EMRFD Message Archive 1982

Message Date From Subject
1982 2008-08-21 19:04:28 rcbuckiii 400 MHz VCO
I want to build a 400 MHz VCO for use with a National LMX2306 PLL chip.
I need roughly +2~3 dB output. I think probably 2 transistors would be
able to accomplish this. Probably a Colpits oscillator followed by a
buffer. I realize I could buy a MiniCircuits part for about $30 but
that is a little steep. And building it will give me some experience in
working at that frequency range. I will use 0603 caps and inductors so
the circuit will be small. Can anyone point me to a schematic on a
website somewhere that I could use as a starting point?

Thanks in advance.

1983 2008-08-22 09:17:50 Dave Re: 400 MHz VCO
1984 2008-08-22 10:13:14 Nick Kennedy Re: 400 MHz VCO
He's not running it at 400 MHz, but here's one by VE3DNL, our buddy here on
the list. It might be your starting point. Some good info on Glen's site.


73--Nick, WA5BDU


[Non-text portions of this message have been removed]
1985 2008-08-22 17:37:17 rcbuckiii Re: 400 MHz VCO
1986 2008-08-23 06:53:52 bobtbobbo Re: 400 MHz VCO
I'm sure most builders will agree that we don't like to spend a lot on
individual parts such as a commercial VCO. One thing I have found
after some 50 years of building is that there are sometimes excellent
trade-offs when using a commercial part instead of cobbling up one's
own circuit. One example is the diode ring DBM. For $5 we get one that
is not only engineered to a fair thee well, but is small, hermetically
sealed and balanced.

I have also found that designing/building problems,headaches
(challenges?) increase geometrically with frequency. Using someones
schematic to build a 7MHz oscillator will probably yield a
satisfactorily working device fairly quickly. I doubt if that would be
the case for a 400 MHz oscillator. At UHF, equally important as the
component values are the component manufacturer, type and, in some
cases, the component manufacturing run of the same part!

This is (and has been) the scenario for giving builders a LOT of
"experience" time. I would suggest that you spend the $, (also look at
Crystek VCOs in Mouser and DigiKey) and put your time into developing
the rest of your design. The $27 is buying a part that probably had
thousands of hours in engineering development, has specified phase
noise, harmonic suppression and output power. The Crystek VCOs are
small SMD parts easily mounted to a DIP header if desired.

I have used 27 MhZ Crystek TCXOs and VCOs, running them into divide by
3 chips in 9 MhZ BFO applications. All worked far superior to many
different discrete designs that I tried. Plus they were much smaller
and far more stable. Good luck!

Bob, K1AO
1991 2008-08-28 13:35:05 rcbuckiii Re: 400 MHz VCO
1992 2008-08-28 17:54:17 bobtbobbo Re: 400 MHz VCO
---> > Bob, K1AO
> >
> Bob,
> I ended up using a ADF4360-7 PLL with a PIC12F508 to set the
> frequency. The VCO is built into the ADF4360-7. It only requires 2
> surface mount inductors to set the frequency range. I have no way of
> measuring the phase noise but Analog Devices says it should be -
> 116dBc at 100KHz.
> I am using this as the clock signal for a AD9951 DDS circuit. I
> could use a 100 MHz clock oscillator and multiply by 4 in the AD9951
> but I think this would yield more spurs than driving it with the 400
> MHz clock. Still in the experimental stage at this point.
> Ray
Sounds good, Ray. I have always has excellent results using AD parts.