Sunday, June 23, 2013
Pipe cap filters for 10GHz
In my case the problem was the metallic enclosure used for the transverter. With the cover removed everything was fine but once the cover was put in its place, the local oscillator was present at the output at nearly 5mW of power: Just unacceptable.
The problem was the cover acts as a mirror, making the radiation from the stripline filter at the LO multiplier to be reflected towards the strip line filter in the TX converter, both working at the same frequency: 9936 MHz. As you can see, unshielded striplines are not a good idea, as they acts as antennas.
My reference was this article from W1GHZ. After reading it I have a clear idea of how a pipe cap filter works. W1GHZ uses 1/2" pipe caps for 10 GHz , so I bought some 12mm pipe caps, as they are the closest ones to 1/2"
But soon I discovered I was wrong. The 1/2" refers to the internal diameter, and my pipe caps have 12mm of external diameter. They were smaller than the one used for 10 GHz, so I ended with some 16mm pipe caps, who have 14mm of internal diameter. The rest of the dimensions were the ones at W1GHZ's article.
Because I need high selectivity in the filter I decided to use two pipecaps in series. This is not a recommended option, because both filters adjustment interacts between them. Anyway, because a 10GHz transverter is almost a single frequency device (10368.2 MHz) I decided to go ahead with this option.
A larger probe will produce low loses, but also wider bandwidth. Shorter probes produce higher loses, but narrower bandwidths. Because my Eyal Gal module has a lot of gain, I was able to sacrifice loses to get a sharper filter, so finally I cut the probes at 2.8mm, this is 0.3mm less than 1/8", the minimum stated at W1GHZ's article.
Once the filter is finished, it's time to tune it. In a pipe cap filter, the screw acts as a quarter wave length antenna that couples energy between probes. So, to peak a filter at 10368.2 MHz and not at 9936 MHz, it is best to start with the screw all outside the cap and turn it down (into) the cap.
Then, I began to turn the screws slowly down, until I saw the first response at 10368 MHz. Then it was easy: Just to peak to maximum signal while alternatively playing with both screws. It was very easy and the peak was easily identifiable. Impossible to make a mistake. Then I adjusted the transverter's power amplifier gain to get again 1 watt output. Only a slightly readjustment was necessary.
What about the local oscillator leakage? Well, it went from about 5mW (+7dBm) down to nearly -40 dBm, producing a total attenuation of almost 70dBc, pretty nice!
Pipe caps filters are easy to construct with readily available (and cheap) materials. They can have significant in line losses, but if you have gain margin (or can add some MMICs to your project) they are a nice solution.