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Old 2nd Jul 2018, 6:04 pm   #1
BackToTheLab
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Post Racal RA17 - 40Mc/s Alignment

Hello one and all,

I am very happy to be joining the vintage radio website, and have been reading through the material here for a number of years... Thank you for all your contributions...

I have recently been gifted a Racal RA17 Mk2 by a colleague who knew that I had a soft spot for them...

The receiver powered up, but it was completely deaf. I don't think that it had been switched on for quite some time, so I started methodically...

I checked the DC voltages on each valve base, and compared to the spec etc. I found a number of issues, particularly in the 37.5MC/s amplifier chain where I found that I was getting very little 37.5MC/s drive to the grid of the 3rd mixer. I rebuilt the circuitry around valves V2, V4, V6, V8, V10 and V9. To cut a long story short, I can now see a ~10Vpp signal at 37.5MC/s for every megacycle detent.
(Thanks to Al / Skywave for providing some useful posts in this subject area...)

This dramatically improved the receiver performance, and I can now inject a 7.5MC/s signal to the antenna input on wideband mode, tune the receiver to the 7MC/s range, and use the kilocycles knob to and resolve a ~5microvolt signal. This took a couple of months on and off, so this was a great relief.

As a matter of course, I also received to rebuild the circuitry around V3, V5 and V7 to ensure that I was getting a nice high 1st VFO signal to the 1st mixer (V5) and harmonic mixer (V4). Again, this improved things, but this time not so dramatically.

I think that the receiver is still quite deaf, so this weekend I decided to characterise the frequency response of the 40MHz BPF. I did this in two ways to ensure that my TE was not introducing any measurement error.

Method 1) Connected a tracking generator to TP2, and measured the response using a homemade high impedance RF probe at TP3.

Method 2) Tuned to the 7MHz Megacycle detent. Set antenna tuning to wide band. Swept the antenna input from 7MHz to 8MHz. Used "max hold" on a spectrum analyser to measure the peak response between 39.5 and 40.5MC/s.

Note - If you're still reading, I commend you... Thanks!!

Both of these methods show that the 40MC/s filter falls off by 15dB from 39.5MC/s to 40.5MC/s. This is obviously not great for receiver sensitivity...

I marked all of the trimmers in the 40MC/s BPF and checked that all of the trimmers influence the frequency response, since I suspected a dodgy silver mica capacitor. They do.

This is the point where I need some help from an expert. The service manual for the RA17 provides no information regarding the process for tuning the filter. Since it is an "overcoupled" filter, it does not tune as intuitively as the 37.5MC/s filter.

Does anyone here know what sequence I should tune the resonators in?
Can I peak each stage individually using a tracking gen / spectrum analyser?
What frequencies should I tune each resonator to?

Any help would be gratefully received.

Kindest regards,

Peter

P.s. Back to the lab...
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Old 2nd Jul 2018, 7:21 pm   #2
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Default Re: Racal RA17 - 40Mc/s Alignment

There is a dirty trick for aligning coupled resonator filters (whether the coupling is spatial, L or C, top or bottom)

Each resonator should resonate at the geometric centre frequency when the other resonators to each side of it are disabled (shorted)

You need to use very very light coupling from your tracking generator and your analyser to do this, but it works. Spectrum analysers have tons of sensitivity, which can be used for very light coupling. With the floating resonators of the 40MHz filter a coupling loop on each of the tracker and the analyser is needed.

There isn't just a mathematical basis to this method, it is actually involved in the design of such filters.

Trying to just twiddle the trimmers on a high order filter is an unstable process, and even with a display of the shape in front of you, settings tend to diverge, not converge.

Racal used preset jigs and all sorts of custom hardware to make setting easy, but the 3dB k and q method only really got going after Zverev in 1967. The older m-derived and image parameter methods were much more tedious to work out and didn't give you an adjustment method as a freebie. There was a bit of a quiet revolution in filter design methods in the mid sixties that came out of Bell labs and Westinghouse. Filter design was a big issue for telecomms firms of the day.

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Old 2nd Jul 2018, 7:54 pm   #3
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Default Re: Racal RA17 - 40Mc/s Alignment

David,

Thank you for the swift reply, and for your "dirty trick"... Apologies in advance for the tidal wave of questions...

I know very little about coupled resonators - does the 40MC/s filter contain mutual-inductance coupled resonators?

When you say geometric centre frequency - do you mean sqrt(39.5 * 40.5)?

Also, I presume that since the filter bandwidth is relatively wide at ~1MHz, each of the resonators needs to be set for a different frequency. Should I evenly space the resonators? Geometrically space them? Do they need to be aligned in increasing or decreasing resonant frequency order? Do you have a specification at all?

When you say "the other resonators to each side of it are disabled (shorted)", do you mean ALL of the other resonators should be shorted? Or just the immediate two resonators either side?

