JPH01320829A - Apparatus for avoiding or suppressing harmful frequency relations in superheterodyne receiver - Google Patents

Abstract

PURPOSE: To prevent, suppress or remove tweet by controlling the frequency of a local oscillator corresponding to difference in frequencies between a reference frequency signal from a phase locked loop(PLL) and an intermediate frequency(IF) signal corresponding to a received radio frequency. CONSTITUTION: A programmable frequency divider 19 outputs a local oscillator signal at the frequency, required for a frequency converter 12 onto a lead wire 29 and generates a signal at the 2nd reference frequency to be supplied through a lead wire 25 to a PLL 21. Then, a phase detector 23 generates a control signal, expressing the difference in frequencies between the 3rd reference frequency signal from the PLL 21 and the IF signal from an IF amplifier 13. This control signal changes the 1st reference frequency output of a main oscillator 18. The change in the 1st reference frequency causes changes in the 2nd reference frequency on the lead wire 25 and the frequency of the local oscillator signal on the lead wire 29. As a resut, the 3rd reference frequency from the PLL 21 is made equal to the frequency of the IF signal on a lead wire 16, and a tweet frequency is turned to zero.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a superheterodyne receiver that uses a frequency converter or a frequency synthesizer to generate the local oscillator frequency required for a mixer in the receiver. The radio frequency (RF) signal received at this receiver is
It relates to what is converted into a corresponding intermediate frequency (IF) signal.

(Prior Art and Its Problems) A relatively common problem in superheterodyne receivers is a harmful output that interferes with the desired output, and the interference is very close to an integer multiple of the receiver's intermediate frequency. This can occur when a receiver is tuned to a signal being transmitted on a carrier frequency. Institute of Electrical and Electronics Engineers (IBE)
(Tnstitute of Electrical
and Electronics Engineer
s)) published on August 10, 1984.
Electronic Standard Terminology Dictionary 3rd Edition (r I EE EStand
ardDictionary of Electric
al and ElectronicsTerms
J), pages 458-459, describes this phenomenon as ``harmonic interference of intermediate frequencies (superheterodyne receivers)''.
. Intermediate frequency signals of radio frequency circuits receive harmonics due to interference. ” is defined. Document 1 “Standard Test Method for Frequency Receiving Broadcasting Receivers” (IEEEF, STD 1
85-1975 ((IEEE/IHF 5tan
darcl), 5standard Method
s ofTesting Frequency Mod
ulationBroadcastReceiver
Page 24, Section 6.22 - Reproduction of s)) states that this type of interference is referred to as [tiyeet J], a term that has been widely used in the receiver industry for many years. It has been used for many years, and will be used in this specification as well. Although this condition can occur in any type of superheterodyne receiver, it is particularly troublesome in amplitude modulation (AM) receivers designed to receive signals in the AM radio broadcast frequency band (535-1605 kHz). Met. However, the present invention is not limited to application only to this type of receiver, but can also be applied to superheterodyne receivers designed for other frequency bands and modulation methods other than AM.

For the sake of simplicity, the following discussion will be based on an audio broadcast receiver, such as an AM radio receiver.

Traditionally, radio receivers have used either a continuous tuning system that allows for continuous tuning over the frequency band of interest, or a push-button device that allows the listener to tune to a preselected station in stages. It was getting worse. In either case, if the receiver is tuned to a signal transmitted on a carrier that is very close to an integer multiple of the receiver's intermediate frequency (IF), then if the receiver's IF circuit and antenna or RF
If there is enough harmful regenerative coupling or feedback with the circuit, unpleasant interference tweets will occur.

Recently, in superheterodyne receiver frequency conversion circuits that convert the received signal frequency to an intermediate frequency, synthetic frequency methods have become increasingly popular in order to generate the local oscillator frequency required by this frequency conversion circuit. It has become possible to do so.

Frequency synthesizers for superheterodyne receivers vary in design, but generally share the following characteristics: That is, 1) it has a frequency-stable master oscillator operating at a first reference frequency from which the desired frequency is derived, and 2) its receiver receives a signal broadcast at an assigned frequency within the frequency band of interest. In order to do so, the frequency converter of a superheterodyne receiver has the ability to step up the local oscillator frequency required.

Frequency synthesizers generally provide more accurate frequencies to frequency converters than earlier tuned systems. In the special case that the received signal frequency is a harmonic of the IF, if the signal carrier is also exactly at its specified frequency, the frequency synthesizer can in theory It is possible to give a composite frequency that becomes an IF. but,
In this case, if the playback path required to generate the tweet is in the receiver, the tweet will be inaudible because it is at frequency zero. However, due to tolerances normally present in receiver components, thermal and aging effects, and tolerances in signal frequency, the above zero-tweet condition is highly unlikely and under normal operating conditions. Notice that it is included in the output of the receiver.

