EMRFD Message Archive 7114
Message Date From Subject 7114 2011-12-28 17:04:04 Baltasar Perez Di... Tuneable 10.7MHz notch filter Hello everybody,
I'm trying to design and build a tuneable notch filter with the following
- Tuneable frequency range: 10.7MHz + - 500KHz (10.2MHz to 11.2MHz)
- Notch depth: > 10dB
- Bandwidth: < 100KHz
and looking through bibliography I've found three approaches:
1) Twin T Notch Filter (double T with resistors and capacitors and a
opamp to increase Q)
2) Bandpass filter with the input and the output connected to a adder/sub
3) Low pass filter + High pass filter paralleled and an adder opamp
I've mounted the first approach a I got a notch but with a very bad Q
even with matched components (double and half value).
The bandwidth is around 1MHz so it's very broad for my case.
I'm now trying second and third approach and I hope to get a better Q.
Do you have any recommendation from your experience?
Thanks in advance
72 de EA8BVP, Bal
7115 2011-12-28 20:30:47 R Wall ML emails Re: Tuneable 10.7MHz notch filter Hi Baltasar,
My old Yaesu FT301 has a tuneable notch that uses a crystal. The variable capacitor has a knob to allow front panel adjustment.
Roderick Wall, vk3yc.
7116 2011-12-29 03:02:57 Roelof Bakker Re: Tuneable 10.7MHz notch filter Hello Bal,
I believe a passive design is the way to go.
Last week, I have build a passive notch filter to notch out BBC4 on 198
I have used the free Elsie filter software:
You can play with all kind of filter types and learn along the way which
one is the best for your application.
7117 2011-12-29 03:12:57 Lasse Re: Tuneable 10.7MHz notch filter I would look at using a hybrid coupler/power splitter...
Put a tunable band-pass filter at the S-port, terminate the other end of
the filter in 50 ohm. . The filter will act as a very high impedance
except for all frequencies with-in the bandpass at the S-port.
Using the two other ports of the hybrid as input/output you will have a
6 dB loss, but also a great notch.
Baltasar Perez Diaz skrev 2011-12-29 00:42:
> Hello everybody,
> I'm trying to design and build a tuneable notch filter with the following
> - Tuneable frequency range: 10.7MHz + - 500KHz (10.2MHz to 11.2MHz)
> - Notch depth:> 10dB
> - Bandwidth:< 100KHz
> and looking through bibliography I've found three approaches:
> 1) Twin T Notch Filter (double T with resistors and capacitors and a
> opamp to increase Q)
> 2) Bandpass filter with the input and the output connected to a adder/sub
> 3) Low pass filter + High pass filter paralleled and an adder opamp
> I've mounted the first approach a I got a notch but with a very bad Q
> even with matched components (double and half value).
> The bandwidth is around 1MHz so it's very broad for my case.
> I'm now trying second and third approach and I hope to get a better Q.
> Do you have any recommendation from your experience?
> Thanks in advance
> 72 de EA8BVP, Bal
7118 2011-12-29 03:16:46 ha5rxz Re: Tuneable 10.7MHz notch filter How about approach number four?
I have a device in my shack called an ANC-4 Antenna Noise Canceller, this has two antenna inputs, a main antenna and a noise antenna. In use the received noise is inverted then added to the wanted signal, the result being a clean signal.
Tap off some of your IF signal and put it through a mixer, a narrow filter at (say) 45 MHz and then a second mixer back down to 10.7 MHz. Invert this signal then add it back into your IF, the result should be a 'hole' in the passband. If you have a 10.7 MHz IF and a 45 MHz filter then you can use a 33.8 MHz to 34.8 MHz oscillator to drive both mixers.
7119 2011-12-29 07:50:41 nt1u Re: Tuneable 10.7MHz notch filter I also built the DeMaw qrn squasher, as he called it. Not sure it would solve the original problem. Here is a link to the original article
I am pretty sure there was a followup with some corrections a few months later. I don't have them handy at the moment.
A notch filter at 10.7M with a Q of 100 should be realizable with a Chebyshev (equal ripple) filter. I have not had much luck pulling crystal filters that much (and especially if it is multiple crystals).
The Maxim ap note might be a good place to start as well. Elsie will of course work, but I think it is instructive (at least for me) to work through the numbers, to understand them.
7120 2011-12-29 10:47:49 Wes Re: Tuneable 10.7MHz notch filter Hello Bal and group,
Perhaps we are making more of this task than is needed. As is often the case, it is worthwhile to step back and ask how this would be done in a simple way.