When you say "light coupling", I presume that a single turn for the tracking generator and spec analyser input should be adequate.

How would you recommend achieving this, given that the proximity of the shielding can will probably affect the resonant frequency of each resonator?

I am not familiar with the work of Zverev - I will look for some online reference material...

Many thanks again for your advice,

Peter

Last edited by BackToTheLab; 2nd Jul 2018 at 8:24 pm. Reason: Made a subtle error in wording
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Old 2nd Jul 2018, 9:34 pm   #4
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Default Re: Racal RA17 - 40Mc/s Alignment

The 40MHz filter characteristic are here https://www.recelectronics.co.uk/ct501.htm

Basically it should be +/-650kHz at -6dB . That uses a CT501 wobbulator, but Racal also used the Rohde & Schwartz Polyskop SWOB II.

Alas because I am a bit of a collector of stuff I've got both of these boatanchors. Since you are only about an hour drive away, if you fancy a trip to sunny Abingdon we can give the unit a going over. It is quite a long time since I ravaged mine, but with an active magnetic loop I had no problem pulling in Radio Australia on a transmitter side lobe (transmitter pointing towards Hong Kong!). So if you come I'll be relearning the guts of the beast.

Craig

Last edited by Craig Sawyers; 2nd Jul 2018 at 9:51 pm.
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Old 2nd Jul 2018, 10:06 pm   #5
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Default Re: Racal RA17 - 40Mc/s Alignment

David,

Apologies, I initially misread your reply.

Please disregard the following in my reply:"Also, I presume that since the filter bandwidth is relatively wide at ~1MHz, each of the resonators needs to be set for a different frequency. Should I evenly space the resonators? Geometrically space them? Do they need to be aligned in increasing or decreasing resonant frequency order? Do you have a specification at all?"

I can see how tuning each resonator to 40MHz works, if the level of mutually inductive coupling between the resonators sets the bandwidth... I presume that this coupling is heavily dependent on the dimensions of the cavity into which the resonators are installed.

What are the benefits of a mutually inductive coupling versus a more traditional capacitive coupling between resonators?

Best regards,

Peter
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Old 2nd Jul 2018, 10:16 pm   #6
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Default Re: Racal RA17 - 40Mc/s Alignment

Craig,

Thank you for your reply....

Thank you also for your kind offer of having a look over the receiver with me... This would be great, but unfortunately I currently have a hectic lifestyle with work etc.

Working on the RA17 for the last couple of months has been pretty rewarding, but I have found that due to the construction techniques, even the smallest of jobs can occupy many hours...

I would be very keen to compare notes with you though, regarding the performance of your RA17...

Kind regards,

Peter
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Old 2nd Jul 2018, 10:48 pm   #7
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Default Re: Racal RA17 - 40Mc/s Alignment

Mine is in the attic, Peter. Anyway, I made the offer.

Craig
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Old 2nd Jul 2018, 11:02 pm   #8
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Default Re: Racal RA17 - 40Mc/s Alignment

The 40MHz filter is a constructed as a string of LC resonators relying on magnetic field coupling between adjacent ones though there will be a little E field as well.

You have the geometric centre right.

Each resonator is set in isolation to the geometric centre, and the coupling to its neighbours pulls it off of the centre. In a neat feat of mathematics the coupling factors not only move the resonances to the right places, they also get the right Q for each resonator to complete the pattern which synthesises the required shape.

The source and load impedances applied to the ends of the filter set the Qs of the end resonators, and the coupling factors pass some of the damping along to the inner resonators. The coupling just happens to get the pattern of the Qs right. Handy Eh?

Zverev wrote the book which set out "Modern Filter Theory" it turned the decision of where to place the roots of the differential equation defining the transfer function of a filter into a science. It's an approximation for most idealised filter shapes, but you can decide how complex a filter you want to make and this defines how close an approximation you'll get.

Zverev's Handbook of Filter Synthesis covers the theory and then gives catalogues of achievable shapes, than gives tables of normalised values for making these shapes.

ust about every filter book written since 1967 cites Zverev. Many books have been written on how to understand Zverev, but they don't put it so directly

Zverev scares a lot of people off. He writes equations of attenuation when equations of transfer function are now the norm so his equations are upside down. His poles are our zeroes, his zeroes our poles.

It's not a book to go into alone. You need a guide to show you what it can do. Otherwise it will seem weird and you may not notice the valuable bits.

A small single turn loop for each of tracker and analyser is right. Ideally you want them on opposite sides of the resonator

David
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Old 2nd Jul 2018, 11:09 pm   #9
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Default Re: Racal RA17 - 40Mc/s Alignment

Ah, I must learn to type faster and think faster a few posts happened while I was writing.

