Author Topic: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?  (Read 6398 times)

0 Members and 1 Guest are viewing this topic.

Offline evb149

  • Super Contributor
  • ***
  • Posts: 1648
  • Country: us
Re: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?
« Reply #25 on: April 08, 2017, 12:28:41 PM »
Looks nice.  Will you share the particular special techniques and overall design details with us?
How would one recreate something similar at a different frequency and using different materials?

Many years ago I designed a very special notch filter for use up at around 1500MHz. It was designed to be quite sharp and had very low loss right up to 6GHz.

The plots below are from a real production filter measured on a VNA and imported into Genesys. You can see how special this is when you look at how good the match is right up to 6GHz and how sharp and deep the notch is.

Divide all the numbers by 10 and it would be in the ballpark of the 144MHz band :)  But it would be very big if scaled to work here. It was designed on some very special PCB material and used special techniques to achieve no re-entry modes right up to nearly 8GHz.
 

Offline cdev

  • Super Contributor
  • ***
  • Posts: 3278
  • Country: 00
  • I love science, stars, nature and electronics.
Re: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?
« Reply #26 on: April 09, 2017, 09:12:41 AM »
An 1/4 wave open  coaxial cable stub can make a fairly sharp notch filter at one frequency. Sharp enough only a few MHz away? Maybe, but I suspect more probably maybe not. A sharper fairly straightforward solution is a helical resonator.

 http://satsignal.eu/wxsat/filters/HelicalNotch.htm

"What the large print giveth, the small print taketh away."
 

Offline Howardlong

  • Super Contributor
  • ***
  • Posts: 4118
  • Country: gb
Re: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?
« Reply #27 on: April 09, 2017, 09:32:11 PM »
An 1/4 wave open  coaxial cable stub can make a fairly sharp notch filter at one frequency. Sharp enough only a few MHz away? Maybe, but I suspect more probably maybe not. A sharper fairly straightforward solution is a helical resonator.

 http://satsignal.eu/wxsat/filters/HelicalNotch.htm

1/4 wave stub will be nowhere near good enough I'm afraid. Here's an example using RG58, 100MHz span, centre 160MHz, somit gives you an idea of what kinf of performance can be achieved.



DIY filters such as the helical you mention will work, but you really need the right skills and tools to build them, and the right test equipment to hand tune. Personally I usually find designs like this, both online and in print, are often difficult to reproduce: the same applies to antennas. If the designer mentions they have specifically paid attention to reproducibility from the ground up then this is a good sign. While reproducibility can have an effect on performance, that benefit usually outweighs the lack of performance from a one-off contrivance that happened to work on a particular day with the right weather.

Size aside, the huge benefits of SAW filters are their reproducibility, and that, once prototyped, there is no further tuning required. With ready-matched 50 ohm SAW filters, it's unlikely you'd need to tune them at all. The downside, in small volumes, is finding a suitable one, as typically their are a limited number of off the shelf designs.
 

Offline cdev

  • Super Contributor
  • ***
  • Posts: 3278
  • Country: 00
  • I love science, stars, nature and electronics.
Re: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?
« Reply #28 on: April 10, 2017, 09:44:05 AM »
Do you know of a good source for a single. 137.5 MHz part with low insertion loss?

"What the large print giveth, the small print taketh away."
 

Offline Howardlong

  • Super Contributor
  • ***
  • Posts: 4118
  • Country: gb
Re: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?
« Reply #29 on: April 10, 2017, 08:30:18 PM »
As already mentioned several times in this thread, Golledge.

For example http://www.golledge.com/pdf/products/specs/mp07202.pdf
 

Offline Howardlong

  • Super Contributor
  • ***
  • Posts: 4118
  • Country: gb
Re: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?
« Reply #30 on: April 10, 2017, 09:45:06 PM »
I called them up and they are sending me some samples this week, but they only had a few in stock. Also bear in mind that I already purchase quite a bit from them so a punter off the street may get a different response.

I'll put one or two on a board and see how they perform on a VNA.
 

Offline eb4fbz

  • Contributor
  • Posts: 46
  • Country: es
Re: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?
« Reply #31 on: April 11, 2017, 05:53:29 AM »
You really need 60dB of gain? What are you using as receiver? Something with 50dB noise figure?  :-//

60dB without filtering is crazy, even without the nearby APRS transmitter.
 

Online PA0PBZ

  • Super Contributor
  • ***
  • Posts: 3455
  • Country: nl
Re: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?
« Reply #32 on: April 11, 2017, 06:15:48 AM »
You really need 60dB of gain? What are you using as receiver? Something with 50dB noise figure?  :-//

60dB without filtering is crazy, even without the nearby APRS transmitter.

I asked the same question about 3 weeks ago, unfortunately the OP never replied.  :-//
Keyboard error: Press F1 to continue.
 

