A MEASURING DEVICE FOR MEASURING THE CARRIER FREQUENCY OF SHORT DURATION PULSES

Abstract: A device for measuring the frequency of a pulsed carrier signal in which the carrier frequency is situated within the frequency range and to give a continuous signal whose frequency is equal to the measured carrier frequency. A frequency discriminator (3) gives an analog voltage via a track-and-hold circuit (4) and a DC-amplifier (15) to one input of a comparator (6) whose second input receives the analog voltage from the discriminator (3). The difference value from the comparator (6) is supplied to a voltage controlled oscillator (8). This is via a directional coupler (9) and a controllable switch (1) connected to the frequency discriminator (3) which alternatively receives the input signal to the device and the output signal from the voltage controlled oscillator (8).

Description: A MEASURING DEVICE FOR MEASURING THE CARRIER FREQUENCY OF SHORT DURATION PULSES TECHNICAL FIELD The present invention relates to a device according to the preamble of the claim 1 for measuring and regenerating the carrier frequency of each of the individual pulses in an incoming pulsed carrier frequency signal. The device according to the invention can, for example, be included as a part in a radar receiver, the carrier frequency being situated within the micro wave range. BACKGROUND ART In, for example, the radar technic there is frequently the- task to be able to measure the frequency of the individual pulses in an incoming pulse train to the receiver. This can cause problems, especially, when every incoming pulse has a very short duration and in the case the carrier frequency varies from one pulse to another. The incoming signal consists, for example, of pulses having mutually different frequency values, which for example, constitutes the signal from different radar stations, the device should after the measuring deliver a signal with the same frequency as the received carrier. Examples of known circuits or systems for this purpuse is described in, for example, the U.S.- patent 3 996 588. DISCLOSURE OF INVENTION The device according to the present invention intends to solve said problem. During the duration of a pulse the value of the carrier frequency of the pulse is measured by means of a frequency discriminator, the thus measured value being supplied to a voltage controlled oscilla¬ tor as a control quantity. This control quantity is allowed to control the oscillator to a frequency approximatively equal to or in the neighbourhood of the received frequency during the short time inter- val of the carrier frequency pulse duration. During the subsequent longer time interval (or part thereof), when no carrier frequency pulse is received, the device works as a closed loop and the voltage controlled oscillator is controlled by the value of a difference signal and is adjusted to a frequency value near the value of. the carrier frequency pulse detected during the'previous time interval " . The novel and the characterizing of the present device is a track-and- hold circuit and an amplifier connected between the frequency discrimi¬ nator and the voltage controlled oscillator. A continuous signal which follows the output signal from the discriminator is delivered on the one hand directly and on the other hand via a track-and-hold circuit and the amplifier to a difference forming circuit and during the time inter¬ val of the pulse duration. The output signal of the difference circuit controls the. osci Llator-.-Duringthe subsequent longer time interval the last measured value from the discriminator is stored in the track-and- hold circuit and the oscillator is controlled in dependence on the difference between a value corresponding to the oscillator frequency and the stored value. The device is then characterized as it appears from the characterizing part of claim 1. By this a continuous control of the voltage controlled oscillator is attained already from the beginning of a carrier frequency pulse, i.e. sudden changes in the oscillator control is avoided which implies a more rapid and reliable measuring and adjusting of the carrier frequency. The object of the present invention is thus to provide a device to rapidly and securely measure and regenerate the carr5er .frequency of received pulses in a pulse train where the carrier frequency can vary from one pulse to another and to give an accurate determined analog counter¬ part to every detected and measured frequency, for example in the form of a direct voltage. BRIEF DESCRIPTION OF DRAWNINGS The invention will be closer described with reference