Use the one resonator at a time process. You do NOT stagger them at all. You tune them all to the geometric centre frequency. One stage at a time with the neighbours each time shorted.

When you take the shorts off, the coupling moves them all onto the right frequencies.

Staggering is automatic. it does it for you itself.

When the full filter is running you will see Chebyshev bumps in it, but if you twiddle the trimmers you'll find that they all interact. It isn't a case of trimmer X moves bump Y. You don't get anything so easy. That's why most trimming attempts of people unfamiliar with a design go astray even with wobbulators spectrum analysers etc.

Cryptography relies on mathematical one-way streets, functions which are easy to do in one direction, but much harder to do in the reverse direction. Filter twiddling is like that. You have to learn to cheat.

David
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Old 3rd Jul 2018, 5:45 pm   #10
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Default Re: Racal RA17 - 40Mc/s Alignment

David,

Thank you for this additional clarification - I tried your one-turn Spec Analyser / Tracking Gen concept last night, and found that it works like a charm. I didn't have time to align every stage of the 40MC/s filter, but I established that at least one resonator is resonating at the correct frequency.

When I get the time, I shall endeavour to align the whole filter.

I presume that the same approach also applies to the 37.5MC/s filter too? If so, I will definitely align the 37.5MC/s filter using this methodical approach...

I presume that the final alignment needs to be done with all the covers on - it'll be quite tricky to get the excitation / sense windings through the aperture in the covers...

Best regards,

Peter
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Old 4th Jul 2018, 12:07 am   #11
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Default Re: Racal RA17 - 40Mc/s Alignment

Suggest replacing the 2k2 resistor which terminates the filter.
Usually high, makes response lumpy.
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Old 4th Jul 2018, 6:27 am   #12
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Default Re: Racal RA17 - 40Mc/s Alignment

The source and termination resistors set the Qs of the end stages of the filter, and via the coupling factors set the Qs of all the other stages.

If the resistance at either end goes high, the Qs go too high for the designed response and the ripple gets big.

Lower the resistances and the Qs fall too low for the designed shape and the filter rolls off.

If you're filter-hacking on a simulator like Spice, you can vary width by scaling the coupling factors, then fiddle with the terminating R at each end to adjust to get the flatness/ripple right.

David
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Old 4th Jul 2018, 7:26 am   #13
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Default Re: Racal RA17 - 40Mc/s Alignment

Peter - have you thought about joining the Racal RA17 forum on yahoo groups?

I take it as read that you have replaced all the paper capacitors.

Last edited by Craig Sawyers; 4th Jul 2018 at 7:33 am.
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Old 5th Jul 2018, 11:58 am   #14
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Default Re: Racal RA17 - 40Mc/s Alignment

A question for Radio Wrangler,
Thanks for your very lucid description of the inner workings of multi-resonator filters and Mr.Zverev.
The ideal 40MHz filter has a passband from 39.5 to 40.5 MHz and zeroes at 39.0 and 41.0MHz. Does the alignment technique you describe give the correct zeroes as well as the correct passband? (assuming the filter has been correctly designed, of course)
Similarly with the 37.5MHz filter, this should have zeroes at 36.5 and 38.5MHz. Alignment is presumably done at 37.5MHz as there is no requirement for a wide passband, so do we expect that the zeroes will fall in the right places?
I have an RA117A under refurb at present, and look forward to checking the filters using this technique.
cheers
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Old 6th Jul 2018, 10:14 pm   #15
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Default Re: Racal RA17 - 40Mc/s Alignment

You might also find it useful to download the EMER 724 RA17 Field and Base repairs document from the VMARS archive web site. It not only includes procedures to align these filters but also how to setup the remainder of the set
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Old 6th Jul 2018, 10:54 pm   #16
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Default Re: Racal RA17 - 40Mc/s Alignment

There's a danger of confusion here with talk of zeroes and Zverev.

Zverev drew equations of attenuation but now we tend to use equations of transfer function. These are reciprocals of each other, so no big deal. But the zeroes of transfer function are the poles of attenuation and vice versa. So Zverev and current usage have the words zreoes and poles swapped.

The tune one stage at a time method works for "All-pole" filters (poles of transfer function) reaised as coupled resonators. The schematic posted earlier of Racal filters as simple L-C tanks spatially separated is such a filter... they have no notches.

This technique does NOT put the poles on the required frequencies. It puts them all on the same frequency. Then it relies on the coupling factors to pull the poles apart to shift them onto the required frequencies. It is rather neat that the same coupling factor does the right frequency pull and sets the right Q both at the same time.

The passband ole positions show up as blunt ripples in the passband top, making them very hard to adjust if you try this way. Better, they appear as much sharper nulls in the return loss. So a tracker and return loss bridge on a spectrum analyser can be used this way.

