New CODD Pick-up Amplifier

Frequency response

Fig.1: Σ channel gain versus frequency.

The frequency response is tailored to compensate for the high-pass effect of a 100pF electrostatic PU electrode looking into a 200Ω resistive load. The resulting pole is at 8.05MHz. As a consequence, the amplifier response has a zero at that frequency (Fig.1). The measurement has been taken with the amplifier directly between the ports of the HP8753D. Power setting is -70dBm, and the amplifier is set for minimum gain. It's difficult to test higher gains because cable leakage from the output to the input causes low frequency oscillations. The low-frequency gain rolls off below 100kHz, as there is little to gain by going lower, and much to loose.

To appreciate gain flatness, some way to simulate the PU electrodes is needed. I chose to use a constant resistance bridged-T having the same frequency response as the loaded PU (Fig.2, 2a). The constant resistance is obtained by setting Z_aZ_b = R², and the frequency response is given by H=R/(R+Z_b). Thus for a high-pass with a pole at 8.05MHz, Z_b should be a capacitor of 396pF and Z_a a coil of 991nH. The coil has been realised as a series connection of a 6-turn toroid coil on an A_l=24.3nH/turn², in series with a toroid of 2 turns on a second core of the same type. (A fixed 1uH coil has an unacceptably low parallel resonance frequency.) The resulting circuit is good to 1GHz or so.

Fig.2: Frequency response test jig.	Fig.2a: Frequency response of test jig.

Fig.3: Amplifier frequency response with simulated electrode.

Fig.3 shows the frequency response of the (Σ) amplifier with the test jig. The NA power setting is now -30dBm. The -3dB bandwidth is 100kHz-56MHz, the upper cut-off being primarily determined by the AD630 VGA. (This measurement doesn't have the 40 MHz low-pass filter at the input yet.)