Frequency converter for varying the frequency of a local signal over a wide band

An input signal of a frequency f.sub.0 is supplied to one end of a photodiode. Optical signals of frequencies F.sub.0 and F.sub.0 +.DELTA.F from two light sources are combined by a semitransparent mirror and the resulting interference light is applied to the photodiode. The interference light is converted by the photodiode to an electrical local signal of a frequency .DELTA.F. The electrical local signal and the input signal are frequency mixed in accordance with the nonlinear characteristic of the photodiode and an intermediate-frequency signal of a frequency corresponding to the frequency difference between them is provided at an output terminal.

BACKGROUND OF THE INVENTION 
The present invention relates to a frequency converter which is suitable 
for converting high-frequency signals, such as micro-waves, into 
intermediate-frequency signals over a wide band. 
FIG. 1 shows a conventional frequency converter. An input signal from an 
input terminal 11 is supplied via a directional coupler 12 to one end of a 
mixer diode 13. The directional coupler 12 is supplied with a local signal 
of a local oscillator 15 from a terminal 14 and the local signal is 
applied to the mixer diode 13. The other end of the mixer diode 13 is 
grounded via a capacitor 16 acting as a high-frequency signal path and, at 
the same time, it is connected to an output terminal 17. 
Thus, the input signal from the terminal 11 and the local signal from the 
terminal 14 are provided via the directional coupler 12 to the mixer diode 
13, from which a component of the frequency difference between the both 
signals, obtained owing to the nonlinearity of the mixer diode 13, is 
provided as an intermediate-frequency signal to the output terminal 17. 
Since the input signal and the local signal are applied via the directional 
coupler 12 to the mixer diode 13, they are restricted by the frequency 
characteristics of the paths from the input terminal 11 to the junction of 
the mixer diode 13 and from the terminal 14 to the junction of the mixer 
diode 13 and it is difficult to operate the conventional frequency 
converter over a wide band. 
It is also difficult to construct the prior art frequency converter so that 
the local oscillator 15 is capable of varying a high-frequency local 
signal over a wide band. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a frequency 
converter which is able to operate over a wide band and capable of varying 
the frequency of a local signal over a wide band. 
According to the present invention, an input signal is supplied to a 
photodetector which has a nonlinear characteristic such as obtainable with 
a PN junction, and two optical signals of different wave-lengths are 
simultaneously applied to the photodetector, from which a frequency 
converted output is taken out. The frequencies of the two optical signals 
are selected such that the frequency difference therebetween is the 
frequency of the local signal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 2 illustrates a first embodiment of the present invention. In this 
embodiment, an input signal from the input terminal 11 is applied to one 
end of a photodiode 21 acting as a photodetector, the other end of which 
is connected to a bias power supply 23 via a high frequency blocking coil 
22, grounded via a high frequency passage capacitor 22 and connected to 
the output terminal 17. An optical signal of a frequency F.sub.0 from a 
light source 25 such as a laser diode and an optical signal of a frequency 
F.sub.0 +.DELTA.F from a light source 26 such as a laser diode are 
combined by light combining means 27 such as a semitransparent mirror, and 
the resulting interference light is applied to the photodiode 21. 
The interference light is converted by the photodiode 21 to an electrical 
signal, which indicates variations in the amplitude (or intensity) of the 
interference light and has a frequency F.sub.0 +.DELTA.F-F.sub.0 
=.DELTA.F. This electrical signal acts as a local signal with respect to 
the input signal from the input terminal 11, and owing to the 
nonlinearlity of the photodiode 21, an intermediate-frequency signal of a 
frequency .vertline.f.sub.0 -.DELTA.F.vertline. which is the difference 
between the frequency .DELTA.F of the above-mentioned electrical signal 
and the frequency f.sub.0 of the input signal is provided at the output 
terminal 17. Incidentally, the high-frequency component is grounded via 
the capacitor 24. For instance, the frequency F.sub.0 is set at 200 THz 
(.lambda.=1.5 .mu.m), the frequency .DELTA.F a maximum of 100 GHz and the 
frequency f.sub.0 a maximum of 100 GHz. 
While in the FIG. 2 embodiment the input terminal 11 and the output 
terminal 17 are connected to the both ends of the photodiode 21, the 
output terminal 17 may also be provided at the same side as the input 
terminal 11. That is, as shown in FIG. 3 wherein the parts corresponding 
to those in FIG. 2 are identified by the same reference numerals, the 
output terminal 17 is connected via a high frequency blocking coil 28 to 
the connection point of the input terminal 11 and the photodiode 21. If 
necessary, the output terminal 17 is grounded via a capacitor 29 as a high 
frequency path. 
FIG. 4 illustrates another embodiment of the present invention which is 
constructed as a balanced frequency converter. Photodiodes 21a and 21b are 
connected in series in the forward direction, one end of the series 
connection is grounded via a resistor 31 and grounded via capacitor 32 
serving as a high frequency path, and the other end is connected via a 
resistor 33 to the bias source 23 and grounded via the capacitor 24 
serving as a high frequency path. The input terminal 11 is connected to 
the connection point of the photodiodes 21a and 21b. An output transformer 
35 has its primary side connected across the series connection of the 
photodiodes 21a and 21b, and the output terminal 17 is connected to one 
end of the secondary side of the output transformer 35, the other end 
being grounded. 
A laser diode 25a as the light source 25 and a laser diode 26a as the light 
source 26 are mounted on low-temperature and high-temperature sides of a 
Peltier element 36, respectively. In the case where the laser diodes 25a 
and 26a are those of the same characteristics, for example, and their 
temperature difference is 10.degree. C., the frequency differency .DELTA.F 
between their optical outputs is about 100 GHz. The optical signals from 
the laser diodes 25a and 26a are combined by an optical coupler 37 and the 
resulting interference light is applied to each of the photodiodes 21a and 
21b. 
The interference light applied to each of the photodiodes 21a and 21b is 
converted to an electrical signal, which functions as a local signal with 
respect to the input signal in each photodiode, generating an 
intermediate-frequency signal of a frequency corresponding to the 
frequency difference between the input signal and the local signal, 
.vertline.f.sub.0 -.DELTA.F.vertline.. The intermediate-frequency signals 
thus produced are combined by the output transformer 35. In this instance, 
since the plus side and minus side of the input signal act in such a 
manner as to compensate its distortion, an intermediate-frequency signal 
with a small distortion is obtained at the output terminal 17 Furthermore, 
input signal components are cancelled each other and do not appear at the 
output terminal 17. 
By controlling the current which is applied to the Peltier element 36, it 
is possible to sweep the temperature difference between the laser diodes 
25a and 26a to sweep the frequency .DELTA.F of the local signal. 
In the embodiments described above, the photodiodes may be replaced with 
phototransistors. 
As described above, according to the present invention, since two optical 
signals are applied to a photodetector and the input signal is also 
provided to the photodetector, there is provided the same state as that in 
which the input signal and the local signals are applied directly to the 
photodetector, and no restrictions are imposed on the frequency 
characteristic of the input signal from the input end to the junction of 
the photodetector and the frequency characteristics of the local signals 
to the junction of the photodetector. Hence the frequency converter 
according to the present invention is capable of operating over a wide 
band. Moreover, the frequency of the local signal can be varied over a 
wide range, since it is the frequency corresponding to the frequency 
difference between the two optical signals. 
It will be apparent that many modifications and variations may be effected 
without departing from the scope of the novel concepts of the present 
invention.