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S. Grigo:

"A Receiver Filter for Wide-Band Matching of the Mixer"

Low noise microwave receivers without selection on the input side, for instance satellite receivers, require a band-pass filter between the low noise amplifier (LNA) and the mixer. That is necessary in order to sup­press the noise of the image frequency band. Figure 1 shows it schematically.
The demands on the filter are not great. An attenuation at 15 dB of the image frequencies is usually suffi­cient. However the reflection of the image band and the local oscillator frequency (LO) at the bandpass fil­ter have a negative effect on the frequency response of the transmission band.
Fig. 1
Fig. 1

The properties of the mixer can essentially be improved, if all appearing spectral frequencies meet with a matching port on the input and the output side of the mixer. A reflection factor at -8...-10 dB is usually suffi­cient for this purpose. At the IF-side that is simple to do by means of the following amplifier stages. At the filter-side it is to do with an attenuator, for instance 3dB (Figure 2).

Fig. 2
Fig. 2

However the attenuation deteriorates the total noise factor of the receiver. It can be remedied by a band-pass filter with wideband-matching properties beyond the demanded selection for the transmission band. The way to realize this wish is simple: two identical band-pass filters are shunted by means of Wilkinson couplers. The necessary phase shift can be obtained with two λ/4-50Ω-transmission lines. The loss of se­lection on 3dB caused by the shunting of the filters is unimportant for this task. Figure 3 shows a realization in microstrip technique with two-resonator side-coupled filters.

Fig. 3
Fig. 3

The development was done for a RT-Duroid 5870 substrate with a thickness of 0.438mm for the Intelsat high-frequency band. The simulation was carried out by the tool HP-MDS. Figure 4 shows the calculated characteristics of a single band-pass filter. The markers show the receiver band 11.45...11.70Ghz. The LO-band was 12.78...13.03GHz. This follows the image frequency band of 14.11...14.36GHz.

Fig. 4

Fig. 4

Figure 5 shows the layout and Figure 6 the measured characteristics S11 and S32 of a realized Wilkinson coupler. For the 100Ω-resistors of the Wilkinson couplers there were chosen SMD-chips with the size 0504. The microwave properties of the chip resistors were not taken into account during the simulation but the T-junctions. For the realization of the length of the 71Ω-transmission lines there was taken the middle length of the curved lines. The measured properties of the Wilkinson coupler are sufficient for our purpose.
Fig. 5
Fig. 5

Fig. 6

Fig. 6

Figure 7 shows the results of the simulation of the entire filter with Wilkinson couplers. The reflection factor at the image frequency band amount to better than -10dB with a selection at 25dB. At the LO-frequencies the matching is better than 8dB.

Fig. 7

Fig. 7

The practical results corresponded well to the simulation at the interesting points (Figure 8). Therefore the development ended successfully with the first realization.

Fig. 8

Fig. 8

Figure 9 shows the filter in a receiver-block of a RF-transceiver.

Fig. 9

Fig. 9
Summary:
In this paper a new way was described to realize a wideband matching of the mixer by a new form of the receiver filter. A realized filter on RT-Duroid 5870 for the 11GHz-band and the measured results were shown.
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