Dishal tuning method question

After the not-so-great bandpass filter I built last week, I built another one this week using a capacitor-coupled resonator topology. Happily, it works pretty much as designed! However, I did discover that the "real world" method of Dishal tuning isn't quite so clean as the literature would have you believe.

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(which, ironically enough, is trying to sell you on laser timming rather than trimmer caps, which is what I used :-) ) have, e.g., figures 3 and 4 where the first and second resonator, respectively, are tuned. Well, I can get figure 3 just fine, but when I go to the second resonator, the two peaks in figure 4 are asymmetrical in amplitude, and I have to re-adjust the first resonator to make them symmetrical again, kind of iterating between centering the second resonator's dip and making the first resonators response symmetric.

As you could imagine, this problem tends to compound itself as you move from resonator to resonator. So, I'm curious:

1) Presumably the assymetries in one section arise due to the additional parasitics present when you un-short the next section? Or perhaps they're due to a little through-the-air coupling between sections? I'm making another board that shields each resonator section to find out... 2) I eventually was able to tune up the entire filter reasonably well by deciding that I was only going to tweak the "current" section and the immediately prior sections, getting things as centered and symmetrical as possible (a somewhat iterative process). Trying to go through multiple prior sections became as exercise in futility, and I rationalized that the further away a section was, the less impacts the parasitics of the current section had on it -- hence the idea of only considering the current section and the one immediately prior. How's that sound for a tuning procedure?

I believe that all the plots in the Johanson paper were computer-generated. Anyone have some experience with tuning real LC filters?

Thanks,

---Joel

Reply to
Joel Kolstad
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In message , dated Tue, 22 Aug 2006, Joel Kolstad writes

Is this filter on just an open board? It's good to build a filter in a U-shaped metal channel that is a waveguide below cut-off at the highest frequency of interest. This attenuates radiated coupling, of course, but you may need to put metal partitions between the sections as well.

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Reply to
John Woodgate

I suspect you may be right about this being a simulation - I ran the circuit in LTSpice and got very close indeed to the Johanson plots.

However, if your results were markedly different then I would suspect undesired coupling. John's suggestion sounds good, also if you are using chip inductors you could try orienting them so they are at 90 deg to their neighbours.

I did find that the interconnecting track between sections can have an effect.

Regards Ian

Reply to
Ian

Hi John,

Yes, with a ground plane on the backside of the board. It's about 3" long by

1/2" wide, with the signal flowing from one end to the other.

That's pretty much one of the things I'll be trying in the next few days (although I was still planning on a "straight" layout).

---Joel

Reply to
Joel Kolstad

Hi Ian,

The amplitude imbalance was probably some 5dB or more, but "leveling" things requires all of about 5-15 degrees of tweaking on the trim caps.

They're air core ("spring") inductors. The board is ~3"x0.5", and the axis of the inductors is perpendicular to the long axis of the board; after shielding each section, I'll try rotating 90 degrees per section (I suppose I could also alternate board sides from section to section). Too bad no one (that I'm aware of) makes torodial air cores (or any core such that the unloaded Q is still, say, >100 at VHF/UHF)...

Good to know I'm not alone. :-)

---Joel

Reply to
Joel Kolstad

In message , dated Wed, 23 Aug 2006, Joel Kolstad writes

Plastic rings. Try your local low-cost 'everything' store.

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Reply to
John Woodgate

micrometals also have a ur = 1 series of toroidal cores (aka formers)

Cheers Terry

Reply to
Terry Given

Note that using solid plastic cores will seriously degrade the tempco and distributed capacitance. The solid plastic will be strong enough to stretch the wire, so that the tempco will go up to twice the CTE of the plastic.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

Thanks for the suggestions on toroidal air (-like) cores. In my case I'm using coils that are about 1/4" long and 1/8" in diameter, so I don't think I'll be pursuing the dollar store ring option. :-) Still, in the future for HF stuff, I like the idea!

