Yo! RF dudes!

Suppose I have a 1206 ceramic-core inductor on a pc board with nothing else nearby. What would be the differential effect on inductance of adding a ground plane on the opposite side of the board? Even more important, how might it affect the TC?

I've got a 50 MHz oscillator, using a 150 nH inductor, some fixed caps, a Maxim flecap (coarse tune), and a varicap (fine tune.) Last board rev, we had it almost perfectly temperature compensated, thanks to some N750 0603 caps that are special-ordered from Japan, almost impossible to get in reasonable quantities and time frames. Some copper was moved on the latest board rev, more copper close to the L, and now the center frequency and TC are different, so I was wondering if copper proximity could explain the difference.

Oh, does anybody know of a place to get stock surface-mount NTC caps?

We can software compensate out the new TC, so the thing works, but I'd rather fix the inherent oscillator TC. If we apply a linear-with-temperature compensation voltage to the varicap, we can get a zero TC near room temp. But the varicap has its own, nasty TC that varies with capacitance, so this compensation winds up with a parabolic TC curve. OK, we could add a compensating polynomial in the software, if we didn't mind spending a week or so getting that right.

RF requires a lot of patience.

John

Reply to
John Larkin
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The effect on inductance would depend on the construction of the coil, but if the coil is wound from end to end a conductor underneath the coil shouldn't affect the inductance that much.

Absent a shield on the coil, such a conductor _would_ capacitively couple to the coil windings, which would change (probably lower) your center frequency. FR-4 has a pretty healthy temperature coefficient, so the added capacitance would be highly variable with temperature; this may be your trouble.

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Tim Wescott
Wescott Design Services
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Tim Wescott

Usually it adds stray capacitance (winding to plane), with whatever TC the dielectric of the PCB material exhibits.

The market for that stuff has become so small that you'd have to rely on expensive boutique parts. Always with the risk that the next call results in "We no longer manufacture...". I most certainly would keep doing that electronically (as you have done).

Allow me the question: Why?

The brute force approach would be to keep that part of the circuitry temperature-regulated. Did that a lot with oscillators in the olden days. I always used a temperature that is guaranteed to be higher than ambient will ever be so no TEC was required, just a resistor as a heater.

Another thing I have done but in precision phase shifters and not oscillators was to use either FET arrays (mostly gone the boutique route by now...) or PIN diodes to pull in a capacitance. Mostly with the HSMP38xx series but nowadays there are some nice PIN diodes in the Infineon BAP series. If you can get them, that is. I found that company to be less than efficient when buying prototype qties RF requires a lot of patience.

Sure does, but the results can be very rewarding and the solution stunningly sparse in parts count. Sometimes :-)

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Oh, forgot a link to a distributor you might want to ask about the caps:

formatting link

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Now just imagine the poor Europeans who have those inductors floating about in some kind of RoHS compliant goo of unknown performance.

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Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Yes, the frequency shift direction and tc are consistant with extra board capacitance. I once measured some FR4 capacitance and got +900 ppm per degree C. The oscillator frequency tc apparently shifted from near zero to -70 ppm when the extra ground was added. I guess I'll measure the critical node capacitance on a bare pcb and do the math, just to see if the numbers are close. Luckily, there's a pattern I can cut out of the backside copper that should seriously reduce C and not trash any vias. Good thing we only bought 50 boards!

Still, I'm curious about the effect of a ground plane on the inductor. I can, and will, measure the effect on L, but it's more difficult to measure the effect on the inductor's TC.

John

Reply to
John Larkin

It's most likely miniscule compared to the capacitance (bottom of winding to plane). It would be easy to measure the added capacitance if you can get something like a HP4191 input pod really close to that point. Dremel away the suspect area, measure, place copper tape over it, measure again.

Question: Is there some headroom to just solder silver wire to the pads and then the inductor to those, so it comes 1/4" or so off the board? Those wires should be beefy and stiff to avoid microphonics, else you'd have an earthquake detector.

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Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

You may have some SR problems at some spectrum..

