Electrically tunable crystal band pass filters

[gf]

I've attached the page from the Icom manual.

The two Icom filters obviously have an ultra-wide flat top and a very steep transition band. Almost brickwall. Then it is certainly possible to obtain a narrow bandwidth by choosing a small overlap.

I've attached the section of Zverer's "Handbook of Filter Synthesis".  The last page shows the filter I want to build as a tunable center frequency, constant BW filter which I can cascade with a similar filter with the opposite shift using an emitter follower to isolate the 2 filters.

I cannot imagine that a filter with only two crystals and a -3dB bandwidth of 1500Hz can have such a steep transition band that a cascade of two frequency-shifted filters can achieve a stopband bandwidth of only (say) 50Hz at -40dB (or whatever stop band attenuation you desire). IMO you need a very high filter order in order to make that possible. I guess even so high that it ends up being impractical.

[mawyatt]

I cannot imagine that a filter with only two crystals and a -3dB bandwidth of 1500Hz can have such a steep transition band that a cascade of two frequency-shifted filters can achieve a stopband bandwidth of only (say) 50Hz at -40dB (or whatever stop band attenuation you desire). IMO you need a very high filter order in order to make that possible. I guess even so high that it ends up being impractical.

This is where a Commutating Filter can likely help, achieving extremely high effective "Qs" (BW/CF), while stable with temp and time, tunable over decades with constant BW, and utilizing available inexpensive components.

Best,   

[rhb]

You must learn to walk before you run.  I just want to demonstrate that one can replicate the behavior of the DSP implementation in analog form.

The competition is the NorCal 40 IF filter.  Just a different configuration of 4 crystals with some emitter followers added,

I'll be using commutation in the final mixer.

Have Fun!
Reg

[mawyatt]

Or replace the entire RF/IF/Filter/Demod chain with a PPM 

Best

[rhb]

You can't replace the TPBT structure with PPM without much more complexity and spurious outputs.

How does your PPM implement a 50 Hz PB without either using 2 PPMs or a 50 Hz filter? The former is more complex and the latter not usable.

The TPBT is one of the most impressive accomplishments in filter theory I've ever seen and I've seen some really amazing stuff.   I 'd like to emphasize that the Icom display is just that, a conceptual display.  Not fact,    The BWs are 250, 500 and 1200 Hz for CW, 1.8, 2.4 and 3.0 kHz for SSB and  250, 500 and 2400 Hz for RTTY.  I'll skip the rest
With regard to the dual crystal filter, I think I have everything sorted except the transformer.  What core and how many turns?  On what basis does one decide?

Reg
Reg

[vk4ffab]

You must learn to walk before you run.  I just want to demonstrate that one can replicate the behavior of the DSP implementation in analog form.

The competition is the NorCal 40 IF filter.  Just a different configuration of 4 crystals with some emitter followers added,

I'll be using commutation in the final mixer.

Have Fun!
Reg

Ok, Cohn Min Loss xtal filter with varactor diodes is something I have done, the biggest issue is finding varactors with a suitable capacitance spread to cover bandwidth needs. With the Cohn Min Loss design, the caps set the bandwidth and impedance matching sets the ripple. By making the IF double conversion you could implement Twin Bandpass Tuning quite easily, with some though to overall receiver gain. Each filter will have 3 to 10db loss, depending on how well you match the crystals, their Q and the bandwidth of the filter, narrower filters have more loss so you wont be using buffers to isolate one stage from the other, but rather gain stages in the order of 10 to 20db gain.



Input and output impedance is typically about 200 ohms and the xtals need to be matched to withing 10Hz of each other. Buy lots of xtals and have fun sorting them LOL and start with a xtal with lots of decimal places in its value, like 4.19543Mhz rather than 5.00Mhz, less variance, greater Q.

[rhb]

I am interested in electrically tuning the 2 xtal circuit on the last page of my the excerpt from Zverev..  I'll use higher order filters when I get that working.

Walk before you run.

reg

[mawyatt]

You can't replace the TPBT structure with PPM without much more complexity and spurious outputs.

Sorry not sure what TPBT means?

How does your PPM implement a 50 Hz PB without either using 2 PPMs or a 50 Hz filter? The former is more complex and the latter not usable.

