DE3733967C2 - - Google Patents

Description

The present invention relates to a narrowband receiver for receiving the signals from Time signal transmitters such as B. the DCF-77 transmitter according to the preamble of claim 1. An example for such a circuit is in the DE 35 40 380 A1 described.

In these known heterodyne receiver circuits one or more mixing stages are arranged, to the input signal on a corresponding Amplify the intermediate frequency without feedback and to be able to filter. Then then e.g. B. demodulated by rectification.

The general state of the art is e.g. B. also presented in the PhD thesis by R. Bermbach "New radio clock concepts through close integration of receiver and microcomputer", Darmstadt dissertation 1985, pages 29 to 50. Common straight-line and superimposed receivers as well as the usual selection principles (coil filters, RC filters, mechanical filters, digital filters) are described there. Other circuit examples can be found in DE-OS 27 30 153 and DE 28 00 253 C2. In the first of these documents it is shown, see FIG. 1 there, how the received signal is divided into two intermediate frequency signals with two mixers and added after a phase shift and filtering. In the second of the cited documents, an overlay receiver is described which works with two antenna rods offset by 90 ° from one another.

Another known method is that of Synchronous demodulation. This mixes the input signal with a mixing frequency that is exactly the Carrier frequency of the signal to be demodulated corresponds. This method has over the first the advantage that with very simple filters high filter effect can be achieved and is already demodulated by mixing. In addition can the superimposition of the useful signal by mirror frequencies excluded with the help of the simplest filter will. However, the disadvantage is Effort (PLL) to generate the exactly coupled Mixing frequency is required. In particular make the reductions in the carrier signal more difficult the construction of a stable PLL circuit.

With all these different known basic circuits the receiver technology are radio clock receivers been realized. The goal of small, reliable and above all very cheap radio clock is with none of the principles used so far been achieved.

Compared to the described methods can by the inventive use of the from the book von Lüke, H. D. "Signalverarbeitung" 3rd edition, Pages 187-192 known quadrature overlay method the benefits of mixing down lowest frequencies are maintained without a special effort for the mixed cycle generation would be required. By different choice of Mixing frequencies can be the various available Time signal transmitters are selected, with what universal time signal receiver possible is.  

Basic principle of quadrature overlay is the mix of the input signal with two Mixed frequency signals that are the same frequency, but a phase shift of 90 ° to each other have.

This procedure is described in the literature on transmission technology, for example in the book by Lüke, H. D. "Signalverarbeitung", 3rd edition, pages 186- 187 as a comfortable but difficult too implementing methods described so that the less complicated straight-ahead and the overlay receiver be brought forward.

The effort for the purely analog quadrature overlay on the one hand arises from the high demands the accuracy of the multipliers, a quadrature overlay in digital representation would, on the other hand, be unrealistically high Processing rates in the signal evaluation circuits require.

Therefore, the quadrature overlay has so far been used not yet used in the time signal receivers, where you don't have a large apparatus Can make effort.

The invention has for its object to design more powerful and at the same time cheaper receiving circuits than they have been known so far. This object is achieved in that a quadrature superimposition circuit is used as the device for mixing and filtering. The fact that the principle of quadrature overlay is first introduced in radio clocks, which ensures the quality or performance, and that scanning mixers are also introduced, the receiver can be realized in a compact and inexpensive manner. This means that with the use of quadrature superimposition in a narrowband receiver according to the invention and in particular with the aid of scanning mixers, a receiver is obtained which is very easy to implement, especially for very narrow reception bands, and nevertheless has all the advantages of the known analog method.

In the following it is first described how through the principle of quadrature overlay AM- modulated, carrier-frequency signals mixed down and be demodulated at the same time. The calculation underlying circuitry corresponds the information in claim 2. The description takes place in the time range in which makes the calculation easy. Subsequently does the calculation of the quadrature scan presented in the order after corresponds to claims 3 to 6.

The quadrature overlay in the time domain

In the following the quadrature overlay is considered in the application to the character signal. If the useful signal y (t) is modulated onto a sine carrier of frequency f ₀, the function h (t) results , see Figure 1

h (t) = y (t) cos (2 π f ₀ t) (1)

This carrier-frequency signal h (t) is now not mixed in a known manner with a single frequency, but it is combined on the one hand with a signal sin (2 π f _m t) and on the other hand with a signal cos (2 π f _m t) multiplied. The frequency f _m can be chosen arbitrarily. The resulting signals are called x ₁ (t) and x ₂ (t) . First, x ₁ (t) is calculated

x ₁ (t) = h (t) sin (2 π f _m t)
= y (t) cos (2 π f ₀ t) sin (2 π f _m t)
= y (t) · (sin (2 π (f _m - f ₀ ) t) + sin (2 π (f _m + f ₀ ) t) / 2 (2)

