Heterodyne Phase Lock System

Heterodyne receivers are widely used in communications and other equipment for converting applied input signals to fixed lower frequency signals. By means of such receivers, an input signal with frequency components that vary over a wide frequency range may be converted to a fixed frequency signal (usually referred to as an intermediate frequency signal) for subsequent detection and measurement. A swept heterodyne receiver of the prior art is shown in FIG. 1. Generally, the greater the number of oscillators used in the receiver to perform the conversion, the more selective the receiver becomes.

To correct for factors such as frequency drift, frequency jitter (residual FM noise) and phase noise which limit the frequency resolution of the receiver, a selected oscillator, typically the first oscillator of the receiver, is phase locked to a spectrally pure reference signal (i.e., a reference signal with a noise level lower than the signal level desired to be detected and measured) by means of a phase lock loop as shown in FIG. 2. Such a phase lock loop serves to correct for residual FM and phase noise introduced into the receiver system by the first oscillator (a voltage controlled oscillator), but does not correct for (i.e., does not substantially eliminate) residual FM and phase noise introduced into the system by the other oscillators of the system (e.g., the second through the n.sup.th oscillators).

Also, it is often difficult to determine the tuned frequency of the receiver (i.e., the exact frequency to which the receiver is tuned). This difficulty arises from the fact that in some very wide tuning receivers, such as spectrum analyzers, many of the oscillators are tuned to microwave frequencies (e.g., 2-4 GHz), which frequencies are difficult to count directly. (It is necessary to count the frequency of each and every one of the oscillators in order to find the tuned frequency of the receiver, because the tuned frequency of a receiver is dependent upon the frequencies to which the individual oscillators are tuned.)

What is needed, therefore, is a system that corrects for frequency drift, residual FM, and phase noise from all of the oscillators in the system, and that provides a means for determining the exact frequency to which the system is tuned without having to count the frequencies of the individual oscillators.

Thus, in accordance with the illustrated preferred embodiment of the present invention, a heterodyne receiver phase lock system is provided having an input circuit, a feedback or control circuit, and a signal producing means comprising a plurality of oscillator circuits at least one of which is a voltage controlled oscillator circuit. A similar plurality of heterodyne (mixer-filter) circuits is included in each of the input and control circuits, with respective heterodyne circuits in the input circuit corresponding to respective heterodyne circuits in the control circuit, thereby forming ordered pairs of heterodyne circuits, each pair being coupled to receive, as a common input signal, the output signal from a corresponding oscillator circuit. The control circuit also includes a harmonic signal generator, and a phase detector or phase lock circuit. A spectrally pure reference frequency signal is applied to the harmonic signal generator and applied as a first input to the phase lock circuit. The output signal from the harmonic signal generator is applied to the first heterodyne circuit in the control circuit, and the output signal from the last heterodyne circuit in the control circuit is applied, as a second input signal, to the phase lock circuit.

In response to an input signal applied to a selected one of the heterodyne circuits of the input circuit, the phase lock circuit of the control circuit applies a phase lock voltage to a selected voltage-controlled oscillator, thereby correcting for frequency drift, residual FM and phase noise introduced into the system by all of the oscillators, and causing the system to produce, for subsequent detection and measurement, an intermediate frequency signal that is an accurate representation of a selected component of the input signal. Also, the frequency to which the system may be tuned can be easily determined without having to count the frequencies of the individual oscillators. For tuning purposes, a tuning circuit comprising a voltage controlled offset oscillator and a voltage summing circuit is included in the system, the voltage summing circuit being responsive to applied tune and sweep signals for producing a DC voltage signal. The voltage controlled offset oscillator applies an offset frequency signal to the last (3.sup.rd) heterodyne circuit of the control circuit, in response to the DC voltage signal and the output signal from the last (3.sup.rd) oscillator.

The heterodyne circuits of the control circuit may also be included in a generator system to correct for noise introduced into the generator system by oscillators of the system.

FIG. 1 is a block diagram of a system of the prior art.

FIG. 2 is a block diagram of the prior art system of FIG. 1, including a phase lock loop.

FIG. 3 is a block diagram of the system of the present invention, showing various circuit elements for receiving and detecting a signal.

FIGS. 4A-4N are graphic illustrations of signals applied to and produced by certain of the circuit elements of the system of FIG. 3.

FIG. 5 is a block diagram of the system of FIG. 3 including a tuning circuit having an offset oscillator circuit component.

FIG. 6 is a block diagram of the system of FIG. 5 showing the offset oscillator circuit in greater detail.

FIG. 7 is a block diagram of the system of the present invention showing various circuit elements for generating a signal.

