GB2135543A - Modulation meters - Google Patents

Abstract

A modulation meter comprises a phase-lock loop receiver with storage means for maintaining lock despite loss of received signal, the loop including a mixer 21 arranged to mix frequency outputs from a comb generator 22 with the received signal 24. The difference frequency from the mixer is passed through gain control circuitry 30-32 and is locked to a predetermined frequency fREF by comparing in a frequency detector 34. The error is converted into a voltage for application to a voltage controlled oscillator 23 to shift the frequencies of the comb generator 22. The direction of the shift is evaluated by using a phase detector 36 to examine the actual phase relative to the predetermined frequency. The voltage and shift direction for application to the comb generator 22 are sampled and held in sample-and-hold units 39 and 37. In this way the modulation meter locks onto a carrier frequency of a received signal and a hold signal 33 is then applied to all sample-and-hold units, so that when the received signal is terminated and reappears the modulation meter is already locked on to the carrier frequency. <IMAGE>

Description

SPECIFICATION Improvements relating to modulation meters This invention relates to modulation meters for use in recognizing radio frequency signals.

It is necessary to test transmitters in order to check that they are functioning correctly. When a transmitter is switched on a number of test modulation tones are initially transmitted. These tones can be examined by demodulating them in a modulation meter, thereby verifying that correct modulation by the transmitter is taking place.

Figure 1 illustrates a modulation meter wherein a received RF signal from an input 4 is mixed, in a mixer 1, with a range of frequencies from a comb generator 2 driven by a voltage controlled oscillator 3. The comb generator produces an equally spaced series of frequencies .... ..... fun. The output of the mixer is passed through a selective filter 5 having a pass band centered around an intermediate frequency suitable for both an automatic gain means 6 to which the filter output is relayed and also for an input 7 of subsequent demodulation circuits (not shown). The meter is locked to the carrier frequency fc of the received RF signal by comparing in a detector 8 the gain means output with a reference intermediate frequency fREF from a source 9.The comparison is used to generate a control signal to alter the voltage controlled oscillator and hence the comb frequencies so that the modulus of fcfx is equal to fREF.

The locking to the carrier frequency requires a finite time and consequently some of the test modulation tones can be lost to the detriment of the test. Naturally, the carrier frequency could be measured before testing by using a frequency meter, and the measured frequency could then be programmed into the modulation meter. This is however inconvenient and requires either a means of measuring the frequency or prior knowledge thereof, before programming the frequency into the meter can be effected.

It is an object of the invention to provide a modulation meter substantially obviating the above mentioned problem of programming a measured carrier frequency into the meter.

According to the invention there is provided a modulation meter adapted to remain locked to a frequency of a received signal when said signal ends comprising means to mix at least one oscillator generated frequency with said received signal frequency in order to produce a difference frequency, means to derive a control signal dependent of said difference frequency, means to alter said at least one oscillator generated frequency in response to the control signal in order to substantially equate the difference frequency with a predetermined intermediate frequency, and means to hold said control signal following a termination of the received signal. In this way the modulation meter locks onto the carrier frequency of the received signal from a transmitter.When the transmitter is switched off and on for a second time the control signal is already present in the meter since it is held in the means to hold. Consequently the meter is already locked onto the carrier frequency. Therefore the test modulation tones are tuned into immediately when the transmitter is switched on again and they are therefore not lost. This permits verification that the transmitter is functioning correctly. The necessity for either measuring or having prior knowledge of the transmitter carrier frequency is eliminated, thereby increasing the flexibility of use for the modulation meter in accordance with the invention.

Embodiments of the invention are now described and illustrated, by way of example only, in the accompanying drawings in which: Figure 1 illustrates a prior art modulation meter; Figure 2 illustrates a modulation meter in accordance with the invention; Figure 3 illustrates a test timing sequence of the meter shown in Figure 2.

Referring to Figure 2, an RF signal having a carrier frequency fc is received at an input terminal 24 and passed to a mixer 21. In the mixer, the construction of which is well known to those skilled in the art, the frequency fc is mixed with a discrete equal spaced series of frequencies ... .... N relayed from a comb generator 22.

The comb generator is driven by a voltage controlled oscillator 23. The series of frequencies from the comb generator can be increased or decreased by applying a positive or negative voltage respectively to a control terminal of the oscillator 23. The output of the mixer is passed through a selective filter 25 to a gain control means 30, the filter having a pass band centered on a predetermined intermediate frequency f,F selected to be suitable for the gain control means and subsequent stages of the modulation meter.

