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
AF4JF

Synthesizer with ADF4351

I have discovered interesting synthesizer modules on eBay:

I have also found an interesting project from Alain, F1CJN on the Internet.

This synthesizer can easily be controlled by Arduino Uno and a Shield with LCD Display:

Final result is here. A synthesizer that can be programmed for many stored frequencies. Following picture shows one of them:

I will upload more about this wonderful synthesizer shortly. Hoping to use it for a small local beacon for 23cm.

73 Herbert

UPDATE 2016-06-07:

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Nothing is perfect. Not even synthesizers. There is a serious problem with spectrum purity when used for frequencies above ~ 1.1GHz. See following pictures.

We need 960MHz reference signal for a Microsource YIG that generates 23GHz. Spectrum around 960MHz seems to be OK:

However, 1296MHz seems to have a problem with spurious signal only 25MHz away and only 39dB suppressed:

Workable solution seems to be to generate the beacon signal on 1/2 frequency and double it. That would require 648MHz. Following picture shows reasonably clean spectrum for that. You can see the 2nd harmonic on the spectrum analyzer already:

Looks like I have to work on frequency locking in my lab - frequencies shown on my RIGOL analyzer are little off the frequencies shown on the Arduino. Luckily, this synthesizer allows (wonder of all wonders) locking to a 10MHz reference!

(to be continued)

73 Herbert

AF4JF

UPDATE 2016-06-10:

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I have retrieved my old 23cm filter that I built long time ago with the idea of using Flex 1500 with IF between 28 and 50MHz in mind. This filter was made per W6PQL description and is described incl. pictures and frequency response in my other posting here.

Wonder of all wonders, it fixed the problem! The 1296MHz signal is a little weaker (6-7dBm) but all unwanted peaks seem to be suppressed more than 55dB:

In fact, I can't identify them in the noise floor anymore. Definitely good enough for a small local beacon! I will follow that idea soon ...

73 Herbert
AF4JF