EMRFD Message Archive 13060

Message Date From Subject
13060 2016-08-09 02:30:56 nothdurftm Phasing exciter all-pass circuits - 10 pF across feedback resistor
I'm laying out a PCB (mostly SMD) for a phasing SSB exciter based on the circuits shown in EMRFD (Revised 1st Ed.) Figures 9.82 and 9.83.  My question has to do with the 10 pF capacitors shown in parallel with the feedback resistors in the all-pass circuits.  What is the purpose of these capacitors.  I thought maybe they were to reduce gain at high frequencies (suppress some oscillation or something?) but there is no discussion RE these capacitors. These aren't present on any of the other similar all-pass circuits used in receivers such as Fig. 9.62 and previous articles on direct conversion receivers.  I modeled this circuit in LTSpice and could see some gain reduction at high frequencies (around 3 MHz as I recall) but that is all.  I'd like to eliminate these capacitors if I can.  Comments anyone?
Mike, K5ESS

13062 2016-08-09 04:30:00 Steve Dick Re: Phasing exciter all-pass circuits - 10 pF across feedback resist
To prevent potential oscillation in the op amp.  It is very important to minimize the capacitance on the summing node of an op amp to prevent introducing an extra phase shift which can cause the op amp to oscillate depending on its phase margin.  The capacitor provides a “zero” in the open loop response which cancels out the “pole” of the resistance and stray capacitance the summing node sees.  Sort of like how scope probe compensation works.  A typical 10X scope probe has a compensating capacitor across the top resistor to compensate for the capacitance across the lower resistor resulting in a flat response instead of an “R-C” response which looks like an added “pole” in the open loop response of an op amp.
“Digital Steve”, K1RF