https://www.febo.com/pipermail/time-nuts/2017-May/105554.html
Our attempt to measure the low frequency noise to determine the possible lock improvement was a failure. We will have to get access to the Loop Gain pot to get a flat signal at low impedance. Beware of low frequency roll off. I found out that the feedback amplifier on the A7 board causes the electron multiplier output to roll off above 19 cps. It places an 8,200 pFd condenser across the 1 M load resistor which causes the roll off. Here is a linear sweep up to 8 kc. http://gonascent.com/papers/hp/hp5061/waveform/wideswee.jpg It shows the 274 cps second harmonic as well as the 16 cps roll off. Our previous posts involving various modulation schemes were correct because we drove the scope directly from the electron multiplier. While the meter driver is a convenient amplifier, you have to disconnect C1 to prevent the low frequency roll off. The feedback amplifier has a gain proportional to frequency. This restores flatness above 19 cps for the loop gain pot. I got a gain of 141 at 137 cps with a lagging phase angle of 90°. R10 is loaded by C1 which drops the level at that frequency by 7:1. Effective gain is about 20:1 on the Loop Gain pot in the hi gain position of S1. Gain is about 7 in the low gain position. In both positions low frequency roll off is about 19 cps. πθ°μΩω±√·Γλ WB0KV _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.