If you spend some serious time reading (warning it's a deep dive) the details we've offered, you should realize the Commutating Filter and PPM both implement a narrow bandpass filter at the commutating clock frequency, so no need for 2 of either.

The PPM achieve this BPF because the shunt C and "leak" effective R form a simple 1st order LPF which is mirrored around the effective clock, thus "looks" like at the RF port, a narrow BPF centered at the clock. We reviewed a IEEE paper for a PhD student at USC which won the best student paper at the ISSCC back in ~2012, this was about utilizing the Bilateral transformation property of the PPM to upconvert a multi-order LPF (added for clarity) baseband I & Q filter, thus from the RF antenna port it "looked" like a much steeper slope BPF filter centered at the clock.

Back in ~80 we implemented a Commutating Filter that could extract tones at RF & Microwave frequencies. We had to detect tones in 4KHz sequential replicating "bands" in Supergroups, for example Fo +-n*4KHz, where n is incrementing by 1 integer and Fo is the center frequency. The tones were displaced at 4KHz -150Hz (3850Hz)  and 0 +150Hz (150Hz) relative to the 4KHz repeated "bands" and recall this filter had something like 10~20Hz -3dB bandwidth at RF & Microwave frequencies!!

The TPBT is one of the most impressive accomplishments in filter theory I've ever seen and I've seen some really amazing stuff.   I 'd like to emphasize that the Icom display is just that, a conceptual display.  Not fact,    The BWs are 250, 500 and 1200 Hz for CW, 1.8, 2.4 and 3.0 kHz for SSB and  250, 500 and 2400 Hz for RTTY.  I'll skip the rest
With regard to the dual crystal filter, I think I have everything sorted except the transformer.  What core and how many turns?  On what basis does one decide?

Reg
Reg

Please enlighten us, don't know what this TPBT actually is??

BTW long ago recall there was some concern amongst researchers about the harmonic response of the PPM. It was discovered the harmonic response follows a N +-1 where N is the number of clock phases, so with an 8 phase clock one would experience a response at 7 and 9 times the center frequency (think we got this correct, you can check).

Anyway, interesting topics.

Best,

[vk4ffab]

You can't replace the TPBT structure with PPM without much more complexity and spurious outputs.

Sorry not sure what TPBT means?

How does your PPM implement a 50 Hz PB without either using 2 PPMs or a 50 Hz filter? The former is more complex and the latter not usable.

If you spend some serious time reading (warning it's a deep dive) the details we've offered, you should realize the Commutating Filter and PPM both implement a narrow bandpass filter at the commutating clock frequency, so no need for 2 of either.

The PPM achieve this BPF because the shunt C and "leak" effective R form a simple 1st order LPF which is mirrored around the effective clock, thus "looks" like at the RF port, a narrow BPF centered at the clock. We reviewed a IEEE paper for a PhD student at USC which won the best student paper at the ISSCC back in ~2012, this was about utilizing the Bilateral transformation property of the PPM to upconvert a multi-order baseband I & Q filter, thus from the RF antenna port it "looked" like a much steeper slope BPF filter centered at the clock.

Back in ~80 we implemented a Commutating Filter that could extract tones at RF & Microwave frequencies. We had to detect tones in 4KHz sequential replicating "bands" in Supergroups, for example Fo +-n*4KHz, where n is incrementing by 1 integer and Fo is the center frequency. The tones were displaced at 4KHz -150Hz (3850Hz)  and 0 +150Hz (150Hz) relative to the 4KHz repeated "bands" and recall this filter had something like 10~20Hz -3dB bandwidth at RF & Microwave frequencies!!

The TPBT is one of the most impressive accomplishments in filter theory I've ever seen and I've seen some really amazing stuff.   I 'd like to emphasize that the Icom display is just that, a conceptual display.  Not fact,    The BWs are 250, 500 and 1200 Hz for CW, 1.8, 2.4 and 3.0 kHz for SSB and  250, 500 and 2400 Hz for RTTY.  I'll skip the rest
With regard to the dual crystal filter, I think I have everything sorted except the transformer.  What core and how many turns?  On what basis does one decide?

Reg
Reg

Please enlighten us, don't know what this TPBT actually is??

BTW long ago recall there was some concern amongst researchers about the harmonic response of the PPM. It was discovered the harmonic response follows a N +-1 where N is the number of clock phases, so with an 8 phase clock one would experience a response at 7 and 9 times the center frequency (think we got this correct, you can check).