The summand of the frequency f _m + f ₀ can be eliminated by filtering with the simplest filters, particularly if the mixed frequency f _m in the order of magnitude of f ₀ has been selected in accordance with claim 2. This particular choice of the mixing frequency also makes it possible to place the image frequencies occurring during the mixing down so close to the useful signal frequency that they become harmless. The filtered residual signal is called x _{1 r} (t) . For the sake of simplicity, the forward gain of the filter function should always be chosen so that any pre-factors of the filtered signal can be shortened to one:

x _{1 r} (t) = y (t) sin (2 π (f _m - f ₀ (t) (3)

The same calculation for x ₂ (t) gives:

x ₂ (t) = h (t) cos (2 π f _m t)
= y (t) cos (2 π f ₀ t) cos (2 π f _m t)
= y (t) · (cos (2 π (f _m - f ₀ ) t) + cos (2 π (f _m + f ₀ ) t)) / 2 (4)

Here too, the summand of the frequency f _m + f ₀ can be eliminated by simple filtering. The residual signal is called x _{2 r} (t) :

x _{2 r} (t) = y (t) cos (2 π (f _m - f ₀ ) t) (5)

If the two signals x _{1 r} (t) and x _{2 r} (t) from equations (3) and (5) are squared and then summed, the terms that contain the difference frequencies add up to the value one:

x _{1 r} ² (t) + x _{2 r} ² (t) = y ² (t) · [sin² (2 π (f _m - f ₀ ) t) + cos² (2 π (f _m - f ₀ ) t) ] = y ² (t) (6)

If one takes the root of this expression, the desired useful signal y (t) results

The following is always the overlay with quadrature overlay of the carrier frequency useful signal with the orthogonal Signals, the subsequent squaring and the summation referred to both components. The following It is not necessary to square the resulting signal, because with binary time signs also from the squared Sum evaluates the time signal.

The multiplication with the phase-shifted mixed signals can be interpreted as complex multiplication in the time domain, where the cosine component is the real part and the Sinus component the imaginary part of the resulting mix represents signals. The summation of the squared components and the subsequent etching then corresponds to the amount education. If the complex components become the Calculated phase, even possible image frequency can be Distinguish signals from the useful signal.

The calculation steps described above are in the electrical Block diagram of the receiver in Figure 2, which the Claim 1 corresponds, illustrated again.

After reception by the antenna and after a preliminary strengthening the signal becomes two mixing stages (Multi plication stages) supplied, which phase with each other shifted signals are operated at the same frequency. The respective output signals are filtered, squared and added.  

The squaring and summation can be done as shown in Figure 3 shown, are already executed by a processor.

1.3. The quadrature scan

In the block diagram in Figure 2 there are two mixing stages that generate the quadrature components as multipliers with a subsequent band pass. That yourself the quadrature overlay, the two analog multiplication levels required, also by quadrature scanning to be replaced should now be explained. in the Block diagram Figure 4, which clarifies claim 3, For this, two scanners with phase-shifted to each other Sampling clocks provided. Just like the ones described earlier The samplers also become mixing stages with a sampling frequency operated in the order of the carrier frequency. As a scanner z. B. so-called SC filter Find use with which are scanned and efficient at the same time can be filtered (DE-PS 34 26 779).

The necessary orthogonality between the scanning signals is generated by a phase or time shift of T _d = t / 4, T indicating the period of the scanning frequency.

We now start again with the time function h (t) from equation (1). It is scanned with a sequence of pulses with an arbitrary phase position T _d . The scanning sequence is named as usual

where δ (t) is the Dirac needle pulse. Then the result x (t) of the sampled function h (t) is as follows

To find the Fourier transforms X (f) , the transforms are first individually determined for the factors h (t) and s (t - T _d ). They are each named with the corresponding capital letters H (f) and S (f) . According to equation 1, two factors are to be considered for H (f) , the transforms of which, as is known, must be folded in the frequency domain. Since the transformation of a sinusoidal oscillation of the frequency f Spektr results in two spectral lines at the frequencies ± f ₀, the spectrum H (f) belonging to h (t ) is obtained by convolution

This is precisely the known spectrum of an AM-modulated signal. The transform to the sampling function s (t - T _d ) in G1. 8 is.

With equations (9), (10) and (11) the Fourier transform of x (t ) results in the convolution of the terms H (f) and S (f):

If the sum is brought forward and the convolution is carried out, the sampling frequency f _abt = 1 / T follows:

The spectrum of the sampled signal is thus determined. Interfering mixed products are now delivered with a bandpass of the resonance frequency f ₀- f _abt , so that only the components for n = 1 and n = -1 can pass through the filter. The remaining part remains

Transformed into the time domain:

Now choose T _d = - T / 4 = -1/4 f _abt for the sine scan. Then you get the time function x _{1 r} (t)

x _{1 r} (t) = y (t) cos (2 π ( f ₀ t - f _abt t ) - π / 2)
= y (t) sin (2 π ( f ₀ t - f _abt t)) (16)

For the cosine scan, T _{d is} chosen to be 0:

x _{2 r} (t) = y (t) cos (2 π ( f ₀ t - f _abt t)) (17)

If the two components are squared individually, added and the square root is finally drawn from the sum, so results as above:

The original signal is therefore also released from the carrier here restored.

