I want to build an LO

[John S]

Ask him!

In usually start with Xc and Xl around 50 ohms. Seems to work, usually.

Yeah, that's better. It becomes a sine wave and the Fo is 9.78Mh for a
calculated tank of 10MHz.

 

[Phil Allison]

"John Devereux"

Mines simpler

<http://ee.devereux.me.uk/uhf-vfo.png>

** Doesn't the UHF band start at 300MHz ??

And how about a part number or two for the JFET ?



.... Phil

 

[John S]

Clearly you fucked up >:-}

Hint, Zo = Impedance of the tank at resonance.

I apologize if I'm using too advanced engineering terms for you guys

I promise I'll try to be nicer if Larkin and John S promise to try to
be smarter >:-}

...Jim Thompson

Well, at resonance that means that Zo = R + j0 which is 1000 ohms. So
essentially, there is a parallel 1000R across the tank or, equivalently,
a series R of .4 ohms in the inductor. Since you say the Q is 50, then
Xl = 1000/50 or about 20 ohms (you don't specify whether that is loaded
or unloaded Q, but I think an unloaded Q of 50 is more realistic than a
loaded Q of 50). This works out to a parallel combo of about 318nH,
795pF, and 1000 ohms. Or, about 318nH with .4 ohms of series resistance
in parallel with about 795pF.

It does not oscillate in LTSpice.

 

[John S]

When you are designing a circuit that requires tuning, most engineers
should be thinking in terms of the tank's resonant frequency, its
impedance at resonance, and its Q... I do. So, during the development
of that model airplane RC chip (I mentioned it here before) I whipped
up a template for a tank...

REFDES = TANK?
TEMPLATE = L^@REFDES %pin1 %pin2 {@Zo/(6.283185*@Fo*@Q)} \n
C^@REFDES %pin1 %pin2 {@Q/(6.283185*@Fo*@Zo)} \n
R^@REFDES %pin1 %pin2 @Zo
PART = TANK
SIMULATIONONLY =
Fo = 10Meg
Zo = 1K
Q = 10

where Fo, Zo, and Q are selectable parameters.

(Users of other Spice variants should be able to easily re-write this
"template" as a subcircuit.)

I also did it for tapped tanks, both L-taps and C-taps.

Why do I have to be constantly recalculating L, C and R values, when
PSpice will do it for me automatically?

I can even have PSpice list the L, C and R values in the parts list,
if I want.

The only reason I can see for Larkin's consternation is that the
method went right over his head, and sycophant John S "proudly"
announced, "Well, Jim didn't furnish L and C values".

Start acting like engineers, folks. Instead of trying to label me the
"buffoon", you should be asking yourself why didn't you understand it,
or is it that your only intent is to be assholes? I think the latter.

...Jim Thompson

The only thing I did not understand was your reference to Zo. I've never
seen that applied to a tank circuit.

And why are you being so discourteous to me? I have been polite. I have
not attacked you. I have not intentionally insulted you. I never
referred to you as a "buffoon".

 

[John Devereux]

"John Devereux"


** Doesn't the UHF band start at 300MHz ??

Does it? Hey, I was (even more) ignorant then, that was a scan I did in
1999 or so, of some old notes. And I don't know where the circuit came
from, AoE perhaps since that was my about my source of all knowledge
then!

And how about a part number or two for the JFET ?

Can't remember for sure, it was 20 years ago. J310 I think?

 

[Phil Allison]

"John Devereux"

"Phil Allison"

Does it?

** Yep.

VHF TV broadcast transmissions extend to over 250MHz.

http://en.wikipedia.org/wiki/Radio_spectrum

But UK television has been on UHF only for a long time now.

Can't remember for sure, it was 20 years ago. J310 I think?

** Should be OK up to 250MHz.



.... Phil

 

[Jamie]

Someone doesn't know how to run a transient analysis on an oscillator.
Was that John S who said it doesn't oscillate in LTspice? Shame on
you!




Liar, liar, pants on fire. I'm trying to be helpful and you keep
trying to stick your nose up my dog's butt.

The discerning student is learning from my posts. But you're not ;-)

Why don't you go sit in the corner, suck your thumb, and STFU?

...Jim Thompson

Yeah, you got that right, people are learning..

Jamie

 

[josephkk]

As long as it doesn't give you the Clapp.

If it crystallizes it may Pierce you. It would also severely limit the
tuning range.

?-)

 

[josephkk]

You flatter yourself, old man... because nobody else will do it. You have made
yourself the group buffoon.

No Larkin that is you.

?-(

 

[o pere o]

It's not if you stick to the KISS principle, for example...

http://www.analog-innovations.com/SED/Primitive_LO.pdf

...Jim Thompson

The 1648 is a nice IC! Does this structure have any advantage compared
to the cross-coupled pair with the tank across the collectors -which I
have used quite successfully to make quick-and-dirty oscillators from 1
to several GHz?

