Introduction

This is a interesting audio filter that was an option in the Icom IC-730 transceiver. The filters in this design are part of a group of filters call State Variable Filters and often referred to as Biquadradic filters, or simply Biquad. This particular filter consists of a Band Pass Filter followed by a Integrator and a Inverter. The output of Inverter is then fed back to the Band Pass Filter.

The IC-730 generates two signals that are used by the EX203 for power and control. When the IC-730 is in receive, it generates the signal R8 (P1-2). This signal is 8 Volts when the IC-730 is in receive. The EX203 uses this signal for two purposes. One is to power the two filter sections and provide bias voltage to the audio switch (U1B). When not in receive, the filter is not powered up.

The other signal is CW8 (P1-1). This signal is also 8 Volts and is generated when the IC-730 is in CW mode. This signal is used to generate the bias voltage (4.5 Volts) for the filter sections and disable the audio switch (U1B) So, when in Receive and CW Mode

• The Center Frequency, F₀ is controlled by R3 and C2, F₀ = 1/(2×PI×R3×C)
• The Gain, A_V is controlled by R1 and R2, A_V = -R2/R2
• AV = -R2/R2
• Q = R2/R3
• C' = C2 = C3
• f₀ = 1/( 2 × PI × R3 × C'
• BW = 1/( 2 × PI × R2 × C'
• R3 = R6 + R7