All posts by Per Magnusson

WordPress gives 403 (forbidden) error for a post with “phpmyadmin” in the title

2016-04-16PCPer Magnusson

When I try to access a post with “phpmyadmin” in the title on my WordPress site, I get a 403 (forbidden) error. After digging around for a bit, I realized that the issue was in the permalink and that replacing “phpmyadmin” with “phpmyadm” made the post accessible again.

After some more thinking I figured out that the reason was that I had set up a rule in apache\conf\httpd.conf to deny access to phpmyadmin from most IP-addresses. The rule looks like this:

<Files "*phpmyadmin*">
  order deny,allow
  deny from all
  allow from 192.168.0.50
  allow from 127.0.0.1
</Files>

So, this rule inadvertently triggered on the permalink and denied access to the post.

Maybe someone else stumbles upon this problem and is able to google up this explanation and solution.

Changing PHP Timeout Setting

2016-04-16PCPer Magnusson

When e.g. updating WordPress, it often happens that the download takes longer than the default 30 second PHP timeout and so the upgrade cannot be completed. The workaround is to increase the timeout (temporarily), which is done by editing the max_execution_time variable in the php.ini file which is located in the php/ directory. 300 seconds is usually enough.

For this to have an effect, it is probably necessary to restart the Apache server.

And don’t forget to revert to 30 seconds once the WordPress upgrade (or whatever slow process) is done.

Tool for designing Butterworth and Chebyshev filters

2016-03-05Electronics, PCPer Magnusson

Designing passive LC-filters typically involves looking up prototype filter component values in a table in reference books like “Handbook of Filter Synthesis” by Zverev or “Design of Microwave Filters, Impedance-Matching Networks, and Coupling Structures” by Matthaei et. al., then transforming the values to produce a filter with the desired impedance and cut-off frequency. This can be a bit tedious and error prone, so when I found out about the classic Matthaei book, which is downloadable via the link above, and that it contains the math required to calculate the prototype filter component value tables, I set out to create a spreadsheet to help with the filter design.

In addition to calculating the (previously) magical prototype tables for Butterworth and Chebyshev (with user-specified pass-band ripple) filters, the spreadsheet also performs the frequency and impedance transformation for filters of orders from 1 to 10.

To make the design process even quicker and better, I added a feature to create LTSpice schematics of the selected filter so that the filter properties can be simulated (and perhaps manually adapted to standard component values and to include parasitics) using LTSpice. I used the SI prefix formatting function I wrote about in the previous blog post to write out the component values in a pretty manner.

The usage of the spreadsheet should be fairly self-explanatory, but there are also usage instructions on the first tab. Basically, the user should fill out the values in yellow cells and leave the rest alone. I did not lock any cells, since I often get annoyed by spreadsheets with locked cells and I encourage others to modify and improve it.

Here is a link to the Excel 2002 file:

[Excel] FilterSynthesis_v1.0.xls