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INTRODUCTION
There are six different VCO options for the ENS-76 system. Only option 1 will be shown here. If you are interested in the other options, you can order the original Electronotes issue EN#75 that contains details on all six options.
VCO OPTION 1: This VCO is pretty much a standard VCO, with the exception that it has a Linear FM input which had been added because it is useful and easy to implement. The basic oscillator was given by Terry Mikulic in EN#62 (13). This option is intended to give the greatest accuracy and range of any of the designs. We used a somewhat simpler exponential stage here. The heart of the design is really the Analog Devices type AD818 NPN matched pair which has been optimized by the manufacturer for log conformance. A small amount of reset compensation (R*) has been added to the design, but basically, we rely on the matched pair to keep the high end up (see the full circuit description for more information). The circuit has ± 10 volt signals throughout the waveshaping circuit, and it is trivial to bring out these signals, although we have chosen here to bring out our standard ±5 volt signals. We have used a "counter-glitch" correction in the saw-to-triangle converter (see the Utility VCO in EN#67), and becasue we have a ± 10 volt triangle available, it is convenient to use the FET type triangle-to-sine converter.
GENERAL FEATURES OF ALL THE OPTIONS
All the VCO's have the standard exponential response of 1 volt/octave with control voltages fed in through 100k. The output levels are ±5 volts, although it is generally possible to convert to a ± 10 volt system with little trouble. For the most part, signals are handled with the type 556 op-amp. In 5 volt systems, it is possible to use the 307 type op-amp if optimum high frequency preformance is not required.
In order to simplify the schematic diagrams, we have not shown specific input
stages but instead will be using as an input for control voltages the structure
shown in Fig. 1.
This could of course be used as is, but we have in mind that in nearly all
cases the builder will use something with more features. In particular, COARSE
and FINE frequency controls are common, and variable input may also be desired.
There should be at least one input that is the direct-in 100k 1% type. A
possible input stage is shown in Fig. 2.
In practice, thermal contact is made by mounting the resistor and the transistor pair or array so that they are touching, and then putting a small amount of heat-sink compound between them. For set-up purposes and testing, an ordinary 2k 5% resistor can be used. The T.C. resistor and heat sink compound can be put in during final packaging and calibration.
The resistors Ra, Rb (a trim pot, probably a multi-turn unit), and Rc are selected so that a one volt change at the input, through 100k, is scaled to an 18mV change at the wiper of the trim pot. At the same time, it is best to maintain a low source impedance (say 1k or lower) so that the base of the converting transistor is held down well. Since the output of the op-amp will never exceed a voltage of much more than a few hundred mV, there is no problem with the op-amp driving such a low impedance. In the circuit diagrams, we will be showing the voltage divider shown in Fig. 4a (Ra = 0, Rb = 100, Rc = 390) which permits the voltage to be scaled from any values between 20mV and 16mV. This works quite well if a multi-turn pot is used, and gives a lot of excess room on the ends. A somewhat tighter divider is shown in Fig. 4b, where Ra = 47, Rb = 100, and Rc = 820. This gives voltages scaled from 17 mV to 19mV. If you are using the precision resistors indicated (the 100k 1% or better), this should work out well. If however you are just setting up with 5% resistors, you may need the extra room given by the circuit of Fig. 4a.
In any case, you will be adjusting the trim pot so that a 1 volt change at the input gives a one octave change of frequency. This should be set first at the lower frequencies (around 100 Hz) and then checked at higher frequencies, and compensated if necessary. Tuning is often best done by ear. As an aid, it is useful to have a precision one volt source which is switched in, and at the same time, a flip-flop is switched into the return line from the VCO. Thus, the pitch of the VCO should go up one octave, but the pitch returning should remain the same, because of the additional flip-flop. The original flip-flop is needed so that only square waves are compared. See Fig. 5 for this setup.
Most of the other adjustments and calibrations that you will need to make are described in the circuit descriptions of the various options. Keep in mind that there are six different circuits, and we built each one only once. If you copy ours exactly, it should work, but you may have to do a little trimmimg to get everything exactly the way you want it. We were not overly careful to adjust the amplitudes to exactly the five volt levels, and made no trim adjustment provisions for amplitude except in one case. You should trim these up if you find it necessary. You can use a scope to do this if you have an accurate one. However, even if you don't have an accurate scope, you can use an accurate meter. You just have to set the VCO frequency to 1/10 Hz or below and read the peak of the meter. For making adjustments of waveshapers, it is best to use a scope, even if it is a relatively poor one. Note however, that when trimming up the sine wave, the best instrument you have is your ear.