Contents :

A collection of thoughts, and my notes about experiments and ideas, technical or otherwise, connected to Amateur Radio, Satellite working and monitoring and other electronics.

Saturday, June 19, 2010

Crystals and oscillators . . .

For some time I just had a few boxes full of crystals and oscillators- some found at ebay, others at rallys etc - not very practical when trying to find a particular one, or know which ones I have.

So the last week I have sorted out a bit, so that crystals with standard values (and in large quantity) have been separated into their own drawers. Others have been grouped in similar frequencies. some are still left unsorted, but that will have to wait.

Now to make a list of what I really have.

The idea is to write a small program for calculating possible uses of the crystals I have - for simple single frequency (or vxo controlled) receiver or transmitter circuits. After all, not all TX/RXs need to be DDS controlled. Yes - I know that there are lists out there with frequency combinations from standard crystals, but I have some additional frequencies in my stock, so will have to figure those out myself.

At least I got a *little* bit more order in my very messy shack ;) - even if I could use some more sorting out - that will have to wait.

DDS-60 has arrived

Just received a DDS-60 "daughter-board".

Now will be the time to look into getting the DDS programmed, there is no controller yet.

For very simple stuff a PIC or PICAXE processor programmed in BASIC will probably do. For more functionality (and more memories) I should probably go for a more advanced processor. IFor that I consider the Arduino project, using AVR ATMEGA processors. It uses a variation of C (with a limited functionality and special commands for controlling I/O pins), something I have not yet done anything with, but is it ever too late to learn ?

Arduino is an open source project, so even the bootloader can be loaded on to a processor without one, unlike the PICAXE bootloader which is proprietary. I now have an Arduino board and a few processors on the way.

More to think about, more to try out ...

This summer I also have a large reading project going on, so a bit less radio building activity is expected.

Sunday, June 13, 2010

Enter the DDS

Just received a kit, the DDS-2 from N3ZI.

The kit contains all parts, including a simple display, for making a functional DDS.

The kit is relatively easy to assemble, all parts were present. The SMD integrated DDS chip is somewhat more tricky to solder without creating solder bridges. I think the pads are a bit wider than the pads for the NE612 SMD mixer on the Rockmite/Warbler kits from Small Wonder Labs.

First the bad news : Tuning the DDS-2, as it is, is an exercise in patience. it is slow, when trying to use the fast function (turning the knob faster), the tuning becomes erratic. The lack of a keyboard entry function is serious with such a slow tuning.

It looks like a modification of the firmware should be attempted. Quite a daunting task, given that (as far as I know) the source code is not available, and my programming skills are not that good.

The good news is that the temperature stability looks fine. the crude "switching on and off the fan" method revealed about 20 - 25Hz drift on 10 MHz, of course relatively less on lower frequencies. For a non- temperature compensated clock oscillator this feels quite good, and I would expect better stability when the DDS is boxed.

Other good new on the DDS-2 : even if tuning is a bit of a pain, there are two redeeming qualities :

- the standard firmware has 10 "memories", acting like the "Band stacking" feature of many modern transceivers. so a multi frequency MEPT with 10 (11) pre-programmed frequencies is quite doable

- the application of the RIT should be quite useful for generating FSK easily, eg for a relatively frequency agile MEPT.

However, for more experiments I did order a DDS60 daughterboard that should be programmable from a PC or an external (PIC, PICAXE etc) microprocessor board.

Sunday, June 6, 2010

More RM30 info

The RM30 was tested a bit this week end before the shack update, and the results look encouraging and disappointing.

The sensitivity of the RM30 looks really good, noise increases more than 30dB when a piece ow wire (in the shack) is connected, indicating sufficient gain for a grabber.

The frequency response shows the unmodified RM30 to have a slightly too low peak frequency, about 1kHz, so next test will be a (variable) capacitor in series with the input crystal, the main problem is mounting it on the PCB.

The frequency stability, however is not nearly as good as I would want. I may have to get some of the clip-on heaters that DB6NT has for sale, but initially the RM30 will be boxed and thermally insulated.

Update :

47pF in series with the input filter crystal brings the peak (rather broad) up to 10140.5 , this is quite acceptable since the bandwidth of the filter is about 1 kHz in the RM-30 setup.

Shack update

Some time ago I moved the shack from the "attic" position, especially because it gets extremely hot up the in the summer and very cold in the winter. Also the mess there is "rather visible".

Today I was digging out some of the test equipment from upstairs, so the shack downstairs is now sort of usable. The soldering iron was , of course there already, but now I added my Lab power supply, oscilloscope, frequency counter is now set up, as weel as an old "Bolometer" for RF power measurements. oh yes, a step attenuator from 1 - 110dB is not bad to have in the shack - and an ancient AF signal generator. An old digital multimeter running off mains seems to be out of service at the moment, but a portable one will have to do for now.

The RF signal generator, running from 10 - 500 MHz is not that good for SSB/CW measurements, but will have to do for now. It seems I should build a DDS or synthesized RF generator and just use the external attenuator.

And finally the heavy equipment : a spectrum analyzer, yes old surplus, but looks like in working order still, all will now have to be powered up some time every day to make sure it does not burn out.