Offline evb149

  • Super Contributor
  • ***
  • Posts: 1648
  • Country: us
Re: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?
« Reply #33 on: April 11, 2017, 08:14:03 AM »
I didn't look at the details of their setups but people claim to be receiving good signals with basically tiny antennae with negligible gains (e.g. small QFHs or such).  So it wouldn't seem likely that 60dB of tuned RF gain is needed otherwise you'd think that people would have to or choose to be using some better gain antennae besides just having a small antenna and ordinary preamp + SDR.

With the strong nearby signal I can believe that he/she may need / want 60dB of relative gain change between the two (suppress the interference signal, boost the desired one) but even then how bad can the selectivity / intermodulation of his RX be?

You really need 60dB of gain? What are you using as receiver? Something with 50dB noise figure?  :-//

60dB without filtering is crazy, even without the nearby APRS transmitter.

I asked the same question about 3 weeks ago, unfortunately the OP never replied.  :-//
 

Offline cdev

  • Super Contributor
  • ***
  • Posts: 3278
  • Country: 00
  • I love science, stars, nature and electronics.
Re: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?
« Reply #34 on: April 12, 2017, 05:31:11 AM »
NOAA and METEOR sats are semi weak signals but a single LNA (or even no LNA) is adequate for me to receive them with occasional noise from pagers, etc. With more selectivity he probably will do fine. QFH antennae are best for that and better for RHCP (NOAA) sats than omni biconical I use but vary as far as pattern, unlike a biconical a tall skinny one has more (not less) gain at horizon but picks up more terrestrial noise too.

"What the large print giveth, the small print taketh away."
 

Online AF6LJ

  • Supporter
  • ****
  • Posts: 2785
  • Country: us
Re: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?
« Reply #35 on: April 12, 2017, 09:19:09 AM »
This looks interesting...
I would go with a band reject cavity or two.
At that frequency they are not hard to make if one is creative.
Sue AF6LJ
Test Equipment Addict, And Proud Of It.
 

Offline cdev

  • Super Contributor
  • ***
  • Posts: 3278
  • Country: 00
  • I love science, stars, nature and electronics.
Re: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?
« Reply #36 on: April 12, 2017, 09:47:29 AM »
It's not easy to get nice clean images if you live in an urban area.
"What the large print giveth, the small print taketh away."
 

Offline Howardlong

  • Super Contributor
  • ***
  • Posts: 4118
  • Country: gb
Re: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?
« Reply #37 on: April 12, 2017, 09:50:58 AM »
In my experience, as I alluded to earlier, practically speaking making such filters that need accurate measurement and manual tuning aren't always particularly easy to fabricate unless you have known reproducible plans, and the right skills and tools at your disposal.

While there are plenty of examples in print and online, finding one that's easily reproducible is a bit of a crapshoot. Any given plans may have worked for an individual on one occasion, but are those plans documented well enough and easily reproducible?

Much the same applies to antennas, by the way. I am sure this lack of reprocibility in many cases is why RF is often thiught of as voodoo!

Those SAW filters arrived today, unfortunately I have some other commitments right now, but I'll be putting them on a board and measuring them over the next few days.
 

Offline David Hess

  • Super Contributor
  • ***
  • Posts: 6314
  • Country: us
  • DavidH
Re: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?
« Reply #38 on: April 12, 2017, 10:22:17 AM »
I did the tuning on my 2 meter helical resonator using an SWR bridge and less than 5 watt transmitter.  In practice with only two sections, adjusting the two piston trimmers which are not shown in the photograph is easy enough with just a receiver and RF attenuator.
 

Offline vk6zgo

  • Super Contributor
  • ***
  • Posts: 3942
  • Country: au
Re: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?
« Reply #39 on: April 12, 2017, 11:56:26 AM »
Where I used to work there was a 1kW pager on 148.3MHz (or thereabouts) co-sited with our TV/FM Broadcasting installation.

Common 1970s/80s mobile 2m Ham radios were only affected by the pager if tuned up towards 148MHz----at 147MHz & lower, there was no discernible breakthrough.

It is fashionable to dismiss superheterodynes as "yesterday's technology", but they are a hell of a hard act to follow.

Almost any Direct conversion receiver is just an "Envelope Detector" to a strong enough & "adjacent enough" signal.
 

Offline Howardlong

  • Super Contributor
  • ***
  • Posts: 4118
  • Country: gb
Re: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?
« Reply #40 on: April 12, 2017, 04:02:01 PM »
I did the tuning on my 2 meter helical resonator using an SWR bridge and less than 5 watt transmitter.  In practice with only two sections, adjusting the two piston trimmers which are not shown in the photograph is easy enough with just a receiver and RF attenuator.

What was the passband and stop band ripple like? How well did it match across the passband?

In addition, you already have a lot of relevent skills and experience to built such a device. I am pretty sure the OP doesn't, and that was what I was trying to convey. I also doubt the OP has a 5W transmitter or SWR meter. That method is exactly the method I've used in the past, but when you take a filter you've tuned that way and put it on a VNA you realise it's not quite as good as you thought!
 

