M
Mike Monett
- Jan 1, 1970
- 0
I've used them before. As transmission lines, they have very low
impedances, ballpark 10 ohms, so to make an instantly-triggerable
oscillator you wind up needing a lot of supply current. And they're
expensive in moderate quantities and tricky to solder down. The lowest
they go is about 600 MHz, and that's physically huge, so you're
talking ECL drivers and prescalers. The whole product uses only 2.5
watts.
An LC oscillator is ideal for what I'm doing. If I could get some 18
pF N1000 caps, the osc TC would be about zero. As is, I spent most of
the day taking temperature data and came up with a polynomial
temperature curve I can use to tweak the varicap bias... all the
hardware is already there... temperature sensor, tweak dac, varicap
stuff. So I guess we'll go with a software fix.
The other thing that would help would be a zero TC inductor!
John
I don't dispute the cost, but that is probably insignificant compared to
the selling price and to the time you spend fixing a problem that you don't
know if it will recur.
Physically huge is a problem. Perhaps the helical versions would help solve
that.
Tricky to solder is a process problem. You have experts that can figure
that out.
I'm not sure about the lower frequency limit. Seems I saw some that went to
300MHz, but I'll have to check. Another divider stage would solve that, and
give lower jitter at 50MHz.
I don't think low impedance is an issue. The 50MHz Colpitts you use now has
an inductive reactance of 50 ohms.
The ceramic resonator acts as a high-Q inductor, so it should behave
exactly the same, except at a higher frequency. The high Q gives less phase
noise (jitter), and the frequency stability means you don't need to add
much varicap control, which helps maintain the low noise.
The divider might be a problem. ECL would definitely burn your power
budget. Is there any reason it has to be so low, such as portable battery-
powered operation?
There should be a way around that problem. I have a simple battery-powered
frequency counter from Radio Shack that goes up to 2GHz. I haven't checked
the power drain, but it goes a long time on a set of alkalines.
Regards,
Mike Monett