HUGH
11-28-2015, 09:08 AM
Most of us have been made aware that ceramic capacitors used for coupling and decoupling can have some inductive reactance as well as having, often unspecified, temperature coefficient problems. I thought the reactance was mostly a problem at high frequencies and at LF, 136khz in this case, would be no problem.
On finishing a high-gain preamp for an LF ferrite rod antenna I required a 170pf capacitor and 25pF trimmer to tweak the tuning. A bundle of silvered mica caps would tune it but the entire was bigger than the amplifier, polystyrene was good but the nearest I found was 180pF, just too large. Tipping out a pile of ceramic capacitors, there were a few 150p discs so I soldered one in. Behold, the amp took off at about 145kHz as a nice square wave oscillator so I checked again and it was 150pF as labelled. Luckily I have a collection of tiny Philips PCB capacitors so, adding a 150pf and a 10pF, the amplifier worked perfectly with sharp tuning on the 25pF preset. I've no idea what the temperature coefficient is but we shall see.
The 150pF discs have all gone in the trash, the reactance must have been quite large and upset the feedback phase as it's an AD811 op-amp and buffer in the amplifier.
On finishing a high-gain preamp for an LF ferrite rod antenna I required a 170pf capacitor and 25pF trimmer to tweak the tuning. A bundle of silvered mica caps would tune it but the entire was bigger than the amplifier, polystyrene was good but the nearest I found was 180pF, just too large. Tipping out a pile of ceramic capacitors, there were a few 150p discs so I soldered one in. Behold, the amp took off at about 145kHz as a nice square wave oscillator so I checked again and it was 150pF as labelled. Luckily I have a collection of tiny Philips PCB capacitors so, adding a 150pf and a 10pF, the amplifier worked perfectly with sharp tuning on the 25pF preset. I've no idea what the temperature coefficient is but we shall see.
The 150pF discs have all gone in the trash, the reactance must have been quite large and upset the feedback phase as it's an AD811 op-amp and buffer in the amplifier.