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Custom Audio Filter designs for Ham AM - Dynamic Bandwidth Control




 
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Author Topic: Custom Audio Filter designs for Ham AM - Dynamic Bandwidth Control  (Read 5413 times)
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K1JJ
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« on: February 08, 2020, 10:14:33 PM »

I've decided to build up some audio filters for transmitting that will dynamically manage bandwidth for several of my homebrew and analog AM transmitters.  (non-SDR) They are low level low pass filters placed in the audio chain, so can be common to all rigs. They basically roll off the extreme highs at a preset point.  Since many transmitters have "wider than normally used audio"  to generate a clean signal with headroom, we need some kind of control to keep our bandwidth in line with band activity and crowding.

I've selected 10th order Butterworth designs with a 600 ohm input and 600 ohm output. (The impedances can be changed to anything else with the software below.)

Being 10th order, the roll offs are quite steep. I want a filter for three kinds of band conditions on AM. For example, a 4KHz filter starts to roll off at 4 KHz of spoken audio.  The total bandwidth is 8KHz because of two sidebands on AM. Here are some roll off examples of the filter designs attached below:

1) 4 KHz  filter =  down -20 DB at 5KHz  (somewhat tighter than AM broadcast bandwidth)

2) 5.5 KHz filter =  down -20 DB at 7.1 KHz

3) 7.5 KHz filter =  down -20 DB at 9.5 KHz


These filters will be built on one board and switched in and out in an instant.

The reasons for three filters:

1) When there is 5 KHz spacing between stations, the band is very busy and the 4KHz filter can be selected. At 5KHz the audio is rolled off -20 DB for the neighboring station's benefit.  (and sharper rolloff at 5.5 KHz, etc.)

2) When the station spacing is 7-8 KHz (common) then the 5.5 Khz filter is selected.

3) When the band is quiet and stations are spread out with 10 KHz spacing, then the 7.5 KHz filter can be run.

*** For these filters to do their job, the transmitter and amplifiers must be clean - low IMD and a flat response is important. ***

The Butterworth filters appear to have the least amount of ripple and are passive filters.

I plan to order the parts and give them a try within a week.  Until there is a DSP audio filter designed for this kind of job, the Butterworth using inductors and capacitors may suit the bill.


The parts are very cheap and small and standard values.  At Mouser Electronics, inductors the size of a pea will work FB cuz there is really little current in the circuit.

Here is the web calculator in case you want to run your own filter calculations:
https://rf-tools.com/lc-filter/


Comments?

Tom, K1JJ

Thanks to Frank / GFZ for his guidance...


* Filter 4.0.png (10.61 KB, 535x187 - viewed 146 times.)

* Filter 5.5.png (10.69 KB, 535x187 - viewed 94 times.)

* Filter 7.5.png (10.62 KB, 535x187 - viewed 72 times.)
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« Reply #1 on: February 09, 2020, 09:00:00 AM »

Actually, an all-in-one processor is in its final design stages. The MAX processor incorporates several stages of envelope processing primarily geared to the AM market:



The optional stages are: Low Cut, All-Pass Filter, Pre-Emphasis, Compression, Limiting, SCAF (Switched Capacitor Audio Filter) for 3, 5, and 7kHz or user selectable), Peak Clipper, and Low Pass Filter.





The bandwidth display shows white noise used as the source and two MAXIM 295 Butterworth SCAF chips in tandem synced with a 555 timer. These are well worth considering for your project.

https://www.maximintegrated.com/en/products/analog/analog-filters/MAX295.html
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K1JJ
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« Reply #2 on: February 09, 2020, 01:03:22 PM »

Outstanding, Clark!

We have been waiting for such a product.  The spectrum scope roll offs look impressive!

I have used switched capacitor audio filter technology before. At least with the chips I tried, (one was a Maxim)  there was a hint of "cell phone" ringing that we often hear when a cell phone gets overloaded. The filter did a tremendous rolloff, but the slight "artificial digital" distortion was not good, so I abandoned it.  Hopefully you have solved this problem or have a different approach to using it.  

I also tried a switched cap commercial product from Indiom? or some name like this that Stu AB2EZ was experimenting with about 15 years ago. He sent it to me to try. It rolled off sharply too as desired, but had artifacts that a critical AMer could hear.

Steve/QIX mentioned he is going to be working on a DSP version of audio filtering too.  I am looking forward to testing that cuz we know DSP works well in the Flex, Anan, etc.

