UHF tank circuits

[Paul Burridge]

Hi guys,

I've just been doing some calculations out of sheer curiosity. It
turns out that to make a tank circuit for 450Mhz (which isn't *that*
high a frequency by today's standards) would take 5pF || 25nF. These
are both *very* low values. 5pF is getting perilously close to being
seriously affected by circuit board stray capacitance and 25nF isn't
much better; little more than a couple of turns of wire, I guess.
What does one do in such circumstances? Should one be thinking in
terms of etching these values out of the PCB by the time one gets of
to these frequencies, or is it still acceptable to make them up out of
discrete components?

p

 

[Leon Heller]

Hi guys,

I've just been doing some calculations out of sheer curiosity. It
turns out that to make a tank circuit for 450Mhz (which isn't *that*
high a frequency by today's standards) would take 5pF || 25nF. These
are both *very* low values. 5pF is getting perilously close to being
seriously affected by circuit board stray capacitance and 25nF isn't
much better; little more than a couple of turns of wire, I guess.
What does one do in such circumstances? Should one be thinking in
terms of etching these values out of the PCB by the time one gets of
to these frequencies, or is it still acceptable to make them up out of
discrete components?

Don't you mean 5 pF || 25 nH? You could probably use a single turn or a
hairpin (1/2 turn) inductor and capacitor. It's quite workable at 450 MHz.


Leon

 

[John Larkin]

Hi guys,

I've just been doing some calculations out of sheer curiosity. It
turns out that to make a tank circuit for 450Mhz (which isn't *that*
high a frequency by today's standards) would take 5pF || 25nF. These
are both *very* low values. 5pF is getting perilously close to being
seriously affected by circuit board stray capacitance and 25nF isn't
much better; little more than a couple of turns of wire, I guess.
What does one do in such circumstances? Should one be thinking in
terms of etching these values out of the PCB by the time one gets of
to these frequencies, or is it still acceptable to make them up out of
discrete components?

p

You can buy surface-mount caps below 1 pF and inductors below 1 nH, so
a discrete LC is perfectly feasible at 450 MHz. Even with a couple of
pF of circuit stuff - say, the gate of a gaasfet or an IC - you should
still be able to make a resonator or a filter up there. A small
trimmer cap or varicap would help nail the resonant frequency.

PCB capacitance for 0805 or 0603-size parts is pretty small, and you
can keep connection inductance in the 1-2 nH range.

Helical resonators are nice, too, as are coaxial ceramic resonators.
450 MHz is sort of in the transition region between discrete and
distributed resonator domains.

John

 

[W3JDR]

Paul,
Once you get to these frequencies, it's often better and easier to use
transmission lines (microstrip) rather than lumped LC components.

What is the application?

Joe
W3JDR

 

[James Fenech]

Hi Paul,

I'm currently experimenting with VCO circuits (had an earlier thread about
varactors). My current prototype uses a MPF5179, a pair of BB909 tuning
diodes and a home made inductor. The inductor is about 4 turns on 0.25"
centre (air cored), spread out to about 0.4" in length, giving about 150nH.
This circuit operates to 350MHz and (if the tuning diode info is OK)
indicates strays of less than 1pF. This is built "dead bug" style, and is
really my first attempt at anything of this sort of high frequency.

I'm sure with a little care 450MHz is quite achievable.

James.

 

[Tim Wescott]

I haven't actually built anything out of it, but the ARRL UHF experamenter's
manual seems to be well written. Get a copy if you're serious.

In general you get to a frequency/power level where you need to toss the
notion of using discrete componants and start using distributed (i.e. some
variation of a transmission line) instead. This happens sooner at higher
power levels (1500W amplifiers start showing plate lines at 2 meters).

One of these days I _am_ going to build some VHF circuits -- really!

 

[Paul Burridge]

Paul,
Once you get to these frequencies, it's often better and easier to use
transmission lines (microstrip) rather than lumped LC components.

What is the application?

I'm just musing with the idea of building a TX for 70cm. Since it's
probably going to end up with a commercial, ready-built 3 stage 40W
module for the PA., I'm just considering what to use for the last
frequency multiplier and buffer stage...

 

[Leon Heller]

I'm just musing with the idea of building a TX for 70cm. Since it's
probably going to end up with a commercial, ready-built 3 stage 40W
module for the PA., I'm just considering what to use for the last
frequency multiplier and buffer stage...

Don't construct it like your FSM, please!

Leon

 

[Joe McElvenney]

Hi,

Try a hairpin loop.

Transmission lines are still quite large at 450MHz and
one less than a quarter-wave will have the inductive
reactance necessary to resonate with your 5pF. The beauty
of them is that you can tap up and down the line quite
easily and variable coupling is a doddle.

Should they work out to be somewhat on the long side for
the board, double them over. Some old ECM transmitters I
came across had what looked like a conventional centre-
tapped tank but which were really rolled up lines.


Cheers - Joe

 

[Paul Burridge]

Don't construct it like your FSM, please!

Hehe! No chance.
On the subject of PA modules for the 400-500Mhz range, would anyone
care to recommend a suitable manufacturer?

 

[ddwyer]

Hi Paul,

I'm currently experimenting with VCO circuits (had an earlier thread about
varactors). My current prototype uses a MPF5179, a pair of BB909 tuning
diodes and a home made inductor. The inductor is about 4 turns on 0.25"
centre (air cored), spread out to about 0.4" in length, giving about 150nH.
This circuit operates to 350MHz and (if the tuning diode info is OK)
indicates strays of less than 1pF. This is built "dead bug" style, and is
really my first attempt at anything of this sort of high frequency.

in the early 80s I built some narrow band VCO employing a 450MHz Toko
helical resonator bandpass filter.
The trick was to get the phase including amplifier and a varicap phase
shifter to zero.
I recall a -125db /rootHZ at 10kHz with a bfr bipolar.
Never got anything better though was narrow band.