Musing on RF filters generally

Properly constructed PLL

Or a lock-in amplifier - which is almost the same thing.

John

An ADC followed by a CPU.

-- john, KE5FX

As a real filter, it sounds like you want a crystal filter, which can be a big deal.

Or as suggested, a PLL, preferably one that uses a very good VCXO.

The best PLL uses a VCOCXO and has distinct acquire and track modes or a very good phase-frequency detector, to minimize the active bandwidth once it finds lock.

I've mounted the OCXO on springs, to make it less vibration sensitive.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Your layout and manufacturing quality has improved vastly over the years, John. Well done!

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Er, can you expand on that a bit? I mean, I know what a PLL does in its typical implementation, but how could it be used as something approaching an ideal filter?

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Am 27.10.19 um 19:26 schrieb John Miles, KE5FX:

Then you have a clean plot, but not a clean carrier.

;-) Gerhard, DK4XP

a superconducting cavity resonator.

yeah that.

at 10MHz a cavity resonator could be quite large.

--
  When I tried casting out nines I made a hash of it.

That is part of the NIF timing system, delivered in the year 2000.

One problem was fan vibration, but a bigger problem was the SMB test connectors. When someone yanked a cable out of the module, the connector would SNAP! and we'd lose phase lock for a second or so. This is a bang-bang phase detector with a fast acquire mode and a slow track mode. The slow mode has super low phase noise and jitter, but is easily knocked out of lock.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Helical resonator, vibration mounted. Maybe the size of a trash can.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Am 27.10.19 um 18:55 schrieb Cursitor Doom:

3 weeks ago, I tried this: < >

input - 50 Ohm to 400 Ohm transformer----- 400 Ohm line ----

400-to50-transformer - output.

From the 400 Ohm line to GND are 2 cheap 10 MHz crystals, one with a

35pF series trimmer,

the other with series trimmer and 4.7uH. The xtal resonances thus are 10 MHz +1.6 KHz and -1.6 KHz.

Transformers are 1:8CT Pulse electronics CX2049LNLT digikey

553-2287-1-nd. Autotransformer feature was not used.

The filter costs next to nothing. It won't turn my MV89A references into BVAs, but gives at least some keyhole insights . There could be easily 5 crystal pairs for strategically chosen frequencies.

For notches closer to the carrier the impedance level must be probably lower (1:4 or 1:2 xformers). That makes the notches less deep, but also less wide, so they cannot interact at the carrier frequency itself.

The filter withstands quite a lot of power because the crystals do not resonate on the carrier frequency. Just make sure you do not sweep them with +30 dBm accidentally.

If you would build a filter that is resonant on the carrier frequency, it would impair phase stability of the carrier and the crystal would blow up at @20 dBm. At -3 dBm (a level that the crystal can survive in resonance), your signal takes a hit from the thermal noise @ -173 dBm.

On the VNA, the notches looked that way:

< >

And, using John's array of ADCs and a virtual Win7 CPU, that was the phase noise result: Impact of the filter on the phase noise of an R&S SMIQ signal generator. SMIQ was chosen as an easy victim.

< >

Blue: without notch filter, red: with notch filter.

cheers, Gerhard

[...]

Vielen dank!

One ancient wave analyzer method is mixing the incoming spectrum with a VFO, then using a crystal filter at a convenient fixed frequency to do the required filtering at a fixed frequency. The filtered signal is restored to original frequency by mixing it with the _same_ VFO.

These days the crystal filter could be replaced with some DSP processing, say at 100 kHz. Some analog front end selectivity is needed ahead of the first mixer and after the second mixer, to avoid image responses.

That VFO needs high phase stability, because the crystal filter adds delay and will do fm-to-am modulation. No free lunch.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

On Sunday, October 27, 2019 at 10:55:59 AM UTC-7, Cursitor Doom wrote:

What is "IRO?" Never heard of it. There is no "best" filter.

You may find the needed Q a little challenging:

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TEXT -768 -1760 Left 2 !.SAVE S11(v1) S21(v1) S12(v1) S22(v1) TEXT -752 -1672 Left 2 !.param Rsrc00 = 1000\n.param Rload00=1000.982 TEXT 320 -752 Left 2 ;2 TEXT 656 -752 Left 2 ;3 TEXT 928 -1488 Left 2 ;7 TEXT 1744 -1488 Left 2 ;9 TEXT 1072 -416 Left 2 ;6 TEXT 1120 -672 Left 2 ;8 TEXT 864 -720 Left 2 ;5 TEXT 1776 -432 Left 2 ;4 TEXT 2048 -1472 Left 2 ;13 TEXT 2384 -1536 Left 2 ;15 TEXT 2000 -800 Left 2 ;12 TEXT 2688 -720 Left 2 ;14 TEXT 2496 -960 Left 2 ;11 TEXT 2208 -1328 Left 2 ;10 TEXT 3232 -1360 Left 2 ;16 TEXT 856 -2256 Left 2 !.param fo=10e6\n.param wo={2*pi*fo}\n.param Li =2.693638n\n.param Lx1=109.536651m\n.param Lx2=116.998399m\n.param Lb =1.367668n\n.param Lx3=111.018960m\n.param Lx4=115.644086m\n.param Lo =2.696283n\n.param QL=0.1e6\n.param Qx=0.1e6\n.param RsLi={wo*Li/ QL}\n.param RsLx1={wo*Lx1/Qx}\n.param RsLx2={wo*Lx2/Qx}\n.param RsLb= {wo*Lb/QL}\n.param RsLx3={wo*Lx3/Qx}\n.param RsLx4={wo*Lx4/Qx}\n.param RsLo={wo*Lo/QL} TEXT -600 -2168 Left 4 ;Crystal Filter - 10MHz LINE 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