JPH09252261A - Frequency conversion circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To convert an RF signal of high purity into an IF signal without deteriorating the signal purity by mixing together the frequency divided local signals and the output signal of an IF filter for execution of the conversion of frequency. SOLUTION: This circuit includes a divider 29 which divides the local signal of a local oscillator 36 and a mixer 17 which mixes together the local signals divided by the divider 29 and the output signal of an IF filter 66 to perform the conversion of frequency. Then the oscillator 36 does not construct a PLL as long as the local frequency Lf2 of the oscillator 36 is stable. Therefore, the frequency Lf2 of the oscillator 36 is divided by a divider 68 into the local frequency Lf3 and the frequency can be stabilized. As a result, the crystal oscillator of a conventional 3rd local oscillator and its peripheral PLL circuit can be omitted. Then an RF signal of high purity can be converted into an IF signal with no deterioration of the signal purity, and a compact and inexpensive frequency conversion circuit is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency conversion circuit for converting a high-purity RF signal into an IF signal without degrading the signal purity when using a multi-stage IF filter.

[0002]

2. Description of the Related Art An example of the prior art will be described with reference to FIG. In the spectrum analyzer shown in FIG. 3, the mixer 10 performs the first frequency conversion of the input signal by the local signal of the 1st local oscillator 31, and the IF filter 60 performs the I frequency conversion.
F output, then second frequency conversion and IF output by the second local oscillator 34, mixer 11 and IF filter 61,
Then, the 3rd local oscillator 35, the mixer 12, and the IF filter 62 perform the third frequency conversion and IF output.

Further, the IF output is fed to a reference oscillator 33.
And the multiplier 49 generate a local signal, and the mixer 13 and the IF filter 63 perform the fourth frequency conversion and IF output,
The IF output is detected by the wave detector 80, and the spectrum is displayed on the display 70.

The time axis generator 30 generates a sawtooth wave and synchronizes it with the sweep oscillator 31 in order to display the spectrum of the input signal as a frequency axis. In addition, the 2nd local oscillator 34 and the 3rd local oscillator 35
Using the frequency stability of the reference generator 33, the phase comparators 51, 52 and the frequency dividers 44, 45, 46, 47 and 4
And 8 form phase locked loops PLL1 and PLL2.

Generally, in spectrum analyzers, network analyzers and heterodyne receivers, multistage IF filters are used to remove image signals and to improve frequency resolution.

When the RF signal and the signal of the local oscillator are mixed by the mixer, the sum and difference outputs are obtained as outputs, and one image signal is removed by the IF filter. Further, the frequency resolution is determined by the resolution bandwidth of the IF filter. Generally, if the IF frequency is high, the image frequency can be easily removed, but it is difficult to realize the narrow band IF filter. Conversely, at low IF frequencies, narrow band IF
Filters are easy to make, but image frequencies are more difficult to remove.

Therefore, as shown in FIG. 3, the conventional spectrum analyzer is constructed by a four-stage multi-stage heterodyne system in order to remove the image signal and to improve the frequency resolution. The signal purity of the frequency-converted IF signal is determined by the signal purity of the input signal and each local signal. Therefore, in order to convert the signal into an IF signal without degrading the signal purity, a local oscillator with good signal purity is required. Therefore, in order to perform sweep oscillation in the past, the 1st local oscillator 31 (4066.42MH)
z to 7666.42 MHz), and a 2nd local oscillator 34 (38) depending on the oscillation frequency band.
40MHz) with a dielectric oscillator, 3rd local oscillator 3
5 (200 MHz) and reference oscillator 33 (10 MH
A crystal oscillator is used for z).

By the way, the 2nd local oscillator 34 and the 3rd
The local oscillator 35 uses a voltage controlled oscillator (VCO) to generate the 2nd local signal Lf2 and the 3rd local signal Lf3, but in order to stabilize the oscillation frequency,
A phase locked loop (hereinafter referred to as PLL) is configured based on the reference oscillator 33 having a stable frequency. The PLL 2 has the phase of the signal of the reference oscillator 33 and the 3rd local oscillator 35.
A phase error signal is detected by the phase comparator 52 with respect to the phase obtained by dividing the oscillation frequency of the frequency divider 47, 48, and the phase error signal is fed back to the 3rd local oscillator 35 to match the phase of the reference oscillator 33 and oscillate. We are trying to stabilize the frequency. further,
The PLL 1 compares the phase of the signal divided by the frequency divider 47 of the PLL 2 and the phase of the oscillation frequency of the signal of the 2nd local oscillator 34 by the frequency dividers 44 to 46 with a phase comparator 51 to output a phase error signal. Detect and detect it as the 2nd local oscillator 34
To stabilize the oscillating frequency by matching the phase of the signal which is fed back to and divided by the frequency divider 47. Further, the reference oscillator 33 multiplies it by 3 to obtain the 4th local signal Lf4.