Anyway, interesting topics.

Best,

TPBT == Twin Pass Band Tuning. Essentially it allows one to move 2 filters in relation to each other to get overlapping of parts of the bandpass. The OP wants to do this analog.

I am interested in electrically tuning the 2 xtal circuit on the last page of my the excerpt from Zverev..  I'll use higher order filters when I get that working.

Walk before you run.

reg

2 Xtals in that configuration will not provide a usable filter, its slope factor will be poor. You need to cascade 3 of those stages before you start to get a reasonable shape, GQRP Club had a filter kit using that design but the transformer cans are unobtanium now days. Its a lot of math for not very much gain, which is why I recommended the Cohn Min Loss, its simple and does not require a lot of math or complex transformers to get a good result, its what is in the nocal 40, sans the series pairs on the in and output.

Also you wont be able to vary the bandpass width with that design using varactors, which I thought was part of your design parameters, being variable width filters with twin pass band tuning.

Cohn works well with crappy computer grade xtals which you dont know or do not have to measure the motional parameters for. it has been shown over and over again, by better builders than I, that these mass produced xtals do not produce better filters than the cohn by using any other method like Dishal et al.

Which is why I recommend cohn filters. Select 4 or more xtals within 10hz of each other, caps are all the same value, transformers use Mix-43 ferrite and and the filter in and out impedance is typically 200 ohm. No math, no stress. Cap sets the filter bandwidth, less cap == wider bandpass, transformers control the ripple. Nothing can be easier. Replacing the caps with varactors that give 30 to 400pF should give filter width variable from 200 to 3000Hz.

https://www.gqrp.com/club_ssb_filter.pdf

[mawyatt]

Thanks!!!

Best,

[rhb]

Twin Pass Band Tuning

The 705 brochure explains it very well at the layman level.

You *CANNOT*  implement a 50 Hz wide BP filter that doesn't ring for about 20-40 ms.  You *CAN* with a pair of partially overlapping,  broader filters achieve the same result..  Major education for me. 

[T3sl4co1l]

Twin Pass Band Tuning

The 705 brochure explains it very well at the layman level.

You *CANNOT*  implement a 50 Hz wide BP filter that doesn't ring for about 20-40 ms.  You *CAN* with a pair of partially overlapping,  broader filters achieve the same result..  Major education for me.

You've mentioned this several times, but haven't provided measurements illustrating the claim.  Surely you can hook up a simple tone burst or ASK signal and measure the radio's output under these conditions?

Tim

[mawyatt]

Twin Pass Band Tuning

The 705 brochure explains it very well at the layman level.

You *CANNOT*  implement a 50 Hz wide BP filter that doesn't ring for about 20-40 ms.  You *CAN* with a pair of partially overlapping,  broader filters achieve the same result..  Major education for me.

We'll also challenge that statement, as "ring" implies the classic underdamped response. Since the Commutating Filter can be implemented as we've shown earlier with an array of simple RC low pass filters which by definition are well over damped and have no "ring" response whatsoever, whether standalone or commutating into a BPF. However they do have a well defined settling time, which is 3.91*R*C for 2% of final value (they are IIR filters wether commutated or not). The PPM behaves in the same manner.

Edit: We checked the Icom IC-705 Basic Manual and found Digital Twin PBT page 4-4, even this layman can understand!! With this understanding, seems nothing more than just moving the upper and lower steep skirts of LP and HP filter sections to achieve a steep skirt bandpass response, is this supposed to be some kind of new filter magic or something? BTW this IC-705 is loaded with features and such and seems to be a classic SDR implementation by moving the ADC right up to the antenna proceeded by a preselection filter, nice radio

Best,

[rhb]

I'm waiting for 500x 5 MHz xtals to arrive from China.  I've been using 40 MHz 3rd overtone xtals for my basic experiments.

The TPBT concept is completely new one on me.  In fact, had I not observed it,  I'd have said it was impossible.

The time domain response of a filter is determined by the type and the BW and order.  Narrow BWs inherently have long time domain responses.

I'll continue further when I have some reswults to show, e.g. the variation in capacitor values for a constant BW filter being tuned over the BW or some 5 MHz xtal filters.

