Wide band flat power detector

Abstract: A wideband detector having a substantially flat response is made by using the signal at the isolation (15) port of a coupler (11,13). As the response or transfer characteristic at the isolation port is substantially linear, a matching network (23) having the inverse or opposite transfer characteristic with respect to frequency may be connected to the coupler. The addition of the matching network transfer characteristic to the transfer characteristic of the coupler in the isolation port produces a superposition. The matching network transfer characteristic compensates for the linear varying transfer characteristic at the coupler isolation port to produce a substantially flat output of the matching network. The detector (25) connected to the matching network output will then produce a detector signal which is substantially flat over the frequency range of the bandspread of interest.

Description: This invention relates to wideband flat power detectors for detecting output power of an amplifier over a wide frequency band. Detectors useful for sensing a voltage or current level indicative of the power output level at an amplifier are known. Additionally, in the field of microwave transmission, micro-strip directional couplers are also known. For example, a micro-strip directional coupler is disclosed in the specification of U.S. Patent No. 5,159,298. Further, microwave filters impedance matching networks, and couplers are disclosed in "Microwave Filters, Impedance-Matching Networks, and Coupling Structures. George L. Matthaei, Leo Young, and E.M.T. Jones; McGraw-Hill Book Company, New York; 1964, Library of Congress Catalog Card No. 64-7937." Such Detectors for measuring amplifier RF power, for example, may be coupled to an RF power output line through a microwave coupler. At microwave frequencies, the coupler may be a micro-strip coupler as disclosed in the specification of U.S. Patent No. 5,159,298. Where the RF power is to be transmitted over a wideband of frequencies, the coupler may exhibit a transfer function varying over that wide frequency band. For example, where the coupler has the characteristics of a bandpass filter, the power out of the coupler at the coupler port may vary, with an increasing power transfer characteristic as frequencies advance in bandpass from the low end with one or more peaks in the center of the bandspread and then decreasing towards the upper end of the bandpass. A matching network may then be used to compensate for that frequency varying transfer function. The purpose of the matching network is to receive the frequency varying output of the coupler and by the additional effect of the matching networks transfer characteristic, produce as substantially a flat power to frequency output response as possible to a detection device such as a detector diode. However, as the coupler transfer characteristic is non-linear, the power frequency transfer characteristic at the diode detector varies nonlinearly with frequency. This results from the difficulty of matching that nonlinear coupler characteristic. Accordingly, the final signal out such as a voltage in the case of the detector diode, varies from a true signal substantially indicative of the RF amplifier power out over the frequencies in the bandpass. International Application WO-A-9101514 discloses a broadband power detector and detection method in accordance with the preambles of Claims 1 and 9. The present invention aims to provide a broadband power detector and detection method which produce an improved output signal which is substantially flat and is indicative of the power input over the bandspread of interest. According to the invention, there is provided, in one aspect, a broadband power detector coupled to a power source and producing an output signal which is indicative of the power of said power source over a finite width frequency band and which is flat with respect to frequency, the detector comprising: a directional coupler having a primary circuit and a secondary circuit, said primary circuit being connected to a load having an impedance matching the output impedance of said primary circuit, and said secondary circuit having a coupling port towards which a primary current propagates in said secondary circuit in the opposite direction to the current flow in said primary circuit and an isolation port towards which a secondary current in said secondary circuit propagates in the same direction as said current flow in said primary circuit; and a matching network having an input port and an output port, said matching network input port being connected to said coupler isolation port; characterised in that said coupler couples said power source to said isolation port so as to produce an output signal at said isolation port which varies substantially linearly with respect to frequency over said finite frequency band, said matching network has an input to output port response which compensates for said substantially linear variation of said isolation port output signal with respect to frequency, and said matching network produces a flat output signal with respect to frequency over said finite frequency band. In