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"I\'m never wrong, once i thought i was, but was mistaken"
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Reply to
Jamie
[...]
[...]

If the oscillator frequency is critical enough to require compensation, why not lock it to a nearby crystal? You've already got the varicap, why spend a week or so fiddling with compensation?

There has to be some crystal oscillator nearby. An hour's work should get the divider and loop filter. Build, debug document and ship in an afternoon.

You do this often enough. Why is there a problem in this case?

Regards,

Mike Monett

Reply to
Mike Monett

Not an RF person but would see the Copper under the coil as the total problem. The tuned circuit is now part tuned by the added yucky 1%decC TC, Q=80, FR material distributed capacitance but in addition this Copper "screening" (by traditional mechanisms) is also dropping the L value by say 5% and the Q value by say 5%.

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Reply to
john jardine

This is a 50 MHz oscillator that is started by an external trigger. There's a DSP servo loop, in an fpga, that does in fact lock it to a quartz reference, but preserves the phasing relative to the trigger. So the frequencies are locked, and the phase is also locked to whatever the phase difference was when the LC started.

The oscillator is coarse-tuned by the flecap at powerup, and then the varicap takes over. We need to keep the varicap pull range fairly small to meet our noise/jitter requirements. What we don't like is that the inherent TC of the oscillator was tuned to near zero, and a minor layout change messed it up enough to lose lock at temperature extremes. Again, I have a trim dac and a temperature sensor on board, and the firmware is tweaking the baseline varicap voltage to mostly null out the temperature drift, but we don't understand what's going on.

I measured the hot node capacitance on a bare board and it's only 1.2 pF, which accounts for only 10 ppm of the 70 ppm/degC we're seeing, so it's still strange.

But who can "Build, debug document and ship in an afternoon."?

John

Reply to
John Larkin

How come? Is noise getting into the varicap drive?

Who knows what else is going on. Eddy currents in the added plane etc. Can't you just raise the inductor by 1/4" and see if the problem will go away, or at least push it outside the critical range?

Some software companies seem to do that 8-D

It's like with bananas and tomatoes, the ripening process happens at the customer.

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Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

And some wines. Aged in transit. Regards,

Mike Monett

Reply to
Mike Monett

Noise *always* gets into varicap drive. And varicaps have rotten TCs, and the TCs change with voltage, which is a fresh bag'o worms. Which is why my linear oscillator TC, compensated by a linear temperature correction voltage, results in a parabolic freq:temp curve!

I might try hacking the ground plane away from the back side. Anything I try takes hours in the temp chamber to evaluate.

John

Reply to
John Larkin

As I heard the story as a youngster (mommy having worked for the brewer in question), a famous St. Louis brewer who was running out of tank brewing space poured malt, hops, water, and yeast into a railway tank car and sent it to Milwaukee where it was turned around and sent back to St. Louis as fermented beer.

Jim

Reply to
RST Engineering (jw)

What? Thunderbird ?:-)

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson
[...]

Sounds like the poor varicap is pushed to the limit. How about a slightly larger one if it would still allow you to meet the phase noise. Or maybe several in parallel. Called Massively-Parallel. Rohde shows examples in figures 5 and 8 of

formatting link
Oscillators.pdf (132KB)

(sorry for the wrap)

Also, if I'm not mistaken, the flecap has pretty low Q. This might also be worth looking at to try to reduce the effect. Other circuit arrangements might improve the phase noise by reducing the loading on the tank.

I found using a simple constant current source in place of the emitter resistor in a Colpits really improves the tank Q under load.

It also helps align the emitter current pulse with the peak of the sine wave from the tank, which improves the phase noise. Hajimiri and Lee discuss this in

formatting link

Also, Rohde patented a simple bias stabilization technique shown in figure 15 of

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(915KB)

I believe Matjaz Vitmar, S53MV, also described it long before Rohde applied for the patent.

I've tried it, and it works very well. Other feedback arrangements to stabilize the bias also help improve the phase noise.

So there's lots to look at that might help.

[...]