Reg

[vk4ffab]

Twin Pass Band Tuning

The 705 brochure explains it very well at the layman level.

 is this supposed to be some kind of new filter magic or something? BTW this IC-705 is loaded with features and such and seems to be a classic SDR implementation by moving the ADC right up to the antenna proceeded by a preselection filter, nice radio

Best,

Not really magic, but useful in certain situations HAHA. The real question is, how narrow do you actually need ones filters to be. For CW/Morse code operating, I have never used a filter narrower than 200Hz. So the 50Hz ringing issue, seems to me to be a non issue as filters that narrow for CW/Morse are irrelevant and if close in stations still remain a problem, switching sidebands and using the filters in ones head are good enough. In my home brew tranceivers the CW xtal filter in the IF is 500Hz and the audio filters about 300Hz. The only time I actually use TPBT is on SSB where moving one skirt up or down can be helpful to remove close in interference.

[rhb]

This is about electrically tuning a crystal band pass filter,  it is not about a TPBT.

Reg

[pienari]

Digi cap will do the job.

[rhb]

I finally got the maths sorted.

The attached PDF shows the variation in capacitance for the 4th order Butterworth design example in EMRFD.  I should like to point out that the variation for a 400 Hz shift in both direction is tens of femptoFarads.  This is a constant 400 Hz BW shifting the center frequencies of the filters.

To keep the crystal matching effort low, I'm going to do a 2 xtal filer first and test tuning it before taking on higher order filters.

BTW What's a "digi cap"?

Have Fun!
Reg

[RFDx]

I've attached the section of Zverer's "Handbook of Filter Synthesis".  The last page shows the filter I want to build as a tunable center frequency, constant BW filter which I can cascade with a similar filter with the opposite shift using an emitter follower to isolate the 2 filters.
It seems to me that if the caps in parallel with the xtals are implemented with varactors one can electrically tune the filter center frequency.  The BW needs to be 1500 Hz  to get good time domain response and ideally symmetric pass bands or anti-symmetric so that the intersection pass band is symmetric.

Changing the parallel capacitance of the crystals will barely move the center frequency and will ruin the passband ripple. A Lattice filter imho isn't the right filter topology for a tunable filter. A low order ladder type filter is easier to tune. The crystals have the same series resonance. Attached is a second order Chebychev ladder filter with 0.1 dB ripple and a BW=1.2kHz. The parallel capacitance of both crystals is compensated. Dercreasing  the crystal series capacitors from 100pF down to 20pF, moves the center frequency up at least 1.2kHz with no impact on filter bandwidth.

Crystal data:
Fs = 5MHz
Ls =  77.93937 mH
Cs = 13 fF
Rs = 15 Ohm
Cp = 4 pF

[RoGeorge]

Lots of reading to do [about Poly-Phase Mixer, AKA N-Path Filter]

Note the name might vary, found it once in a paper called 'Harmonic rejection mixer', 'Network transfer functions', 'Passive mixer-first receiver', 'Sampled data filters', etc.  You may want to watch these first:

N-Path Filters
ISSCC Videos
https://youtu.be/MP7m5OjXWUg

N path filters: basics & demo
icdutwentenl
https://youtu.be/L3wJ1XedpSo

The Realization of Narrow Band-Pass characteristics Using Sampled Data Filters (PDF thesis free to download, has all the math:  https://macsphere.mcmaster.ca/handle/11375/15508)


Back to the tunable filter, I think there is a tacit assumption in this:

Twin Pass Band Tuning

The 705 brochure explains it very well at the layman level.

You *CANNOT*  implement a 50 Hz wide BP filter that doesn't ring for about 20-40 ms.  You *CAN* with a pair of partially overlapping,  broader filters achieve the same result..  Major education for me.

You've mentioned this several times, but haven't provided measurements illustrating the claim.  Surely you can hook up a simple tone burst or ASK signal and measure the radio's output under these conditions?

Tim

rhb, this is about the sinx/x lobes, right?  (the "inescapable ringing", is it about a brick-shape band pass filter when the filtered signal is translated back, from frequency domain to time domain?)

If so, this can be avoided by choosing another shape for the bandpass filter (instead of the assumed brick that will ring in time domain).  For example, I've read a Gaussian shape translates also into a Gaussian shape (when transforming from t to F domains, or the other way around).  So a very narrow bandpass filter of a Gaussian shape will produce a time domain pulse with no ringing (the pulse will be also Gaussian in shape).  Or at least that's how I remember it.