[Schematic is unclear, but you know what I mean ]

| |
R R
| |
*----TANK----*
| |
C---- -----C
B--- X ---B
E E
------*-------
Re
|
GND

There are variants of this, most notably with L/2 instead of each of the
R and the cap between collectors. They seem to be very popular in
integrated VCOs nowadays...

Pere

 

[o pere o]

The 1648 is a nice IC! Does this structure have any advantage compared
to the cross-coupled pair with the tank across the collectors -which I
have used quite successfully to make quick-and-dirty oscillators from 1

oops: that shoud be:
from 1 MHz to several GHz

Pere

to several GHz?

[Schematic is unclear, but you know what I mean ]

| |
R R
| |
*----TANK----*
| |
C---- -----C
B--- X ---B
E E
------*-------
Re
|
GND

There are variants of this, most notably with L/2 instead of each of the
R and the cap between collectors. They seem to be very popular in
integrated VCOs nowadays...

Pere

 

[o pere o]

On Sat, 8 Jun 2013 13:50:45 -0700 (PDT), "M. Hamed"

Hello the most knowledgeable electronics group on earth!

I am trying to build a few LC oscillators for experimenting with mixers. I do not want to use Crystals, Frequency Synthesis, DDS, etc. Let's say 2 MHz and 100 MHz. I don't know where to start!

I have a few RF books and the treatment of oscillator can be one of the following:

1- Too much theory on the analysis of oscillator circuits, phase noise, IMD, etc. No circuits to build. Just demo circuits for illustration, sometimes only the small signal model.
2- Circuits to build but designed at very specific frequencies with very specific components. Very little on how to generalize things into different frequencies. (The ARRL handbook 2010 and Experimental Methods fall under this).
3- Books that give me a design procedure with S parameters but I can't find any S parameter files for the devices I'm interested in using.
4- Cookbooks that have the same problem as (2).

Why is it so hard to build an oscillator at an arbitrary frequency?

It's not if you stick to the KISS principle, for example...

http://www.analog-innovations.com/SED/Primitive_LO.pdf

...Jim Thompson

The 1648 is a nice IC! Does this structure have any advantage compared
to the cross-coupled pair with the tank across the collectors -which I
have used quite successfully to make quick-and-dirty oscillators from 1
to several GHz?

[Schematic is unclear, but you know what I mean ]

| |
R R
| |
*----TANK----*
| |
C---- -----C
B--- X ---B
E E
------*-------
Re
|
GND

There are variants of this, most notably with L/2 instead of each of the
R and the cap between collectors. They seem to be very popular in
integrated VCOs nowadays...

Pere

Jim's circuit has one end of the tank at RF ground, which can have advantages,
like when using an asymmetric trimmer cap or a varicap. And it's easier to
extract the output signal. Q is potentially higher, too, since it doesn't have
those R things across the LC.

True. In the above circuit I had to use back-to-back varicaps to make a
VCO. And the output is best processed differentially too, but this may
not be an issue depending on the application.

Currently I am playing with another variation of this circuit, with
transmission lines in place of the L/2 inductors. Trying to observe and
find an use for the multiple natural resonant modes that arise. Perhaps
injection locking at several frequencies...

Pere

 

[Fred Abse]

The only thing I did not understand was your reference to Zo. I've never
seen that applied to a tank circuit.

Traditionally called "Dynamic Impedance at Resonance"

 

[josephkk]

Dynamic?

Do you have some issue associating dynamic with motional?

?-)

 

[Fred Abse]

I didn't know it had a specific name ;-)

I just found the impedance method useful when designing, rather than
just pulling some number out of the air, like 50 Ohms.... >:-}

Radiotron Designer's Handbook calls it "Resonant Impedance".

 

[Fred Abse]

Dynamic?

Quite so. Sometimes just called "The Dynamic".

In the case of a parallel circuit, with loss resistance only in the
inductor, and Q high enough to neglect the effect of R on resonant
frequency, it reduces to:

Zd = L/CR

Radiotron Designer's handbook gives a more thorough treatment in terms of
admittance, including capacitance losses, and resistive effects on
resonant frequency.

What it all boils down to is: keep the L/C ratio as high as possible.

I threw this simulation together to illustrate the effects of varying L/C
ratio. 1MHz resonant frequency, constant Q, and the ratio L/C varied from
0.1 to 10. Zd varies from 40 ohms to 400 ohms.

There is a plot file after the .asc file, which plots Zd.

