EMRFD Message Archive 7571

Message Date From Subject
7571 2012-07-10 15:03:39 g3wie Mixed form bandpass filter with different impedances
Hi All,

I've just completed a design for a 70MHz transceiver which uses the mixed-form version of bandpass filters from the EMRFD 2008 program set. This has produced filters with nice symmetrical passbands, useful for protecting against broadcast band II (88 - 108MHz in Eu). I use one of these to follow a classic mosfet receive preamp whose output impedance (about 1250 ohms) I step down to 50 with a transformer on a binocular ferrite to feed the filter, then attenuator, then diode mixer.

I made an attempt at changing the filter input stage to match directly to 1250 ohms by carving out a pi-network and refactoring it whilst keeping the output capacitor the same as originally carved out. Unsurprisingly, this spectacularly didn't perform! So I'm here to ask the people who really understand these filters. Is there a methodology for designing them with different input and output impedances?

Chris G3WIE
7572 2012-07-10 18:50:52 Ashhar Farhan Re: Mixed form bandpass filter with different impedances
In the emrfd chapter on filters you will find details on using the
bisection theorm to simply transform impedances using the classic low
pass filter from one impedance to another.
One could start there and then replace shunt and series impedances
with resonating equivalents.
I have spent idle hours working out similar filters on tonne's elsie
and xlad.exe (I am not too sure if that's the one I am referring to).
These programs will help you quickly arrive at a filter by playing
around with the values. I know that starting with a particular
polynomial is the best way to do it, but we hams can afford to arrive
at a design by serendipity as well.
- Farhan

On 7/11/12, g3wie <g3wie@crystalhobs.org.uk> wrote:
> Hi All,
>
> I've just completed a design for a 70MHz transceiver which uses the
> mixed-form version of bandpass filters from the EMRFD 2008 program set. This
> has produced filters with nice symmetrical passbands, useful for protecting
> against broadcast band II (88 - 108MHz in Eu). I use one of these to follow
> a classic mosfet receive preamp whose output impedance (about 1250 ohms) I
> step down to 50 with a transformer on a binocular ferrite to feed the
> filter, then attenuator, then diode mixer.
>
> I made an attempt at changing the filter input stage to match directly to
> 1250 ohms by carving out a pi-network and refactoring it whilst keeping the
> output capacitor the same as originally carved out. Unsurprisingly, this
> spectacularly didn't perform! So I'm here to ask the people who really
> understand these filters. Is there a methodology for designing them with
> different input and output impedances?
>
> Chris G3WIE
>
>
>
>

--
Sent from my mobile device
7574 2012-07-10 19:59:21 Wes Re: Mixed form bandpass filter with different impedances
HiChris,

Interesting problem. Can you tell us a bit more about what you were trying. What was the design BW at 70 MHz, and what sort of inductor were you using? How many resonators in your filter?

This mixed form filter is quite a bit different than the traditional, for the coupling cap between resonators has a relationship to the nodal C that differs from the simple ratio found when inductors are all grounded. The result was that a N=2 filter had the coupling capacitor changing by 2 as you shifted from 50 Ohms to a 1200 Ohm filter. That will depend upon relative bandwidth too.

Trying the same thing with a N=3 filter with 10% bandwidth produced a "divide by zero" error with the program TTC08. However, I was able to do a 10% bandwidth filter with a 200 Ohm load on one end.

The reasons for this difficulty becomes more clear when you study the equations used for the design. These are illustrated in the article
7575 2012-07-11 08:45:10 g3wie Re: Mixed form bandpass filter with different impedances
Thanks for reminding me of Mr Bartlett's theorem. I'm still unsure how that should be applied in my case. See the G3WIE file folder for the schematic (arac2 receive converter).

Chris

7576 2012-07-11 09:20:35 g3wie Re: Mixed form bandpass filter with different impedances
Hi Wes,

In the G3WIE files folder you'll find the schematic (arac2 receive converter) which shows the filter following the preamp. I've also uploaded the file produced by ttc3 (ttc2-5.. mixed), and the result of my transforming to substitute the 0.5pF coupling Cs and fiddling in gpla08to get some standard values (ttc2-5...opt).

The design bw was 2.5MHz with CF 70.3MHz, and the network analyser scan (NA scan) showed 3MHz which I'm quite happy with. The little peak in the passband centre is handy as it makes tuning easy with no test equipment. Peaking for maximum at 70.3 did the job.

The coils were 9t of 18swg enamelled copper wire. The change from 22swg in an earlier versi
7582 2012-07-12 19:03:34 Wes Re: Mixed form bandpass filter with different impedances
Hi Chris and gang,

I spent some time playing with various filters on the computer and it looks to me like the mixed form filters will work with unequal terminations at the ends. The existing design program should work fine. I added an appendix to the web piece that I have on the subject. It's at http://w7zoi.net/mixed-bag/mixed_bag.html This tends to work better with narrower filter bandwidth such as the 4% or so that you used in your transceiver. Also it is better to use higher L values. The examples that I used in the added appendix were for 50 MHz filters with a bandwidth of 2 MHz. The terminations considered are 50 and 1000 Ohms.

Good luck with your transceiver.

73, Wes
w7zoi