Agilent Technologies 2948, 2712A Low-Pass Filters Service Sheet, Audio Amplifier Service Sheet

Model 8904A Service

Low-Pass Filters (Service Sheet 2)

Precise construction of the analog waveform from the DAC requires careful filtering. Since the samples from the DAC are clocked through at approximately 1.67MHz, sampling theory dictates that a low-pass filter of approximately 840 kHz or less is required; in practice the 3 dB corner will be less than this but it must be greater than 600 kHz, which is the upper frequency limit for a sine wave.

The output from the Track-and-Hold Circuit is filtered by the Sharp Cutoff Filter (an elliptic function filter) for sine waves, noise, and some types of complex waveforms (when automatically selected). The filter is flat within f O . O 1 dB to 600 kHz and has a 3 dB corner at 670 kHz. Above 670 kHz the filter rolls off rapidly but ripples in the stopband. Adjustments to the filter are made to keep the stopband attenuation more than 68 dB beyond 1.07 MHz. For most other waveforms, the Low Overshoot Filter (a Gaussian type filter) is automatically selected. The filter prevents ringing but limits the useable frequency range to 50 kHz.

The Sharp Cutoff Filter is switched by relay K11, the Low Overshoot Filter by K10. The filters are switched in complement-one or the other is always in. The filter control signal on line FLC is inverted by peripheral driver U8A, which controls K10. The output of U8A is inverted by U8B, which controls K11. A high on FLC switches in the Low Overshoot Filter.

The filters can be isolated and tested manually by connecting a signal source to 55 and moving jumper 53 to connect point 2 to point 3, and by connecting a signal analyzer to J6 and moving jumper 57 to connect point 2 to point 3.

Audio Amplifier (Service Sheet 2)

The Audio Amplifier has a precise gain of 8.9 (19 dB).It must pass signals from dc to 600 kHz and maximum levels of 8.9 Vpk with minimum introduction of ripple, distortion, noise, or dc offset. The amplifier is a discrete design of a standard inverting operational amplifier. The gain is the ratio of the parallel combination of R48 and R231 to R47.

The Audio Amplifier has three stages: (1)an input stage (U9, Q14, and QE), (2) an intermediate, high-gain stage (Q20 and Q21), and (3) an output driver stage (Q18 and Q19). The virtual ground of the amplifier is the junction of C40 and R49.

In the input stage the ac component of the input signal flows through C40 and R56 into the base of Q14. DC current flows through R49 and is amplified (and inverted) by U9. U9 functions as an integrator providing large gain for dc and low gain for ac. The output of U9 is fed into the emitter of Q14 through R53. Q14 differentially sums the ac and dc signals. Components C45, C46, and R57 form a lag-lead (at 1MHz and 10 MHz) equalization critical for stabilizing the amplifier.

Q20 is a common-collector amplifier which drives common-base amplifier Q21to form the intermediate stage. Q21 drives the collector of Q16 (through Q17) which presents a high-impedance load to Q21 and gives the intermediate stage high gain.

The output stage has unity gain. The complimentary transistor pair Ql8 and Q19 provide sufficient current to drive the load. Q17 sets and thermally compensates the idle current in Ql8 and Q19. Bias current for Q17 is generated by Q16. CR6 thermally compensates Q16.

Sine X/X Compensation(Service Sheet 2)

The Sine X/X Compensation filter compensates the natural roll-off of the sample-generatedwaveforms up to 600 kHz. L11 is adjusted for best flatness at 550 kHz. The parasitic load capacitance is accounted for in the network’s design.

6, 12, and 24 dB Attenuators (Service Sheet 3)

Three pi-section attenuators provide 6, 12, and 24 dB of programmable attenuation. Each attenuator has an input impedance of 1 kR. C232 and C504 introduce a load capacitance that is constant for all combinations of attenuation settings.

Principles of Operation (A3) 8-27