At this time I see no products on the mkt that will cover ham AM audio ranges, just CW/SSB stuff at 2.8 KHz, 1.2 KHz, etc., so I am rooting for you...

I will experiment with my passive Butterworth filters until one of you guys come out with something better. Let me know if you need any beta testing. I would be more than happy to help and test it on various AM rigs..

T


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« Reply #3 on: February 09, 2020, 03:27:02 PM »

I run a stock Heathkit DX-100 with an original crystal D-104 non-amplified microphone.(high impeadance)
would your homebrew filter designs work for my application?
you mentioned 600 ohms.
AG5UM
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K1JJ
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« Reply #4 on: February 09, 2020, 06:18:40 PM »

I run a stock Heathkit DX-100 with an original crystal D-104 non-amplified microphone.(high impeadance)
would your homebrew filter designs work for my application?
you mentioned 600 ohms.
AG5UM

Sure, they should work FB. The filters work best when the input and output impedance is matched. (less ripple, etc.)

For the DX-100, find a stage AFTER the mic and preamp that is at least a few volts of swing. Determine the input and output impedances and run the calculator using these values. The calculator is easy enuff to use after 10 minutes of playing around.  Be sure the capacitors are rated high enough in voltage for tube environments. 

Maybe later you will lose the D-104 and go in with a processor into a later stage. Then it will make it easier to slip a filter in the low level solid state 1 volt area.

Before you do anything, run some tones thru the DX-100 to insure it is clean without IMD splatter problems.

T


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« Reply #5 on: February 09, 2020, 08:04:29 PM »

K1JJ,
sounds interesting, keep us updated on your progress. The calculator is easy to use,very cool.
I have done scope tests,etc. on the DX-100, I like and plan to keep the D-104.
I think I remember reading an article that you wrote that was very good about audio testing,what I remember
is the warning about overdriving the audio input and damaging the mod tfrm. Good article.
I think K4KYV,Don in Tenn. has one of the best sounding signals on the air, from what I gather he uses a
D-104 into a push-pull tube audio input.
I thought your idea is interesting for times when the bands get crowded.
73's
AG5UM
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w9jsw
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« Reply #6 on: February 09, 2020, 10:14:28 PM »

Tom,

Be glad to whip out a PCB for you. You going to relay switch the 3 filters?

John
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K1JJ
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« Reply #7 on: February 10, 2020, 12:06:33 AM »

Tom,

Be glad to whip out a PCB for you. You going to relay switch the 3 filters?

John

Super, John!!!

I was just thinking of how I would do a point-point prototype board, but this would be great.

Leave big pads for the outputs and inputs.  I'll have to pick out the inductors to use, probably no bigger than a pea.

Interesting idea on the relays. I was going to use six toggles, three on each end of the board for input/output selection.   Not sure yet.  I will decide by Monday.

Thanks for the offer, OM.

How's the new rig coming along?  I made another 813s + parts   "offer he can't refuse" to Daryl, KD2AFL on the air the other day. I still have  four tubes and parts left.  He owns a modified T-368 and was talking about buying a KW-1. I asked him why not build a pair of 813s X 813s?  He is thinking about it. I mentioned your thread here on AMFone.  I told him you are approaching about 70% completion with a very FB rig build.

T
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« Reply #8 on: February 10, 2020, 06:36:11 AM »

SMT or thru hole, I can do both. 1206 size SMT is really not very hard to solder.

I can use a footprint that can use either 5V or 12V signal relays (or 24V for that matter). That way you have options on how to wire it. I was thinking in my case, I could tie it into the Arduino sequencer. I have a touch screen on it that I could enable to select the bandwidth. Another guy could just wire up a simple rotary switch to select one of 3. Simple.

What did Frank say about his little op-amp board. Does it fit in this scenario?

Yes, around 70-80% of physical assembly. We both know that there is still a LOT of work after that on bring up. I have just about finished the mechanical work and will then wire up the RF deck. Takes a lot of time to get all of those big components mounted just right.

John
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K1JJ
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« Reply #9 on: February 10, 2020, 01:04:30 PM »

John,

I like the idea of thru holes so we can modify things without lifting SMT pads.

The relay holes option that can covert to a rotary switch or toggles is a good idea for versatility.