another aspect, the invention provides a method of producing a power indicative signal which is flat with respect to frequency over a finite width frequency band, comprising the steps of: connecting a primary circuit of a directional coupler to a source of power having frequencies varying over a finite width frequency band; connecting an output port of said primary circuit to a matched impedance; connecting a matched impedance to a coupling port of a secondary circuit of the coupler, towards which coupling port current in said secondary circuit flows in the opposite direction to current flow in said primary circuit; and connecting a matching network to an isolation port of said secondary circuit of said coupler towards which isolation port current in said secondary circuit flows in the same direction as current flow in said primary circuit; characterised by coupling said isolation port to said power source so as to produce a signal at said isolation port which varies substantially linearly with respect to frequency over said finite frequency band, transmitting said signal at said isolation port to said matching network which has a linear transfer characteristic with respect to frequency and which compensates for said linearly varying signal at said isolation port, and producing a flat output signal with respect to frequency over said finite frequency bandwidth, at the output of said matching network. Advantageously, the transfer characteristic for detector voltage to RF power over the frequency bandspread (in the preferred embodiment measured at the output of a power amplifier) is made substantially flat by terminating the coupling port of the RF coupler with a matched impedance and connecting the coupler isolation port to the detector through a compensating linear matching network. With this arrangement, the coupler transfer characteristic varies linearly with respect to frequency within the bandpass of the coupler. A compensating linear matching network can thus be connected to the linearly coupling network to produce a substantially flat RF power output with respect to varying frequency. The transfer characteristic of the coupler at the isolation port, varying linearly with frequency, may have a positive slope over the frequency bandspread of interest. For a positive slope, more power is coupled to the detector network as frequency is increased. The matching network, may then be designed for a negative slope linear transfer characteristic with respect to increasing frequency. The super-position or addition of the characteristics of the coupler and the matching network then produces substantially flat response at the output of the matching network and to the detector. Accordingly, the power from the matching network into a sensing device such as a detector diode, for example, would then be essentially flat over the RF power frequency bandspread of interest. As a result, the voltage produced by the detector diode would in the preferred embodiment be substantially unaffected by the variations in the RF frequency and would be a substantially accurate representation of RF power out. Preferably, a detector diode is used to produce a voltage indicative of RF power. That diode detector has an internal junction capacitance and a resistance. In the design of the matching network, the junction capacitance may be included within this part of the matching network. The invention will now be described, by way of example with reference to the accompanying drawings in which: Figure 1 shows a conventional system for producing a voltage responsive to RF power. Figure 2 shows the coupled port response within the coupler of Figure 1 with respect to frequency. Figure 3 shows the matching network response with respect to frequency of the matching network shown in Figure 1. Figure 4 shows the detector diode incident power response with respect to frequency for the detector shown in Figure 1. Figure 5 shows the preferred embodiment of an RF power detector according to the inventive principles. Figure 6 shows the isolation port response for the coupler shown in Figure 5 with respect to frequency. Figure 7 shows the matching network response for the matching network shown in Figure 5 with respect to frequency. Figure 8 shows an improved and essentially flat response for the detector diode incident power response with respect to frequency for the detector shown in Figure 5. Referring to the drawings, a circuit schematic for a known (prior art) wideband power detector is shown in Figure 1. The power detector 10 is shown in Figure 1 by including a micro-strip directional coupler having a primary 11 and secondary 13. The isolation port for the secondary of the coupler is shown at 15 connected to a matching impedance 17. The coupled port 19 is shown connected at input 21 to a matching network shown as 23. A detector diode 25 is shown producing a detector voltage output at 27. In operation, the direction of propagation in the primary 11 of the coupler is shown by arrow 31. The primary direction of current propagation in the secondary 13 of the coupler is shown by arrow 33. A secondary direction of current propagation in the secondary 13 of the coupler is shown by arrow 