GB2369512A

GB2369512A - Signal separation of spectrally close frequencies

Info

Publication number: GB2369512A
Application number: GB0024645A
Authority: GB
Inventors: Anthony David Newton
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2000-10-07
Filing date: 2000-10-07
Publication date: 2002-05-29
Also published as: GB0024645D0

Abstract

A circuit and method for signal separation of a composite signal containing two signals with spectrally close frequencies by filtering and cancellation mixing (110, 120, 130, 140, 150) with a frequency midway between the two signals to be separated. This provides the advantage of achieving similar filtering performance as given by ceramic filters, without requiring such filters.

Description

CIRCUIT AND METHOD FOR SIGNAL SEPARATION

Field of the Invention This invention relates to signal separation, and particularly though not exclusively to FM subcarrier separation in TV signals.

Background of the Invention In the field of this invention it is known that in"B" and"G"stereo/dual television an extra frequency modulated sub-carrier signal at 5.74 MHz is added to the composite video signal which already contains the monophonic 5.5 MHz sound subcarrier. In stereo mode the original sound subcarrier is modulated with combined (left + right) channels (and is therefore compatible with monophonic television receivers), while the new subcarrier is modulated by right information only. A matrix operation on the detected FM signals allows recovery of separate left and right signals. In the dual mode the two subcarriers are modulated by independent information such as dual languages.

However, this approach has the disadvantage (s) that, since in the receiver the two subcarriers are normally separated by multi-pole ceramic filters, and then demodulated by quadrature FM detector stages, in order to fully integrate the above functions it is necessary to resolve the problem of separating the two subcarriers

(spaced only 240 KHz apart) with on-chip active filters.

The stereo mode can tolerate considerable cross channel interference but the dual language mode requires much better separation. To achieve the same filtering performance as given by ceramic filters is not practical just by using on-chip active filters.

It is an object of the present invention to provide circuit and method of FM signal separation wherein the abovementioned disadvantage (s) may be alleviated.

Statement of Invention In accordance with-a first aspect of the present invention there is provided a circuit for signal separation as claimed in claim 1.

In accordance with a second aspect of the present invention there is provided a method for signal separation as claimed in claim 8.

Brief Description of the Drawing (s) One circuit and method for FM signal separation incorporating the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 shows a block schematic circuit diagram of a circuit for separating FM audio signals in an integrated circuit for TV signal processing; and FIG. 2 shows a graph illustrating the frequency response of filters within the circuit of FIG. 1.

Description of Preferred Embodiment (s) In an integrated circuit 100 for processing TV signals, TV sound intermediate frequencies of 33.4 and 33.16 MHz are applied to a first down-conversion stage 110 where they are down-converted to the standard baseband video subcarrier frequencies of 5.5 and 5.74 MHz using a local beat signal at the standard IF frequency of 38.9 MHz.

Cancellation mixing is used to reject the image frequency around 44 MHz.

In this technical field of radio signal processing"downconversion mixing"and"up-conversion mixing"are used widely to describe changing a signal's frequency either to a lower or a higher frequency by multiplying the signal by a local oscillator frequency. Multiplying two frequencies together produces two resultant frequencies, a sum frequency and a difference frequency, and by selecting one or the other resultant up-or downfrequency conversion is achieved. Selection is usually done with an appropriate bandpass filter.

The term"mixing"is often used in place of"multiplying" since the multiplication process is often approximated by adding together the local oscillator signal and the signal whose frequency is to be converted, and applying them to a non-linear device (mixer).

"Cancellation mixing"is a well-known method of separating the resultants of the multiplication without the aforesaid bandpass filter. A signal is multiplied by a local oscillator signal of phase zero and simultaneously by the, local oscillator at phase 90 degrees (I/Q mixing), and one multiplication output shifted by 90 degrees is added to or subtracted from another unshifted multiplication output, resulting in the either the sum or difference resultant being cancelled, leaving only a single resultant. This process can be demonstrated mathematically as follows : Multiplying a signal (sine) by a local oscillator signal (sins) produces sinA * sinE = 0. 5 (cos (A-B)-cos (A+B)) (1) and multiplying the signal (sima) by the local oscillator signal shifted through 90 degrees (cost) produces sinua * casE = 0. 5 (sin (A+B) + sin (A-B)) (2) Shifting the right side of equation (1) by 90 degrees (such that'cas'becomes'sin') produces : sinA * sinS = 0. 5 (sin (A-B)-sin (A+B)) (3) Adding or subtracting equation (3) and equation (2) cancels either the A+B'or the'f-B'term respectively, leaving only the desired single resultant.

It will be understood that similar results can be obtained without the 90 degree phase shift by utilising two stage mixing (two local oscillators) and six mixers.

