RF amplifier and filter for 10.7 MHz

The RF amplifier is similar to the one used in the 2. 5MHz amplifier. At a frequency of 10MHz the capacitances of a power MOS-FET become significant. Noiseless feedback by the use of transformers is no longer straightforward. Intermodulation and overtones are caused by the variation of the capacitances with the voltage. Performance is adequate if a

MOS-FET with high enough channel resistance is selected (with associated small capacitances). The input IP3 of the RF amplifier is in the order of +45dBm which is high enough to make the 10. 7 to 2. 5MHz converter dominated by IM3 from the mixer. The entire unit has an input IP3 of about +40dBm. The gate transformer, TR8, is wound on a ferrite toroid core from Ferroxcube (Philips). Material 4C65, type TN 14/9/5. The gate winding is 6 turns, the source winding is 3 turns and the input winding is 7 turns. The wire dimension is uncritical. The drain transformer, TR10, is wound on a ferrite toroid core from Ferroxcube (Philips). Material 4C65, type TN 14/9/5. The winding is 6 turns with three twisted wires. The wire dimension is uncritical. The 10. 7MHz band pass filter has a flat response over about 500 kHz. The frequency response is shown in figures 2 and 3. The main purpose of the 10. 7MHz filter is to suppress the mirror image 2. 5 MHz above the local oscillator. Since the mixer is sensitive at harmonics of the local oscillator, higher frequencies also have to be well suppressed. The mirror frequency (15. 7MHz) is suppressed by more than 80dB. There is a notch filter at this frequency, without the notch the mirror frequency would have been attenuated only by about 65dB. The notch is made with low cost standard inductors. They form resonators at 15MHz so they do not produce third order intermodulation problems for strong...

Leave Comment

characters left:

New Circuits