Filters - Page 1
Forum section: electronics / beginners

author: M0BSW
author_rank: Frequent Contributor
timestamp: March 30, 2013, 03:07:52 pm
content: I've been building a bandpass filter for my amateur radio, is there a way I can check the roll off, with a frequency counter and oscilloscope, I want to use it for 5MHz band weve been allowed to use with permission, from Ofcom & MOD, and I want to make sure anything pass the upper frequency is stopped dead in it's tracks. And really not sure how to do this.

author: fbs
author_rank: Newbie
timestamp: March 30, 2013, 03:37:17 pm
content: What you want to do is measure the ratio between the input and output voltage at each frequency(step). The scope can measure the input & output voltage, but you need something to generate the input signal with. If you dont have a signal generator adds synthesizermight be an option. AD9850 boards are quite cheap on ebay and good for 20MHz or so.

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 30, 2013, 03:41:53 pm
content: The filter works both ways, so you can use your receiver to check the attenuation. If your S-meter is reasonable correct you can even see how many dB's it is attenuating on the frequencies of interest.

author: M0BSW
author_rank: Frequent Contributor
timestamp: March 30, 2013, 05:17:23 pm
content: Quote from: fbs on March 30, 2013, 03:37:17 pmHi Yes I do have a signal generator which goes up to 10mhz,so I'll use that.

author: M0BSW
author_rank: Frequent Contributor
timestamp: March 30, 2013, 05:21:50 pm
content: Quote from: PA0PBZ on March 30, 2013, 03:41:53 pm,Yes it is correct , never thought of that staring right at me, thank you, isn't it funny you look for a technical way , when the most simply and effective way is right there.Thank you fellow radio amateur

author: M0BSW
author_rank: Frequent Contributor
timestamp: March 30, 2013, 05:23:57 pm
content: Still going to do it both ways, as I've never done this before with lots to learn , thank you to both replies, both with different views brilliant, very helpful.

author: KJDS
author_rank: Super Contributor
timestamp: March 30, 2013, 05:58:54 pm
content: You'll probably want to check it above 10MHz so your sig gen won't be quite up to the job. The largest unwanted signals you want to filter out are likely to be the second and third harmonic at 10 and 15MHz respectively. Using your receiver will give an idea of how well it works, however I'd trust an S meter about as far as I could throw it.

author: kg4arn
author_rank: Unknown
timestamp: March 30, 2013, 06:15:41 pm
content: Paul,The W7ZOI power meter is probably the most used piece of test equipment in my shack.It's fairly easy to build and use.73 Edhttp://www.kangaus.com/Documentation%20files/Power%20Meter%20Documentation%20May%202011.pdfhttp://www.cliftonlaboratories.com/Documents/Calibration_Report_W7ZOI_Power.pdfhttp://w7zoi.net/Power%20meter%20updates.pdf

author: M0BSW
author_rank: Frequent Contributor
timestamp: March 30, 2013, 06:18:01 pm
content: Quote from: KJDS on March 30, 2013, 05:58:54 pmI took a look on ebay as mentioned the DDS it goes up to 30mhz, cheap Chinese, I'm half tempted to buy one there cheap and I don't know how accurate they are, may be worth buying just to see.http://www.ebay.co.uk/itm/1setx-AD9850-DDS-Function-Signal-Generator-0-30MHz-DDS-Source-SCM-DDS-Module-/261191578274?pt=LH_DefaultDomain_0&hash=item3cd03b32a2

author: KJDS
author_rank: Super Contributor
timestamp: March 30, 2013, 07:35:26 pm
content: Quote from: M0BSW on March 30, 2013, 06:18:01 pmIt's been a long time since I've played with DDSs, but their big drawback is that they produce a lot of almost random spurious frequencies as well as the wanted signal. It would probably be ok looking at the filter output on a scope, but if you were using a power meter then it's likely that a spurious in your passband would dominate the power measured at the output rather than the filter roll off.As you have a frequency counter i'd be tempted to build a simple Colpitts oscillator with an output buffer from a couple of transistors, add the ability to switch in a little extra capacitance to the tuned circuit to broaden the sweep and use that as a source.

author: M0BSW
author_rank: Frequent Contributor
timestamp: March 31, 2013, 08:41:17 am
content: Quote from: M0BSW on March 31, 2013, 08:38:26 am,It's Sunday morning so sorry for late reply, found a circuit in our RSGB Year book, that looks an OK build, thank you for the advice.Paul

author: M0BSW
author_rank: Frequent Contributor
timestamp: March 31, 2013, 08:43:25 am
content: Quote from: kg4arn on March 30, 2013, 06:15:41 pmThat to looks an OK build thank you Ed