Reply to
Joel Kolstad

In message , dated Thu,

24 Aug 2006, Phil Hobbs writes

I think that's a bit alarmist. I don't suppose the OP needs 0.1% resistance stability and military temperature range.

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Reply to
John Woodgate

In message , dated Thu, 24 Aug 2006, Joel Kolstad writes

No, use beads from your small daughter's toy necklace. Replace them with real pearls. (;-)

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Reply to
John Woodgate

Hello John,

I think Phil was looking more at the change in inductance. But it won't be a lot either since wire tends to spring back a little after winding, leaving some slack towards the PVC "core".

Now if you ran this setup in L.A. and the smog gums up the PVC surface that's a whole 'nother matter.

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

The tempco of an air-core copper coil is approximately twice the CTE of the copper, because the inductance goes as r**2. If you wind the wire tightly on the plastic, the plastic now controls the thermal expansion because it stretches the wire as it heats up, and the tempco is then about twice the CTE of the plastic. The CTEs are:

Copper: 16 ppm/K

Polystyrene: 50-80 ppm/k

Polypropylene: ~200 ppm/K

Teflon: nonlinear, 125 to 1000 ppm/K

So the typical tempco of an air-core coil is about 32 ppm/K, vs 400 ppm/K if you wind it on polypropylene.

Length effects may change the numbers somewhat, but the ratio will still be on the order of 10x.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

In message , dated Thu,

24 Aug 2006, Phil Hobbs writes

OK, but you didn't say tempco of inductance and you still haven't explicitly. 400 ppm/K is still pretty low, even though it's 10 times what you get with an air core. And you only get 400 ppm/K if you wind it tightly.

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Reply to
John Woodgate

In message , dated Thu, 24 Aug 2006, John Woodgate writes

I should have added, 'Bring back N750 capacitors!' (Ceramic capacitors from WWII to around 1970 had a tempco of capacitance of -750 ppm/K unless you paid more for something else). These COK things don't compensate.

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Reply to
John Woodgate

Well, I think 400 ppm/K is pretty high if you're building filters or oscillators, don't you? At 30 MHz, that's 6 kHz per degree!

And if you don't wind it pretty tightly, it'll squirm around unless you're using really heavy wire. If the wire is heavy enough, heating after winding will probably cause enough slump and shrinkage to relax the fit, which could be a reasonable technique for hand-winding. (Of course one could also melt the plastic core completely, but styrene monomer is not my favourite thing to breathe.)

Anyway, this is all getting too ARRL Handbook-ish. Winding toroids on shower rings might be sort of fun in a weird way.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

In message , dated Thu,

24 Aug 2006, Phil Hobbs writes

I meant to add the bit about N750 caps. And it depends entirely on what performance is wanted, whether the TC is acceptable or not.

What are the CTE's of ferrites? Anisotropic, I expect.

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Reply to
John Woodgate

Since they're sintered powders, they should be pretty isotropic, I think. For gapped toroids, the gap will expand with temperature at the same rate as the core, and will usually dominate the reluctance, so the TC of inductance ought to be

TC_L ~ (2*CTE) - CTE + TC_mu

total xsection gap permeability.

or CTE + TC_mu.

For ungapped ones, there's no gap to expand, and the inductance should go as the volume, so TC_L ~ 3*CTE + CT_mu. I don't know how the CTE and TC_mu compare, but it's sort of interesting that for a CT_mu = 0, the TC_L of an ungapped toroid comes out to be triple that of a gapped one.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

Hello Phil,

Not to forget the microphonics when doing high-Q filters up there. Many of us have heard that telltale "boink" in the audio when tapping the chassis.

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Regards, Joerg

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

The micrometals "formers" are phenolic. I did a quick google, but CTE depends on the exact material, however I did find one type of material (X7101) whos CTE was close to Aluminium....

Teflon would make a pretty crap former!

Cheers Terry

Reply to
Terry Given

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