The Beehive

[DrStrangelove]

I think only critical for precision analogue 'scopes?

Very probably. It's likely to have come from something about scope magnetic shields in some magazine or other decades ago.

It's odd the irrelevant stuff one remembers.

Then again I can barely remember what I had for lunch yesterday.

[Mike Watterson]

Maybe you had mince pies?

[BusyBee]

Well I've finally got back to things. It was not the best of New Years with me but I have been keeping going, at a much reduced rate.

I have now built 1j24b and 1j29b Russian rod valve audio amplifiers. I gave them basic tests and determined that I was getting amplification. The resulting construction, although sound, ended up being a little untidy. I can see that the method would work well, with modification, for higher frequency work and Manhattan style / surface mount but I think it unlikely that I will continue this approach at audio / MW / LW. Next I will test the amplifiers with my audio amplifier. See photograph.

Another (sub)project I have been thinking of for some time is to use an Arduino with LCD display for frequency / wavelength display with a superhet receiver (effectively oscillator frequency). I did a test and determined that it was accurate enough. I used software readily available online. This works to good accuracy up to around 4MHz and to some extent higher (bear in mind the clock frequency is 16MHz so the limit will be well below that). Shown here is an input amplifier I built. The input is protected by a diode ladder rather than zeners. The main reasons for that is that I don't have any surface mount zeners but do have more than enough diodes. The actual amplifier is from a FET applications datasheet but is running well below optimum with a 5V supply. It did appear usable on simulation and initial tests are promising.

I have also designed a 6.3V to 5V regulator such that I can make use of my valve heater supply (I use DC) rather than have to use a separate supply. On simulation it appears to work well so next comes construction.

       

[BusyBee]

I have been busy with a lot of other things lately, not all desirable, but I have finally got a working frequency / wavelength display unit, as mentioned in my last post.  I have used an Arduino Uno along with the input amplifier, mentioned above, and a 74LS74 double flip flop chip to provide a divide by four function. This enables the unit's range to extend well beyond 10MHz which is all I intend at the moment. It is fairly easy to change a module if required though.

For the programming I used software which is available online via this link: https://www.gammon.com.au/timers . All I have done is add functionality for calculation and modification of frequency data and for the LCD display. As such I won't show my code as other implementations will require similar, but possibly different, adaptation.

The photos shown give output using a precision input source (crystal oscillator with oven) although it's not been calibrated except as it was supplied (Chinese via Ebay). They show results plus internal layout.

1: The unit with an early version of the program which shows frequency input in Hz and the correct frequency (I/P x 4). I do notice and error of 3Hz but this may be rounding (check needed but not too much of an issue).

2: This shows the display using version two of the program which rounds output to KHz. Wavelength is calculated not using the usual approximation (300) and may be rounded lower or be more correct but needs checking. I do feel that wavelength is probably only required with radio tuning though so the displayed information may well be changed. Early days yet!

3: The display when there is no input. When the oscillator frequency is too low to be sensible for radio the frequency is still shown but all others show N/A. I may change the display deleting the oscillator wavelength and show oscillator frequency in HZ as well as kHz such that lower frequencies below 1KHz can be displayed. For normal radio tuning though 1kHz steps are gentler.

4: The internal layout.

               

[ppppenguin]

I wonder if the OCXO might be as much as 3Hz out. That's a fairly large error for a decent 10MHz OCXO but this may not be a very good one or if it's adjustable 3Hz would be well within the normal range.

[BusyBee]

No, I am referring to the first photo (with initial program) which shows the measured input, which is the true frequency divided by four because of the flip flops, and the resultant when multiplied by four by the Arduino. 2500047 x 4 = 10000188 whereas the Arduino shows 10000191.  This is obviously a calculation error which I only noticed when I looked at the photos. As I said, probably not a problem in this application.

I have no easy way of checking the crystal oscillator. I imagine the indicated error is more the Arduino although it appears to have a crystal clock rather than the ceramic of earlier models. Just or reference, I bought two of these boards. Some time back when I was checking possibilities I compared the two boards with the same input (1 or 2 MHz crystal oscillator). There was about a 50Hz difference between them.

For the moment I have taken the offset from 10MHz (191 Hz in this case) and used it as an error adjusted for in calculation, although it's low enough not to show in 1kHz steps anyway.

As you will know, there is a lot of information needed to properly explain even a simple project like this which I didn't get into a short explanation. Much of it is in my mind, not on paper anyway, which is never ideal, particularly for follow on work at a later date. Perhaps a good use of a forum to promp things