Rainbow-electronics AT86RF211 User Manual download pdf (Page 12)

AT86RF211

1942C–WIRE–06/02

Figure 11. Schematic Input of the LNA

Figure 12. Schematic Output of the Mixer

The first mixer translates the input RF signal down to 10.7 MHz or 21.4 MHz as chosen

by the user. The local oscillator is provided by the same synthesizer which will generate

a local frequency 10.7 MHz or 21.4 MHz away from the Tx carrier frequency.

The output impedance of the mixer is 330Ω with a 20% accuracy, so that low cost, stan-

dard 10.7 MHz ceramic filters can be directly driven. Other IFs may be chosen thanks to

the high bandwidth (50 MHz) of the mixer.

IF1 filtering A popular ceramic filter is used to reject the second image frequency and provide a first

level of filtering.

The IF1 filter can however be removed; it leads to a sensitivity reduction of about 3 dB

(the substitution coupling capacitor should be > 100 pF).

IF1 Gain and Second Mixer The input impedance of the IF1 amplifier is naturally 330Ω to match the input filter. The

voltage gain, i.e. gain at 10.7 MHz or 21.4 MHz added to the conversion gain at 455 kHz

is typically 14 dB when loaded by 1700Ω. The second mixer operates at a fixed LO fre-

quency of 10.245 MHz or 20.945 MHz. Its output impedance is 1700Ω in parallel with 20

pF.

RXIN

IF1OUT

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Rainbow-electronics AT86RF211 User Manual Page 12