to the accompany¬ ing drawings in which Figure 1 shows a block diagram of the device according to the present invention. Figure 2 shows a timing diagram in order to explain the function of the track-and-hold circuit which is included in the device according to the invention, Figure 3 shows a block diagram equivalent with Figure 1 during the time interval the device forms an open control loop, Figure 4 shows a circuit diagram equivalent with figure 1 during the time interval the device forms a closed loop. MODE FOR CARRYING OUT THE INVENTION The block diagram according to Figure 1 shows a device according to the invention. The block 1 designates a * du path : controlled switch one output across which a signal Uo appears shown in Figure 2 on the top and which consists of a pulsed carrier frequency signal whose frequency f. , f 2 ... from one pulse to another should be measured. In the case the device is used in, for example, a radar receiver the frequencies fm , f ? are situated within the micro wave range. The other input 1b of the switch 1 is connected to a duplexer 9. The output 1c is connected to a frequency discriminator 3 via an amplifier-limiting stage 2. Across the output of the discriminator 3 an analog quantity U3 is obtained whose value depends on the frequency value of the incom¬ ing microwave signal U2 from the amplifier stage 2. The discriminator 3 is supposed to be linear, i. e. the value of the analog.. signal U3 is directly proportional to the detected frequency with a proportion factor Kd (V/MHz). The output of the discriminator 3 is connected on the one hand to the input of a track-and-hold circuit 4 and an the other hand to the minus input of an operational amplifier 6. The track-and-hold circuit 4 is with its output connected via a DC-amplifier 5, with the amplification Km, to the pulse input of the operational amplifier 6, the output signal (DC-voltage) from the amplifier 5 being designated Uc, and forms the amplified value of the signal U,. The operational amplifier 6 then constitutes a difference forming circuit or a compa- rator, which compares the quantities U, and U,-, a difference signal being formed which via a second DC-amplifier 7 with the amplification K2 is supplied to a voltage controlled oscillator (VCO) 8. This is supposed, like the discriminator 3, to be linear and is characterized by the factor K y (MHz/V ) ,a signal U R being delivered whose frequency is situated within the microwave range and the value being independent of the value of the analog quantity U 7 from the amplifier 7. The signal U g is delivered via the duplexer 9 on the one hand to the second input of the switch 1 and on the other hand to the output of the device. In the diagram according to figure 1 thus microwave signals appear to the left of the dashed line a-a and analog signals in the form of DC-voltages to the right of the line a-a. Before describing the operation of the circuit, briefly the function of the track-and-hold circuit 4 will be described with reference to the timing diagram according to figure 2. At the time t and during the time interval t -t. the control signal Tlsw to the switch 1 is such that the contacts are in the position 1a-1c, U1 being equal to Uo. The control signal Ts is then such that the track-and-hold circuit 4 is fully conducting, i. e. U4 is equal to U3. The input signal Uo con- sists, for example, of a pulsed carrier frequency signal as shown in • figure 2 having a low value of the .dury cycle ' of the magnitude 1/1000). The input signal U2 to the frequency discriminator thus con- sits of a pulse train with short duration pulses, where the carrier frequency can vary from one pulse to another. The leading edge of the pulse coincides mainly with the instant t while its trailing edge appears at the instant t_. or at some instant after that. During the time t -t.. the carrier frequency is measured by the discriminator 3, the output signal U3 consisting of an analog signal whose value is proportional to the frequency of the pulse. The signal U3 is after that conducted unchanged through the closed track-and-hold circuit 4. At the time t. a control pulse Ts is delivered to the track-and-hold circuit which implies the value of the signal U, at the time t,. "is frozen" (stored), so that the output signal U, consists of an analogue signal with a constant value during the interval 1 -t' (equal to the value U3 at the time t 1 >. At the same time, t. , the switch 1 -is changed by the signal Tlsw to the position 1b-1c. At the time t ? the track-and- hold circuit is switched so that the signal U3 is completely let through (as at the time t and the process is repeated. The circuit 4 can as known consist of a series-parallel-circuit of a switch (for example a transistor) in the series path and a capacitor in the parallel g When the switch is closed, the capacitor is charged to the value of the signal U3 (the interval t -t„) and when the switch is disconnected o 1 ( the interval t.