Depends on how much you can cut and paste from previous projects. Betcha Joerg does it all the time:)

Regards,

Mike Monett

Reply to
Mike Monett

You might have to shield that portion with the varicaps and the loop for them. Just had to do the same thing. A laser diode driver where where a fraction of a microamp of noise would throw a whole system off the rocker.

If it's lots of boards that might be more work than raising them.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Ulrich Rohde's book "Communications Receivers" is one that I find indispensable for such noise critical jobs. If it just didn't have such an ugly green cover after the dust cover fell apart.

I try not to ;-)

The only true race I did was when a client's service technician got seriously sick and almost a dozen big machines had piled up before I got there. That was true pit stop action. The worst was one machine that I had to repair over the lunch hour because the trucker was waiting outside the building to take it back, with his Diesel running.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Joerg wrote:

Aha - an expert! I've done some work in start/stop oscillators for data recovery in hard disk drives. How do you like this 50MHz version? The oscillator amplitude is set by the R2 which sets the current source in the emitter. The initial amplitude is set by the R1 from the collector of Q1 to ground.

The constant current source gives very low loading on the tank and helps stabilize the oscillator amplitude, much like Rohde's bias stabilization patent. It also helps align the emitter current pulse to the peak of the base sine wave in accordance with Hamijiri and Lee. So it helps meet several different requirements at the same time.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Version 4 SHEET 1 880 708 WIRE -32 -128 -160 -128 WIRE -32 -112 -32 -128 WIRE -160 -80 -160 -128 WIRE -400 -32 -448 -32 WIRE -256 -32 -320 -32 WIRE -224 -32 -256 -32 WIRE -256 -16 -256 -32 WIRE -32 -16 -32 -32 WIRE -448 32 -448 -32 WIRE -256 64 -256 48 WIRE -160 64 -160 16 WIRE -160 64 -256 64 WIRE -128 64 -160 64 WIRE -112 64 -128 64 WIRE -256 80 -256 64 WIRE -448 128 -448 112 WIRE -160 144 -160 64 WIRE -256 192 -256 144 WIRE -224 192 -256 192 WIRE -256 208 -256 192 WIRE -160 256 -160 240 WIRE -112 256 -160 256 WIRE -96 256 -112 256 WIRE -160 368 -160 336 WIRE -160 464 -160 448 FLAG -160 464 0 FLAG -128 64 Vosc FLAG -448 128 0 FLAG -32 -16 0 FLAG -112 256 Vem FLAG -256 208 0 SYMBOL voltage -32 -128 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 0 13 11 Left 0 WINDOW 3 -9 57 Left 0 SYMATTR InstName V1 SYMATTR Value 5V SYMBOL npn -224 -80 R0 SYMATTR InstName Q1 SYMATTR Value 2N2369 SYMBOL cap -272 -16 R0 WINDOW 0 35 18 Left 0 WINDOW 3 38 47 Left 0 SYMATTR InstName C1 SYMATTR Value 5n SYMBOL res -176 240 R0 SYMATTR InstName R7 SYMATTR Value 15k SYMBOL cap -272 80 R0 WINDOW 0 41 17 Left 0 WINDOW 3 37 45 Left 0 SYMATTR InstName C2 SYMATTR Value 140p SYMBOL ind -304 -16 M270 WINDOW 0 32 56 VTop 0 WINDOW 3 5 56 VBottom 0 WINDOW 39 -16 53 VBottom 0 SYMATTR InstName L1 SYMATTR Value 20u SYMATTR SpiceLine Rser=10 SYMBOL npn -224 144 R0 SYMATTR InstName Q2 SYMATTR Value 2N2369 SYMBOL voltage -160 464 R180 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 0 8 11 Left 0 WINDOW 3 -7 58 Left 0 SYMATTR InstName V2 SYMATTR Value 2v SYMBOL voltage -448 16 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 0 8 11 Left 0 WINDOW 3 -17 56 Left 0 SYMATTR InstName V3 SYMATTR Value 2.5V TEXT -440 328 Left 0 !.tran 0 250u 0 5n TEXT -408 -168 Left 0 ;'My 50MHz StartStop Current Source Colpitts