[0009]

As described above, 3
Since the rd local oscillator 35 oscillates at a high frequency (200 MHz), a crystal oscillator of 9th order overtone mode or the like is used, resulting in high cost. Further, if a dielectric oscillator is used as the 3rd local oscillator 35, the size becomes large and the cost is often increased, which is inconvenient in practical use.

The present invention has been made in view of these problems, and an object thereof is to provide a frequency conversion circuit for converting a high-purity RF signal into an IF signal without degrading the signal purity. I am trying.

[0011]

That is, the invention according to claim 1 made in order to achieve the above object is provided with a local oscillator 36 for generating a local signal in a multistage frequency conversion circuit, and the local oscillator 36 is provided. A mixer 16 that mixes the local signal of 36 and the input signal to perform frequency conversion is provided, and an IF filter 66 that receives the converted signal of the mixer 16 and removes an image signal is provided to change the local signal of the local oscillator 36. A frequency divider 29 for frequency division is provided, and the local signal frequency-divided by the frequency divider 29 and the I
The gist of the frequency conversion circuit is that a mixer 17 for mixing the output signals of the F filter 66 and converting the frequency is provided.

Further, according to a second aspect of the present invention made to achieve the above object, the local oscillator and the circuit for dividing the local signal of the local oscillator and outputting the other local signal are phase-locked. The frequency conversion circuit according to claim 1 is constituted by a loop.

The invention according to claim 3 made to achieve the above object has as its gist a measuring instrument comprising the frequency conversion circuit according to claim 1 or 2.

Further, a fourth aspect of the present invention, which has been made to achieve the above object, has as its gist a receiver comprising the frequency conversion circuit according to the first or second aspect.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described in the following examples.

[0016]

【Example】

(Embodiment 1) Hereinafter, Embodiment 1 of the present invention will be described with reference to FIG. As shown in FIG. 1 (a), the configuration is a heterodyne type spectrum analyzer using a four-stage IF filter, and the 2nd local signal Lf2 of the conventional configuration is used.
And the method of generating the 3rd local signal Lf3 and the 4th local signal Lf4 is changed.

That is, in the spectrum analyzer shown in FIG. 1A, the 1st local oscillator 31 has the same YI as in the conventional case.
Although it is a sweep oscillator using a G oscillator, the 2nd local signal Lf2 from the local oscillator 32 forms a phase locked loop PLL3 using the reference oscillator 33 as a reference source.

Here, the PLL 3 divides the phase of the signal of the reference oscillator 33 and the signal of the local oscillator 32 into frequency dividers 40 to 40.
The phase divided by 43 is detected by the phase comparator 50 as a phase error signal, which is fed back to the local oscillator 32 so as to match the phase of the reference oscillator 33 to stabilize the oscillation frequency. Further, the signal of the local oscillator 32 is frequency-divided by the frequency divider 40 and the frequency divider 41 to output as the 3rd local frequency Lf3, and the frequency-divided signal of the frequency divider 42 is output as the 4th signal.
It is output as the local frequency Lf4.

The signal purity B of the divided signal is
When a signal having a signal purity of Adbc / Hz is input to a 1 / N frequency divider, it is generally obtained by the following equation. B≈A−20 log N (dBc / Hz) From the above equation, it is understood that the signal purity BdBc / Hz of the frequency divider output is improved over the signal purity AddB / Hz of the input signal.

Therefore, even if the signal of the local oscillator 32 is divided to generate the 3rd local frequency Lf3 and the 4th local frequency Lf4, the signal purity does not deteriorate.

In the first embodiment (a) of the present invention, the local oscillator 32 is composed of the PLL circuit by the reference oscillator 33 for the purpose of stabilizing the frequency. However, FIG.
As shown in, the local frequency Lf2 of the local oscillator 36 is
Is stable, the local oscillator 36 does not form a PLL, and the local frequency Lf2 of the local oscillator 36 can be divided by the frequency divider 68 to obtain the local frequency Lf3.

