OT: CH4 Trap.

Tim

-- Seven Transistor Labs Electrical Engineering Consultation Website:

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Are you looking for a methane detector, filter mask, or measuring device?

If detector, this is about as cheap as they come:

A dust filter mask won't work for methane. You'll probably need a respirator and air/oxygen bottle.

If you're trying to measure the methane level, there are plenty of "combustable gas" detectors available:

I didn't know you kept cows.

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Methane or TV?

TV Channel 4 rejection/trap filter >:-} ...Jim Thompson

TV ...Jim Thompson

I like it.

Cable or OTA (over the air) channel 4?

I'll assume OTA, where CH 4 is 66-72 Mhz. Also 75 ohms.

Since you mentioned "good steep-sided", such filters are usually rack mounted and used in conjunction with RF insert modulators, where the channel reject filter drops a channel and replaces it with local content. Manufacturers don't like to stock these as there are too many possible frequencies involved. So, you order them tuned to whatever you need. You probably won't like the prices.

Unfortunately, I could have shipped one to you as I had a rack full of them with no real application. So, I recycled them. Sorry. Some of them are the 8 black boxes near the bottom of the rack with more in a box nearby.

as usual, it's better to prevent the problem than to cure it. Avoid eating too many beans...

That is quite wide relative bandwidth, so making a good bandpass/bandstop filter is quite hard, especially if steep slopes (and presumably low passband loss) is required.

One way to deal with such large stop band could be a combination of 66 MHz LPF and 72 MHz HPF.

What kind of offending signal is on Ch4 ? If analog, most of the signal power is concentrated around the video and audio carriers, so a notch on the video carrier and steep notches on audio carrier(s) will eliminate much of the power from Ch4. Attenuating the strongest carrier with say 10 dB, will drop the third order intermodulation products by 30 dB in the stages after the filter. These intermodulation product would typically fall on nearby channels, causing reception problems to these channels.

Not a bad idea.

It'll drop the IM3 by 20 dB, assuming the dominant signal is (CH4)**2 * (something else). To drop it 30 dB you'd have to reduce the other signal by 10 dB as well.

I'm a fan of coax stubs for that sort of job, with a small cap in series at the tee-connector end. You figure out how long the coax should be, then adjust by putting a thumbtack through the shield into the centre conductor. Works great.

Cheers

Phil Hobbs

Wake up Jim, Give us some more details.

Mikek

It's you who are asleep. Did you miss this message in the thread?

Subject: Re: OT: CH4 Trap. Date: Sun, 13 Jul 2014 20:01:55 -0700 Message-ID: ...Jim Thompson

You did not specify the required stop band rejection, maximum allowed passband insertion loss and what you mean by "steep". Is it enough to have a -3 dB in middle of Ch4 or the response dropping to -60 dB one MHz from the channel boundary ?

I'm looking for sources, then I'll sweat the details. ...Jim Thompson

The ARRL Handbook.

The ARRL handbook is a good source, if you try to get rid of some point frequencies such as using big resonators to prevent your own transmission frequency from blocking your receiver frequency i.e. a duplex filter.

However, notching out a relatively wide (fmin:fmax) frequency band is more difficult. You either have to use the LP/HP idea as in my previous post, use several big high-Q resonators with very tight coupling between resonators or use multiple high-Q notches tuned at different frequencies (staggered tuning), but the insertion losses might be quite high.

Or simply buy it...

...Jim Thompson

I assumed from the start a channel 4 trap. I wanted to know what you are trying to do.

How do I use that message-ID? What do I put it in to see the message?

Mikek