(Summary of the Invention) Therefore, an object of the present invention is to detect the frequency relationship that causes tweets in a superheterodyne receiver having a synthetic frequency tuning means, and automatically adjust the TF in the direction of preventing, suppressing, or eliminating the tweets. It is important to provide a means to change the

The present invention provides a superheterodyne receiver that combines a received radio frequency (RF) signal with an intermediate frequency (
It can be applied to prevent or suppress harmful frequency relationships that may exist between IF) signals.

The receiver has a local oscillator (LO) that can control the specific real frequency at various selected frequencies.
first means for providing a signal and a second signal representative of a nominal intermediate frequency.

The receiver also has second means for providing the received RF multiplied signal and third means for combining the RF multiplied signal and the LO multiplied signal to generate an IF multiplied signal within a predetermined IF band.

The receiver also includes fourth means for generating a control signal responsive to the second signal and the IF multiple signal indicative of the existence of a detrimental frequency relationship between the RF multiple signal and the IF multiple signal. Finally, the receiver also has means for controlling the LO times signal frequency in addition to the first means with its control signal in order to prevent or suppress harmful frequency relations.

Next, explanation will be given with reference to the attached drawings.

(Example) The operation of the circuit shown as a block diagram in FIG. 1 will be explained first, and then a mathematical analysis will be performed to prove the operation of the present invention.

In FIG. 1, the main oscillator 18 of the frequency synthesizer 17 is
A source of a first reference frequency signal is connected via conductor 27 to programmable frequency divider 19 . This frequency divider is the local oscillator (LO) required by the receiver's frequency converter 12 to tune to a signal from a broadcast station operating at an assigned frequency within the frequency band of interest to the receiver. ) can be programmed to provide one frequency at a time.

A control signal provided from program source 24 to frequency divider 19 is output from the frequency divider on lead 29.

Determine the frequency of the 0 output body symbol. There are a variety of devices suitable for generating the programming control signals provided to frequency divider 19. For example, it may be manually operated as part of the receiver's tuning device to tune in steps from channel to channel. Alternatively, it may automatically scan from one receiving station to other stations (or only preselected stations). It is also possible to use a combination of these methods. Whichever method is used, the device 24 for controlling the frequency divider 19 and channel selection via the converter 12 is typically combined with a tuning device for the antenna 10 and/or radio frequency (RF) circuit 11. It will be done.

Blocks 10-14 of FIG. 1 operate in the usual manner for a superheterodyne receiver. RF signal is antenna 10
and is fed to an RF circuit 11, which typically includes a bandpass filter and an amplifier. Frequency converter 12
The RF multiplied signal is converted into an IF multiplied signal corresponding to the RF multiplied signal, and this IF multiplied signal is amplified by an IF amplifier 13 and supplied to an AM detector 14 .

The detected signal is supplied via conductor 15 to an audio frequency amplifier, if necessary, and then to an audio reproduction device such as a speaker.

What has been described above is a conventional superheterodyne receiver with frequency synthesis tuning means. control unit 20
changes the first reference frequency of the master oscillator output on conductor 27 in a direction that prevents, suppresses, or eliminates tweets that may occur when the received signal frequency is an integer multiple of the nominal IF. In order to do so, the necessary functionality is provided to generate a control signal which is supplied via conductor 26 to the main oscillator.

The programmable frequency divider 19 outputs L× the frequency required by the frequency converter 12 on a signal conductor vA 29 and also outputs a phase-locked loop (PLL) via a conductor 25.
21 to generate a signal at a second reference frequency.

The second reference frequency is a divisor of IF in FIG. The automatic frequency of the oscillator of PLL 21 is equal to the nominal IF, and the divisor (n) of frequency divider 22 is an integer equal to the ratio of the nominal IF to the nominal second reference frequency. Such circuits are well known to those skilled in the art, and it will be appreciated that the PLL oscillator is locked to a frequency (n) times the second reference frequency.

The output from the PLL 21 is a signal of the third reference frequency,
This is supplied to a phase detector 23 and a frequency divider 22. This phase detector is supplied with I from the output terminal of the IF amplifier 13.
F times signal is supplied. The phase detector 23 generates a control signal representing the difference in frequency between the third reference frequency signal from the PLL 21 and the IF multiplied signal from the IF amplifier 13 .

The main oscillator 18 is a stable, frequency controlled oscillator (such as a voltage or current controlled oscillator). The control signal generated by phase detector 23 is applied by lead 26 to the frequency control input terminal of the master oscillator, thereby varying the first reference frequency output of the master oscillator on guide vA 27. A change in the first reference frequency results in a change in the second reference frequency on lead 25 and the LO times signal frequency on lead 29. As a result, P
The third reference frequency from the LL 21 and the f times signal frequency on the conductor 16 are equal to each other in a preferred embodiment, and change in the direction of being approximately equal to each other in other embodiments. When these two signals become equal, it is shown that the tweet frequency becomes zero.

To facilitate the mathematical analysis of the working of the invention, the symbols representing frequencies at various points in FIG. 1 and their interrelationships in the 31 sets of states are defined in the following table.

