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patrick-turner patrick-turner is offline
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Default Tuning gang C value for Wien bridge oscillator

In any Wien bridge oscilator equipeed with a variable tuning capacitor, there
are limits on performance at higher H above about 200kHz unless the capacitance input at oscilator amp input ports is very low, and the stray capacitance is very low.
Consider a typical oscillator with a typical broadcast band dual gang tuning cap with each gang having 400pF maximum and 20pF minimum.
Suppose you wanted 100kHz when C = 400pF, then Wien bridge R value = 3,975r.
In theory, if C = 20pF, then F = 2MHz, but you'll never get that in practice because of the stray C and device input C and maybe you'd only get 250kHz before the oscillations stopped, because the NFB input became larger than the PFB input.
But one might add some trimmer C across both gangs so minimum C might become say 40pF, and in theory you'd get 1 MHz with R = 3,975. But then the maximum C becomes 420pF and lower Fo drops from 100kHz to 95kHz, and if you reduced R a bit then you'd get low Fo to 100kHz, but then that moves the theoretical high Fo to about 1.06MHz.
One can spend days guessing R and C values and twiddling trimmer cap screws and never get a nice decade change of F.

So, how to set up what is needed?

If you are using solid state devices, try to used j-fets at the differential inputs which have low C, say less than 10pF at each. With tubes, a 6DJ8 as a diff input isn't bad and C is fairly low, because the common cathode signal voltage is large compared to the anode Vo of opposite phase, so Miller C is small.

If you are aiming to get highest Fo = 1MHz, then you can try using a Wien bridge network with fixed R&C of say 100pF and 1,590r. If you find it oscillates OK with a planned 6Vrms output, then you may find Fo is only 800kHz.
This may be due to 20pF stray and other C in parallel with 100pF, but usually the stray C affects the Wien bridge top and bottom C values assymetrically and it just won't oscillate properly. Ideally, if you get is working fine with RC giving 1kHz network with NFB FB adjusted just right, then it should work fine when you switch up the F ranges to the very top Fo and the NFB setting should not need any re-adjustment.

Some oscillators, even the HP200A, have 4 gang caps with each gang having 500pF max C and 20pF minimum. The actual C change in the gang for each network C = 1,000pF down to 40pF.
But we really want C mimimum to be 1/10 of C maximum.
Consider the formula, Ct = ( C1 - [ 10 x C2 ] ) / 9,
Where Ct = trimmer C value + stray C + input C,
C1 = maximum tuning gang C,
C2 = minimum tuning gang C.

Consider the 4gang tuning C with C1 = 1,000pF, C2 = 40pF.
What is added Ct for decade F change?

Ct = ( 1,000 - [ 10 x 40 ] ) / 9 = 600 / 9 = 66.6pF = say 66pF.
In theory, you'd get 1,066 Pf max C down to 106pF minimum.

But the trimmer C you'd need to actually add might be a fixed 39pF plus a 4-60pF trimmer C, and hopefully, when you try for 1Mhz, you will get it, with R = 1,500r.

But the circuit may still not want to allow you get such a high F without a Vo level roll off of maybe -6dB unless you have sufficient open loop gain even with the R&C compensation network in place to stop the circuit oscillating parasitically at above 1Mhz, at maybe 5MHz, which can be typical.

in my recent experiments while trying to get a tubed RC coupled amp to work with parallel tuning gangs, I tried a fixed RC network with 220pF and with R = 390r.
F max still varied while varying the R&C HF compenstation network of the amp, and as F rises the whole caboodle becomes more interactive and problematical and the only solution seems to be to increase the minimum C so you get reliable oscillations at 1MHz without parasitics also present, or any squegging, with oscilations stopping and starting. I found 220pF would be an absolute minimum C value.
I am using two 3gang caps each 1,200pF to 39pF and this gives 2,400pF to 78pF for the change of tuning C. The range of actual C will be 2,580pF down to 258pF with added C = 180pF, including whatever stray C is there. R value should be 616 ohms for 100kHz to 1MHz.
It means also that for 1Hz to 10Hz I'd need 61.6Meg ohms, which is far too high because of biasing problems with tubes, ie, the minute grid input currents.
So I shall settle for 10Hz to 100Hz being the lowest F range with R = 6.16M with the tuning cap F control. I can do 1Hz to 10Hz range with 50k log tuning pot with added 5k6k in series and 2.86uF fixed C, approx, subject to final R adjusrtment.
The resulting 2 dials should give readings within +/- 3%, and stable operation if I fit reduction drives.
Patrick Turner.
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