35. An electromagnetically induced current in the secondary will produce a primary flow 33 in the opposite direction to the primary flow 31 in the coupler primary 11 and a secondary or counterflow 35 in the coupler secondary 13 which flows in the same direction as the of flow of current, in the primary 11, as shown by arrow 35. The coupled port 19 transfer characteristic or response with respect to a varying RF frequency is nonlinear as shown in Figure 2. This response may be nonlinear as shown or may be nonlinear, having several peaks, depending on the characteristic of the coupling network. The coupler isolation port 15 would have a matching or terminating resistor 17 and the coupling coefficient can be varied by the physical location or proximity of the secondary 13 with respect to the primary 11. As the desired or preferred response of the detector 25 is flat with respect to RF frequency, it is necessary to design a matching network that compensates for the nonlinearity at the coupling port 19 as shown in Figure 2. For example, a matching network 23 response compensating the nonlinearity of the coupler may be as shown in Figure 3. However, attempting to produce a substantially flat detector response starting with the nonlinear coupled port 19 response, at best attainable was a nonlinearly varying response as shown in Figure 4 by a ripple. In the embodiment, shown in Figures 5 through 8, a substantially flat detector diode incident power response to frequency is accomplished by reversing the secondary of the coupler and by using the secondary flow of current 35 in the coupler secondary 13 to produce a signal indicative of RF power out. As shown in Figure 5, and where the same numerals are used to show the same or similar parts as in Figure 1, RF current is made to flow within the primary of the coupler 11 as shown by arrow 31 from RF IN to RF OUT. As stated above, a primary current shown by arrow 31 in primary 11 induces primary current 33 in the secondary 13 of the coupler. A secondary or counterflowing induced current in secondary 13 is shown by arrow 35 flowing opposite to that of the primary induced current 33. This secondary current 35 flows in the same direction as the current 31 in primary 11. The secondary current 35 should be zero or a minimal, under optimum design conditions, where the load impedance at the output of the coupler primary 11 matches the coupler output impedance. Under this impedance matching condition, a current reflected from the coupler primary 11 output port should be zero or minimal. Under the optimum impedance matching conditions, the induced current 35 flowing in the coupler secondary 13 toward the isolation port 15 should have no or minimal component due to any reflection in the coupler. As shown in Figure 5, the coupled port 19 is connected to terminating resistance 17. The termination or isolation port shown as 15 is coupled to the input 21 of the matching network 23. The direction of current flows in the secondary define the coupling and isolation ports. As a result, the counter or secondary induced current 35 flowing in the coupler secondary 13 is transmitted to the matching network 23 and through the matching network 23 to the detector diode 25, producing a detector voltage output 27. The embodiment, shown in Figure 5 and, with a matching primary load impedance as described above, the induced current in coupler secondary 13 produces a substantially linear response at the isolation port 15 with respect to frequency, as shown in Figure 6. It is easier to compensate a linear varying response as shown in Figure 6, than the nonlinearly varying response as shown in Figure 2. In the embodiment, the matching network 23 response may be designed to have a substantially linear transfer characteristic with respect to increasing frequency. In the preferred embodiment, where the response of the coupler with respect to increasing frequency is shown as substantially linear with a positive slope, the matching network 23 response over the same frequency range would be substantially linear and with a negative slope. Accordingly, the output of the coupler at the isolation port increases substantially linearly as the frequency of the RF current through the coupler primary is increased. However, as shown in Figure 7, the matching network response is made to decrease with respect to increasing frequency. The addition of the transfer responses of the coupler at its isolation port 15, as shown in Figure 6, to the response of the matching network 23, as shown in Figure 7, produces a substantially flat Detector Diode Incident Power Response with respect to frequency, as shown in Figure 8. The construction of the matching network 23 incorporates within it the junction capacitance of the detector diode. What is disclosed is a way of producing a substantially linear transfer responses with respect to frequency so an output signal is substantially flat with respect to a broadband frequency range. The particular design of the coupling network at microwave or other frequencies or of the matching network or the use of a sensor such as a detector diode for detecting incident power is known, therefore is not shown in detail here.

