Friday, June 10, 2016

Filters, filters and more filters

Exact, stable frequency is very important in ham radio. Especially if you intend to use digital modes, such as WSJT. I have built a GPS disciplined oscillator to generate 10MHz reference signal and used it in my 1296MHz transverter. It worked nicely ... until the moment when I connected a 40W power amplifier. Wow ... I intend to run 1500W for EME some day, so my GPS should withstand much more than 40W!

What is a solution? A filter, of course. The 23cm band uses 1296MHz while GPS uses 1575MHz. Should be easy to separate.

I have found an eBay seller named "iseeabluewhale" who sends packages signed "GPIO Labs" from Ontario, Canada. This seller offers reasonably cheap filters. One of them is a bandpass for 1575MHz (GPS frequency L1):


I have measured the frequency response. Looks quite good, 1296MHz suppressed nearly 60dB:

Time to test the GPSDO with this filter again. I will post the results later.

Second filter that I needed was 432MHz bandpass between my new 10GHz and 24GHz transverters and IF radio. One of them will output IF on- and below 432MHz, the other above 432MHz. Luckily, GPIO Labs make both filters, see the pictures:

Nice filters! It would be nice to find something like that for the 1296MHz frequency. There was one in their eBay listing, but the bandwidth is too narrow for the weak signal portion of the 23cm band:


What can we do? Obviously - get a filter elsewhere. Maybe make it ourself! For example using this design from W6PQL. I have made one of these filters (very easy to build) and here is the result:


Yes, this filter is quite good. Definitely cheap and easy to make. And the suppression of unwanted signals goes below 80dB!

I have recently used that filter to suppress unwanted spurious signals from ADF4351 synthesizer in the posting below. Quite a game changer there!

73 Herbert

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