Referring now to FIG. 3, a heterodyne receiver phase lock system is shown having an input circuit 11, a feedback or control circuit 13 and a signal producing means 14 comprising three oscillators 15, 17, 19, oscillator 15 being a voltage controlled oscillator, (i.e., an oscillator, the frequency of whose output signal is controllable by an applied voltage), and oscillator 17 being shiftable in frequency. Input circuit 11 comprises a first heterodyne circuit 21, a second heterodyne circuit 23, and a third heterodyne circuit 25 arranged in cascaded fashion, with each heterodyne circuit containing a mixer and a filter. Control circuit 13 also comprises first, second and third heterodyne circuits 39, 41, 43, arranged in cascaded fashion, with each heterodyne circuit containing a mixer and a filter. Control circuit 13 further comprises a harmonic signal generator 57 and a phase detector or phase lock circuit 59. Phase lock circuit 59 comprises a phase comparator 61 and a loop or integrating amplifier 63.

A spectrally pure reference signal (e.g., a signal from a crystal oscillator, such as the 20 MHz signal shown in FIG. 4A) is applied to harmonic signal generator 57 and to phase comparator 61 of phase lock circuit 59. An input signal (as shown in FIG. 4B) whose center frequency or fundamental component is to be measured or detected, is applied to mixer 27 of first heterodyne circuit 21 of input circuit 11. The system converts the input signal, which may vary over a wide frequency range, to a fixed or intermediate frequency (I.F.) signal, for subsequent detection and measurement. The conversion is performed by mixing the input signal with signals from oscillators 15, 17, and 19, and by filtering out unwanted mixing products. Oscillators 15, 17, 19 are tuned to different frequencies, each of which frequency bears a predetermined constant offset relationship to the fundamental frequency component of the input signal. Harmonic signal generator 57, in response to application of the spectrally pure reference frequency signal, applies a stable spectrally pure, precise frequency comb (harmonics of the reference signal which lie in the signal input bandwidth of the system as shown, for example, in FIG. 4C) to mixer 45 of circuit 39. An output signal (for example, a signal as shown in FIG. 4D) from first oscillator 15 is also applied to mixer 45. Mixer 45 mixes or heterodynes the signal (including frequency drift, residual FM, and phase noise) from oscillator 15 with the frequency comb, and produces therefrom a mixed signal as shown in FIG. 4E. This mixed signal is then filtered by filter 47 (e.g., a filter with center frequency of 2 GHz) to produce a first control signal as shown in FIG. 4F. The first control signal is applied to mixer 49 of heterodyne circuit 41, where it is combined (heterodyned) with the output signal from oscillator 17, (an output signal including residual FM and phase noise of oscillator 17 as shown, for example, in FIG. 4G), and the combination is filtered by filter 51 (e.g., a 260-280 MHz filter), to produce a second control signal as shown in FIG. 4H. This second control signal is then applied to mixer 53 of heterodyne circuit 43, where it is combined with the signal (as shown in FIG. 4I, including frequency drift, residual FM, and phase noise) from oscillator 19, and the combination is filtered by filter 55 to produce a third control signal as shown in FIG. 4J. This third control signal represents a "pilot I.F. signal" into which the harmonics of the reference frequency signal, which are close in frequency to the fundamental component of the input signal, are converted. (This third control signal has a frequency in the range of 10-30 MHz, corresponding to the width of filter 55, when not phase locked, and a frequency of 20 MHz, corresponding to the reference frequency, when phase locked.)

The third control signal, together with the reference signal, is then applied to phase comparator 61. Phase comparator 61, in response to the applied reference and third control signals, produces a DC voltage signal proportional to the frequency difference between the reference and third control signals. This DC voltage signal is then applied to loop (integrating) amplifier 63 which produces therefrom, a correction or phase lock voltage signal. This phase lock voltage signal is then fed back and applied to VCO 15, causing VCO 15 to adjust for differences between the reference frequency signal and the third control signal, thereby adjusting for frequency drift, residual FM, and phase noise contained in the third control signal from all of the oscillators 15, 17, 19 in the system. Once the phase lock or feedback loop is locked, i.e., once the third control signal and the reference frequency signal are found by comparator 61 to be of the same frequency, the DC voltage signal from the comparator, and hence the phase lock or correction voltage signal applied to VCO 15 from amplifier 63, is maintained unchanged, causing VCO 15 to cancel (i.e., to limit to the extent of the spectral purity of the reference frequency) all frequency drift, residual FM, and phase noise introduced into the system by the oscillators 15, 17, 19. VCO 15 cancels the frequency drift, residual FM, and phase noise by producing in its output signal (FIG. 4D), components which are equal in amplitude, but opposite in phase (i.e., 180.degree. out of phase) to the frequency drift, residual FM, and phase noise.