Offline David Hess

  • Super Contributor
  • ***
  • Posts: 6314
  • Country: us
  • DavidH
Re: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?
« Reply #41 on: April 12, 2017, 07:01:44 PM »
I did the tuning on my 2 meter helical resonator using an SWR bridge and less than 5 watt transmitter.  In practice with only two sections, adjusting the two piston trimmers which are not shown in the photograph is easy enough with just a receiver and RF attenuator.

What was the passband and stop band ripple like? How well did it match across the passband?

In addition, you already have a lot of relevent skills and experience to built such a device. I am pretty sure the OP doesn't, and that was what I was trying to convey. I also doubt the OP has a 5W transmitter or SWR meter. That method is exactly the method I've used in the past, but when you take a filter you've tuned that way and put it on a VNA you realise it's not quite as good as you thought!

When I designed it, the only information I was missing was how large to make the coupling window so I made it 1/3rd the width and height which as it ends up made it massively overcoupled but this had some advantages.  Loss in the center of the band was like 0.05dB, there was no measurable ripple, and it covered most of the 4 MHz wide 2 meter band.  It was perfect for removing pager interference.

I never had access to a VNA to test it or I would have lowered the coupling.  I did come up with a design for a variable coupling window but this one worked so well that I never proceeded with it.

This was actually the 2nd one I designed and built.  The first helical resonator was not nearly as well constructed and used film trimmers which lowered the Q significantly preventing operation at even 5 watts because they would break down.  After that I found someone selling piston trimmers at the ham radio swap meet which made all of the difference.

The design process for both was interesting.  I started with the fixed size of the wire for the helical elements and worked out the dimensions going backwards.  The center frequencies with the trimmers were right on.  The final product was definitely not an optimal design but it was and still is way more than needed to protect a receiver from out of band overload and the extremely low loss has advantages.

Incidentally, this project was also how I discovered that RF coupling into a gas discharge bulb is incredibly efficient.  A neon bulb inserted into the helix with 1/2 watt is way too bright to look at.  If I ever build a gas laser, this is how I will do the excitation.
 

Offline Howardlong

  • Super Contributor
  • ***
  • Posts: 4118
  • Country: gb
Re: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?
« Reply #42 on: April 15, 2017, 11:51:53 PM »
Today I made up a board for the SAW filter and tested it on the VNA, these are the results.

144.8MHz is 44dB down, and insertion loss between 137 and 138MHz is 1.5 to 1.6dB.

There is no matching required, although I left space on the board for L matches if necessary at both input and output. I used 1nF caps in series with the input and output in place of the L match.

Total time designing and making the board was an hour, fifteen minutes to populate it, then a further fifteen minutes to test it. There was no tuning to do :-)

Marker 1 is 137.0MHz
Marker 2 is 138.0MHz
Marker 3 is 144.8MHz in the S21 plot
Marker 3 is 137.5MHz in the S11 plot








« Last Edit: April 15, 2017, 11:55:40 PM by Howardlong »
 

Offline cdev

  • Super Contributor
  • ***
  • Posts: 3278
  • Country: 00
  • I love science, stars, nature and electronics.
Re: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?
« Reply #43 on: April 16, 2017, 01:09:01 AM »
Very nice! You could sell that as a kit with no additional work!
 1090 MHz too.
« Last Edit: April 16, 2017, 01:13:55 AM by cdev »
"What the large print giveth, the small print taketh away."
 

Offline Howardlong

  • Super Contributor
  • ***
  • Posts: 4118
  • Country: gb
Re: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?
« Reply #44 on: April 16, 2017, 02:06:55 AM »
Very nice! You could sell that as a kit with no additional work!
 1090 MHz too.

Coincidentally, I designed and built a 1090MHz one a few years ago already as part of a bigger TCAS system I was working on. I'd have to dig out the schematics, but ISTR that was a native 50 ohm SAW too,  ut a different package.
 

Offline evb149

  • Super Contributor
  • ***
  • Posts: 1648
  • Country: us
Re: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?
« Reply #45 on: April 16, 2017, 04:03:29 AM »
The SAWs look nice.
It might be a decent idea to use two of them in series if one is really trying to cut the out of band noise if the level is very high.

I don't recall what the modulation scheme is, but if it involves a bandwidth over which there can be much phase shift then maybe some phase equalization could be necessary to compensate for the phase response of the filter over the passband (which might show some large shifts particularly near the band edges and maybe elsewhere).

 

Offline Howardlong

  • Super Contributor
  • ***
  • Posts: 4118
  • Country: gb
Re: Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)?
« Reply #46 on: April 16, 2017, 04:55:53 AM »
I haven't done VHF NOAA reception myself for many years, but ISTR it's an FM modulated carrier with an IF bandwidth of about 40kHz. That bandwidth requirement was quite a problem for commonly available VHF receivers because their hardware filters were generally designed for narrower bandwidths at VHF. Nowadays, with SDR, that turns into a software problem.

(Some years ago I designed a simple and reproducible turnstile antenna for VHF NOAA reception which made it to print in one of the newstand mags over here in Blighty).
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf