SineCore Modulator
About the SineCore Modulator
The SineCore Modulator is based on a design by Barrie Gilbert from 1977. It uses a series of cascaded differential amplifiers to smoothly change the polarity of the output signal as the input voltage is increased. This became the basis for the AD639 "Universal Trigonemtric Function Converter" IC from Analog Devices, Inc. The SineCore Modulator can produce +/-720 degrees of a relatively low distortion sinewave. Over the first +/-90 degrees, the THD is around -80dB, making it very useful for triangle or sawtooth to sinewave conversion. If the amplitude of the input signal is modulated, the number of reversals increases, essentially acting as a quadruple wavefolder. When a sawtooth wave is applied, it can be summed with a DC offset to produce sinewave Phase Modulation (PM) with through-zero capability.
How to use the SineCore Modulator
Pinout
The module comes in a 14P Wide-DIP (0.7" spacing) format. The pins are labeled on the board from 1 to 7, top to bottom, on 2 headers: J1 and J2. As this is confusing and non-standard (sorry about that, not sure what i was thinking), they are re-labeled below in the standard 1 to 14 format. The pins are as follows:
Pin |
Function |
||
J1 |
1 |
(1) |
Positive input - base of modulator transistor |
2 |
(2) |
Shield - connect to ground if used |
|
3 |
(3) |
Shield - connect to ground if used |
|
4 |
(4) |
Alternate bias current control - emitters of modulator transistors |
|
5 |
(5) |
Negative input - base of modulator transistor |
|
6 |
(6) |
Negative outout - collectors of modulator transistors |
|
7 |
(7) |
Positive output - collectors of modulator transistors |
|
J2 |
7 |
(8) |
width current control - emitter of reference transistor |
6 |
(9) |
Width current control - base of output transistors |
|
5 |
(10) |
Width current control - supply voltage input |
|
4 |
(11) |
Bias current control - emitters of reference and output transistors |
|
3 |
(12) |
Bias current control - base of output transistor |
|
2 |
(13) |
Bias current control - base of reference transistor |
|
1 |
(14) |
Bias current control - collector of reference transistor |
|
Bias current control
The bias current controls the amplitude of the output signal. This can either be fixed or varied. It's good to keep the bias current between 100uA to 200uA for optimal distortion performance, but it can be as high as 1mA without too much degredation. The bias current can either be set with the onboard reference and output transistor, or with an external source tied to pin4 (J1-pin4). The bias and output transistor can be used as a simple current mirror for linear control, or set up as an exponential pair for exponential control. The output transistor can also be driven directly with an opamp for linear control.
Width current control
The SineCore Modulator implements the function Vout = sin(A*Vin). The width current sets the gain factor 'A' in this equation, which is effectively the number of folds per volt of input signal. Increasing the width current, increases the input amplitude required to reach the next fold. Lower width currents give lower distortion, but also make the circuit more temperature dependent. If width currents are set too high, the output of the modulator will no longer be a sinewave, but rather a trapezoidal wave. A width current of around 100uA is best for most applications, and there really isn't a need to modulate the width current, as it has a similar effect to modulating the input signal amplitude, but introduces distortion and can not be modulated down to zero. The width current can be set by placing a voltage at the base of the width transistor, or by driving the current directly with an opamp around the width transistor. The reference resistor inside of the SineCore Modulator is 51k, so 5V or 6V across it should give ~100uA.
Input signal
The input signal should be low impedance (<100ohm) and preferably differential, although a single sided signal can also be used if one of the inputs is grounded. A high impedance, single sided source will have higher distortion than a low impedance, differential source. Bandwidth of the input signal is also critical, as any peaks which become rounded off will cause distortion.
Output Signal
The output signal is a differential current source. A differential amplifier or a current mirror and a transimpedance amplifier can be used to convert this to a single sided voltage source. The summing junctions for these amplifiers must be kept above the highest voltage at the input to keep them from saturating.
Shield pins
The shield pins connect to a shield plane on the SineCore Modulator PCB. This plane does not make any electrical connections to the components on the board, and does not need to be connected. If you do choose to use it, connect it to analog ground in your circuit, to help keep noise out of the SineCore Modulator.
Over-voltage Protection
There is the risk of damage to the module if the Vbe of the modulator transistors are reverse biased more than 6V. I have routinely driven them well past this without damage (up to 20V), but if you want to be safe, the positive and negative inputs on pins 1 and 5 (J1 pins1,5) should never differ by more than 6.6V. There are tradeoffs between various protection schemes between current consumption during the over-voltage period, steepness of the transition region, and the bandwidth of the circuit. For this application, speed is necessary as the sharpness of the triangle or saw wave determine its accuracy. Any rounding off of the peaks will cause distortion. For this reason, if you want to put protection in, i would reccomend running the opamps that drive those pins at +/-5V rails to clamp the voltage.
Temperature Dependence
Because the SineCore Modulator is based on the exponential function of the transistor, it is dependent upon temperature. This causes slight changes in distortion and amplitude with temperature. The distortion change is not very noticeable, but the amplitude change can be as much as 10%. Luckily, the human ear isn't too sensitive to amplitude, so this might not be an issue unless the output of the SineCore Modulator is being used a CV. If these variations are a problem, they can be minimized in 2 ways. First, use opamps to hold the bias and width currents stable with temperature. This will require a temperature stable reference voltage. If it is still a problem, then use a thermistor or Vbe multiplier on the input signal to increase the input amplitude linearly with absolute temperature. This second technique is the exact same one used on exponential converters in VCOs.
Example Schematics
