The active filter cont.

Piotr Wyderski · 2017-08-20T22:13:49+00:00

So I've finally built the device (8th order bandpass based on LTC1562-2) and it works. It turns out that my intuition was, unfortunately, correct: the gain of 36dB built into the filter caused the device to be extremely unstable. It started to oscillate whenever it could, sometimes without any apparent cause. Fortunately, the gain in this application could be set by the value of just one resistor, so when I set it to be 0dB, the thing became super-stable. After the filter I have an opamp buffer stage on MCP6022E (to buffer both Vdd/2 and Vout, as the output is differential). Fortunately, in the very last moment I've changed the PCB layout to contain the place for two more 0805 resistors, so I could choose whether the opamp should work as a follower or a non-inverting amplifier. Then I moved those missing 36dB into the output stage. What I don't understand: 1. The filter is still stable. What difference does it make if the gain is in the filter or its DC-coupled direct successor stage? 2. I don't have a decent spectrum analyser, so I can't check for sure, but after the mixer stage (with 76.5kHz LO) the output signal contains a lot of noise, way beyond the 2.6kHz bandwidth of the input signal. I don't know if it comes from the mixer or the input signal. 3. I wasn't able to obtain similar results with the analog resources of a PSOC5LP (LNA on an opamp, the mixer, 4th order Sallen-Key low-pass). The various internal couplings make the device to oscillate very wildly. But the PSOCs are there *exactly* for mixed-mode applications. My analog frontend design was very conservative, so I doubt that the fault is on my side. Very disappointing. Funny thing: my Farnell order shipment was blocked: they wanted me to make a statement that the device will not be exported to Russia. What the heck, is the LTC1562 part so high-tech? :-))) Best regards, Piotr

Aug 20, 2017 21 Replies

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George Herold 8 years ago

(to Piotr) Is there some reason you can't put the gain before the filter?

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Piotr Wyderski 8 years ago

One more:

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Best regards, Piotr

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