Therefore, according to the present invention, the crystal oscillator of the conventional 3rd local oscillator and the peripheral PLL circuit are unnecessary, and a high-purity RF signal can be transmitted to an IF without degrading the signal purity.
It becomes a frequency conversion circuit that can be converted into a signal, and that is small in size and low in cost.

(Second Embodiment) A second embodiment of the present invention will be described with reference to FIG. 2 in the case of a heterodyne receiver. The configuration is a two-stage heterodyne configuration as shown in FIG. The signal from the antenna is amplified by the amplifier 20, the first frequency conversion is performed by the mixer 14, the IF output is performed by the IF filter 64, the second frequency conversion is performed by the mixer 15, and the IF output by the IF filter 65 is detected. Bowl 8
0 is detected and further amplified by the amplifier 21 and the speaker 25
To output audio.

The 1st local signal Lf1 from the local oscillator 22 forms a phase locked loop PLL4 using the reference oscillator 23 as a reference source for stabilization.

That is, 1 oscillated from the local oscillator 22.
The st local signal Lf1 is divided by the frequency divider 26 into the 2nd local signal Lf2, and the phase of the signal divided by the frequency divider 27 and the signal of the reference oscillator 23 are divided by the frequency divider 28. The phase is detected by the phase comparator 24 to detect the phase error signal,
It is fed back to the local oscillator 22 and matched with the phase of the reference oscillator 23 to form the PLL 4.

Therefore, in the second embodiment of the present invention, the crystal oscillator of the 2nd local oscillator and the peripheral PLL circuit are not required, and a high-purity RF signal can be converted into an IF signal without degrading the signal purity. The frequency conversion circuit can be made compact and inexpensive.

[0027]

The present invention is embodied in the form described above and has the following effects. Therefore, in the frequency conversion circuit having two or more stages, the signal of one local oscillator is divided by the frequency divider to generate the signal of the other local oscillator, so that the crystal oscillator of the local oscillator and the peripheral PLL circuit are connected. Reduced and high purity R
It is possible to convert the F signal into an IF signal without degrading the signal purity, and it is possible to obtain a frequency conversion circuit that is small in size and low in cost.

[Brief description of drawings]

FIG. 1A is a block diagram of a spectrum analyzer according to a first embodiment of the present invention. (B) It is a block diagram of the present invention.

FIG. 2 is a block diagram of a heterodyne receiver according to a second embodiment of the present invention.

FIG. 3 is a block diagram of a conventional spectrum analyzer.

[Explanation of Codes] 10, 11, 12, 13, 14, 15, 16, 17 Mixer 20, 21 Amplifier 22 Local oscillator 23 Reference oscillator 24 Phase comparator 25 Speaker 26, 27, 28, 29 Divider 30 Time axis Generator 31 1st local oscillator 32 Local oscillator 33 Reference oscillator 34 2nd local oscillator 35 3rd local oscillator 40, 41, 42, 43, 44, 45, 46, 47, 4
8 frequency divider 49 multiplier 50, 51, 52 phase comparator 60, 61, 62, 63, 64, 65, 66 IF filter 70 indicator 80 detector

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[Procedure amendment]

[Submission date] December 2, 1996

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Claim 2

[Correction method] Change

[Correction contents]

Claims (4)

[Claims] 1. A multi-stage frequency conversion circuit, wherein a local oscillator (36) for generating a local signal is provided, and a mixer (16) for mixing the local signal of the local oscillator (36) and an input signal to perform frequency conversion is provided. An IF filter (66) for receiving the converted signal of the mixer (16) and removing an image signal, and a frequency divider (29) for dividing the local signal of the local oscillator (36), The local signal divided by the divider (29) and the I signal
A frequency conversion circuit characterized in that a mixer (17) for mixing the output signals of the F filter (66) and converting the frequency is provided, and the above is provided. 2. The frequency conversion circuit according to claim 1, wherein the local oscillator and the circuit for dividing the local signal of the local oscillator and outputting the other local signal are constituted by a phase locked loop. 3. A measuring instrument comprising the frequency conversion circuit according to claim 1. 4. A receiver comprising the frequency conversion circuit according to claim 1.