The received interference signals, the frequency of which is a multiple of the carrier frequency f ₀, must be damped for the quadrature scanning by a simple pre-filtering.

At the available carrier frequencies of the different Time signal transmitter, which one z. B. from R. Bermbach's dissertation "New radio clock concepts through the close integration of the recipient and Microcomputer "(Darmstadt dissertation 1985, Pages 3-7), this contradicts but not the demand for universality.

Quadrature scanning can also be done from a fast Microprocessor or a signal processor with Using two A / D converters with the required data width be made directly. This was also in the essay by R. Bermbach and J. Wietzke "Einsatz digital signal processors in time signal receivers ", appeared in the book Funkuhrtechnik, Oldenbourg Verlag 1988, pages 127-140. For this purpose, in picture 5 Claim 4 two with phase-shifted sampling clocks described A / D converter used.

The sampled signals can then be further processed purely digitally the processing steps of Arrangement of the block diagram in Figure 4 correspond. Of course, only an A / D converter with a scanner can be used find the two in time division Digitized quadrature components (claim 5, Figure 6).  

With the described arrangements, various pleasant properties can be achieved:
With the methods of quadrature superimposition and quadrature scanning, mixing can take place at any frequency and, at the same time, demodulation. There are only stability requirements for the short-term stability to maintain the quadrature phase, the long-term stability is largely irrelevant for demodulation.

Specular frequencies that could also pass through the filters are made harmless by the fact that the mixed frequency close to the carrier frequency to be demodulated is placed. In contrast to the known synchronous demodulation here is neither a phase nor a frequency coupling of the mixed signal with the carrier signal required.

If the mixed frequency is chosen so that the useful signal resulting from the superimposition has a very low resulting carrier frequency (f ₀- f _m - <0), the simplest RC elements are sufficient to filter out undesired signal frequencies. A large gain in quality for the filters can be achieved by mixing down the useful signal. This is described in detail in the article by J. Wietzke "Funkuhr mit Stabantenne" in the book Funkuhrtechnik, Oldenbourg Verlag München 1988, pages 163-178. (All of the above-mentioned articles in the book Funkuhrtechnik were produced at the same time as the patent application, but were only published at a later point in time than when the patent was filed.)

One sampling stage and one filter can be used be replaced by a compact SC filter, whereby a largely identical behavior of both Quadrature channels is reached.  

The entire quadrature overlay can be digital Area z. B. with the help of a signal processor realize. Then there are only one weak pre-filtering and amplitude adjustment required as analog circuit components.

In summary one can determine:
With quadrature scanning, a suitable method is obtained in order to enable a compact and, at the same time, flexible analog reception preamplifier for a largely digital universal receiver. The analog circuitry is limited to three operational amplifiers and two SC filters, even when using simple processors. This makes it possible to implement receivers the size of a matchbox.

Basically, you can even use the quadrature scan Receiver can be realized, the analog part only consists of an antenna and an amplitude adjustment. Corresponding to the different block diagrams Prototypes realized.

Claims (7)

1. Narrow band receiver for receiving the signals from time signal transmitters containing an antenna, preamplifier, devices for mixing, filtering, amplification and signal processing, characterized in that a quadrature superimposition circuit is used as devices for mixing and filtering. 2. Narrow band receiver according to claim 1, characterized in that the antenna signal after preamplification in a preamplifier (V) is simultaneously fed to two mixing devices (X) in which it is mixed with sinusoidal oscillator signals with the same frequency but a phase different by 90 ° that the resulting intermediate frequency signals are individually filtered and amplified in band filters (BP) , that the output signals of the band filters are squared in squarers (X) and added in an adder, which results in the demodulated squared useful signal. 3. Narrow band receiver according to claim 2, characterized characterized in that the oscillator mixing frequency is equal to or close to the carrier frequency the carrier frequency is.   4. Narrow band receiver according to one of claims 1 to 3, characterized in that for the two Mixing devices scanners are used and that the sampling frequency is about the carrier frequency corresponds to the antenna signal, the sampling clocks of the two sampling mixers a time shift of a quarter period to each other exhibit. 5. Narrow band receiver according to claim 4, characterized in that for the two sampling mixers the scanners of two A / D converters are used and that the subsequent filtering and arithmetic operations in digital representation of a processor are made. 6. Narrow band receiver according to claim 5, characterized in that as a sampling mixer instead of the Scanning two A / D converters, only one A / D Transducer with scanner used in multiplex mode is. 7. Narrow band receiver according to one of claims 1 to 6, characterized in that the mixers or sampling mixers optionally different, on the different time signal transmitters tuned Mixing or sampling frequencies supplied are.