JPH01152374A - Mixer circuit - Google Patents

Abstract

PURPOSE:To allow the lower limit of the range of frequency to be measured to approach a CD as near as possible, by inputting a local oscillation input signal to adjust the phase and level thereof and adding the output thereof to the output signal of a mixer to negate the leak signal at the output terminal of the mixer. CONSTITUTION:The voltage applied to a variable capacity diode D1 is adjusted so that the phase thereof becomes reverses to that of the leakage signal of LO terminal input appearing at an RF terminal. Further, the level of said signal is adjusted on the basis of the resistance value of a PIN diode D2. Generally, since the leakage signal (LO leakage signal) from the LO terminal of a mixer 2 to the RF terminal is attenuated by several tens of dB as compared with an LO input level, the same attenuation is also required in level adjustment due to the PIN diode D2. The voltage applied to the variable capacity diode D1 and the PIN diode D2 is given from a processor through inductances L1, L2, L3. By the signal adjusted as mentioned above, the LO leakage signal can be negated before inputted to a BPF amplifier 4.

Description

[Detailed description of the invention] [Industrial application field] - The present invention includes:
This paper relates to improving the lower limit of the measurement frequency range in superheterodyne spectrum analyzers and network analyzers.

[Prior Art] FIG. 3 is a block diagram showing an example of a conventional superheterodyne spectrum analyzer. In FIG. 3, 1 is a low-pass filter that has the characteristic of passing a signal S in the frequency range O to f0 measured by the spectrum analyzer and cutting off frequency components higher than f0.
When an input signal of frequency ft (usually f includes many frequency components) is added, an output signal of frequency fL is output. 2 is a mixer, the output signal of the low-pass filter 1 is applied to one input terminal IP, and the output signal of the voltage controlled oscillator H (hereinafter referred to as VCO) 3 is applied as a local oscillation input to the other input terminal LO. 1 Compared to the usage of a normal mixer, the RF terminal and the This connection is used because it is desired to handle the output frequency fL up to DC. A sawtooth wave as shown in FIG. 4 is applied from the sweep oscillator 4 to the VCO 3.
It outputs a frequency fv that changes depending on this applied voltage.

This sawtooth wave is also applied to the horizontal axis of the CRTIO and used for frequency sweeping.

The mixer 2 sends an output signal having a frequency f that is the difference between these frequencies fv and fL to the next stage bandpass filter (hereinafter referred to as BP).
F) is added to the BPF amplifier 4. This BP
The F amplifier 4 selects and amplifies a frequency band centered around a certain frequency f, and applies the amplified frequency band to one input terminal of the mixer 5. The output signal of the oscillator 6 is applied to the other input terminal of the mixer 5, and the output signal of the mixer 5 is applied to the BPF amplifier 7. As a result, a double superheterodyne system is constructed which can increase frequency selectivity and gain gain. The amplitude of the output of the BPF amplifier 7 is detected by a detector 8, noise components are removed by a video filter 9, and the output is added to the vertical axis of the CRTIO.

In such a configuration, the frequency ft-1 applied to the BPF amplifier 4 is the frequency fL, which is obtained by removing unnecessary components (components not to be measured) of the input frequency ft, shifted by the frequency fv. Here, if the measurement range of the spectrum analyzer is 0 to fo, then the variable frequency range fv of the VCO 3 is set to fv=f, ~(fz+fo>,
The output frequency fL of the low-pass filter 1 is fL=o~fo
, the center frequency of the BPF amplifier 4 is set to fl. Therefore, if the frequency of the VCO 3 at a certain moment is, for example, fvl, the frequency fM is a mixture of this frequency fVt and the frequency fL that includes frequency components from 0 to fo, so the frequency fM is the frequency component from (fvl fo) to fV. It will include. Among the frequencies fM having such a band, only the frequency value corresponding to the center frequency f1 of the bandpass filter in the BPF amplifier 4 is selected and sent to the next stage, and the frequency that can pass through the BPP amplifier 4 is the VCO 3.
is shifted by sweeping the output frequency fv of .

With this configuration, the frequency spectrum waveform is displayed on the CRTiO.

