Frequency converting circuit

A frequency converting circuit is provided comprising a constant current source, an oscillation circuit including an amplifier so connected to the constant current source as to operate in the unsaturated operation region owing to the output current of the constant current source, an operation circuit which receives an input signal and the output signal from the oscillation circuit and produces an output signal corresponding to the product of both the received signals and a frequency converter for converting the frequency of the output signal of the operation circuit.

The invention relates to a frequency converting circuit. 
Various frequency converting circuits used in receivers have been 
developed. One istance, as shown in FIG. 1, comprises: a doubly balanced 
type differential amplifier circuit including a differential amplifier 
stage, which is constructed by a couple of transistors TR1 and TR2, for 
receiving an input signal V.sub.g and a couple of differential amplifier 
stages having transistors TR3 to TR6; an oscillation circuit including a 
differential amplifier section constituted by transistors TR7 and TR8 and 
a tank circuit constituted by a coil L1 and a capacitor C1, and connected 
to apply an output signal to the bases of the transistors TR3 and TR6; and 
a tank circuit, which includes a coil L2 and a capacitor C2, connected 
between the collectors of the transistors TR4 and TR6 and a power source 
terminal V.sub.cc. In this frequency converting circuit, the doubly 
balanced type differential amplifier circuit produces an output signal 
corresponding to the product of the input signal V.sub.g and the output 
signal of the oscillation circuit (the output signal has a frequency which 
is the sum or difference of or between the frequencies of both the 
received signals). The output signal of the doubly balanced differential 
amplifier circuit is frequency-converted by the tank circuit comprised of 
the components L2 and C2 and then is taken out from an output terminal 
V.sub.out. 
In the oscillation circuit, the differential amplifying circuit comprised 
of the transistors TR7 and TR8 amplifies the resonant signal from the tank 
circuit having the components L1, C1 and the amplified resonant signal is 
fed back to the tank circuit for effecting the oscillation. In this case, 
these transistors TR7 and TR8 are often operated in the saturated 
operation region. Therefore, the oscillating frequency would be restricted 
to a relatively low frequency. Further, when the frequency converting 
circuit is fabricated in the integrated circuit form, a parasitic pnp 
transistor is formed so that the oscillation signal is easily leaked to 
exterior, thus adversely affecting other circuits. 
Accordingly, an object of the invention is to provide a frequency 
converting circuit with an oscillation circuit capable of producing a high 
frequency signal with a constant amplitude. 
According to one embodiment of this invention, there is provided a 
frequency converting circuit comprising a constant current source circuit, 
an oscillation circuit including an amplifier section and a resonant 
section; means for biasing the amplifier section of the oscillating 
circuit thereby to cause the amplifier section to operate in the 
unsaturated operation region; and an operation circuit receiving the 
output signal from the oscillating circuit and an input signal to be 
converted and producing a signal corresponding to the product of both the 
received signals.