As shown in FIG. 3, the output signal from VCO 15 is applied to mixer 27 and to mixer 45. Mixer 27 combines this output signal with the applied input signal (FIG. 4B) and produces therefrom a heterodyned signal which is filtered by filter 29 to produce a first stage signal as shown in FIG. 4K. This first stage signal and the output signal (FIG. 4G) from second oscillator 17 are then heterodyned (combined) by mixer 31, and the combination filtered by filter 33 to produce a second stage signal as shown in FIG. 4L. The second stage signal and the output signal (FIG. 4I) from third oscillator 19 are then combined by mixer 35, and the combination filtered by filter 37 to produce an intermediate frequency (I.F.) or third stage signal as shown in FIG. 4M. This I.F. signal may then be detected and measured by a detector 65.

To enable the system to be continuously tuned, even to frequencies between the comb lines (i.e., to frequencies between the harmonics of the reference signal), a spectrally pure voltage controlled offset oscillator 67 of a known frequency is included in the system, as shown in FIG. 5. FIG. 5 shows tuning circuit 75 to include voltage summing circuit 69 and offset oscillator circuit 73, where offset oscillator circuit 73 comprises offset oscillator 67 and mixer 71. Frequency tuning and sweep is performed by sweeping or tuning offset oscillator 67. Voltage summing circuit 69 is responsive to applied tune and sweep signals for producing a DC output voltage signal (the applied tune signal being a DC voltage that is proportional to the frequency to which the system is tuned, and the applied sweep signal being a DC voltage that is proportional to the width of a desired frequency window). In response to the DC voltage signal from voltage summing circuit 69, offset oscillator 67 produces an output signal (representing a variable offset frequency F.sub.00) that is proportional to the DC output voltage signal. Mixer 71 combines or heterodynes this output signal with the output signal from third oscillator 19 and produces therefrom an offset signal, with frequency, F.sub.OFF, as shown in FIG. 4N. (Frequency F.sub.OFF represents the frequency of the output signal from mixer 71, which frequency is proportional to the DC voltage applied to offset oscillator 67, given that the output signal from oscillator 19 has a frequency of 280 MHz.) This offset signal is then applied to mixer 53. This permits the frequency to which the receiver is tuned to be varied without having to vary the reference signal, thereby enabling the system to be continuously tuned to frequencies between the comb lines. The frequency offset F.sub.OO represents an amount or frequency range by which the output signal from first oscillator 15 may be offset. This offset or frequency range is equal to the spacing between the harmonics of the reference frequency plus the desired sweep range. (The output signal from first oscillator 15 is offset by an amount equal, but opposite in polarity, to the frequency range represented by F.sub.OO.) Thus, first oscillator 15 is indirectly tuned or swept by offset oscillator 67. Because offset oscillator 67 can be tuned to a low frequency (e.g., 1-100 MHz), it is usually stable and introduces little noise into the system. To avoid difficulty in performing the phase lock operation when the system is tuned to a frequency located midway between two harmonics of F.sub.REF, offset oscillator 67 may be tuned to a fixed offset frequency F.sub.00 equal to 1/2F.sub.REF. This will cause one of the two harmonics to move into the center of the passband of filter 55 for frequency F.sub.C3. To cancel undesirable harmonics of the comb that may be introduced into the control circuit 13 of the system, when offset oscillator 67 is tuned over its range (.+-.1/2F.sub.REF), the output signal from second oscillator 17 is shifted by an amount equal to 1/4F.sub.REF.

In some prior art wide-band swept receiver systems, the first oscillator is generally tuned over a wide range (2-4 GHz), and phase locked to a high (2000.sup.th -4000.sup.th) harmonic of the reference frequency. Also, the reference frequency is generally kept low (usually about 1 MHz) and, to preserve signal amplitude accuracy, the tuning range of one of the other oscillators in the system (typically, the second oscillator) is generally kept small, i.e., not larger than the range or space between harmonics of the reference signal. However, because the first oscillator is phase locked to a high harmonic of the reference frequency, and because phase noise increases with harmonic number, the first oscillator introduces a substantial amount of phase noise into the system.