Column A of the table shows that the main oscillator 18 of the frequency synthesizer 17 is exactly at the design center frequency, and the received RF signal frequency is
It shows the relationship that exists when the frequency is exactly equal to the frequency assigned to a particular radio broadcast station and is exactly equal to an integer multiple of the IF. The relationship under these conditions exists whether or not the invention is operative.

The relationship in Example B of the table applies when either or both of the main oscillator frequency and the received signal frequency are not the design center frequency or the specified frequency, and (for example, when the phase detector 23 does not operate), the main oscillator frequency and the received signal frequency are This is the relationship when the invention is not working. In this case, the IF is not a divisor of the RF signal frequency and there are tweets.

With the conditions described above for column B of the table, the relevant relationships are as shown in column C of the table, except that the present invention is utilized here. The invention works so that the IF is a divisor of the frequency of the received signal, thereby reducing tweets to zero frequency.

RF signal frequency rSB
, rH* According to the function (action) of the present invention, R
It will be demonstrated mathematically below that an integer relationship (k3) exists between the F signal frequency (f;) and the intermediate frequency (fτ).

The following mathematical analysis shows that the tweets are indeed reduced to zero frequency by the operation of the present invention.

From the above equation (4c), when the present invention operates, it is as follows.

(51fl = 4f'za Also, due to superheterodyne operation, (6) fi
−fLo−fS. Substituting equation (5) into equation (7) yields.

Substituting equations (1c) and (2c) into equation (8) yields θ21 fLo=fs+fI Substituting equation (b) into 2〇〇 gives the following equation.

In the table, the operation of the frequency divider 19 of the frequency synthesizer 17 causes the signal frequency and the main oscillator frequency to deviate from their specified values, k, It should be noted that and 2 are fixed constants. However, k3 is a constant integer only for the special situation described above for column A, as shown in column C when the invention is in operation, as shown mathematically below. That's right. Also, k4 is a constant constant only for the states shown in columns A and C. k
4 is the divisor (n) of the frequency divider 22 in FIG.

To reduce the tweet frequency to zero, phase detector 23 of the preferred embodiment of FIG.
1 and the IF multiplied signal on conductor 16 to lock to the same frequency. In this case, the control signal output from the phase detector 23 on the conductor 26 contains a DC component, and until the frequencies of the two inputs to the phase detector 23 are equal, the AC
There are also ingredients. If only the AC component of the control signal output of the phase detector 23 is supplied to the control input terminal of the main oscillator 18, the phase detector becomes equivalent to a frequency detector instead of a phase detector, and the tweet frequency becomes Even if it is reduced,
It doesn't necessarily reach zero. The reason for this is that the frequencies of the two input signals to the phase detector 23 are not necessarily locked, and a slight frequency difference may remain. However, the difference frequency is small and the tweet frequency is in the sub-audible frequency range.

Although the invention has been described in connection with the AM receiver embodiment of FIG. It can also be applied to suppress or eliminate it. Furthermore, application of the principles of the present invention is not limited to the circuit of FIG. For example, as mentioned above, the frequency synthesizer can be designed to provide a second reference frequency equal to the nominal IF frequency, thereby eliminating the need for P L L 21 and frequency divider 22 in FIG. can do. The essence of the invention is to generate a control signal that can reduce the tweet frequency to zero in a preferred embodiment, or to a frequency low enough to be inaudible or at least not jarring. That's true.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an AM receiver embodying the present invention in one form. 10-1---Antenna, 15.16.25.26.
27.29---Conductors, 20---Control unit.

Claims (1)

[Claims] 1. In a superheterodyne receiver having a predetermined intermediate frequency (IF) band and a nominal IF, local frequencies of various selected nominal frequencies can be used to control the specific actual frequency. first means for providing an oscillator (LO) signal and a second signal representative of the nominal IF of the receiver; second means for providing a received RF signal; I in the IF band by combining
third means for generating an F signal; in response to said second signal and said IF signal, generating a control signal indicative of the existence of said detrimental frequency relationship between said RF signal and said IF signal; fourth means; means for providing said control signal to said first means to control the frequency of said LO signal to prevent or suppress said harmful frequency relationship;
) A device characterized in that it prevents or suppresses harmful frequency relationships that may exist between the signal and the resulting actual IF signal. 2. The apparatus of claim 1, wherein the second signal is a submultiple of the nominal IF and is derived from the same source as the LO signal. 3. The apparatus of claim 1, wherein the second signal is a signal having a frequency equal to the nominal IF and derived from the same source as the LO signal. 4. The apparatus according to claim 2 or 3, wherein the first means comprises an oscillator from which the LO signal and the second signal are obtained, and the control signal controls the frequency of the oscillator. 5. The fourth means: responsive to the second signal, generating a reference signal having a selected frequency within the IF band and a phase related to the phase of the second signal; 2. The apparatus of claim 1, further comprising means for detecting a frequency and phase relationship between an IF signal and said reference signal and generating said control signal representative thereof. 6. The apparatus of claim 3, wherein said reference signal is generated with a frequency equal to said nominal IF.