[Problems to be Solved by the Invention] In the spectrum analyzer shown in FIG. 3, the isolation from the LO terminal to the RF terminal of the mixer 2 is, for example, about 30 to 40 dB. Therefore, the LO terminal input causes a slight leakage to the RF terminal (or IP terminal).

As a result, the leakage component v of the VCO 3 is present at the RF output terminal of the mixer 2 in addition to the fL±fv component due to the normal mixing operation. As shown in Figure 5, V
When CO3 is subtracting the frequency around fv~f, the leakage component v of VCO3 is the next stage BPF#! ! Width board 4
, and as the frequency of the VCO 3 moves away from f%, the leakage component v is attenuated by the BPF amplifier 4.

If the measured signal level of the spectrum analyzer is considerably smaller than the leakage component level of the VCO 3 within the band f, ±BW of the BPF amplifier 4, when the BPF amplifier 4 attempts to amplify the measured signal, the amplifier will fail due to the large leakage component. saturates, making it impossible to amplify the signal under test.

Therefore, the signal cannot be measured within the bandwidth Bw of the BPF amplifier 4. In other words, when VCO3 starts frequency sweep from f, the signal cannot be measured until ft + BW.If we consider the input signal, it is from DC to Bw[H
The signal up to zl becomes unmeasurable. Generally, the center frequency of the BPF amplifier 4 is several 100 M) (z), and it is not easy to narrow the bandwidth Bw. As a result, the lower limit of the measurement frequency range of the spectrum analyzer is BW
[It becomes Hzl.

The present invention has been made to solve the above problems, and an object of the present invention is to realize a mixer circuit in which the lower limit of the measurement frequency range of a spectrum analyzer is brought as close to DC as possible.

[Means for Solving the Problems] The present invention relates to a mixer circuit that mixes an input signal to be measured and a local oscillation input signal, and its feature is that the phase and level of the local oscillation input signal can be adjusted manually. The present invention is characterized in that it includes an adjustment circuit that adjusts and adds its output to the output signal of the mixer, and is configured so that the leakage signal of the local oscillation human signal contained in the mixer output is canceled by the adjustment circuit.

[Example] Hereinafter, the present invention will be explained in detail using the drawings.

FIG. 1 is a block diagram showing an embodiment of a mixer circuit according to the present invention, and the same parts as in FIG. 3 are denoted by the same reference numerals. A 50Ω attenuator is connected at one end to the three terminals of the mixer 2 to reduce reflections, and the attenuator 14 is connected to the other end of the attenuator 12 to input the VCO3 signal as a local oscillation input and output a signal to cancel the leakage signal. This is an adjustment circuit that adds to the other end of 13. IP, LO, and RF in parentheses represent the terminal names of the mixer circuit. In tM*@Route 14,
C1 is a capacitor whose one end is connected to the LO terminal and cuts the DC component, R1 is a phase adjustment resistor whose one end is connected to the other end of this capacitor C1, and D is a capacitor whose cathode terminal is connected to the other end of this resistor R1. A phase adjustment variable capacitance diode L, which is connected and whose capacitance value changes depending on the applied voltage, is an inductance for high frequency signal blocking, one end of which is connected to the anode terminal of the variable capacitance diode D, and the other end is connected to the processor. C2 is a DC cut capacitor whose one end is connected to the anode terminal of the variable capacitance diode D and the other end is connected to the common, and R2 is the capacitor for DC cutting whose one end is connected to the cathode terminal of the variable capacitance diode D and the other end is connected to the common. Connected inductance for blocking high frequency signals, D2
is a PIN whose cathode terminal is connected to the cathode terminal of the variable capacitance diode D1, and whose resistance value changes depending on the applied voltage.
A diode, Lm, is an inductance for blocking high frequency signals, one end of which is connected to the anode terminal of this PIN diode D2, and the other end is connected to the processor, and C3 is an inductance, one end of which is connected to the anode terminal of the PIN diode D2, and the other end of which is connected to R.
This is a DC cut capacitor connected to the F terminal.

The operation of the mixer circuit configured as described above will be explained next.

The phase of the signal branched from the LOt4A terminal is adjusted by the capacitance values of the resistor R1 and the variable capacitance diode D1.