I think Frank's board will stay separate until we can test these filters out first. They will have some phase shift and I want to make some careful measurements to see if it is tolerable. All filters have some phase shift and more as they get more poles.   DSP is the way to go, but we are not there yet.

I think these filters will work FB when needed for crowded condix, and when conditions are quiet, just bypass them completely and do as we have done in the past - use a Desser, roll off the EQ, etc.  But I do like the idea of switching in a known filter set of roll off numbers to know exactly how far out we are transmitting vs: a pot knob that estimates bandwidth.

I'll order up the parts later today. I gotta see who has values near the suggested ones and their physical size for you to work with.  I did request standard values in the software run, but who knows what is really out there.

I've been adding a lot of bells and whistles to the shack lately.  I am going to wire Steve/QIX's REA mod monitor to show received signal pos and neg peaks.  I found a good 455 KHz tap on the FT-1000D and will buy a tiny amplifier to bring it up to 20 V p-p. This is a valuable service to give out receiving reports, IE what are my actual pos and neg peak numbers?   Also, Jeff/W2NBC sent me an SDRPlay RSPdx  SDR receiver which arrived today. Also, the filters we have been discussing.  I am also waiting for a Chinese mic cartridge to mount on a headset. That should be cool to be able to walk around away from the mic boom.

I also put a Henry 2K on line driven by the FT-1000D for easy, bandswitching 150 watt AM. Also added back in my dual 4-1000A linear amplifier for medium power service.  I even have a new laptop in the shack for all this stuff.   I am looking at multiple "power levels" for my rigs. Instead of having five 100 watt boat anchors to chose from, I have five rigs that gradually rise in power levels to match conditions and my mood... :-)    Fun days.

Later -

T
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« Reply #10 on: February 10, 2020, 03:28:37 PM »


Tom,

Sent you an email.
There are tons of products on the market that do exactly this job, very well,
and are inexpensive, work on line level audio... Cheesy

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« Reply #11 on: February 10, 2020, 07:43:06 PM »

Outstanding, Clark!

We have been waiting for such a product.  The spectrum scope roll offs look impressive!
......................

T


Here is a bit more information about the audio processor that Clark / N1BCG and I have been developing over the past few months.  We are on the fourth revision of PCB artwork, and all that remains is testing and fine-tuning.  On-air testing has proven very rewarding.

As Clark mentioned above, it includes modules for Low Cut, All-Pass Filter, Pre-Emphasis, Compression, Limiting, SCAF (Switched Capacitor Audio Filter) for 3, 5, and 7kHz or user selectable), Peak Clipper, and Low Pass Filter.  Each of these functions may be enabled or disabled, either by panel-mounted controls, or set by on-board jumpers.  The modular design is thus very flexible.  

In addition, several inputs are provided for balanced, low impedance studio microphones, or single-ended high impedance units such as the Astatic D-104.  Balanced and unbalanced line inputs and outputs are provided for interfacing other signal sources and driving multiple transmitters.  Utility outputs for signal monitoring or a VU meter are included.  LED indicators are included for normal and peak input, as well as showing whether positive or negative peaks are predominant.  A special feature is included in the compressor module to eliminate the pumping effect when the operator's voice is paused, thus automatically adjusting the compressor level only when speech is detected.  A pad with AC coupling is provided, such that no transceiver modification is required to interface with microphone inputs.

Views of the PCB are attached.  In the very near future, more information will be shared as on-air testing is completed.


* 20200210_183204.jpg (3099.74 KB, 3264x2448 - viewed 95 times.)

* 20200210_183226.jpg (3175.48 KB, 3264x2448 - viewed 71 times.)
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Rick / W8KHK  ex WB2HKX, WB4GNR
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« Reply #12 on: February 10, 2020, 11:30:08 PM »

Actually, an all-in-one processor is in its final design stages. The MAX processor incorporates several stages of envelope processing primarily geared to the AM market   ...

The MAXIM chips are the cat's MEOW. I didn't a little fooling around with them myself. I must, say though, your project looks awfully good. Would a 180 degree phase switch be appropriate?

Oh, one more thing. I had a problem with spurs. Are you having a problem with spurs? I seem to see some in the spectrum display. Maybe they're an illusion.

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« Reply #13 on: February 10, 2020, 11:39:06 PM »

Actually, an all-in-one processor is in its final design stages. The MAX processor incorporates several stages of envelope processing primarily geared to the AM market   ...

