Single sideband generator with leakage signal cancellation

Suppressing the L.O. leakage signal in the second converter section of a gle sideband generator by utilization of a generator bypass circuit which supplies a signal at the generator output which in turn cancels the leakage L.O. signal.

DESCRIPTION OF THE BEST MODE AND PREFERRED EMBODIMENTS 
One of the more serious design problems in microwave single sideband 
generators (S.S.B.G.) is that of satisfactorily suppressing the unwanted 
local oscillator (L.O.) leakage signal in the second converter section 
over even moderate frequency bandwidths, L.O. power variations and 
temperature extremes. FIG. 1 shows a block diagram of the basic 
arrangement. This invention disclosure solves this problem by utilizing a 
generator bypass circuit 100. The purpose of this circuit is to supply a 
L.O. signal to the generator 22 at the generator output which cancels the 
leakage L.O. signal. 
The undesired leakage signal results from a combination of phase and 
amplitude assymmetries in diode reflection properties and hybrid leakage. 
Since the lines within the generator are used to phase out the "other 
sideband signals", no phase control is available to reduce the L.O. 
leakage. The magnitude of L.O. signal used to bias the diodes must be 
extremely high relative to the IF or UHF signal for reasons of 
intermodulation products and compression performance; therefore, even 
small diode reflection assymmetries and hybrid leakage result in a L.O. 
leakage signal comparable to the desired output (i.e., .perspectiveto.-5 
dB: measured data). 
The bypass or cancellation circuitry 100 (FIG. 1) when considered as a 
"black" box, must exhibit similar insertion phase and amplitude properties 
to the generator itself. The absolute phase must be .DELTA.180.degree. 
from the generator. The more similar these properties, the more effective 
the cancellation circuit. 
This circuit can have many different forms. One configuration consists of 
two directional couplers, a length of transmission line and a phase 
adjustment circuit. The directional couplers are used to extract signal 
from the L.O. input line and to feed the bypass circuit signal to the 
generator output. The directional coupler values, the phase adjuster and 
the transmission line phase length are empirically set for optimization of 
performance over the desired frequency bandwidth, temperature and L.O. 
power ranges. 
FIGS. 2 and 3 show the inner conductor circuitry of different second 
converter sections of a single sideband generator. The bypass circuit in 
FIG. 2 consists of the parts described above, while FIG. 3 has a more 
refined circuit for improved variation. The refinement includes a 
diode-hybrid circuit plus an amplitude as well as phase trimmer. 
FIG. 2 is a schematic diagram of the first embodiment of the by-pass 
circuit. A directional coupler 10 is used to couple off a portion of the 
local oscillator (L.O.) input signal to the single sideband generator 22. 
Such coupled signal is passed via a transmission line section 12, a 
variable phase shifter 14, an attenuator 16, and a second length of 
transmission line 18 to a second directional coupler 20. The latter is 
provided to inject a properly phased and attenuated replica of the L.O. 
signal on the output line from the single sideband generator 22 where it 
is used to cancel the L.O. leakage signal from the latter. The 
transmission line sections 12 and 18 are provided to simulate the path 
length through the single sideband generator 22 thereby to obtain the 
optimum bandwidth from the by-pass circuit. The variable phase shifter 14, 
which is here a trombone stripline section, is provided to adjust the 
phase of the L.O. signal from the by-pass circuit to be 180.degree. out of 
phase with the L.O. leakage signal out of the single sideband generator 
22. The attenuator 16, which may be a stripline pill attenuator from EMC 
Technology, Inc., Philadelphia, Pa., is provided to control the magnitude 
of the L.O. signal from the by-pass circuit such that it is of equal 
magnitude to the L.O. leakage signal from the single sideband generator 
22. 
Illustrated in FIG. 3 is a schematic diagram of the alternate embodiment of 
the by-pass circuit. Here a directional coupler 30 is used to couple off a 
portion of the L.O. input signal and pass it to a so-called "Magic-Tee" 
hybrid 32. A first one of the output ports (arms) of the latter is shown 
to include a variable phase shifter 34 and a diode 38, while the remaining 
output port also contains a variable phase shifter 36 and a diode 40. The 
diodes 38, 40 are tied together to ground and here act as variable loads. 
Again, the variable phase shifters 34 and 36 may simply be trombone 
sections of stripline. It should be appreciated that by adjusting one of 
the phase shifters 34 or 36 relative to the other, one may control both 
the phase and the amplitude of the output signal from the "Magic-Tee" 
hybrid 32. The output signal from the latter is injected by a directional 
coupler 42 into the output arm of the single sideband generator 22 where, 
as before, it is used to cancel the L.O. leakage signal from that device. 
One advantage of the alternate by-pass circuit is that it offers L.O. 
leakage suppression over a wider bandwidth as a result of more similar 
tracking properties between the former and the single sideband generator 
22. 
The by-pass circuit of FIG. 3 contains a mixer with diodes 38 and 40 biased 
to the same level as the generator diodes. This feature offers L.O. 
suppression over a wider bandwidth than here-to-fore due to more similar 
tracking properties between generator and by-pass circuitry. All generator 
diodes can now be biased with at least 3 db more L.O. drive which means 
better I.M.P.'s. This is possible because diode phase changes due to 
increased L.O. occur in both by-pass and generator circuit. The 
producibility of the single sideband generator (S.S.B.G.) is improved 
since diode lot to lot changes affecting the generator circuitry leakage 
have an identical effect on the by-pass circuitry leakage, hence relative 
tracking properties between the two remain the same. The isolation phase 
and amplitude leakage signals of the mixer hybrid rings are compensated 
for by those of the by-pass hybrid. The phase trimmers can also be used to 
adjust by-pass circuit amplitude as well as phase. This fact alone most 
certainly will improve device producibility.