That is, the voltage applied to the variable capacitance diode D is adjusted so that it has an opposite phase to the leakage signal input to the LO terminal appearing at the RF terminal. Furthermore, level adjustment is performed by the resistance value of the PIN diode D2. Generally, the leakage signal from the LO terminal to the RF terminal of mixer 2 (hereinafter referred to as LO leakage (No. 8)) is several 10 dB (for example, 30 to
40 dB), the level adjustment using the PIN diode D2 requires the same amount of attenuation. The voltage applied to the variable capacitance diode D and the PIN diode D2 is applied to the LO leakage signal before inputting it to the BPF amplifier 4 by a signal that is adjusted to 9 or more and is given from the processor via the inductance L, , R2, and R3. can be canceled out.

Furthermore, since the LO leakage signal changes depending on the input frequency of the LO@, the temperature of the mixer 2, etc., the processor determines the voltage applied to each diode in response to these conditions. The processor sets the frequency of the VCO 3 to the center frequency f1 of the BPF amplifier 4, monitors the LO leakage signal, and determines the voltage applied to the diode so as to minimize it. In this way, the LO leakage signal can always be canceled to a minimum.

According to the mixer circuit having such a configuration, it is possible to cancel the leakage component of the LO terminal input signal of the mixer, so that the BP
Even when the VCO 3 sweeps around the center frequency of the F amplifier 4, saturation of the amplifier due to leakage components does not occur, and the input signal can be measured up to a frequency closer to DC. Therefore, the lower limit measurement frequency of the spectrum analyzer can be improved.

FIG. 2 is a configuration block diagram showing a second application example of the mixer circuit according to the present invention, which is applied to a network analyzer. A sawtooth wave signal outputted from the sweep oscillator 15 causes the VCO 16 to sweep frequencies from 0 to f, and one of the outputs is applied to the object under test 17 . VCOl
The other output of 6 is mixed in a mixer 18 with the output of one oscillator of 1 at a frequency of f1 to f0.
It becomes a signal of t. This signal passes through the object to be measured 17 and is affected by its characteristics, and mixer circuit 1 similar to that shown in FIG.
After the signals are mixed at 20 and passed through a BPF amplifier with a center frequency f, the transmission characteristics of the object under test 17 are displayed on the CRT 22. In the mixer circuit 20, 201 is a mixer similar to 2 in FIG. 1, and 202 is a phase and level adjustment circuit similar to 14. According to such a network analyzer, the implementation of FIG. The lower limit measurement frequency can be improved in the same manner as in the example.

In each of the above embodiments, a VCO signal whose frequency changes is used as the local oscillation input of the mixer circuit, but a signal with a constant frequency may also be used.

Further, as the adjustment circuit, a circuit having an arbitrary configuration that can make the input signal have an opposite phase and adjust the level can be used.

In addition, the display section is not limited to CRT, but also liquid crystal and EL.
Other indicators such as the following may also be used.

[Effects of the present invention]

As described above, according to the present invention, a mixer circuit that can bring the lower limit of the measurement frequency range of a spectrum analyzer or the like as close to DC as possible can be realized with a simple configuration.

[Brief Description of the Drawings] Fig. 1 is a block diagram showing an embodiment of the mixer circuit according to the present invention, and Fig. 2 is a block diagram showing an example of application of the device shown in Fig. 1 to a network analyzer. , Fig. 3 is a configuration block diagram showing an example of a conventional spectrum analyzer, Fig. 4 is a diagram showing a sawtooth wave, and Fig. 5 is a block diagram showing an example of a conventional spectrum analyzer.
FIG. 3 is a diagram for explaining the operation of the device. 2.201...Mixer, 14.202...Adjustment circuit, 20...Mixer circuit, St...Input signal to be measured. X v

Claims (2)

[Claims] (1) A mixer circuit that mixes an input signal under test and a local oscillation input signal, which includes an adjustment circuit that inputs the local oscillation input signal, adjusts the phase and level, and adds the output to the output signal of the mixer. A mixer circuit characterized in that the mixer circuit is configured such that a leakage signal of a local oscillation input signal included in the output is canceled by an adjustment circuit. (2) The mixer circuit according to claim 1, wherein the amount of phase adjustment and the amount of level adjustment are controlled in response to at least one of ambient temperature and local oscillation input frequency.