Version 4
SHEET 1 880 680
WIRE 144 80 -16 80
WIRE 256 80 144 80
WIRE 144 128 144 80
WIRE 256 128 256 80
WIRE -16 160 -16 80
WIRE 256 224 256 208
WIRE -16 336 -16 240
WIRE 144 336 144 192
WIRE 144 336 -16 336
WIRE 192 336 144 336
WIRE 256 336 256 304
WIRE 256 336 192 336
FLAG 144 336 0
FLAG 192 336 0
SYMBOL ind 240 112 R0
SYMATTR InstName L1
SYMATTR Value {1u*sqrt(N)}
SYMBOL cap 128 128 R0
SYMATTR InstName C1
SYMATTR Value {25.33n/sqrt(N)}
SYMBOL res 240 208 R0
WINDOW 3 36 52 Left 2
SYMATTR InstName R1
SYMATTR Value {w*1u*sqrt(N)/Q}
SYMBOL voltage -16 144 R0
WINDOW 123 24 108 Left 2
WINDOW 39 24 120 Left 2
SYMATTR InstName V1
SYMATTR Value ""
SYMATTR Value2 AC 1
SYMATTR SpiceLine Rser=1000
TEXT 336 56 Left 2 !.param w=2*pi*1e6
TEXT 336 136 Left 2 !.ac dec 1000 300k 1.8meg
TEXT 336 88 Left 2 !.step param N 0.1 10 1
TEXT 336 112 Left 2 !.plot V(n001)
TEXT 336 72 Left 2 !.param Q=100


[AC Analysis]
{
Npanes: 1
{
traces: 1 {524290,0,"V(n002)"}
X: ('M',1,300000,200000,1.8e+006)
Y[0]: ('m',0,0,0.07,0.7)
Y[1]: (' ',0,-100,20,100)
Log: 0 0 0
GridStyle: 1
PltMag: 1
}
}

 

[Fred Abse]

There is a plot file after the .asc file, which plots Zd.

No, it doesn't. Sorry.

This one does:

[AC Analysis]
{
Npanes: 1
{
traces: 1 {524290,0,"V(n002)/I(R2)"}
X: ('M',1,300000,200000,1.8e+006)
Y[0]: (' ',0,0,40,400)
Y[1]: (' ',0,-280,20,-80)
Log: 0 0 0
GridStyle: 1
PltMag: 1
}
}

 

[Bill Sloman]

If someone is not already credited on this, you should be.

It's a very old joke. I first heard it in 1969 as "amplifiers you design oscillate and oscillators don't".

Many times the result is a very poor, inefficient one at that, as
compared to dedicated (intended to be such at design time) oscillator
circuits.

Obviously. But at least they oscillate.

 

[josephkk]

I associate motional with motion, like a quartz resonator.

Consider your vocabulary expanded.

?;-)

 

[Fred Abse]

What it all boils down to is: keep the L/C ratio as high as possible.

I did another spin of the circuit I posted, using a current source,
instead of a voltage, so as to remove external damping. This demonstrates
that the bandwidth doesn't change with L/C ratio, only the resonant
impedance. Rd now swings from about 800 to 2500 ohms, for a swing in L/C of 100:10.

Plot file after the .asc file.

Version 4
SHEET 1 880 680
WIRE 144 80 -16 80
WIRE 256 80 144 80
WIRE 144 128 144 80
WIRE 256 128 256 80
WIRE -16 160 -16 80
WIRE 256 224 256 208
WIRE -16 336 -16 240
WIRE 144 336 144 192
WIRE 144 336 -16 336
WIRE 256 336 256 304
WIRE 256 336 144 336
WIRE 144 368 144 336
FLAG 144 368 0
SYMBOL ind 240 112 R0
SYMATTR InstName L1
SYMATTR Value {1u*sqrt(N)}
SYMATTR SpiceLine Rser=0
SYMBOL cap 128 128 R0
SYMATTR InstName C1
SYMATTR Value {25.33n/sqrt(N)}
SYMBOL res 240 208 R0
WINDOW 3 36 52 Left 2
SYMATTR Value {(w*1u*sqrt(N))/Q}
SYMATTR InstName R1
SYMBOL current -16 160 R0
WINDOW 123 24 92 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName I1
SYMATTR Value ""
SYMATTR Value2 AC 1
TEXT 336 56 Left 2 !.param w=2*pi*1e6
TEXT 336 136 Left 2 !.ac lin 1000 300k 1.8meg
TEXT 336 88 Left 2 !.step param N 10 100 10
TEXT 336 112 Left 2 !.plot V(n001)
TEXT 336 72 Left 2 !.param Q=40
TEXT -24 384 Left 2 !.measure tmp max mag(V(n001))\n.measure BW trig mag(V(n001))=tmp/sqrt(2) rise=1 targ mag(V(n001))=tmp/sqrt(2) fall=last

Plot file:

[AC Analysis]
{
Npanes: 1
{
traces: 1 {524290,0,"V(n001)/I(I1)"}
X: ('M',1,300000,200000,1.8e+006)
Y[0]: ('K',1,0,300,2700)
Y[1]: (' ',0,-280,20,-80)
Log: 0 0 0
GridStyle: 1
PltMag: 1
}
}