While tuning my Oakley and other homer VCO synth modules I originally used to use and old £45 Maplin "Precision Gold" multimeter of 1990 vintage and use the surprisingly accurate frequency counter on there to set the VCOs to the A440 international tuning standard. However I'd seen a prototype Moog module in the booklet of the Switched On Bach CD box set which Bob Moog once made as a Christmas Present for Wendy Carlos. I thought it was time for a crystal controlled reference oscillator design. The requirements were standard crystal stability (easily good enough for analogue synths), sine wave output and extreme simplicity and cheapness.
Schematic Walkthrough - to02-01
A440 Test Oscillator Schematic test_oscillator01-01.sch
The frequency source is a standard
1MHz crystal oscillator based around a 4069 inverter. The output
from this is squared up by a second inverter in the package to
drive the inverting clock input of the 74HC4040 counter and the
clock input of the first D-type latch. The counter counts
upwards, during which time the 2k2 pull-up is always held low
through one or more of the diode-OR paths. Only when the count
of 1135 is reached does the pull-up and the input to the D-type
rise at the falling clock edge. This high logic propagates to
the counter reset input on the next rising clock edge resetting
the counter and holding it low, meanwhile the count of zero has
made the data input to the D-type low. So the next rising clock
edge is required to clear the reset and allow the counter to
start again. This D-type reset arrangement allows consistant
resetting of the counter which would happen rather
asynchronously if the pull-up was fed directly into the counter
asynchonous reset. The extra cycle accounts for the N-1
selection of the counter reset value on the diodes.
This circuit is a cheap replacement for the 4059 programmable
divider counter in many applications. The 4059 is becoming rare
and only ever seems to come in a 0.6" width package for
historical reasons. A divide ratio of 1136 from 1MHz takes us to
880.28Hz and the further D-type divide by 2 brings us down to
440.14Hz. This I judge to be close enough to 440Hz, being out by
0.03%. The second D-type also crucially provides a square wave
output rather than the very rectangular reset output from the
counter. The square 440Hz signal is d.c. blocked, R-C low pass
filtered and fed via a gain control pot into a high Q gyrator
circuit. This is tuned for maximum output then the gain trimmer
adjusted to provide 0dBrms output. The result is a good looking
sinewave.
Scope Picture
This shows the 440Hz 5V square wave and the 2Vpp sine wave after filtering.
Afterthoughts
Of the four remaining 4069 inverters
two of them are used in linear mode as a condenser mic pre-amp.
There is enough gain so that the outputs eventually saturate and
the two remaining buffers drive an LED which is connected to the
440Hz square wave output. The LED only lights when the outputs
are in opposite states so represents an XOR or mixer funtion.
This way, you can have your synth note coming out on your studio
monitor speakers and being picked up on the internal microphone.
Then without having to do any plugging, the LED will flash at
the beat frequency and finally show a very slow beat when you
have got your synth nicely tuned. You could also plug the synth
output directly into this input if you wanted. It provides a
quick and easy check for tuning as you do your composition,
without you having to switch your brain into 'tuning' mode.You
need to use a low current high brightness LED, but this works
fine.
