Gio mentioned.......
"I'm soldering on in retirement...", same for me and I'm happy to say
farewell to the SMD tip and reinstall the old 1/4" nail.
I build a wien bridge osc. in the flower power era, today I find those dual
gang wire wound pots are reliable as the osc. is still working. For the
frequency pot Colvern's are used, not Compton's, but I doubt if that makes a
difference.
These pots seemed to wear out all too easily. Trying to get GOOD log pots isn't easy. I tried a 20k dual Alps Black log pot but the distribution of F along the dial was really bad, stretched out on one section and bunched up in another.
So I put in a dual gang 12 pos switch with carefully calculated and made up resistors to get F based on multiples of 1, 1.25, 1.6, 2.0, 2.5, 3.2, 3.9, 4.7, 5.6, 6.8, 8.2 10.0. The old oscill went from 1Hz to 1MHz in six ranges but there was trace bounce with switched R - partially because my switch is break before make type. Between F the amp goes beserko and then when resettling there's bounce, with a lamp globe as the NFB govener. So, I pulled out the BWD 141 amp and put in my own discrete opamp with 5 bjts and diff input pair using 2SK363 to get high input Z. Bounce remained, so I gave up on a lamp and have installed an MPF102 which has a negative gate voltage increased as V0 rises. This makes the drain to 0V resistance move between about 2 ohms and 600 ohms to the more NFB is applied if V0 rises. I've used it on another oscillator with an opamp, but that only does 2Hz to 200kHz. The MPF102 should work with this new amp to get 1Hz to 1MHz without any bounce because the R value isn't shocked into changing value and then causing LF oscillations until settlement.
I have a single F oscillator to make 1.0kHz, with tuning cap to get +/- 20Hz. I
used a single tiny globe that is 60r cold and about 200r with 2.5Vac across it. I got 8.3Vrms out with NE5534 with +/- 15.6Vdc rails and THD = 0.004%.
Getting the BDW141 sine/square converter back to work properly can't be too
difficult. THD specs for the BWD141 look allright (-60dB), so I wonder why
you gutted it if not for a defective dual gang Compton.
Pot was fuct, so I replaced is but I didn't like the result. There was no room to install radio tuning gangs which give much less bouncy trace. But vari caps are limited at LF and I want 1Hz.
Did you find the documentation for the BDW141?
http://goughlui.com/?p=1343bbbbbbb
http://www.kevinchant.com/uploads/7/..._wave_gen..pdf
Thanks for the link to BWD141. The schematic there was what I had. I have not removed the schmitt trigger sq.wave amp yet. I want the square wave to make the same 4.5 peak voltage as the sine wave gene. I want LOW output Z from the unit, under 500r max on all ranges, and the attenuator must give the same sine&sq-wave peak voltages on each setting of attenuator. I want the attenuator to give 0dB, -10dB, -20dB and - 30dB, not the -20dB at each setting.
There probably isn't room to install a bjt emitter follower buffer AFTER the attenuator and feeding the output terminal through 47 ohms, 1/8 Watt, with diode protection against stray input.
The 141 seemed to have attrocius DC offset on all outputs, quite intolerable!, so rather than just fix this peice of junk, it better to entirely re-make it. I should end up with a square wave at 1MHz which has its highest F present of about 5MHz, OK for audio where ringing due to square waves needs to be examined.
Unlike the BDW my DIY osc. has a linear dual gang pot which results in an
"unusual" frequency dial, years later a (salvaged) frequency counter module
was added, based on an ICM7216(D), so I could scrap the DIY
paper-on-aluminium f-dial.
One Kikusui Japanese oscillator I modded has a nice radio type dial with vertical pointer on a slide, and a tuning gang. When adjusted right at both ends, the readings along the dial were within 3% and easy to read, and nicely spaced out.
For general tests I use a HP3325A nowadays wich has surprising low THD for a
function generator (-65dB singular H), but for distortion testing the wish
for a (very) low THD osc. remains.
My present low THD 1kHz oscillator makes less than 0.001% at 1Vrms out.
It is the NE5534 oscillator with following active opamp + R&C high slope low pass filter stage.
An old Elektor publication describes a wien bridge osc. with 2H at -85dB and
3H at -80dB, but before I start building I'm curious what kind of figures
you obtain with your circuit(s)?
Good enough figures for me. If you want variable F AND low THD 0.001%, then it gets much more difficult and complex.
But you can make 3 sets of oscillators and following filters, say for 70Hz, 700Hz and 7kHz. The 70Hz is chose because its away from 50Hz and 60Hz mains F, and the harmonics of mains produced by PSU.
Its probably better for a home diyer to make 3 sets of gear than try to have say 2 opamps and switchable R&C values. The fewer switch contacts the better.
One could spend years fiddling around with oscillators, and my time is limited and I have many amp projects to commplete.
I did make a tunable bandpass filter for between 1.4kHz and 11kHz, with Q of 50so its easy to tune to any F in the band using a single pot. It has 3 opamps, a bit complex, but if I want to examine 2,3,4,5,6,7,8,9,10H of 1kHz, then I can easily do it, or I can search for IMD H with say 3kHz + 5kHz. One should find two IMD F, 8kHz, and 2kHz, as sum and difference F. These cannot be H products of either 3kHz or 5kHz.
Usually, if I apply all the necessary zeal required to get THD and noise low and bandwidth wide and Rout low and unconditionally stable then the amp doesn't need much testing, THD will do fine at 1kHz. IMD with standard 4:1 input sig combo, say 1V 70Hz and 0.25V 7kHz will be maybe 3 times THD value at just under clipping with rated load R, and where THD 0.3%.
The high Q filter is online someplace. The BWD schmitt trigger amp is also online if you Google and click 'images' and lots of schematics come up.
In the old Kikusui POS with tubes I fed the sine wave to a pentode amp which has a output R followed by zener diode voltage clamp. Its primitive, but works OK.
Better is my discrete opamp as a schmitt trigger and it makes a passable looking sq.wave at 470kHz. But the slightest C shunt at output soon rounds the corners of waves as higher F are shunted. For best HF from all test gear making test signals you need to have all outputs rated for 50 ohms output resistance.
Patrick Turner.
Regards,
Gio Re