WO2002029425A1

WO2002029425A1 - Rf power measurement

Info

Publication number: WO2002029425A1
Application number: PCT/GB2001/004378
Authority: WO
Inventors: Oliver James Hilton
Original assignee: Racal Instruments Limited
Priority date: 2000-10-04
Filing date: 2001-10-02
Publication date: 2002-04-11
Also published as: EP1322969B1; ATE286258T1; CN1190666C; MXPA03002904A; GB2367633A; DE60108143D1; EP1322969A1; GB0024325D0; CN1468376A; AU2001292049A1; KR20030053512A; JP2004510987A; US20040130333A1

Abstract

A RF power measurement circuit for measuring RF power of a pulsed RF input signal has a sigma-delta ADC circuit and an AND gate for controlling delivery of clock pules to a clock input of the ADC circuit. The clock pulses are delivered to the clock input only within the duration of RF pulses of the pulsed RF input signal enabling a low speed ADC circuit to be used.

Description

RF POWER MEASUREMENT

This invention relates to the measurement of RF power, and the invention relates

particularly to a RF power measurement circuit and method employing an over-

sampling analogue-to-digital conversion (ADC) circuit. As known to those skilled in

the art, an over-sampling ADC requires a finite number N of clock cycles (where N

> 1) to complete a single conversion.

Figure 1 of the accompanying drawings is a block diagram showing a known RF

power measurement circuit. It comprises a RF input 11, a load 12, a Schottky diode

13, an over-sampling ADC circuit in the form of a sigma-delta ADC circuit 14 and a

clock generator 15.

In use, Schottky diode 13 functions as a detector which outputs voltage related to RF

power reaching the RF input 11. The ADC circuit 14 converts this voltage to digital

format giving a measure of RF power which can be stored for further processing.

The clock generator 15 supplies clock pulses to the clock input C_τ of the ADC circuit

14, and these pulses define the conversion rate of the ADC circuit. The conversion

time T_C0NV of the ADC circuit is given by the expression:

T

¹ CONV = ^N

F ^x CLOCK where N is the number of clock pulses required to complete a single conversion and

F_CL0CK is the frequency of the clock generator 15. Hitherto, it has been customary to

use a high speed ADC circuit having a conversion time T_C0NV which is much shorter

than the RF pulse length, and so several power measurements can be derived from

each RF pulse. Generally, a sigma-delta ADC circuit has been preferred since high

ADC resolution can be realised at lower cost than other conversion techniques.

It would be desirable to use a low speed over-sampling ADC circuit which is

significantly cheaper than a high speed over-sampling ADC circuit and yet is still

capable of achieving high ADC resolution leading to improved measurement

accuracy. However, a low speed over-sampling ADC circuit presents a significant

technical problem when the conversion time of the ADC circuit is longer than the RF

pulse length.

According to one aspect of the invention there is provided a RF power measurement

circuit for measuring RF power of a pulsed RF input signal comprising detector means

for converting said input signal to voltage, over-sampling analogue-to-digital

conversion means for deriving a measure of RF power from said voltage, clock

generation means for generating clock pulses and means for controlling delivery of

said clock pulses to a clock input of said over-sampling analogue-to-digital conversion

means. In a preferred implementation, said means for controlling causes said clock pulses to

be delivered to said clock input only within the duration of RF pulses of the pulsed RF

input signal and, preferably, after the start of respective RF pulses. Since, with this

arrangement, clock signals are delivered to the clock input of the ADC only within the

duration of the RF pulses, it is possible to derive a single power measurement from

two or more successive RF pulses. Accordingly, this arrangement enables use of a

low speed over-sampling ADC having a conversion time longer than the RF pulse

length.

According to another aspect of the invention there is provided a method of measuring

RF power of a pulsed RF signal comprising the steps of converting the pulsed RF

signal to voltage, using over-sampling analogue-to-digital conversion means to derive

a measure of RF power from said voltage and controlling delivery of clock pulses to

a clock input of said over-sampling analogue-to-digital conversion means.

An embodiment of the invention is now described, by way of example only, with

reference to the accompanying drawings of which:

Figure 1 is a block diagram showing a known RF power measurement circuit,

Figure 2 is a block diagram showing a RF power measurement circuit according to the

invention, and Figures 3(a) to 3(d) are timing diagrams useful in understanding operation of the RF

power measurement circuit shown in Figure 2.

Referring to Figure 2, many of the circuit components are the same as those used in

the circuit of Figure 1, and have been ascribed like reference numerals.

As in the case of the circuit shown in Figure 1, a RF input signal R_m (shown in Figure

3(a)) is received at the RF input 11, terminated by the load 12 and converted by

Schottky diode 13 to voltage N_D (shown in Figure 3(b)) related to RF power reaching

the RF input 11. The sigma-delta ADC circuit 14 then converts the detected voltage

V_D to digital format and thereby generates data representing a measure of RF power

at the converter output.

