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APPLICATION INFORMATION

AM Section

RF Amplifier and mixer stages

The RF amplifier stage (pin 2) is connected directly to the secondary winding of the ferrite rod antenna or

input tuned circuit. Bias is provided at pin 4 which must be adequately decoupled. The RF amplifier provides

stable performance extending beyond 30 MHz.

The Mixer employed is a double - balanced multiplier and the IF output at pin 3 is connected directly to the

IF filter coil.

Local oscillator

The local oscillator is a cross coupled differential stage which oscillates at the frequency determined by the

load on pin 1.

The oscillator resonant circuit is transformer coupled to pin 1 to improve the Q factor and frequency stability.

The oscillator level at pin 1 is about 100 mV rms and the performance extends beyond 30 MHz, however

to enhance the stability and reduce to a minimum pulling effects of the AGC operation or supply voltage

variations, a high C/L ratio should be used above 10 MHz.

An external oscillator can be injected at pin 1. The level should be 50 mV rms and pin 1 should be connected

to the supply via a 100W resistor.

IF Amplifier Detector

The IF amplifier is a wide band amplifier with a tuned output stage.

The IF filters can be either LC or mixed LC/ceramic.

AM detection occurs at pin 7. A detection capacitor is connected to pin 6 to reduce the radiation of spurious

detector products.

The Audio output is at pin 9 (for either AM or FM); the IF frequency is filtered by an external capacitor which

is also used as the FM mono de-enphasis network. The audio output impedance is about 7K

Ω and a high

impedance load (~ 50K

Ω) must be used.

AGC

Automatic gain control operates in two ways.

With weak signals it acts on the IF gain, maintaining the maximum S/N. For strong signals a second circuit

intervenes which controls the entire chain and allows signal handling in excess of one volt (m = 0.8).

At pin 8 there is a carrier envelope signal which is filtered by an external capacitor to remove the Audio and

RF content and obtain a mean DC signal to drive the AGC circuit.

TDA1220B