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here is the PLT file:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [Transient Analysis] { Npanes: 2 { traces: 1 {524290,0,"V(vosc)"} X: ('µ',0,0,3e-005,0.00025) Y[0]: (' ',1,0,0.5,6) Y[1]: ('_',0,1e+308,0,-1e+308) Volts: (' ',0,0,1,0,0.5,6) Log: 0 0 0 GridStyle: 1 }, { traces: 1 {34603011,0,"Ic(Q1)"} X: ('µ',0,0,3e-005,0.00025) Y[0]: ('m',1,-0.0006,0.0002,0.0012) Y[1]: ('_',0,1e+308,0,-1e+308) Amps: ('m',0,0,1,-0.0006,0.0002,0.0012) Log: 0 0 0 GridStyle: 1 } }

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ This version shows the phase error during startup. Look at the crossings through 2.5V. The phase difference is pretty low.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Version 4 SHEET 1 880 708 WIRE 304 -192 224 -192 WIRE 336 -192 304 -192 WIRE 448 -192 400 -192 WIRE -432 -176 -512 -176 WIRE -400 -176 -432 -176 WIRE -288 -176 -336 -176 WIRE 448 -176 448 -192 WIRE -288 -160 -288 -176 WIRE 304 -160 304 -192 WIRE -432 -144 -432 -176 WIRE 224 -144 224 -192 WIRE -512 -128 -512 -176 WIRE -64 -96 -112 -96 WIRE 96 -96 16 -96 WIRE 160 -96 96 -96 WIRE -800 -80 -848 -80 WIRE -640 -80 -720 -80 WIRE -576 -80 -640 -80 WIRE 96 -80 96 -96 WIRE 448 -80 448 -96 WIRE -640 -64 -640 -80 WIRE -288 -64 -288 -80 WIRE 304 -64 304 -80 WIRE -432 -48 -432 -64 WIRE -112 -32 -112 -96 WIRE -848 -16 -848 -80 WIRE 96 0 96 -16 WIRE 224 0 224 -48 WIRE 224 0 96 0 WIRE 256 0 224 0 WIRE 272 0 256 0 WIRE -640 16 -640 0 WIRE -512 16 -512 -32 WIRE -512 16 -640 16 WIRE -480 16 -512 16 WIRE -448 16 -480 16 WIRE 96 32 96 0 WIRE -640 48 -640 16 WIRE -112 64 -112 48 WIRE -848 80 -848 64 WIRE 224 80 224 0 WIRE -512 96 -512 16 WIRE 96 128 96 96 WIRE 160 128 96 128 WIRE -640 144 -640 112 WIRE -576 144 -640 144 WIRE 96 144 96 128 WIRE -640 160 -640 144 WIRE 224 208 224 176 WIRE -512 224 -512 192 WIRE 224 304 224 288 WIRE -512 320 -512 304 WIRE 224 400 224 384 WIRE -512 416 -512 400 FLAG -512 416 0 FLAG -480 16 Q1E FLAG -848 80 0 FLAG -288 -64 0 FLAG -640 160 0 FLAG -432 -48 0 FLAG -512 -176 Q1C FLAG -640 -80 Q1B FLAG 224 400 0 FLAG 256 0 Q3E FLAG -112 64 0 FLAG 448 -80 0 FLAG 96 144 0 FLAG 304 -64 0 FLAG 224 -192 Q3C FLAG 96 -96 Q3B SYMBOL voltage -288 -176 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 0 13 11 Left 0 WINDOW 3 -111 169 Left 0 SYMATTR InstName V1 SYMATTR Value PULSE(0 5 10n 5n 1n 1 1 1) SYMBOL npn -576 -128 R0 SYMATTR InstName Q1 SYMATTR Value 2N2369 SYMBOL cap -656 -64 R0 WINDOW 0 35 18 Left 0 WINDOW 3 38 47 Left 0 SYMATTR InstName C1 SYMATTR Value 2.546E-10 SYMBOL res -528 208 R0 SYMATTR InstName R7 SYMATTR Value 1.2k SYMBOL cap -656 48 R0 WINDOW 0 41 17 Left 0 WINDOW 3 37 45 Left 0 SYMATTR InstName C2 SYMATTR