Claims: A broadband power detector (10) coupled to a power source and producing an output signal which is indicative of the power of said power source over a finite width frequency band and which is flat with respect to frequency, the detector comprising: a directional coupler having a primary circuit (11) and a secondary circuit (13), said primary circuit (11) being connected to a load having an impedance matching the output impedance of said primary circuit, and said secondary circuit (13) having a coupling port (19) towards which a primary current propagates in said secondary circuit in the opposite direction to the current flow in said primary circuit and an isolation port (15) towards which a secondary current in said secondary circuit propagates in the same direction as said current flow in said primary circuit; and a matching network (23) having an input port (21) and an output port, said matching network input port (21) being connected to said coupler isolation port (15); characterised in that said coupler (11,13) couples said power source to said isolation port (15) so as to produce an output signal at said isolation port (15) which varies substantially linearly with respect to frequency over said finite frequency band, said matching network (23) has an input to output port response which compensates for said substantially linear variation of said isolation port (15) output signal with respect to frequency, and said matching network (23) produces a flat output signal with respect to frequency over said finite frequency band. A power detector (10) as claimed in Claim 1 wherein said input to output response of said matching network (23) over said finite width frequency band is the opposite to, or the inverse of, said linearly varying output signal at said isolation port (15). A power detector (10) as claimed in Claim 1 or 2 wherein said coupler has a matched impedance (17) connected to said coupling port (19). A power detector (10) as claimed in any one of Claims 1 to 3 wherein a detector means (25) is connected to said matching network (23) output port and said detector means (25) produces a signal indicative of power from said power source in response to said matching network (23) output signal at said output port. A power detector (10) as claimed in Claim 4 wherein said detector means is a diode detector (35) having an internal junction capacitance and said matching network (23) includes said junction capacitance. A power detector (10) as claimed in any preceding claim wherein said secondary current in said secondary circuit (13) of said coupler (11,13) propagates substantially in the direction of said current in said primary circuit (11), of said coupler (11,13) as a reflected wave. A power detector (10) as claimed in Claim 6 wherein said output signal at said isolation port (15) increases with respect to increasing frequency and said matching network (23) input to output port response is a decreasing response with respect to increasing frequency. A power detector (10) as claimed in claim 7 wherein said matching network (23) response decreases at a rate which is substantially that of the rate of increase of said isolation port (15) output signal with respect to increasing frequency. A method of producing a power indicative signal which is flat with respect to frequency over a finite width frequency band, comprising the steps of: connecting a primary circuit (11) of a directional coupler (11,13) to a source of power having frequencies varying over a finite width frequency band; connecting an output port of said primary circuit to a matched impedance; connecting a matched impedance (17) to a coupling port (19) of a secondary circuit (13) of the coupler (11,13), towards which coupling port (19) current in said secondary circuit (13) flows in the opposite direction to current flow in said primary circuit (11); and connecting a matching network (23) to an isolation port (15) of said secondary circuit (13) of said coupler (11,13), towards which isolation port (15) current in said secondary circuit (13) flows in the same direction as current flow in said primary circuit (11); characterised by coupling said isolation port (15) to said power source so as to produce a signal at said isolation port (15) which varies substantially linearly with respect to frequency over said finite frequency band, transmitting said signal at said isolation port (15) to said matching network (23) which has a linear transfer characteristic with respect to frequency and which compensates for said linearly varying signal at said isolation port (15), and producing a flat output signal with respect to frequency over said finite frequency bandwidth, at the output of said matching network (23). A method as claimed in Claim 9 including the step of using a matching network (23) having a decreasing slope linear transfer characteristic with respect to increasing frequency where the output at said isolation port (15) increases linearly with increasing frequency. A method as claimed in Claim 10 including the step of connecting a diode detector (25) to said output signal at the output of said matching network (23) to produce said power indicative signal. A method as claimed in Claim 11 including the step of making the junction capacitance of said diode detector (35) part of said matching network (23). A method as claimed in Claim 12 including the step of inducing a current flow in the secondary circuit (13) of the coupler (11,13) and transmitting the current induced in the secondary circuit (13) of said coupling means to said matching network (23).

Inventors: LEANES JAMES A, WU WANG CHIU, LEANES, JAMES A., WU, WANG CHIU
Applicants: HARRIS CORP, HARRIS CORPORATION
Application year: 1993
Main classification: G01R21
IPC codes: G01R21, H01P5, H01P1