The gain control means is adjusted by a feedback loop from a level detector 31 monitoring the control means output level. In this way the level detector output can be adjusted to and maintained close to a predetermined level suitable for subsequent stages of the modulation meter. The feedback loop comprises a sample and hold unit 32 which samples the level detector control level while it is fed back to an automatic gain control terminal of the gain control means.

Under the control of a hold signal on a line 33, the unit holds the most recent sample and passes the held sample to the gain control terminal.

The difference frequency appearing at the level detector output is the modulus of fcfx which lies in said pass band centered on f,p It is desired to lock the detector output to the predetermined frequency fIF. In order to do this a comparison between f,F and modulus fcfx is evaluated by feeding the level detector output to a frequency detector 34 which compares this output with a reference intermediate frequency fREF on a line 35 which has a value equal to f,F. The difference is converted into a voltage control signal V having a size which when applied to the voltage controlled oscillator 23 shifts frequency fx such that the modulus of fcfx is equal to fgp However the shift can be either an increase in frequency or a decrease, dependent on whether f,F is greater or smaller than the level detector output frequency.

Therefore the polarity of the voltage V is dependent on the direction of the shift required.

Consequently the detector 31 output is relayed to a phase detector 36, also receiving the reference intermediate frequency on line 35. The phase detector evaluates whether the detector output advances or lags with respect to the reference frequency. The phase detector produces a polarity signal to alter the frequency detector output to the required positive or negative voltage as described hereinabove.

The polarity signal from the phase detector is sampled by a sample and hold unit 37, whilst being passed to a polarity device 38 receiving the frequency detector output voltage V. The voltage is consequently made positive or negative by the polarity device. The voltage value and sign of V is sampled by a sample and hold unit 39 whilst being passed to the voltage controlled oscillator 23, which shifts the series of frequencies f1 ... fx ... fN in accordance with the sign and size of the voltage. Under the control of a hold signal on line 33 the units 37 and 39 hold the most recent sample and pass them to the polarity device and controlled oscillator respectively.

Referring to Figure 3, in use the modulation meter is used to test a transmitter. Initially the input 24 is connected to receive transmitted signals and the transmitter typically transmits five 100 millisecond tones after being switched on at time t,. The modulation meter adjusts to lock on to the carrier frequency and after a short time period achieves a locked state. However of the five tones transmitted the first two, for example, are missed as illustrated by the dotted lines in Figure 3d). At a time T, after locking has occurred a hold signal is applied to all the sample and hold units. Therefore when the transmitter is switched off at a later time t2 the locked state is retained and consequently when the transmitter is switched back on at a still later time t2 all five tones are recovered and testing of the transmitter is successfully achieved.

Clearly the modulation meter described hereinabove can be employed automatically with different transmitter frequencies without the necessity of prior knowledge of and programming of the transmitter frequency. It will be apparent that the sequence of events illustrated in Figure 3 can be achieved either manually or automatically.

Claims (7)

1. A modulation meter adapted to remain locked to a frequency of a received signal when said signal ends comprising means to mix at least one oscillator generated frequency with said received signal frequency in order to produce a difference frequency, means to derive a control signal dependent on said difference frequency, means to alter said at least one oscillator generated frequency in response to the control signal in order to substantially equate the difference frequency with a predetermined intermediate frequency, and means to hold said control signal following a termination of the received signal.

2. A meter as claimed in claim 1 wherein said means to derive a control signal comprises a frequency detector and phase detector each receiving said difference frequency, the frequency detector providing a signal indicative of the error between the difference frequency and the predetermined intermediate frequency and the phase detector being responsive to whether the difference frequency is greater or smaller than the predetermined intermediate frequency to indicate the polarity of said error.

3. A meter as claimed in claim 2 wherein said means to hold comprises a first means to hold said signal indicative of the error and a second means to hold said indication of the polarity of the error.

4. A meter as claimed in any preceding claim wherein said means to mix includes an automatic gain control means and said means to hold further includes a means to hold an output of said gain control means, the output to be applied to a control terminal of said gain control means.

5. A meter as claimed in any preceding claim wherein said means to hold is user operated.

6. A modulation meter as claimed in any preceding claim wherein said at least one oscillator generated frequency is generated from a comb oscillator.

7. A modulation meter substantially as herein described with reference to Figures 2 and 3.