Function generator - signal quality opinions?

[c4757p]

That's a good idea. I don't have the headroom, but I can make it - I have more than enough G-BW in the output amplifier to reduce the signal level in the VCO and boost it back up at output.

I think it's time to spin a new PCB. God, I hate doing that. I might play around with replacing the comparator, too. LM7171's not a comparator, it's an op amp, and it won't give a very clean output above 1.5 MHz or so. I used it because it was all I had on hand that would even go that fast. The 3312A uses a discrete-transistor comparator that should work fine, I might do that too.

And there's nothing I love more than etching PCBs, other than matching transistor pairs...

[ElectroIrradiator]

Also try using JFETs for Q1 and Q2. The single BJT CCS isn't that great, as you have found out, due to the influence of the base current contribution to the emitter current. A JFET would help with the CCS performance, even when you go cascode. Depending on the emitter/source voltage, you may have to use JFETs with a low pinch-off voltage, though.

[ElectroIrradiator]

Maybe this is Captain Obvious, but remember not to use random junkbox ceramic caps for the low values. A low voltage cap with Y5V dielectric could ruin your waveform, even when run within specs. Use C0G/NP0 dielectric caps for anything tuning/integrating.

[sync]

FYI: This is the Enertec 4433 VCO current source. The 280R emitter resistors are for the highest frequency range (20MHz). The 2.49k for all other ranges. The op amps are LF356. It uses 1N4244 "ultra fast switching diodes" (t_rr<750ps, C<0.8pF).

[c4757p]

Thanks for the ideas. Yep, I am using good caps.

I tried JFETs in the current source. An N FET in the sink improved the performance. Unfortunately, an N FET in the source predictably degraded the performance, and P FETs are quite hard to come by these days... I'd rather not use parts that are a few months away from unobtainium.

Thanks for the schematic! I'll have a look later.

Unfortunately, it might be a couple days before I can resume serious work on this - heaps of schoolwork...