-t" ) the capacitor charge (-voltage) constitutes the holding value. The circuit 4 thus "tracks" the input signal during the time interval t -t.., t_-t, and "holds" (stores) during the inter¬ val 1 ~t' the value of the input signal which last appeared. The function of the frequency measuring circuit will now be closer described with reference to Figures 3 and 4 where Figure 3 shows the circuit during the time interval t ~t. , t ? -t, (open loop) and Figure 4 shows the circuit during the interval t 1 ~t' (closed loop). The discriminator 3 and the voltage controlled oscillator 8 is assumed to be linear with the factors Kd (V/MHz) and Kv (MHz/V), respectively. The amplifiers 5 and 7 are supposed to have each a linear characteris¬ tic with the amplification factor K,. and K->, respectively. Then following is vaLid: U4 = U3 = Kd x fi (fi is the frequency of the carrier wave pulse) U5 = K,. x Kd x fi U6 = Kd x fi x ( K.j-1), K 1 # 1., the frequency fv of the output signal U8 from the voltage controlled oscillator 8 being: fv = Kx(K 1 ~1) x fi ... (1) if Kv x K 2 x Kd = K. During this time interval a signal U3 is obtained to the track-and-hold circuit 4 which varies according to the frequency of the input signal and thus a Likewise varying input signal U7 to the oscillator 8. The frequency fv thus varies but is proportional to the frequency fi of the input signal U1. Figure 4 shows the circuit during the time interval t ^ .-t". The loop is now closed and the track-and-hold circuit 4 delivers a constant signal to the amplifier 5, which is fed to the puls input of the amplifier 6. Across the minus input a signal U3 is obtained, which is proportional to the frequency fx of the voltage controlled oscillator. The following relations can be set: fx = Kv x K 2 x( ,. x Kd x fi - Kd x fx) = K x K,, x fi - K x fx K x <, which gives fx = Λ + v— x ^^ If K., = 1 + 1 /K , it. is obtained fx = fi This implies that the frequency of the voltage controlled oscillator has been controlled to exactly the same value as the received carrier frequency. Putting the value I . ■= 1 + 1/K into the equation (1) it is obtained: fv = fi i.e. also the frequency of the output signal from the oscillator 8 during the interval t ~t<. is exactly the same as the received carrier frequency. By means of the described device thus it is attained that the oscillator already from the beginning of the measuring is controlled to the desired value and that this is carried out without any control error and that at the output of the amplifier 5 the signal , x K , x fi which thus constitutes an ' accurate analog signal proportional to the incoming carrier frequency.

Claims: WHAT WE CLAIM IS : A device for measuring a carrier frequency which varies from one pulse to another in an incoming pulsed carrier frequency signal and for delivering a continuous signal whose frequency corresponds to the measured frequency, the device including a frequency discriminator which gives an analog value corresponding to the frequency of the signal coming to the discriminator, a memory unit to store the analog value, a voltage controlled oscillator (8) to deliver said continuous signal in dependence on the magnitude of a control signal, and a controllable switching unit (1) to connect the frequency discriminator (3) to the input of the device during a first time interval, and to connect the discriminator to the voltage controlled oscillator during a second time interval, characterized in that said memory unit consists of a track-and-hold circuit (4) which is controlled so that during said first and second time interval [(t -t) and (t,.-t'), respectively]] the analog value obtained from the discriminator (3) is let through and stored, respectively, and that said control signal is created from the difference signal from a comparator circuit (6) which during said time interval compares the analog value obtained from the discriminator (3) with the value obtained from the track-and-hold circuit, an amplifier (5) with an amplification ≠ 1 being connected between the output of the track-and-hold and one input of the comparator circuit.

Inventors: MORWING B
Applicants: ERICSSON TELEFON AB L M, MORWING B
Application year: 1980
Main classification: G01S13
IPC codes: G01R23, G01S7, G01S13