A single series tuned circuit forms a notch filter when it is connected as a parallel element. We might put several of such elements in parallel to get a sharper response, but that reduces to merely changing the L/C ratio with a single resonator. To parallel several elements, we need to isolate them. This is easily done with a low pass filter element -- a pi.
I just loaded an example to the files section under the title "tunable LC notch filter." The circuit started with a 12 MHz Low Pass filter. Then two series tuned circuits were placed within the filter, but were isolated from each other by 90 degrees of synthetic transmission line. That is, they were not on the same node. You want high L and low C for the series tuned circuits. Clearly, the two capacitors can be elements of a dual section variable cap to realize a tuning knob. If you have a three or four secti
7122 2011-12-29 14:11:43 William Carver Re: Tuneable 10.7MHz notch filter > Recall the simplest way to design a bandpass filter is to build a lowOK, I know that.
> pass that has a cutoff equal to that of the bandwidth you want. Then
> you resonate the low pass elements "in a bandpass way" to achieve the
> desired bandpass response. This is not always the most practical way
> to achieve a bandpass response, but is a conceptual solution.
> Turns out, you can do a similar thing to realize a well defined bandI have to think there's a hole in my background. If I ever knew that, it
> stop or notch response. Design a High pass filter with a cutoff
> equalling the desired bandstop width. Then resonate the elements in "a
> bandstop way." See EMRFD Fig 3.2.
had slipped away. ........THANK YOU, WES.
Bill - W7AAZ
7133 2012-01-02 16:19:44 Baltasar Re: Tuneable 10.7MHz notch filter Hello Lasse,
Thanks very much for reply.
You have pointed another interesting approach. I haven't considered this one. Very clever.
When I have finished a tunable narrow bandpass filter for 10.7MHz, it should be easy to try this approach.
Happy New Year!
72 de EA8BVP, Bal
7134 2012-01-02 16:19:44 Baltasar Re: Tuneable 10.7MHz notch filter Hello Roderick,
Thanks for reply and sorry for my delayed reply (end/begin of the year family commitments).
I've been reading FT-301 manual and I found out that it uses a crystal as filter (9000KHz +/- 1.5KHz). It's a good idea but it only moves 3KHz. A xtal as a filter has deep notch and very good Q, just what I need :) but it moves only a few KHz.
An idea could be a bank of switchable xtals to cover 10.7MHz +/- 500KHz but I think it would be a bit bulky.
Thanks again and Happy New Year!
72 de EA8BVP, Bal
7135 2012-01-02 16:19:45 Baltasar Re: Tuneable 10.7MHz notch filter Hello Roelof,
Thanks for reply and thanks for recommendation. I also think Elsie is a good tool to design filters. I will play with it to deduce which is the better topology but I'm afraid LC type filters are low Q so the bandwidth will be too broad.
Happy New Year!
72 de EA8BVP, Baltasar.
7147 2012-01-03 20:15:14 Baltasar Re: Tuneable 10.7MHz notch filter Hello Peter,
Thanks for reply and sorry for my delayed reply (family commitments)
Well, I think your approach is more or less similar to my approach number two. Here we have a bandpass filter with its input and output
connected to a subtracter operational amplifier (opamp output=input_from_bandpass_filter - output_from_bandpass_filter).
Adjusting opamp gain to compensate filter inserti
7148 2012-01-03 20:15:15 Baltasar Re: Tuneable 10.7MHz notch filter Hello Dave,
Thanks very much for your reply.
I've been reading the DeMaw qrn squasher, very instructional. I think it's more or less like ANC-4 Antenna Noise Canceller (Peter HA5RXZ recommendation). It adds the signal with a out of phase copy of it (180º) to eliminate the undesired signal (noise). In someway it's similar to approach number 2 where you add/sub the signal after it passes through a bandpass filter.
Thanks for the Maxim AN, good note. I have to look
7149 2012-01-03 20:15:16 Baltasar Re: Tuneable 10.7MHz notch filter Hello Wes,
Thanks for reply and for making the effort in designing the notch filter. You have made it very simple.
With this filter, I'm more worried about filter's Q than depth notch because I intend to eliminate/mitigate random interferences like tones inside the passband (10.7MHz +/- 500KHz) before it reaches the FPGA Analog to Digital Converter. My aim is to mitigate the interference around 10 to 20dB to relieve ADC.
My desired signal occupies 8KHz but it isn't in a fixed position inside the passband. Also the interference (tone) can be next to the desired signal or very far, so filter Q is important to not affect too much the desired signal.
I guess that putting more series tuned filter will sharp the curve.
I was also thinking in opamp to naturally increase filter Q.
Thanks again and Happy New Year
72 de EA8BVP, Baltasar.