A tuning and calibration unit for synthesiser oscillators
CALTUNE has been specifically designed for calibration and tuning of modern synthesisers. It contains a crystal controlled oscillator to generate switched output frequencies of 220, 440, 4400 and 8800 Hz. The lower two outputs are used for scaling the lower frequency end of voltage controlled oscillators as well as providing the normal tuning frequencies. The upper frequencies are for scaling oscillators which have a high frequency trim.
The unit is self contained, having a built-in amplifier and speaker and it may be operated from batteries. Alternatively, it may be run from an external +5V supply or with the addition of a few components from any positive supply within a range of about +7 to above +15 volts.
The output from the oscillator being tuned or calibrated is plugged into one side of a ring modulator contained in the instrument and compared against the internal standard frequencies. If there is a large difference between the two frequencies the sound produced is a complex mixture of harmonics but as frequency matching approaches the overall sound is quieter and there is a very pronounced amplitude modulation which disappears completely at perfect matching. This technique avoids the difficulties experienced by many constructors in using the 'beat frequency' method. Tuning of a synthesiser can be achieved in seconds without any additional equipment.
The calibration method is as accurate as using a digital frequency meter or oscilloscope and has an advantage over the latter in so far as final tuning may also be accomplished.
The block diagram for the calibrator/tuner is shown in Figure 1 and the complete circuit in Figure 2.
An 11MHz crystal is used for the oscillator built around IC2. The output of which is divided down by 74LS90 decade counters (IC's 3 to 7) which may be connected to divide by two or five, or by two and five to make a divide by ten. The required divisions to obtain outputs of 220, 440, 4400 and 8800Hz are shown in Figure 1. These four outputs are routed to individual bandpass filters in order that the residual signal is principally the fundamental frequency which will approximate to a sinewave. The filters are constructed around IC's 9 and 10. They have a nominal Q of 10 and, to allow for component variations, a trimmer is provided which is subsequently adjusted for peak amplitude which corresponds to the best waveform shape.
Any of the four outputs may now be selected by the rotary switch, S2, while switch S3 is used to channel the selected signal to either one input of a ring modulator or direct to the internal amplifier. Whether the instrument is being used for calibrating or tuning the ring modulator route will be selected. If, however, one is calibrating an oscillator from scratch it is best to set its initial frequency in the region of the required calibrating frequency of, say, 440Hz. This can be done aurally by comparing the output of the 440Hz waveform from the built-in amplifier with the output of the VCO (or synthesiser) connected to another amplifier.
The ring modulator, IC10, is an SG3402 amplifier/modulator. While it is not the lowest cost device it can be operated from a single supply and requires few external components and little setting up. From the filters the signals enter the SG3402 via the attenuator R23/R24. The external signal, which comes from the oscillator or synthesiser being calibrated or tuned, enters via jack socket, J1, into the attenuator R25/R26. The resistors have been selected for a 10V p-p signal to give approximately 60mV at pin 12 of IC 10. While the external input signal can be somewhat higher or lower than ten volts a much lower signal will reduce the aural modulation effects. The value of R25 and/or R26 should therefore be altered to suit the signal level of specific instruments. Thus for a 2.5V p-p signal R26 should be reduced to 47R while for a 1V p-p input R25 would be reduced to 12k and R26 to 47R. RV5 is used to minimise the feedthrough of the internal frequency standards. The output of the ring modulator is amplified by IC11 and it is connected by a jack socket, J2, to the driver amplifier, IC12, with volume control RV6 and speaker SP1. J2 allows the use of an external amplifier and in fact the circuit may be terminated at this point if an amplifier is readily available, for example, within the synthesiser so as not to detract from the portability of CALTUNE.
All IC's are operated from a nominal +5V supply and in order to cope with AC signals through the op. amps. a level shifting voltage is applied to their non-inverting inputs, for example, R7/R8 provides the bias voltage for IC8. If an external +5V supply, or a supply consisting of batteries at about +6V, is used for the supply then R1 and IC1 are not installed and a wire link (shown by a dotted line in Figure 2) bypasses this part of the circuit. The PCB allows for the installation of these components together with a 19°C/W heatsink. R1 is used to reduce the heat generated in IC1 and thus if the instrument is connected to an external power supply the following resistors are recommended:
|+15V supply||47R 2W|
|+12V supply||27R 1W|
|+9V supply||10R 1/2W|
The PCB supplied with the kit is printed with a component overlay and so the overlay is not shown in this article. The PCB is preferred since lay-out of the components is quite critical. On the other hand wiring is not critical and so long as it is done neatly normal connecting wire may be used throughout.
The instrument is shown mounted in a 190 x 138 x 68mm Vero battery case which accepts four 1.5V AA cells. The PCB screws onto lugs in the lid of the case while the speaker is bolted to the base. Five 6.5mm holes are drilled into the base underneath the speaker and the case is mounted on rubber feet so that the speaker will be fully effective.
