Monday, 8 September 2014

DIY aluminium washers for cavity resonator

DIY aluminium washers for cavity resonator

In many cavity resonators, the input and output coils are rotatable to adjust the degree of coupling and/or transmission losses. Most mount the coaxial connector for the coil on a large washer, so it can rotate, then use screws to hold it in position. Sounds so easy, but where do you get the washers??

After wasting a few hours in off-line shopping at bolt and plumbing shops, I found that I would have to make them. Working with thin metal sheet is usually not easy. However, I devised a simple technique to quickly make them. Any metal that can be cut by a hole saw or step drill could be used, aluminium, brass, copper, even thin steel, (per my post on using galvanised steel buckets)

The photo shows the process.
  1. Using the hole saw, cut the first blank. Subsequent blanks are started with the sheet over the hole in the wood made by the first and drilling from underneath (hands away from hole), then cutting the blank from above.
  2. Use three "TEK" self-treading screws to hold the blank in place, and use a step (Christmas-tree) drill to cut a hole for the coaxial connector, taking care to get the correct diameter for the first. Subsequent ones use the hole in the wood as a guide.
  3. Remove finished washer and clean off burrs. For thin sheet, hand files are sufficient and don't damage the washer, unlike angle grinders or what ever is at hand!
The finished washer and rotatable connector is shown in place on the cavity resonator, along with a fixed connector and a spare washer. The two TEK screws holding the washer are loosened a little to rotate the coil inside. The second cylinder top shows the size of hole needed for the connector to rotate.

The step drill was used to make the holes for the other cavities in the duplexer, although with 6 mm plate, the holes need to be drilled from both sides.

The general form of the duplexer with just the first cavity working. I used just one cavity for experiments; more in another post.

Sunday, 7 September 2014

Cheap GPS-disciplined 10 MHz oscillator- preliminary-updated

Cheap GPS-disciplined 10 MHz oscillator- updated 5/10/2014

This post is for a cheap GPS-disciplined 10 MHz oscillator. It is a work in progress, while the links are still active; life is a work in progress...

I had earlier set up a Trimble reference, but they are a bit stone-age , but work well! Modern Trimble gear is too expensive for my purposes.

Update 5/10/2014:


I first came across the idea from I am not sure it can be accessed unless you belong to the Yahoo group. Edit: Via the sdr-radio-com group, a page on using the device:
Basically it uses three components, a GPS module, a USB interface for the module and the manufacturer's software to set it up:
The devices per the eBay seller's (goodlucksell) site (note: the four wires between the two devices are not direct and need to checked)
The software gives comprehensive GPS data and mapping as well. Useful as GPS receiver alone, with a serial NMEA stream. Interfaces with Google Earth.
The interface with no device connected:

From the NEO-7M documentation, the device normally outputs a 1 pps pulse (at the point shown in the screen snip), but can be programmed to output a harmonic-rich 10 MHz. Presumably once programed, is standalone oscillator. 
I have ordered the bits ($33 from China)  and had a quick look at the software and data sheets. All quite neat, simple and cheap. The thing should be accurate enough for most purposes.
A lot simpler, cheaper and compact compared to an old Trimble GPSDO, per my earlier post.
The only trouble is antenna needs to see the sky. It may possible to use a different external antenna. The other way I was thinking to get an inside GPS signal was to use an external GPS antenna and run coax to a little dipole in the ceiling above where I wanted the signal. I haven't tried it, but it should work.
To be updated in a couple of weeks when the bits arrive.

Update (late September)

Bits still not here.
Spirited discussion on, mainly about merits of different frequency references.
A site that has already setup the board for 10 MHz reference:
There is a NEO evaluation board with the NEO-7N, with TXCO, SMA antenna connector and built-in USB port for ~$82. It has a point marked "time" in photo.
Serendipitously, there is an article in the latest issue (Sept-Oct) of the ARRL QEX (just arrived by post in Australia!) on the "Calibration and monitoring of frequency standards-Phase method". It makes for good reading for those interested in frequency references. It does stress the difficulty in measuring accuracy of references.
Table 2, p14 is relevant to this discussion:

Relative Accuracy of Various Frequency References
Reference     Modification                  Accuracy Range
Crystal                                                  1 to 100 ppm
Crystal         TCXO                            ~ 0.1 ppm
Crystal         Ovenized OXCO             0.001 - 0.1 ppm
Crystal         Double Oven                   ~ 50 ppt (parts per trillion)
Crystal         GPS supervised OXCO  ~ 5 ppt
Rubidium                                            ~ 50 ppt
Rubidium     GPS supervised              ~ 5 ppt   
Cesium                                               0.01 to 0.1 ppt
Hydrogen Maser  Passive                  1 ppt
Hydrogen Maser  Active                      0.0007 ppt
ppt= parts per trillion, one part in 10(-12) = ns