In the system of the present invention, the tuning of offset oscillator 67 does not change the frequencies F.sub.S1, F.sub.S2, F.sub.IF in the input signal conversion path, and does not change the reference frequency F.sub.REF, and since the pilot or control signals path does not require precise amplitude flatness, it can have a much larger frequency tuning range than the small tuning range of, for example, the second oscillator in certain prior art systems. This makes it possible to have a higher reference frequency (e.g., a reference frequency greater than 1 MHz) and, hence, less phase noise in the system. For example, if a reference frequency of 20 MHz is used, about a 26 dB improvement in phase noise is possible over the system's performance using a 1 MHz reference frequency. Also, unlike certain prior art phase lock receiver systems, the desired mixing product of the output signal from the 1st oscillator and the comb is not a DC component, but a signal at microwave frequency (approximately 2 GHz). This indicates that lower harmonic numbers of F.sub.REF can be used, since the harmonic of the reference frequency which is being converted is in the same frequency range (<2 GHz) as the input signal, rather than being in the frequency range (>2 GHz) of the 1st oscillator as in prior art phase lock systems. When the system of the present invention is tuned to 20 MHz, for example, the fundamental of the reference frequency is the signal which the 1st oscillator 15 is locked to. In a prior art phase lock system, the 1st oscillator may be locked to the 2000.sup.th harmonic of the reference frequency. The resulting difference in phase noise performance of these two systems could be as much as a 66 dB improvement in phase noise of the system of the present invention over that of the prior art.

Also, the system of the present invention, for example, as shown in FIG. 5, provides an easy and accurate means of determining the frequency to which the system is tuned. This represents a significant improvement in system performance when compared to the difficulty encountered in counting microwave frequencies of the oscillators in receiver systems (e.g., spectrum analyzers) of the prior art. For example, in the system of FIG. 5, the frequency to which the system is tuned (i.e., the fundamental component or center frequency of the input signal as shown in FIG. 4B) may be easily calculated for known values of the reference frequency (F.sub.REF), the offset frequency (F.sub.00) and the I.F. frequency (F.sub.IF), as indicated by the following mathematical relationship:

where, F.sub.IN represents the frequency to which the system is tuned, F.sub.REF(H) represents the frequency of the harmonic of F.sub.REF that produces a response in the frequency F.sub.C3, F.sub.00 represents a variable offset frequency of the output signal produced by offset oscillator 67, F.sub.C3 represents the frequency of the third control (pilot IF) signal, and F.sub.IF represents the frequency of the IF signal to be detected. (This IF signal frequency is the frequency of the input signal which gives the maximum system response and which is equivalent to the center frequency of the filter with the narrowest passband in input circuit 11 of the system.)

Equation (1) above is derived as follows:

where F.sub.01, F.sub.02 and F.sub.03 represent respective frequencies to which the first, second and third oscillators are tuned.

Substituting F.sub.03 -F.sub.00 for F.sub.OFF in equation 3, equation 3 then becomes:

Solving for F.sub.01 -F.sub.02 -F.sub.03 in both equations (2) and (5), we get, from equation (2):

and from equation (5):

From equations (6) and (7), it can then be seen that:

and,

Thus, from the foregoing, it can be seen that the frequency F.sub.IN to which the system is tuned can be determined independent of (i.e., without having to count) the frequencies (F.sub.01, F.sub.02, F.sub.03) of the individual oscillators 15, 17, and 19. This frequency F.sub.IN can be easily determined since frequencies F.sub.00, F.sub.C3 and F.sub.IF, which are generally low (i.e., less than 50 MHz), can be easily counted by means of conventional counter circuitry, and F.sub.REF(H) is a selected harmonic of known reference frequency F.sub.REF.

In the event tuning circuit 75 is omitted from the system, as shown in FIG. 3, the term F.sub.00 would be omitted from equation (5) above, and equations (2) and (5) could be combined to produce the relationship:

In which event, F.sub.IN could be determined solely from the frequencies F.sub.REF(H), F.sub.C3 and F.sub.IF.

It should be noted that, whereas FIGS. 3 and 5 show a receiver system comprising three heterodyne (mixer-filter) circuits 21, 23, 25 in the input circuit 11, three heterodyne circuits 39, 41, 43 in the control or feedback circuit 13, and three oscillators 15, 17 and 19, the system could just as well have comprised N heterodyne circuits in the input circuit, N heterodyne circuits in the control circuit, and N oscillators, where N is an integer, representing a plurality of two or greater. Generally, the greater the number of oscillators utilized in the system, the more selective the receiver becomes and the easier it is to filter-off unwanted mixing products.

It should also be noted that whereas FIGS. 3 and 5 show a receiver type system, the combination of heterodyne circuits in the input and control circuits may be utilized in a generator type system, in a similar manner as described hereinbefore, to correct for noise introduced by the oscillators into the system. For example, as shown in FIG. 7, in a generator system, an input signal may be introduced into the system at heterodyne circuit 25, and an output signal with multiple frequency components may be generated by the system from heterodyne circuit 21, the heterodyne circuits in the control loop serving to correct for noise introduced by the oscillators of the system.