air seperated variable capacitors in circuits

[Deathwish]

A bit of an odd one to ask, but if a circuit for say a pulse or sig gen uses a series of switchable capacitor values for duration or such could you put a variable cap on the front panel for such use.

[Psi]

i think you will find that the variable cap has to be as big as a house to get the capacity values and range you want.

[Deathwish]

Possibly so , but how about if merely used in series with the switchable caps for "fine" tuning. We all know most caps are out of value from the off so being able to fine tune with such ??

[TimFox]

If you are going to "tweak" a fixed capacitance value with a variable capacitor, you put the variable in parallel with the fixed, not in series.  A 1000 pF air variable can be huge:  typical ham transmitters used a 3 x 365 pF "receivinig" variable with the units in parallel, and that was typically 4 inches long.

[vk6zgo]

If you are going to "tweak" a fixed capacitance value with a variable capacitor, you put the variable in parallel with the fixed, not in series.  A 1000 pF air variable can be huge:  typical ham transmitters used a 3 x 365 pF "receivinig" variable with the units in parallel, and that was typically 4 inches long.

In Australia & the UK,Broadcast variables commonly had maximum capacitances of around 415-465 pf,& could be quite small.
Hams usually used older,larger variable caps to get better voltage breakdown ratings.

Variable caps were used in some older test equipment,but reset accuracy relied on the operator,so they went out of fashion,replaced by fixed capacitors with trimmers across them,which could be set once & forgotten.

In modern digital equipment,most things are basically crystal locked,& any remaining errors can be removed in software

[AlfBaz]

A bit of an odd one to ask, but if a circuit for say a pulse or sig gen uses a series of switchable capacitor values for duration or such could you put a variable cap on the front panel for such use.

Of course you can

[Deathwish]

Love it AlfBaz , my kind of party 

[TimFox]

The air variables in AlfBaz' post look like dual 365 pF capacitors.  Note that the frames included provision for mica compression trimmer capacitors on each of the two sections (empty screw hole).  These are essentially the same capacitors used in the classic -hp- Wien bridge audio oscillators.

[Deathwish]

The last time I played with one of those it was in an old radio many years ago, if memory serves me correctly I was a young child and tore it to bits wondering what it was, my dad was not very happy 

[German_EE]

Ah, happy memories. As a young child growing up in the UK I used to go to things called 'Jumble Sales' (now called flea markets) with my parents and spend my pocket money on an old tube radio or two. They were all pulled apart which is a shame because some of them would now be quite valuable.

[Deathwish]

It is funny how when young you think nothing of such items except how they work and how to get it apart, I recall hiding my radio under my blanket from dad and listening to such as charles dickens stories on radio 2 or such, something I have not done in a great many years, of trying to tune in an old crystal radio with a god awfull ear phone and other such things. 1/2 crown coins, thrupences and tanners. Oh well. Now I just ask where did it all go too in our march to modernisation in an ever increasing race to the future that outpaces man and the speed he really is designed to live at.

[seriouscoinage]

You could also use a varactor diode with a voltage reference and a potentiometer to get a variable capacitance in a much smaller space than an air variable capacitor. However, this might be difficult to set up if the capacitor you want to add this to in your circuit is floating and neither end is connected to ground.

[T3sl4co1l]

I was going to say I have an Eico 377 which does precisely that, but I see Alf beat me to it, with what looks like a more authentic HP clone!

You can also use varactor diodes, but you don't get much voltage swing before ugly things happen (distortion, poor frequency control, rectification or breakdown, etc.).

Tim