Logging Section:     (Pins 8, 9, 10, 11, 13, 15)

The log adjusts, pins 10 (log 7 adjust) and pin 9 (log 1 adjust) function the same way as they did on the L-17C, and there is a new log adjust, log 6, pin 13. L1 adjust uses R11; L6 Adjust uses R19; L7 adjust uses R18. If you observe that adding the adjustment resistors causes oscillations, place a 1 nf capacitor in parallel with the adjustment resistor.

The linear extension, pin 11 no longer needs a 400-ohm resistor to ground as this is now internal to the chip. The linear adjust resistor on the board is RLIN. As mentioned earlier, the linear extension now consists of 3 log stages so that extended dynamic range now can be accommodated on a single IC (up to about 60 dB with a Schottky only!) To do this, connect the Schottky to pin 2 as usual, and take a buffer, or emitter follower from the Schottky to pin 11 with suitable attenuation and, if necessary, a DC shift.

S+ and S-, the summing junctions, pins 8 and 15, function as before, except that they are now taken from a low-impedance point in the IC, and are not sensitive to capacitance. Do not try to slow the IC down by putting a capacitor across these pins, as they are taken from the emitters of transistors and are consequently not suitable for this purpose. Aside from offering one method of adding the outputs from two L17-Ds, these pins are needed for another purpose, which is described in the output amp section.

Output Amplifier:     (Pins 16, 17 18, 19, 20 and 21.)

Please note that the behavior of this amplifier depends on the rail voltages and when running above +/-6V, certain components, noted below must be modified.

Select the feedback resistor (R27), from pin 18 to 19, to give the required output slope. (To get 50 mV/db requires about 1.9K.)

Note: as mentioned in the pin description section, it is not necessary to put a resistor to ground from the positive input to the output amp (pin 21). This is now taken care of internally.

Before setting up the output amplifier it is necessary to zero pin 21. To do this, first make sure that A1, A2 and A3 are zeroed. Then take equal value resistors from S+, R20 and S- R21 to VCC or VEE as needed. Going to VCC will lower the voltage and going to VEE will raise it. In practice the voltage at pin 21 is high, so the resistors should be taken to VCC using R20 VCC and R21 VCC.

You might have to put a 5 pF comp cap on the output amp compensation, pin 20 to ground, before doing this correction, or you may have oscillations at pin 21. We also recommend a 47 pF cap from pin 21 to ground. It gives a smoother output pulse shape. For higher rail voltages this cap may need to be as high as 125 pf. Select the value by looking at the pulse shape on the monitor point for pin 21.