RF FILTER WITH REDUCED INSERTION LOSS

Abstract

The invention relates to an RF filter with reduced insertion loss. The filter (F) includes a first bandpass filter (BPF1) having a passband, a circuit unit (SE) having an undesired excitation at a critical frequency (f.sub.s) and a reflector (R) that reflects RF signals of this frequency before the circuit unit is undesirably excited and the power is lost as a result.

Claims

1. An RF filter (F) having reduced insertion loss, comprising an input port (P.sub.in), a first bandpass filter (BPF1) having a first passband, a circuit unit (SE), in which RF signals of the frequency f.sub.s cause undesired excitations, a reflector (R) that reflects RF signals of a frequency f.sub.s, wherein the first bandpass filter (BPF1) and the circuit unit (SE) are connected to the input port (P.sub.in), the frequency f.sub.s is within a first passband and the reflector (R) is connected between the input port (P.sub.in) and the circuit unit (SE).

2. The RF filter according to the preceding claim, wherein the circuit unit is an RF filter.

3. The RF filter according to either of the preceding claims, wherein the bandpass filter (BPF1) and/or the circuit unit (SE) operate using acoustic waves.

4. The RF filter according to any of the preceding claims, wherein the reflector (R) is a low-pass filter, a high-pass filter or a band-stop filter.

5. The RF filter according to any of the preceding claims, wherein the reflector (R) operates using acoustic waves or includes LC links.

6. The RF filter according to any of the preceding claims, wherein the reflector (R) rotates the phase of the RF signal of the frequency f.sub.s in such a manner that the reflected signals constructively overlap the RF signals on the first bandpass filter (BPF1) and reduce the insertion loss of the RF filter (F).

7. The RF filter according to any of the preceding claims, wherein the undesired excitations are excitations of frequency f.sub.s, excitations of intermodulation products from signals of the frequency f.sub.s, excitations from harmonics, excitations caused by non-linear effects, broad-band excitations or excitations caused by volume waves.

8. The RF filter according to any of the preceding claims, additionally comprising two or more cascaded base units, each having a reflector (R) and a circuit unit (SE).

9. The RF filter according to any of the preceding claims, in which the reflector (R) is connected in a signal path, in which the RF filter (F) includes additional reflectors (R) that are connected in series to the reflector (R) in the signal path, in which these reflectors (R) are either all high-pass filters or all low-pass filters, in which the reflectors (R), sorted in the signal path according to their cut-off frequencies, are cascades in series.

10. The RF filter according to any of the preceding claims, in which the first bandpass filter (BPF1) is connected in a parallel path and itself has undesired excitations at a frequency f, in which an additional switching segment (IS) is connected in parallel to the first bandpass filter (BPF1) and in transmission creates an opposing signal to an undesired excitation of the first bandpass filter (BPF1) that destructively interferes with the undesired signal of the first bandpass filter (BPF1).

11. The RF filter according to any of the preceding claims, in which the first bandpass filter (BPF1) is connected in a parallel path and itself has undesired excitations at a frequency f, in which an additional filter having a stop band around the frequency f is connected in series after the first bandpass filter (BPF1).

12. A filter component having an RF filter according to any of the preceding claims.

13. A front-end circuit having a filter component according to the previous claim.

Description

[0037] The RF filter and its underlying functional principles, as well as any possible embodiments are described in detail in reference to the schematic figures described below.

[0038] Shown are:

[0039] FIG. 1: the basic scheme of the RF filter F,

[0040] FIG. 2: one possible form having filters,

[0041] FIG. 3: an illustration of the problem that leads to a higher insertion loss in conventional circuits,

[0042] FIG. 4: a transfer of the principle to circuits having a plurality of parallel-connected filters,

[0043] FIG. 5: the additional possibility for reducing interference modes using a parallel circuit element,

[0044] FIG. 6: the possibility for reducing additional interference modes using an additional, serial circuit element.

[0045] FIG. 1 shows the fundamental operating mode for improving the insertion loss of a RF filter F. Filter F has an input port P.sub.in, into which is supplied a signal S having a specific intensity. A first bandpass filter BPF1 is connected to input port P.sub.in. The first bandpass filter BPF1 has a passband around a frequency f.sub.s. A circuit unit SE is connected in parallel to first bandpass filter BPF1 and to input port P.sub.in. Circuit unit SE can be excited at frequency f.sub.s (symbolized by the hatched triangle). Signal S supplied to input port P.sub.in reaches first bandpass filter BPF1. Full power can be output at output port P1.sub.out. The reflector R is connected between first bandpass filter BRF1 and circuit unit SE and reflects corresponding RF power of frequency f.sub.s back to first bandpass filter BPF1.

