From: info@turneraudio.com.au   
      
   Phil mentioned the drive mech for the variable tuning cap in the wien bridge   
   oscilator he has....   
      
   > The shaft is driven by a backlash free, 60:1 worm reduction drive and a    
   > 200 point scale.    
   >    
   > Verrry silky.    
      
   ** Ambiguity fix.    
      
   The 200 point scale is engraved on a 4.4 inch dia steel disk that rotates    
   with the worm drive, making 6000 setting points per range. 30 turns x 200 =   
   6000.   
      
   Well Phil, your dial certainly seems nice by what you say, but how accurate   
   are the 6,000 points on the dial? It would seem that the dial length around   
   the 4.4 inches is roughly 13.8 inches long, 351mm, so distance between each   
   "point" is 351 / 6,000 = 0.   
   058mm and too small to be useful.   
      
   And so often, a maker produces a dial in a prototype and then the cap used for   
   production is slightly different to the prototype and dial is the same, and   
   maybe a bit wrong.    
      
   I like to see no more more or less than 3mm between easy increments in   
   frequency, and so the total number of markings around a fully used dial or   
   351mm would be 117, not 6,000, but then maybe I have a metal picture quite   
   different from what a photo of    
   your oscillator might show. Unfortunately, r.a.t does not allow images,   
   probably because if we were allowed, the porn pedlers would saturate this web   
   service with zillions of terrabites of cunts, arsoles, and megalitres of spoof   
   sprayed on ladies' faces.    
      
   So, to escape the excessive and banal sexual ways of the world at large we all   
   need our own website so that pictures of oscillators can be displayed with   
   minimal interference.   
      
   I also like a frequency dial to have equal distance between octaves of F.   
   In my last effort with a 1H-1MHz WB oscilator, the F is controlled with a 12   
   pos switch for 12 F per decade, based on these numbers, 1, 1.25, 1.5, 2.0,   
   2.5, 3.2, 3.9, 4.7, 5.6, 6.8, 8.2, 10.0.   
   Now this seems awkward, because there are hardly any whole number F stops. But   
   why do ppl insist we have convenient whole numbers? Well, its because much in   
   electronics varies according to a logarithmic rate, and once anyone adopts the   
   thinking behind    
   Standard Resistance Values, then one begins to understand you don't need to   
   depend solely of linear scales at all. Minds of our recent ancestors gave   
   values some thought.    
   Standard R values are 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 5.6, 6.8, 8.2,   
   10.0. That's 12 values and sure you could use this number sequence instead of   
   my sequence. But I wanted at least 4 nearly close octaves, 1.0, 2.0, 3.9, 8.2   
   per decade, and if    
   you read a 6dB drop between 1kHz and 2kHz, then you immediately can suspect a   
   first order LPF.   
   So I have slightly adjusted my resistance values to get there.    
      
   Having a 24 position make-before-break switch would be better! Maybe DACT make   
   something suitable, but to get R values you start with a number just over 1.0   
   and then when that is multiplied by itself 23 times consecutively, the total   
   of 24 numbers are    
   used as factors for the R value that gives the lowest F and the 24 number   
   should give 10 times the lowest R value. Then you have to carefully series or   
   parallel R to get within 1% of the R values. Its all a lot of work.    
      
   But consider a well made dial with 12 marked stops on the F I have suggested.   
   Its not difficult to place additional dial marks at say 0.1 or 0.2 or 0.5   
   increments and highlight Whole Numbers with a longer marking, and this takes   
   the guess work out of reading the dial. But with ONLY 12 F per decade range,   
   you don't need to know any    
   other number than I've suggested to plot a response, and you just have a sheet   
   of paper with all F marked along a LOGARITHMIC sacle where magically, all my   
   numbers appear about EQUALLY spaced, and much less than an octave apart so a   
   reponse graph for 1Hz    
   to 1MHz is a sequence of up to 72 dots, although some repeat at 1 & 10 so   
   actually 66 dots are used, and when the dots are joined you get a smooth   
   enough curve. You know what F you have by just reading the number on the dial.   
      
   But a variable cap does not have an exactly equal dial spacing as I have, and   
   a circular dial or sliding horizontal scale like a radio set must be   
   calibrated according to the F achieved when the unit is properly adjusted and   
   the dial will only suit that    
   tuning cap and no other. Every time you use such a thing you never repeat   
   exactly the same F, and just what F you do get strains your eyes to   
   interpolate what you have. An F meter is a godsend for those who don't trust   
   the dials or F stops with a switch.    
   My F meter is a 1Hz to 50Mhz kit I built about 15 years ago, I got fed up with   
   guesswork, especially when using a function gene with all chips inside and   
   with a crummy dial F control where markings could be 20% in error.    
      
   But today, I tested the tube stages I have created around a phase change   
   network with 3 x R and 3 gang tuning cap each 30pF to 400pF. Well, I was most   
   dissapointed. The darn thing was far more critical to adjust than anyone   
   online has described, and the    
   F stability was approaching a theremin. At all F above 50kHz, making voltage   
   readings stopped oscillations. The NFB and PFB had to be ever so carefully   
   adjusted, and a full decade F variation was impossible.   
   It was similar when I changed R from 3 x 1k5 to 3 x 150k.    
   So, the more I experiment with phase shift oscillator, the more difficulties   
   arrise to be solved, and I think that explains why the Wien Bridge oscillator   
   reigns supreme because it can be made to work so much more easily and with   
   less parts.    
   The fundemental problem with a phase shift oscillator is that the voltage   
   obtained after the 3 x RC H-pass sections is one that rises in amplitude id F   
   goes a little lower, and falls if F goes a little higher. Now this is a far   
   more difficult starting    
   premise than if you have a wien bridge because at F0, the network output   
   either side becomes lower. and at Fo, the is 0 degrees of phase shift, and at   
   all F the ratio of Vo to Vin = 1/3. Then you can have a flat response more   
   easily, and with a NFB    
   network using a j-fet or LDR as a variable R to ensure the oscillator Vo   
   remains at a constant level.   
      
   The LC oscillator for all F above 50kHz is a good option because L does not   
   vary much and nor does the F value of a tuning cap. The LC has a higher Q than   
   any RC arrangement, unless you have a twin T RC network or bridged T RC network   
      
   [continued in next message]   
      
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