Current switching with low charge injection above 1 MHz? - Page 1
Forum section: electronics / projects

author: c4757p
author_rank: Super Contributor
timestamp: September 11, 2013, 01:49:51 pm
content: I've been sick as a dog lately, so I thought I'd tackle a simple, "fun" project and try to build a replacement for my crappy old analog function generator, using the traditional triangle-wave VCO design. For the unaware, that's just a capacitor which is alternately charged and discharged between two thresholds using voltage-controlled current sources. Now.... how is the switching usually accomplished? I'm currently using a few steering diodes to divert the current (the top current source gives 2xmA and the bottom gives -xmA - either both feed the capacitor with a sum ofxmA, or the top one is diverted to ground and the capacitor receives -xmA).The high frequency range requires a small capacitor, in my case 68 pF. With such a low capacitance, there is significant charge injection from the comparator through the steering diodes into the capacitor, enough to distort the waveform by up to 20% of its amplitude. Ferrite beads on the diodes seem to smooth off a bit of the sharp peaks, but there's no amount of inductance that will smooth the peaks without ruining the underlying waveform.I realize I could increase the capacitanceandthe drive current, but if the current gets too high I start having problems switching it fast enough, and things start to heat up too.... My old generator (Simpson 422, by the way) uses less than half the capacitance (33pF), so I doubt 68pF should be too low for a 0.5MHz-5MHz range.The generator I'm trying to replace uses a similar design andalsosuffers from this problem. Is this actually common in commercial function generators? And if not... how do they usually do this?

author: c4757p
author_rank: Super Contributor
timestamp: September 11, 2013, 07:57:57 pm
content: And as usual, I solve the problem myself a few minutes after asking... seems like I only ask for help with things I don't actually need help with...In my sick stupor I've been shuffling between the workbench and my laptop, doing some work with real components and some with SPICE. I tried most of my ideas in both places - and it turns out that the one that worked was one of the few that I only bothered with in SPICE (100 ohm resistor in series with one of the diodes) - and it did not simulate accurately.Works beautifully now, humming along up to about 7 MHz, still looking mostly triangular, just oscillating on the 6pF-ish gate capacitance of the buffer JFET plus parasitics.I'll put up full schematics and other information when I complete everything else (sweep, output amplifier, frequency counter) - it's pretty cheap and performs well, might make a good circuit for someone else to build. Easily outperforms my old Simpson.

author: edavid
author_rank: Super Contributor
timestamp: September 11, 2013, 08:05:35 pm
content: You might look at the HP 3312A, it's a nice refined example of this design.

author: c4757p
author_rank: Super Contributor
timestamp: September 11, 2013, 08:15:01 pm
content: Yep, the 3312A's are nice. We have a few at school.I'm quite happy with my VCO now, though, don't think I'll be redesigning it much.Basic schematic attached, if anyone wants to see. I omitted the crap in the current sources and symmetry adjustment because it's just boring op amp adders and subtractors and followers, nobody wants to see that