Ring modulator

A ring modulator , also known as a ring mixer , product modulator or balance modulator , is an electronic circuit that is used as a symmetrical mixer in heterodyne receivers and for amplitude modulation . Two incoming alternating voltages u _x and u _y are multiplied together, and the voltage u _a is obtained at the output :

Ring modulator

${\displaystyle u_{\mathrm {a} }=u_{\mathrm {x} }\cdot u_{\mathrm {y} }}$

Four semiconductor or tube diodes that are as similar as possible are arranged as a diode quartet in a ring (see circuit diagram). In contrast to the rectifier bridge circuit , all diodes are oriented in the same direction of rotation.

functionality

Signals on the ring modulator when controlled with square-wave voltage

If a rectangle with a large amplitude compared to u _{x is} selected for the signal u _y , the result is a much simpler observation. The voltage of u _y determines which diodes conduct. In normal operation, the following applies here . ${\displaystyle {\hat {u}}_{\mathrm {y} }\gg {\hat {u}}_{\mathrm {x} }>{\hat {u}}_{\mathrm {a} }}$

For a transformer with center tap and a transmission ratio of 1: 1 (i.e. L ₁ = L _{2a + 2b} and L ₄ = L _{3a + 3b} ), the following applies:

condition	Result
${\displaystyle U_{\mathrm {y} }\geq {2\cdot U_{F}}}$	V 1 and V 2 conduct
${\displaystyle {-2\cdot U_{\mathrm {F} }}<U_{\mathrm {y} }<{2\cdot U_{\mathrm {F} }}}$	no diode conducts
${\displaystyle U_{\mathrm {y} }<{-2\cdot U_{\mathrm {F} }}}$	V 3 and V 4 conduct

Here, U _F , the forward voltage ( English : forward voltage ) of the diodes. Since Schottky diodes , unlike pn diodes, have a lower forward voltage, Schottky diodes are always used. ${\displaystyle U_{\mathrm {F} }\approx 0{,}3\,{\rm {V}}}$

Currents in the ring modulator

Internal structure of the ADE-1 ring modulator.

If the voltage u _y and the current i _{y have} a positive value, a current flows in equal parts via the center tap of T _1/2 via the windings L _2a and L _2b , so that the diodes V ₁ and V ₂ conduct. The current then reaches T _3/4 , where it flows through the center tap to ground. Both T _1/2 as well as T _3/4 no voltage is induced because the currents flow in opposite direction and the associated magnetic fields neutralize.

${\displaystyle i_{L\mathrm {3a} }=i_{L\mathrm {2a} }={\frac {1}{2}}\,i_{y}}$

${\displaystyle i_{L\mathrm {3b} }=i_{L\mathrm {2b} }=-{\frac {1}{2}}\,i_{y}}$

${\displaystyle i_{L\mathrm {3a} }=-i_{L\mathrm {3b} }\,}$

A current i _x is now fed in via winding L ₁ of T _1/2 . The following applies:

${\displaystyle i_{L\mathrm {3a} }=i_{L\mathrm {2a} }={\frac {1}{2}}\,i_{y}+i_{\mathrm {x} }}$

${\displaystyle i_{L\mathrm {3b} }=i_{L\mathrm {2b} }=-{\frac {1}{2}}\,i_{y}+i_{\mathrm {x} }}$

By comparing the currents, in addition to the push-pull signal, there is also a common-mode component (GL).

${\displaystyle i_{L\mathrm {3GL} }={\frac {i_{L\mathrm {3a} }+i_{L\mathrm {3b} }}{2}}=i_{\mathrm {x} }}$

Since the superposition of the currents in the two winding halves of T _3/4 results in a value not equal to zero, current i _a also flows on the output side L ₄ , and a voltage is induced.

${\displaystyle i_{\mathrm {a} }=i_{\mathrm {x} }\,}$

Functional diagram of a ring modulator with a bandpass at the output

Now the square wave voltage u _{y changes} its polarity ( i _y also), then the diodes V ₃ and V ₄ conduct . After the calculation steps analogous to the previous analysis, the result is:

${\displaystyle i_{\mathrm {a} }=-\,i_{\mathrm {x} }}$

Thus a simple form of multiplication takes place with square wave voltage:

${\displaystyle i_{\mathrm {a} }=\operatorname {sgn} {\left(u_{\mathrm {y} }\right)}\,i_{\mathrm {x} }}$

A basic distinction is made between up and down mixers for mixers. In the case of the up-converter , an IF signal s _{x is} fed to the input _and multiplied by the local oscillator signal s _y . In the case of the down mixer , an RF signal is fed to the input and multiplied by the local oscillator signal.

Ring modulator as an up-mixer

Ring modulator (without RLC - bandpass )

The circuit shown on the right generated at the secondary winding of T _3/4 a so-called double-sideband signal u _a contains, which both sidebands of the modulated carrier, but not these themselves. This suggests a separating sharp filter a by means of SSB signal produced.

Ring modulator as a down mixer

When down-converter ( English : down converter ), the input signal u _x of the frequency f _x by using the local oscillator voltage u _y with the frequency f _y on the voltage u _a with the so-called. Intermediate frequency f _a converted. The following applies here:

${\displaystyle f_{\mathrm {a} }=\left|f_{\mathrm {x} }-f_{\mathrm {y} }\right|}$

The output signal is by means of a RLC - bandpass freed at the output of unwanted frequency components resulting from the conversion.

Applications

In earlier decades, ring modulators were also occasionally used to convert a direct voltage into an alternating voltage proportional to it, which can be better amplified with a precisely defined factor because alternating current amplifiers have no zero point drift (see chopper amplifiers ). Such an arrangement was used, for example, to control the long-range rocket V2 .

The ring modulator circuit has long been obsolete for most applications, since integrated circuits have been able to produce excellent analog multipliers or mixers with low distortion and good suppression of the input signals since around 1960. Although a diode ring and transformers are no longer used here, the old designation is often retained.

Ring modulators, on the other hand, are important tools in electronic music . Quasi-ring modulators are often used in electronic musical instruments , especially in synthesizers . In contrast to the other applications, the carrier and signal are in a similar frequency range, so that negative frequencies can occur in the lower sideband . Because of the non-harmonic overtone characteristics , you can create bell-like sounds from simple signals .

In amateur radio still ring modulators as so-called high-level mixer with today LO -Pegeln of +7 dBm (1.41 V _ss ) to +23 dBm (8.91 V _ss ) used in large-signal strength receivers and transceivers. Well-known examples of high-level mixers are the types SBL-1 (H), IE-500 or TUF-1 (H).

Otherwise, ring modulators are rarely used in radio technology and have been replaced by other circuits. In particular, the Gilbert cell should be mentioned, which is a more cost-effective, yet high-quality mixer. The passive FET mixer , which achieves better characteristics, can be understood as a direct successor .

Web links

Commons : Ring modulation  - collection of images, videos and audio files

Commons : Ring modulator (effect unit)  - collection of images, videos and audio files

• Data sheet SBL-1 (PDF; 280 kB)

Individual evidence

1. ^ Script on mixer. (PDF)