After wiring up the adjustments required are simple and uncritical. First set S3 to the amplifier position and S2 to 220Hz and adjust RV4 for the purest sound output. The current drain by the amplifier does cause some distortion — it is like many small transistor radios; when the volume control is turned up loudness only increases at the expense of distortion at higher settings. Thus for this step it is preferable to keep the level control, RV6, at a low setting or even use an external amplifier. As stated earlier, however, it is not critical. Repeat the above step for frequencies 440Hz, 4400Hz and 8800Hz and adjust RV1, 3 and 2 respectively.
Lastly, set S3 to (ring modulator) and S2 to 440Hz then adjust RV5 for minimum output from the speaker.
The first step is to ensure that the external input to the ring modulator suits the signal levels of the VCO/synthesiser, as discussed in the circuit description. The normal connection to a synthesiser will be its output provided for an external amplifier. If, on the other hand, there is a direct connection to the VCO then this would be the preferred output since it will not require any alteration to synthesiser settings. The handbook, or construction notes, for the synthesiser (or VCO) will provide information on access and settings for tuning and calibrating and these should be followed.
A. Tuning. Simply plug the output of the synthesiser/VCO into the external signal input, J1, of CALTUNE, set S2 to 440Hz and S3 to R.M., and press A=440Hz on the keyboard. Keep the key held with a high sustain level on the envelope generator or else put the VCA into the 'open' position if one is provided. The writer prefers to use a triangle output from the VCO but again it is not critical. If the synthesiser is perfectly in tune at 440Hz then the sound generated will be predominantly a 880Hz signal. If, however, it is slightly out of tune a distinct pulsing effect will be heard, mostly due to amplitude modulation, and the further it is out of tune the faster the effect and the more raucous the sound. Turn the 'fine' tune control for the VCO until no pulsing is obtained or at least until the effect is only occurring at intervals greater than one second. The actual length of time achieveable between these modulating beats will depend on the stability of the synthesiser/VCO. Now select 220Hz with S2 and press note A=220Hz. What should happen now is that the pulsing should only occur at a similar rate to that achieved at 440Hz. If there is a more rapid pulsation when switching to 220Hz then the VCO requires calibrating.
B. Calibration. The procedures for calibration have been discussed in Nov. '82 and Jan '83 issues of E&MM and instructions are also provided with the kit. For a re-calibration the VCO is going to be near to the correct frequency and thus the procedure is similar to tuning except that the 'scale adjust' pre-set for the VCO is adjusted until the rate of pulsating is virtually the same when switching back and forth from A=220Hz and 440Hz and pressing the appropriate keys. Similar considerations apply when calibrating the VCO using an external voltage instead of a keyboard.
For the situation in which a VCO is being calibrated for the first time using an external voltage it will be desirable to set the VCO near A=440Hz as the starting point. For this situation the output of the VCO is taken to an amplifier and compared by ear with the 440Hz output from CALTUNE. Matching is by no means critical but the VCO should be closer to 440Hz rather than 220Hz or 880Hz otherwise the ring modulator may latch onto one of the latter frequencies. Such a mistake is unlikely to occur to those with any musical ability and even if it does occur it is not a disaster but simply that the VCO may end up being calibrated at a lower or higher range than is usual.
As far as the 4400Hz and 8800Hz outputs are concerned they are mainly provided for bench adjustment of the high frequency trim of VCO's fitted with this facility. If, however, the synthesiser has a one foot range then its high frequency accuracy may be checked by pressing C=4434.9Hz and adjusting the fine tune until a match is achieved with the instruments 4400Hz.
Then a note is pressed an octave higher and compared with 8800Hz. Remember, however, that factors affecting VCO stability (power supply; droop from keyboard sample and hold; very small errors in keyboard accuracy; and so on) all become much more obvious at these higher frequencies and so it is preferable to make high frequency trim adjustments in a bench calibrating environment.
There is a distinct difference when CALTUNE is used at higher frequencies. As the external input becomes matched to the internal reference the output will tend to go significantly quieter since the predominant frequency is double the input and as frequency increases the 'loudness' perception of the ear decreases. Nevertheless, there are sufficient harmonics to provide a very audible modulation. Obviously the worst case is at 8800Hz since the predominant frequency of 17.6kHz is too high for normal hearing. If any difficulty is experienced with hearing the amplitude modulation of the harmonics then there are three courses of action available:
(a) Use a square wave input from the synthesiser/VCO which will result in sharper changes in modulation.
(b) Turn RV5 very slightly clockwise which will let through more of the 8800Hz reference signal and aid hearing the amplitude variations. The small adjustment will not lessen the effect at other frequencies.
(c) Leave the frequency switch at 4400Hz even though the synthesiser/VCO is increased to 8800Hz. CALTUNE will lock onto a harmonic and the effect of matching is still very clear except that one does not get the significant quietening effect referred to earlier.
The technique utilised in this project is accurate and fast and even though it relies on aural matching this does not require any special skill or ability from the user.
The PCB and kit of components (less case, knobs and batteries) are available from E&MM, (Contact Details). The price is £34.95 including postage and VAT. For those wishing to use an existing amplifier/speaker the relevant components may be omitted (please specify this option) and the inclusive price becomes £29.95.