The MAXIM chips are the cat's MEOW. I didn't a little fooling around with them myself. I must, say though, your project looks awfully good. Would a 180 degree phase switch be appropriate?


Yes, there is a phase inverter in the signal path, which may be enabled either by jumper connection or a SPDT front-panel switch.

In earlier versions of the artwork, we did notice the 50-X clock signal was not fully suppressed.  Two stages of LPF following the SCAF clean it up, and there are no apparent spurs produced by the SCAF.  It is my assumption that artifacts others have experienced with the Maxim chips may also be clock signal feed-through.
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« Reply #14 on: February 11, 2020, 12:10:11 AM »

Hi Rick,

Tnx for the email.  Those pictures above - wow, that is some effort!

You may be right about the Maxim chip and clock feedthrough / artifacts.  A LP filter afterwards could be missing with the SCAF products I tested before. It may be the equivalent to Steve/QIX's anti-aliasing LP filter in the PDM generator.

After you added the LP filter did you hear any delay/overload sounds like a cell phone makes?  If that is gone then that would be FB.

T
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"You know my name"  https://youtu.be/noGjJyEDm5s?t=135

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« Reply #15 on: February 11, 2020, 12:18:36 AM »

Actually, an all-in-one processor is in its final design stages. The MAX processor incorporates several stages of envelope processing primarily geared to the AM market   ...

The MAXIM chips are the cat's MEOW. I didn't a little fooling around with them myself. I must, say though, your project looks awfully good. Would a 180 degree phase switch be appropriate?


Yes, there is a phase inverter in the signal path, which may be enabled either by jumper connection or a SPDT front-panel switch.

In earlier versions of the artwork, we did notice the 50-X clock signal was not fully suppressed.  Two stages of LPF following the SCAF clean it up, and there are no apparent spurs produced by the SCAF.  It is my assumption that artifacts others have experienced with the Maxim chips may also be clock signal feed-through.
Yes. The clock was getting through for me too, I see now, having found my sketchy notes. The clock or some mixing product from the clock was causing the spurs I had. You solved it the same way I did. I bet you figured it out a whole lot faster, too.

Your project really does look good.

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« Reply #16 on: February 11, 2020, 12:43:41 PM »

Hi Rick,

Tnx for the email.  Those pictures above - wow, that is some effort!

You may be right about the Maxim chip and clock feedthrough / artifacts.  A LP filter afterwards could be missing with the SCAF products I tested before. It may be the equivalent to Steve/QIX's anti-aliasing LP filter in the PDM generator.

After you added the LP filter did you hear any delay/overload sounds like a cell phone makes?  If that is gone then that would be FB.

T

Tom and Jon, thanks so much for the kind words. 

I don't recall hearing any delay or overload sounds emanating from the SCAF stages.  The SCAF chips are post-compressor in our implementation, so the range of signal levels to the SCAF was well-controlled.  The sound was crisp and clean, and scope display did not reveal any anomalies. 

Our LPF post-SCAF is just a simple RC filter in conjunction with existing op-amps, no additional stages were needed.
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« Reply #17 on: February 11, 2020, 02:47:21 PM »

Folks:

The "MAX" audio processor looks quite good. Wonder of a kit will be sold? Looking forward to hearing it on the air.

On the SCAF (I used a Maxim Elliptical filter chip-two in series), I also found that feeding it well controlled audio level wise was the best as it's S/N is not as high as an op-amp or L/C filter, but for the SCAF the brick-wall filtering was excellent. The phase shift near the cutoff point will be high, but the clipper afterwards will take care of that.

Tom: it will be interesting to hear how you like the LC filter. I bet it'll work FB!

73,
Dan
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« Reply #18 on: February 11, 2020, 05:07:11 PM »

The MAX processor was bandwidth tested on 1540kc at night with an SDR on an antenna and the transmitter on a dummy load. What appears as spurs in the bandwidth comparison image above aren't in any of the other tests (see below).

The SCAF chips do use a 555 timer to generate a clock frequency but that's between 100 and 300kc thus easily filtered out. Also, the bandwidth filter is after the limiter and before the clipper, which is followed by an LPF.

It's important to note that the limiter stage is very tight and prevents the clipper from getting over-driven, thus avoiding the clipping artifacts that have given this function a bad rap.

I've been using the prototype on 75M for most of the Fall and have gotten good reports.