As before, the RF power measurement circuit 14 has a clock generator 15, but differs

from the circuit of Figure 1 by provision of AND gate 16 connected between the clock

generator 15 and the clock input of the ADC circuit 14. The AND gate 16 has an

output terminal connected to the clock input, a first input terminal connected to the

clock generator 15 and a second input terminal connected to receive an externally

generated 'Pulse-on' signal P_0N in the form of a pulse train (shown in Figure 3(c)).

The AND gate 16 functions, in effect, as a switch enabling clock pulses to be

delivered to the clock input of the ADC circuit 14 while the switch is closed (when

P_ON is "high") and preventing such delivery when the switch is open (when P_0N is "low") . The timing of the 'Pulse-on' signal is related to the timing of the RF pulses

of the RF input signal and its effect is to enable delivery of clock pulses to the clock

input C_j of the ADC circuit 14, but only within the duration of the RF pulses (as

shown in Figure 3(d)). Thus, pulses will only be delivered to the clock input when a

RF pulse is present.

As shown in Figure 3(b), Schottky diode 13 has a significant time constant causing

the detection voltage N_D to rise and fall exponentially in response to leading and

trailing edges respectively of each RF pulse. To take account of this, each pulse of

the 'Pulse-on' signal P_0Ν is generated a short time after the start of the associated RF

pulse thereby allowing the detector voltage sufficient time to settle before any clock

pulses are delivered to the clock input of the ADC circuit. In practice, to enable

accurate power measurements to be made, the detector time constant should be much

shorter than the RF pulse length.

Since the clock signals are delivered to the clock input of the ADC circuit 14 only

within the duration of the RF pulses, it is possible to derive a single power

measurement from two or more successive RF pulses; accordingly, a low speed ADC

circuit can be used, having a conversion time which is much longer than the RF pulse

length.

Alternatively, a high speed ADC circuit could be used; in this case, the timing of the 'Pulse-on' signal P_0N will eliminate power measurements that would otherwise be

made while the detector voltage V_D is rising or falling, thereby improving accuracy

of the measurement.

It will be appreciated that although the RF power measurement circuit described with

reference to Figure 2 includes a sigma-delta ADC circuit, other forms of over-

sampling ADC circuit could alternatively be used.

Similarly, although an AND gate 16 and a Schottky diode 13 have been used other

components capable of performing substantially the same switching and detection

functions could alternatively be used e.g. a synchronous clock enabling circuit could

be used for the switching function and a field effect transistor could be used for the

detection function.

It will also be appreciated that RF power may be measured over any desired part of

the input waveform, which need not necessarily include a RF pulse.

Claims

1. A RF power measurement circuit for measuring RF power of a pulsed RF input

signal comprising detector means for converting said input signal to voltage, an over-

sampling analogue-to-digital conversion means for deriving a measure of RF power

from said voltage, clock generation means for generating clock pulses and means for

controlling delivery of said clock pulses to a clock input of said over-sampling

analogue-to-digital conversion means.

2. A measurement circuit as claimed in claim 1 wherein said means for

controlling is connected between said clock generation means and said clock input and

is responsive to a control signal whose timing is related to the timing of RF pulses of

the pulsed RF input signal.

3. A measurement circuit as claimed in claim 1 or claim 2 wherein said means for

controlling causes said clock pulses to be delivered to said clock input only within the

duration of RF pulses of the pulsed RF input signal.

4. A measurement circuit as claimed in claim 3 wherein said means for

controlling causes said clock pulses to be delivered to said clock input after the start

of respective said RF pulses.

5. A measurement circuit as claimed in claim 1 wherein said means for

controlling comprises switching means responsive to control pulses.

6. A measurement circuit as claimed in claim 5 wherein the timing of said control

pulses is related to the timing of RF pulses of said pulsed RF input signal.

7. A measurement circuit as claimed in claim 6 wherein the leading edge of each

control pulse follows the start of a respective said RF pulse.

8. A measured circuit as claimed in any one of claims 5 to 7 wherein said

switching means is an AND gate.

9. A measurement circuit as claimed in any one of claims 1 to 8 wherein said

detection means is a Schottky diode.

10. A measurement circuit as claimed in any one of claims 1 to 9 wherein said

over-sampling analogue-to-digital conversion circuit is a sigma-delta analogue-to-

digital conversion circuit.

11. A measurement circuit as claimed in any one of claims 1 to 10 wherein said

over-sampling analogue-to-digital conversion circuit has a conversion time longer

than the RF pulse width.

12. A method of measuring RF power of a pulsed RF signal comprising the steps

of converting the pulsed RF signal to voltage, using over-sampling analogue-to-digital

conversion means to derive a measure of RF power from said voltage and controlling

delivery of clock pulses to a clock input of said over-sampling analogue-to-digital

conversion means.

13. A method as claimed in claim 12 wherein said controlling step causes said

clock pulses to be supplied to said clock input only within the duration of RF pulses

of the pulsed RF signal.

14. A method as claimed in claim 13 wherein said controlling step causes said

clock pulse to be delivered to said clock input after the start of respective said RF

pulses.

15. A RF power measurement circuit substantially as hereindescribed with

reference to Figures 2 and 3 of the accompanying drawings.

16. A method of measuring RF power substantially as hereindescribed with

reference to Figures 2 and 3 of the accompanying drawings.