Value 8.488E-11 SYMBOL ind -704 -64 M270 WINDOW 0 32 56 VTop 0 WINDOW 3 5 56 VBottom 0 WINDOW 39 -16 53 VBottom 0 SYMATTR InstName L1 SYMATTR Value 1.5915E-07 SYMATTR SpiceLine Rser=1.66 SYMBOL npn -576 96 R0 SYMATTR InstName Q2 SYMATTR Value 2N2369 SYMBOL voltage -512 416 R180 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 0 8 11 Left 0 WINDOW 3 -7 58 Left 0 SYMATTR InstName V2 SYMATTR Value 2v SYMBOL voltage -848 -32 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 0 8 11 Left 0 WINDOW 3 -14 59 Left 0 SYMATTR InstName V3 SYMATTR Value 2.5V SYMBOL schottky -336 -192 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName D1 SYMATTR Value 1N5817 SYMATTR Description Diode SYMATTR Type diode SYMBOL res -448 -160 R0 SYMATTR InstName R1 SYMATTR Value 3.3k SYMBOL voltage 448 -192 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 0 13 11 Left 0 WINDOW 3 -120 173 Left 0 SYMATTR InstName V4 SYMATTR Value PULSE(0 5 217.14687n 5n 1n 1 1 1) SYMBOL npn 160 -144 R0 SYMATTR InstName Q3 SYMATTR Value 2N2369 SYMBOL cap 80 -80 R0 WINDOW 0 35 18 Left 0 WINDOW 3 38 47 Left 0 SYMATTR InstName C3 SYMATTR Value 2.546E-10 SYMBOL res 208 192 R0 SYMATTR InstName R2 SYMATTR Value 1.2k SYMBOL cap 80 32 R0 WINDOW 0 41 17 Left 0 WINDOW 3 37 45 Left 0 SYMATTR InstName C4 SYMATTR Value 8.488E-11 SYMBOL ind 32 -80 M270 WINDOW 0 32 56 VTop 0 WINDOW 3 5 56 VBottom 0 WINDOW 39 -16 53 VBottom 0 SYMATTR InstName L2 SYMATTR Value 1.5915E-07 SYMATTR SpiceLine Rser=1.66 SYMBOL npn 160 80 R0 SYMATTR InstName Q4 SYMATTR Value 2N2369 SYMBOL voltage 224 400 R180 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 0 8 11 Left 0 WINDOW 3 -7 58 Left 0 SYMATTR InstName V5 SYMATTR Value 2v SYMBOL voltage -112 -48 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 0 8 11 Left 0 WINDOW 3 -14 59 Left 0 SYMATTR InstName V6 SYMATTR Value 2.5V SYMBOL schottky 400 -208 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName D2 SYMATTR Value 1N5817 SYMATTR Description Diode SYMATTR Type diode SYMBOL res 288 -176 R0 SYMATTR InstName R3 SYMATTR Value 3.3k TEXT -248 -256 Left 0 !.tran 0 1.5u 0 200p TEXT -304 -320 Left 0 ;'50MHz StartStop Colpitts Measure Phase

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here's the PLT file (watch for line wrap):

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [Transient Analysis] { Npanes: 1 { traces: 3 {524292,0,"V(q1c)"} {524290,0,"V(q1b)"} {524291,0,"V(q3b)"} X: ('µ',1,0,2e-007,1.5e-006) Y[0]: (' ',1,0,0.5,5) Y[1]: ('_',0,1e+308,0,-1e+308) Volts: (' ',0,0,1,0,0.5,5) Log: 0 0 0 GridStyle: 1 } }

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Regards,

Mike Monett

Reply to
Mike Monett

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