This idea (of a Gaussian-shape bandpass filter in order to avoid ringing in time domain) was inspired to me after reading an old Tektronix service manual, where they were explaining that the Y amplifier with a Gaussian response will not ring (the raise time = 0.35/band is also from there, derived for a Gaussian response amplifier, not true for nowadays oscilloscopes).  I'm not a ham, never tested how well the Gaussian filter idea will perform to implement a, say 50Hz wide or so, CW filter that will not ring at all.


The idea of two crystal resonators with variable coupling between them (to vary the bandpass) seems very attractive (I think it was T3sl4co1l who brought it in this topic).  It's the same phenomena as the "line-splitting" from physics.

The main idea is that two resonators coupled together will influence each other.  Frequency and/or band can be changed by changing the frequency of only one of the resonator, or by changing the coupling factor.  No matter if the resonators are crystal, LC, orbitals in atoms, etc, if they are coupled, the resonance bandwidth can be changed by changing the coupling factor between them.

Did once a small simulation to convince myself this is true, here with 2 LC resonators and "k" as variable coupling factor of the coils, see how the bandpass widens while coupling them tighter, or how they "push" each other frequency:



My guess is, that for very narrow band signals as a single tone CW, the resulting bandpass will be flat enough, or would not matter.  A flat response might matter for wider bandpass filter, but won't matter much for a single tone CW.

So, a simple variable capacitor between two crystal resonators (or maybe a variable resistor? - not sure about the best way to couple the two crystals) might just do the job:   obtaining a variable bandpass crystal filter, with adjustable band by changing the coupling factor.

[rhb]

Let's suppose for convenience that the pass band of each filter is a 500 Hz trapezoid.  As a consequence the effective passband if they have a small overlap is a narrow trapezoid.

Naive examination would assume that the effective pass band governed the time domain response.  Narrow filters are wide in the time domain.  Doesn't matter if it's a Gaussian.  Narrow Gaussian in frequency is a wide Gaussian impulse response.  I would have told you it couldn't work if you'd asked over lunch at work.  I was wrong.

Some definitions:

d(w) - delta function in frequency

B(w) - transfer function in frequency of the individual 500 Hz filters.

* - convolution

. multiplication

D = B(w) * d(w-w1) . B(w)*d(w+w2)  // shifted pass band


When you compute D the transfer function is B(w)**2*d(w-w1+w2).  The delta term in frequency is a complex exponential in time.

I invite you to create a 50 Hz wide Gaussian filter centered on 600 Hz and transform it to the time domain.

Have Fun!
Reg

[RoGeorge]

Gaussian shape was proposed as a measure against pulse ringing in the time domain, not against pulse widening in time.  The Gaussian pulse will be wide in time, just that it won't ring.

Never mind, I've read again the OP, and it seems I have misunderstood the request.  You need a constant width band pass filter with adjustable center frequency, while I was offering with those coupled resonators an adjustable bandwidth filter with constant center frequency, sorry.

[rhb]

I'm trying to make the analog equivalent of the DSP in the Icom 705.

BTW  Gaussian filters very definitely ring in the time domain.  You're thinking of the symmetric zero phase time domain response.  But reality is minimum phase. I suspect the DSP version is zero phase, but an analog version can't be.

[T3sl4co1l]

But this is still strictly based on what your ears think "ringing" sounds like (or the lack thereof, as the case may be)?

Tim

[RoGeorge]

I suspect there might be one of those interdisciplinary naming issues, where the same word is used to denote different things.

For me, ringing in time domain means the oscillation that appears after an edge.  It takes some time before the signal settles to the correct value, as in this figure where, instead of a clean step signal, there is an edge followed by a dampened oscillation: 

Image from https://en.wikipedia.org/wiki/Ringing_(signal)

That oscillation right after the edge, I call it 'ringing', and it was not part of the original signal.  The original signal was a clean step.

Now, (and here I might be wrong, didn't have the time to simulate yet) I expect that if we pass a brick pulse through a brick-shape band pass filter, the output after the filter (in time domain) will show ringing, or at least some overshooting after each edge of the pulse.  However, if we pass the same signal through a Gaussian-shape band pass filter, the output pulse will show no ringing, only mellowed edges but no oscillations.  Am I wrong in these expectations?