[0046] FIG. 2 shows a possible form of the filter in which reflector R is designed as a low-pass filter LP. Circuit unit SE is designed as a bandpass filter, in this case next to first bandpass filter BPF1 as second bandpass filter BPF2. Signals of critical frequency f.sub.s are output via first bandpass filter BPF1 at its output P1.sub.out. Signals of the frequency f.sub.2, the operating frequency of second bandpass filter BPF2, are transmitted practically unaltered by low-pass filter LP and are made available at output port P2.sub.out of second bandpass filter BPF2. Reflector R or low-pass filter LP reflects signals that are dissipated in bandpass filter BPF2 or converted into interference signals but admits signals of the operating frequency of second bandpass filter BPF2 unchanged.

[0047] Depending upon the frequency setting of the operating frequency of the bandpass filter and the situation of the interfering excitation and depending upon the order of the two bandpass filters as seen from input port P.sub.in, either a low-pass filter or a high-pass filter is advantageous as a reflector. Alternatively a band-stop or a band-pass filter can also be used as a reflector.

[0048] FIG. 3 illustrates the problem of conventional RF filters. The intensity of input signal S splits into two parts. The majority S.sub.1 passes through the first bandpass filter. A second part S.sub.2, however, passes into the circuit unit and effects an excitation at critical frequency f.sub.s. This intensity is lost. The insertion loss for signals around critical frequency f.sub.s of the RF filter increases in an undesired manner.

[0049] FIG. 4 shows an application of the principle of reflection on an RF filter having more than one bandpass filter and one circuit element, symbolized by the three dots. For each circuit having possible critical excitation or dissipation, a reflector can be provided, represented in FIG. 4 as low-pass filters LPF1, LPF2. The order of the parallel paths having bandpass filters or circuit units must be selected to correspond to the situation of the operating frequencies and the critical frequency. Low-pass filters or high-pass filters cascaded in the signal path and sorted according to their crossover frequency can then be used as reflectors.

[0050] FIG. 5 shows an inverter circuit IS that can be connected in parallel to a bandpass filter or a circuit unit. If first bandpass filter BPF1 (or another corresponding element in a parallel path) itself has a critical frequency in which interference signals are excited or desired signals dissipated, an inverter circuit IS can be provided that creates an output signal on this critical frequency that is equal to the inverted interference signal. This leads to an overlap on the interconnected outputs of the corresponding partial circuits, so that the negative impact of first band-pass filter BPF1 is nullified.

[0051] FIG. 6 shows an additional or alternate possibility for eliminating interference signals. If the power of the interference signal is not otherwise necessary, a simple low-pass filter F (or a high-pass filter or a stop-band filter, as appropriate) can be connected in series after bandpass filter BPF1.

[0052] The RF filter is not limited to the illustrated exemplary embodiments and described embodiments. The filter can include additional circuit elements, impedance matching circuits, circuit units for correcting phase responses, additional bandpass filters and the like.

LIST OF REFERENCE NUMERALS

[0053] BPF1: First bandpass filter

[0054] BPF2: Second bandpass filter

[0055] F: RF filter

[0056] F: Filter connected in series for interference signal suppression

[0057] f.sub.2: Operating frequency in the passband of the second bandpass filter BPF2

[0058] f.sub.3: Frequency of a possible interference in the first bandpass filter

[0059] f.sub.s: Critical frequency of the undesired excitation

[0060] HP: High-pass filter

[0061] IS: Inverter circuit

[0062] LP: Low-pass filter

[0063] LPF1: First low-pass filter

[0064] LPF2: Second low-pass filter

[0065] P1.sub.out: Output port of the first bandpass filter

[0066] P2.sub.out: Output port of the circuit unit

[0067] P.sub.in: Input port of the RF filter

[0068] R: Reflector

[0069] S: Intensity of the RF signal

[0070] S1: First intensity

[0071] S2: Intensity that, absent the reflector, would be dissipated through the undesired excitation in the circuit unit

[0072] SE: Circuit unit