* SCAF295_3kHz.jpg (3788.67 KB, 4032x3024 - viewed 55 times.)

* SCAF295_6kHz.jpg (4825.12 KB, 4032x3024 - viewed 47 times.)
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« Reply #19 on: February 11, 2020, 08:36:46 PM »

Seems like the “Max” offers the important tools for effective AM control.
I have listened to N1BCG and can report his signal as clean (no audible artifacts), and loud!

Thanks for your efforts and looking forward to the W8KHK/N1BCG-type accepted version!!

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« Reply #20 on: February 11, 2020, 11:46:42 PM »

I am ready for the "MAX" to put in in front of my Bauer. And hopefully the Collins 21-E soon. Please keep us updated on availability
KC4MNE
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« Reply #21 on: February 12, 2020, 01:41:53 PM »

From a few emails I have received, I believe that the SCAF CAN be made to work well with no problems as well as long as the LP filtering and clock feed thru issues are addressed.  I look forward to comparing the Butterworth L/C filters to the SCAF for IMD, etc...  John / W9JSW and I are working on parts and he is doing a board layout, so it will be soon.

T
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« Reply #22 on: February 12, 2020, 03:05:39 PM »

*** UPDATE:

I've decided to order parts for just ONE 5.5 KHz Butterworth L/C filter to mount on a prototype board. I will test it and use it on the air.  At that point I want to build and test a SCAF Maxim circuit to compare and decide which one is best to use.

Other than the data sheet circuit, does anyone have a MAXIM SCAF circuit that has the LP filter and other precautions we have discussed here to eliminate the distortion?  Maybe Rick will show me his SCAF-related circuitry since I may be able to improve on it and it will become public anyway in the future once the MAX is released in kit form.

T
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« Reply #23 on: February 12, 2020, 06:15:22 PM »

*** UPDATE:

I've decided to order parts for just ONE 5.5 KHz Butterworth L/C filter to mount on a prototype board. I will test it and use it on the air.  At that point I want to build and test a SCAF Maxim circuit to compare and decide which one is best to use.

Other than the data sheet circuit, does anyone have a MAXIM SCAF circuit that has the LP filter and other precautions we have discussed here to eliminate the distortion?  Maybe Rick will show me his SCAF-related circuitry since I may be able to improve on it and it will become public anyway in the future once the MAX is released in kit form.

T

Tom, I sent email to you this afternoon.....
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« Reply #24 on: February 12, 2020, 06:35:48 PM »

Thanks, Rick!  That is a very generous offer and I believe everyone interested in good bandwidth control will benefit in the end. I will get back soon.  BTW, the schematics and artwork are very impressive and I can see you guys did a pro job.


I received an email today backing up our current beliefs about the Maxim SCAF:

K6JEK with his permission:  "I started with [xxx] mods to the SCAF-1 which uses the MAXIM chips. An HP distortion analyzer showed quite a lot of distortion. It was from the LM386 which [xxx] left in line -- completely unnecessary for line level. [xxx] also defeated the LP filter that was in the original SCAF-1 design which led to the clock getting through and mixing it up with the audio signal and causing all sorts of havoc. Fixing those two things resulted in no distortion I can hear."

I am encouraged as ever to try one again...  I was thinking that maybe my L/C filters could be switched in after the Maxim SCAF (replacing the existing LP  R/C filter) as an additional brick wall, or at least experimenting to see how effective it becomes. I notice the MAX processor's rolloff is sharp, (as shown in the screen shots posted earlier) though not as sharp as a DSP circuit. Maybe a more elaborate LP filter would help.

My new SDR RX (compliments Jeff/W2NBC) is working FB and I am always impressed and able to spot SDR transmitters on the air by their brickwall bandwidth. It's so intense that I can tell the difference between two SDRs running 3 KHz or 3.5 KHz ssb.  Amazing control.  Wouldn't it be cool to be able to do the same with all the big BC transmitters, the plate modulated homebrew rigs, modified riceboxes and the thousands of 100 watt boatanchors of the world?  Right now most AM bandwidth on the band is like the wild west but could be as smooth as driving the Autobahn.   Grin

T
 

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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz,  +-6.0 KHz or  +-8.0 KHz when needed. 

"You know my name"  https://youtu.be/noGjJyEDm5s?t=135

There's nothing like an old dog... a puppy... a dog in its prime... or ANY dog!
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