A frequency converting circuit according to the invention, which is shown 
in FIG. 2, is comprised of a constant current source including a couple of 
npn transistors TR11 and TR12 which are connected at the bases to a DC 
power source terminal v.sub.B1 and an oscillation circuit including a 
series resonance circuit having a coil L11 and a capacitor C11, which is 
connected between the collectors of the transistors TR11 and TR12 and an 
amplifier section including a couple of npn transistors TR13 and TR14 
which are connected at the emitters to the collectors of the transistors 
TR11 and TR12, respectively. The base and collector of the transistor TR13 
are coupled with the collector and the base of the transistor TR14, 
respectively. The collectors of the transistors TR13 and TR14 are coupled 
in transmitting relation with the emitters of npn transistors TR15 and 
TR16 which form a first differential amplifier and the emitters of npn 
transistors TR17 and TR18 which form a second differential amplifier, 
through resistors R11 and R12, respectively. The bases of the transistors 
TR15 and TR18 are connected to one terminal of a signal source V.sub.g and 
the bases of the transistors TR16 and TR17, to the other terminal of the 
signal source V.sub.g. The collectors of the transistors TR15 and TR17 are 
connected commonly to a power source terminal V.sub.cc while the 
collectors of the transistors TR16 and TR18 commonly to an output terminal 
V.sub.out. The frequency converting circuit further includes a parallel 
resonance circuit, which is comprised of a coil L12 and C12, inserted 
between the power source terminal V.sub.cc and the output terminal 
V.sub.out. 
The explanation to follow is the operation of the frequency converting 
circuit shown in FIG. 2. 
In operation, the transistors TR13 and TR14 are alternately rendered 
conductive, that is, those transistors have alternately the minimum value 
of resistance. The oscillator circuit constituted by transistors TR13 and 
TR14, coil L11 and capacitor C11, oscillates at the frequency, 
##EQU1## 
Assume now that the transistor TR13 is rendered conductive. Then, the 
collector current of one transistor TR11 of the constant current source 
flows through the collector-emitter path of the transistor TR13 and the 
resistor R11 while the collector current of the other transistor TR12 
transiently flows through a path including the series-resonance circuit 
having the components C11 and L11, transistor TR13 and resistors R11. 
Therefore, the current flowing through the resistor R11 is approximately 
two times the current through the transistor TR11 or TR12. Then, electric 
energy stored in the coil L11 and capacitor C11 causes current to start 
flowing in the reverse direction so that the current flowing through the 
resistor R11 starts decreasing. Accordingly, the voltage drop across the 
resistor R11 becomes small while the base voltage of the transistor TR14 
rises. As a consequence, the transistor TR14 is rendered conductive and 
the transistor TR13 is rendered nonconductive. Through the conduction of 
the transistor TR14, the current flowing through the resistor R12 becomes 
double the current of the transistor TR11 or TR12. In this manner, the 
oscillation circuit continues its oscillating operation. The output of the 
oscillator is applied through the resistor R11 and R12 to the first and 
second differential amplifier circuits. The first amplifier circuit 
receives at the emitters of the transistors TR15 and TR16 the oscillation 
signal from the oscillator and at the bases of the transistors TR15 and 
TR16 an output signal from the signal source V.sub.g and produces at the 
collector of the transistors TR15 and TR16 an output signal corresponding 
to the product of both the received signals. Similarly, transistors TR17 
and TR18, constituting the second differential amplifier, receive at the 
emitters an output signal from the oscillator and at the bases an output 
signal from the signal source v.sub.g and produce at the collectors an 
output signal corresponding to the product of both the received signals. 
The output signals of those differential amplifier circuits are applied to 
the parallel tank circuit of the components L12 and C12 where they are 
converted into a signal with a given frequency, and the fixed frequency 
signal is taken out from the output terminal V.sub.out. 
It should be noted here that the current value fed from the constant 
current source and the voltage drops across the resistor R11 and R12 cause 
the transistors TR13 and TR14 to operate in the unsaturated operation 
region, that is, those values are so selected as to permit the transistors 
TR13 and TR14 to effect a current mode switching operation. For example, 
the resistance values of the resistors R11 and R12 are so selected that 
the peak-to-peak values of the collector output voltages of the 
transistors TR13 and TR14 are 150 to 300 mV. 
A frequency converting circuit shown in FIG. 3 is the same as the one in 
FIG. 2, except that a series resonance circuit including a crystal 
resonator X and a capacitor C13 is used in place of the resonance circuit 
of the components C11 and L11 and that buffer transistors TR19 and TR20 
respectively are inserted between the resistors R11 and R12 and the first 
and second differential amplifiers. The transistor TR19 is connected at 
the collectors to the emitters of the transistors TR15 and TR16 and at the 
emitters to the resistor R11. The transistor TR20 is connected at the 
collector to the emitters of the transistors TR17 and TR18 and at the 
emitter to the resistor R12. Those transistors TR19 and TR20 are connected 
at the bases commonly to a bias source terminal V.sub.B2. 
Owing to the use of the transistors TR19 and TR20, the voltage variations 
at the emitters of the transistors TR15 and TR16 and of the transistors 
TR17 and TR18 due to the variation of the output signal from the source 
V.sub.g do not affect the base bias voltages of the transistors TR13 and 
TR14, through the resistors R11 and R12. 
The use of the series-resonant circuit by the crystal resonator X and the 
capacitor C13 improves the accuracy of the oscillation frequency, compared 
with the FIG. 2 circuit. 
The feature of a frequency converting circuit in FIG. 4 resides in that a 
variable capacitor C14 is connected between the junction of the capacitor 
C11 and the coil L14 and ground. The oscillating frequency in the 
converter may be changed by adjusting the variable capacitor C14. 
The frequency converting circuits thus far mentioned are well adapted for, 
for example, the frequency converter of a super heterodyne receiver, the 
second frequency converter in a double super-heterodyne receiver, a beat 
down circuit of the pulse count type or the phase locked loop type 
receiver, and the frequency offset circuit for a CB tranceiver. 
While the invention has been described referring to some examples, the 
invention is not limited to such examples. For example, a combination of a 
crystal resonator or a ceramic resonator X and a capacitor C as shown in 
FIG. 5 may be used in place of the combination of the coil L11 and 
capacitor C11 in FIG. 2, resulting in improvement of the frequency 
accuracy of the oscillation signal. Further, in the FIG. 2 example, the 
coil L11 or capacitor C11 may be of variable type. Constant current 
elements such as resistors may also be used in place of the constant 
current source by the transistors TR11 and TR12. Switching elements such 
as MOS FETs, instead of bipolar transistors may be used for the 
differential amplifiers. The frequency converter in FIG. 3 may be modified 
such that, with omission of the transistors TR15 and TR16, the collector 
of the transistor TR19 is directly coupled with the power source terminal 
V.sub.CC. 
When the resultant emitter resistance of the transistors TR15 and TR16 and 
the transistors TR17 and TR18 may be set larger than the series resonant 
impedance of the components L11 and C11, resistors R11 and R12 are 
omissible.