyway
01:44 AM rue_bed: no, I'll go the variableinductor route
01:44 AM rue_bed: I'm sure some of the power ferrite I have will cause a 100:1
01:45 AM mrdata: seems easier
01:45 AM rue_bed: I didn't think of it, thanks
01:45 AM rue_bed: so colpitts it is then
01:46 AM rue_bed: I really wish I had my really old radio book
01:47 AM rue_bed: they went thru how to make most of the components
01:48 AM rue_bed: I was also thinking of an insane series-parallel array of tuning diodes
01:48 AM mrdata: the ARRL handbook?
01:48 AM mrdata: i bet there are online copies
01:48 AM rue_bed: yea, have that, but they dont show as much about how to make your components
01:50 AM mrdata: coils for this stuff are always DIY
01:50 AM mrdata: there are calculators
01:50 AM * mrdata looks up some links
01:51 AM mrdata: here's one for single-layer coils, http://hamwaves.com/antennas/inductance.html
01:51 AM mrdata: and you can push a ferrite core in/out with a screw methinks
01:52 AM mrdata: here's a calculator for coils that have a core, http://www.dicks-website.eu/coilcalculator/index.html
01:52 AM mrdata: both these come recommended
05:14 AM z64556 is now known as z64555
08:52 AM Zeratul is now known as horohoro
09:10 AM rue_: or I can use a linear thing to move a core, I have a feeling it'll have to move a lot
09:11 AM rue_: hmm interesting point, it u is more than 100 I should be set
03:02 PM KimK is now known as kibitzer
03:04 PM kibitzer is now known as KimK