Filters with real-axis zeroes give stopband nulls... EG Elliptic == 'Complete Cauer' designs. these are usually designed by the transformed lowpass technique and suffer from its limitations.

I've played around with narrow bandpass filters with notches achieved by basing things on pseudo-crystals.

David
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Old 6th Jul 2018, 10:58 pm   #17
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Default Re: Racal RA17 - 40Mc/s Alignment

In the 1980s I bought a brand new unused unopened RA117. I sort of still wish I had it. Everything inside was shiny!

But space was a problem. SSB was poor as I didn't have an adaptor and the front end was softer than I liked.

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Old 14th Jul 2018, 11:17 am   #18
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Default Re: Racal RA17 - 40Mc/s Alignment

I think the method David is describing is the Dishal method and this predates the classic Zverev book by about 15 years or so. I've used the classic Dishal methods for setting up multi resonator BPFs many times in my younger days but usually up in the GHz region. For example, in my first job an old RF guru showed me how to set up an interdigital filter up at several GHz using this method.

I had been struggling to set up the filters as it involved adjusting trimmer screws and tightening locknuts for each resonator and each filter had about 7 sections to adjust. At the time I was moving between labs to get work experienceand was spending a few weeks in the 'microwave' lab to learn how to make reliable measurements and I was struggling to get a good passband response and fairly symmetrical S11 and S22 return loss at each port.

But this old guru smiled at my feeble attempts and finally showed me how to do it just at the centre frequency (not swept) and a network analyser looking at the phase of S11 at the centre frequency and how it flips as each initially shorted resonator is freed and tuned in turn. With this method, you only have to adjust and lock each resonator once and the results are amazingly good in terms of filter response once finished. I was able to set up about a dozen filters perfectly in less time than it took me to do one filter reasonably well. The return loss on both ports was close to perfect and was symmetrical. It would take ages (days?) to replicate this performance if tuning the filter manually the conventional way but the Dishal method took maybe 2 minutes per filter once I gained confidence in it.

However, if you don't have a network analyser and the filter has to be tuned 'in situ' then there is another variation of the Dishal method that just uses a high Z detector (could be a scope or a spectrum analyser) and a sig gen at the centre frequency. This time you measure forward gain (but only at the centre frequency) and either tune for a minimum or a maximum depending on which resonator is being tuned. But just like before, you start with all the resonators shorted or deliberately/badly mistuned.

This works nearly as well as the S11 phase method and just like before, you only have to adjust each resonator once. But you do need to be able to get close to the first resonator to sniff off some energy from it with the high Z detector. I can try and demonstrate both procedures on an RF simulator on a youtube video if you want? It really does look like magic and I can still remember being amazed at the S11/phase method showed to me by that old RF guru many years ago. On a filter with maybe 7 to 9 sections there's no way you could hope to compete by just tuning/peaking each section manually. You might get lucky and get close but try doing it on a dozen filters and you soon realise how powerful these old methods are. It will also work on lumped BPF designs using LC resonators down at HF/VHF/UHF and it doesn't just apply to exotic interdigital filters up in the GHz region.

I tried googling for the Dishal method(s) and sadly, I couldn't find any links that describe how to do this in practical terms with a real demo and this classic method of filter tuning is slowly becoming a lost art...
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Old 14th Jul 2018, 12:11 pm   #19
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Default Re: Racal RA17 - 40Mc/s Alignment

This discussion on filter alignment has been really interesting. I've done this sort of thing with spectrum analysers, or period wobbulators, and this suggests other methods, or algorithms for adjustment.

But alas the OP joined on 2nd July and last posted on 3rd July 24 hours later.

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Old 14th Jul 2018, 1:28 pm   #20
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Default Re: Racal RA17 - 40Mc/s Alignment

OK thanks. As others have mentioned already, the best method would probably be to follow the recommended/documented procedure for tuning the RA17 40MHz filter anyway.

But the Dishal method really is impressive when first demonstrated to someone who is unaware of it. Back in my youth, I thought the old guru was pulling my leg when he told be how to do it. Having spent hours trying to set up a few interdigital filters on a swept network analyser I found it difficult to believe it could be done by just adjusting each resonator 'once' in a sequence looking at the phase of S11 at just the centre frequency alone. But when the last resonator is adjusted with this method and the results viewed on a network analyser, it really is like seeing a magic trick unfold.

For maximum wow factor on a typical (multi resonator) bandpass filter I'd suggest using a fixed frequency sig gen, a coupler and an old HP8405A vector voltmeter and tune for S11 phase flips at the centre frequency.

Then when the Dishal method is complete, connect the filter to a swept analyser to see how it is now tuned up almost perfectly across the entire bandwidth for passband response and port VSWR even though only the centre frequency was used to set it up on the VVM.

However, this method might not be suitable for the Racal RA17 filter anyway. But when used on a suitable BPF design it is impressive to see the results
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