The down-conversion stage 110 includes an automatic gain control (AGC) amplifier 111 which receives the combined signals at intermediate frequencies of 33.4 and 33.16 MHz. The AGC amplifier 111 is controlled by an audio detector 112 for enabling the AGC amplifier when audio is detected in known manner. The output of the AGC amplifier 111 is applied to mixers 113 and 114 which also receive a local beat signal at the standard IF frequency of 38.9 MHz derived from a 155.6 MHz signal through a divider 115. The outputs of the mixers 113 and 114 are applied via respective differential-to single-ended converters 116 and 117 to combiner 118 to produce the down converted signals at frequencies of 5.5 and 5.74 MHz.

The two FM sound subcarriers at 5.5 and 5.74 MHz are then applied to a second down-conversion stage 120 where they are further down converted to 1.12 and 0.88 MHZ respectively by mixing with a 6.62 MHz local signal. The down-conversion stage 120 includes mixers 121 and 122 which receive the combined signals at 5.5 and 5.74 MHz and also receive a local 6.62 MHz signal derived from a 25.48 MHz signal through a divider 123. The outputs of the mixers 121 and 122 are applied via respective differential-to single-ended converters 124 and 125 to combiner 126 to produce the down converted signals at frequencies of 1.12 and 0.88 MHz. This increases the ratio of channel spacing to carrier frequency, thus easing subsequent filter requirements. Again,

cancellation mixing is used to reject image signals around 12 MHz. Separation of the two FM signals starts in a first filter stage 130 having two active notch filters 131 and 132 at 0.88 MHz and 1.12 MHz respectively. The frequency response of the active notch filters 131 and 132 are shown in FIG. 2 by the lines 201 and 202 respectively.

Following the 0.88 MHz notch filter 131 is a bandpass filter 133 centered around 1.12 MHz. Following the 1.12 MHz notch filter 132 is a bandpass filter 134 centered around 0.66 MHz.

In order to compensate for component variations that would cause the filters to be off tune, all filters are voltage tuned and have the same type of structure as a VCO that is phase-locked to a 1 MHz signal (which is midway between the required signals). The filter tuning voltages are derived from a VCO tuning voltage. The VCO stage 140 includes a voltage controlled oscillator (VCO) 141 connected with a flip-flop 142 and a charge pump 143.

Further separation is then realised in a final separation stage 150 by cancellation down mixing each separated channel with the same 1 MHz local signal (which, as mentioned above, is exactly midway between the two signals). Since each signal is the image frequency of the other, a further degree of separation is achieved. The separation stage 150 includes pairs of mixers 151,152 and 153,154 which are also receive a 1 MHz signal derived from a 4 MHz signal through a divider 155. The outputs of the mixer pairs 151,152 and 153,154 are

connected through differential-to single-ended converter pairs 156, 157 and 158, 159 to combiners 160 and 161.

The separated signals, now both at 120 KHz, are then limited by respective limiters 162 and 163, and taken to respective quadrature FM detectors 164 and 165, followed by respective low-pass filters 166 and 167.

It will be understood that the circuit and method of FM signal separation described above provides the advantages of achieving similar filtering performance as given by ceramic filters, without requiring such filters.

Claims

1. A circuit for signal separation, comprising : means for receiving a composite signal containing first and second signals having respectively first and second spectrally close frequencies; cancellation mixing and filtering means for cancellation mixing and filtering the composite signal to separate therefrom the first and second signals, the cancellation mixing and filtering means being arranged to utilize a signal having a frequency between the first and second frequencies.

2. The circuit according to claim 1, wherein the frequency between those of the first and second frequencies is substantially equal to the mean average of the first and second frequencies.

3. The circuit according to claim 1 or 2, wherein the signal having a frequency between the first and second frequencies is also arranged to calibrate the cancellation mixing and filtering means.

4. The circuit according to claim 1,2 or 3, wherein the cancellation mixing and filtering means comprise means for down converting to a lower frequency in order to increase the ratio of signal spacing to carrier frequency.

5. The circuit according to any preceding claim wherein the first and second signals are FM signals.

6. A TV signal processing circuit including a circuit for FM signal separation according to claim 5.

7. An integrated circuit comprising a circuit according to any preceding claim.

8. A method for signal separation, comprising: receiving a composite signal containing first and second signals having respectively first and second spectrally close frequencies; cancellation mixing and filtering the composite signal to separate therefrom the first and second signals, the cancellation mixing and filtering utilizing a signal having a frequency between the first and second frequencies.

9. The method according to claim 8, wherein the frequency between those of the first and second frequencies is substantially equal to the mean average of the first and second frequencies.

10. The method according to claim 8 or 9, wherein the signal having a frequency between the first and second frequencies is also arranged to calibrate the cancellation mixing and filtering means.

11. The method according to claim 8,9 or 10, wherein the cancellation mixing and filtering step includes down converting to a lower frequency in order to

increase the ratio of signal spacing to carrier frequency.

12. The method according to any one of claims 8-11 wherein the first and second signals are FM signals.

13. A circuit for FM signal separation substantially as hereinbefore described with reference to the accompanying drawings.

14. A method for FM signal separation substantially as hereinbefore described with reference to the accompanying drawings.