An excellent Frequency Meter Project

[top204]

I was just going through my favourite web site for the sake of nostalgia and I came across a project that shows how much simpler things have become since microcontrollers became available, and how much more boring things have become as well. :-)

See page 296 of the once wonderful "Practical Television" magazine from 1971, and you will see what I mean:

Practical Television - 1971-05.pdf

But I do like the idea of using some analogue meters to give the Ones, Tens, Hundreds and Thousands values for a microcontroller frequency meter using hardware or software PWM for the simple DACs required. Or using four small, inexpensive, colour LCDs to imitate analogue meters. I did a project about 12 year ago to imitate an analogue meter on a Samsung KS0108 (64x128) LCD, but now LCDs have advanced enough, and become inexpensive enough, to make them look like real meters, but you still cannot beat a "real" analogue meter for nostalic reasons. :-)

I made my first digital frequecy meter out of a load of 74 series TTL chips on a large piece of vero-board when I was a teenager, and the 5 Volt power supply to it had to be quite large because they were very power hungry devices and CMOS chips were too expensive at the time. I still remember having to save up for the prescaler chip because they were very expensive. i.e. A very fast divide by n chip, because TTL devices were only good up to approx 50MHz (if you were lucky). :-) Does anyone remember the, "almost magical", 7447 chip, or the even more "magical" 7441 chip if expensive nixie tubes were used instead of 7-segment LED displays?

I would have read that magazine when I was a small boy because dad used to buy it every month on his way back from work, and I'd wait for him to finish reading it in his chair beside the fire, then I would read it and "try" to understand what it had in it, and ask him lots and lots and lots of questions. LOL. He used to buy the "Practical Wireless" and "Practical Television" magazines for him and the "EveryDay Electronics" magazine for me, at a newsagents named "Stampsies". :-)

Wonderful times that we take for granted at the time, and sadly missed now.

[John Lawton]

Hi Les,

wow, I'm sure I bought that issue of Practical Television. The frequency meter was a nice project. The thing about analogue meters is that you can see the nature of any variations in the reading due to noise etc. Try doing that with digits.

In my teens, with a friend we were trying to make a computer, sadly we only had core planes and second hand flatpack gates and flipflops available so it was a huge task and in the end we failed. Sadly were were a few years too early, before the first micros came out.

John

[John Drew]

Hi Les and John,
That's all very high Tech.
My first frequency measuring device was a home made set of Lecher Lines.
Bang two short lengths of floor board onto the end of a 4 foot length of 3"×2". On top of each end vertical, hammer in 2 nails about an inch apart.
Remove the lacquer from the heaviest wire I could get out of an old trannie and string two parallel wires between the nails. One end being open, the other with a 150mm half loop - like a long U.
Next a razor blade was fastened to an old ruler - a handle for the shorting blade.
Frequency meter finished.

To use it a pea lamp was coupled into the final tank of the transmitter and the loop of the frequency meter was also coupled loosely into the TX tank circuit.
Now to measure. Starting from the far end slide the shorting blade along the line until there is a dip in the pea lamp brightness. Mark the spot. Then move the shorting bar further along the wires to find the second dip. Mark the second spot, measure the distance between the two spots. That is a half wave.
Say it was 541mm.
Wavelength = 521×2 = 1042mm = 1.042m.
Frequency = 300/1.042 = 288MHz which was the bottom edge of the 1metre amateur band in the late 50s.

Parts: pea lamp, 3 pieces of wood, 4 nails, a razor blade no longer needed by Dad, some re-purposed wire, an old foot ruler for the handle and Mum's tape measure.

Sorry about the mixed units but parts used Imperial (pre decimalisation in 1965) but Mum's tape had both Imperial and Metric - good old Mum.

Cheers from Oz
John

PS It was a requirement of my amateur radio licence I had to possess a frequency meter.
The Radio Inspector checked my station in 1958 and was satisfied I could check I was operating inside the band with my lecher lines.

[top204]

Wow John. That's a new one on me. :-)

It sounds similar to an early type of Grid Dip Meter that measures absorption?

I made a Grid Dip Meter when I was a young-un, after reading about them in the "Practical Wireless" magazine and asking dad about them, but it was a bugger to calibrate with all the different coils required and the variable capacitor values for the different frequencies, so I eventually gave up on it. :-) But that method you stated sounds like a form of radio trigonometry on a triangle. :-)

[HAL]

Hello

Wow! Lecher lines (lecher wires in the US), I have not heard that name for many years.  I am also a Radio Amateur since 1964.   I would also mention that John has The neatest most well laid out station that I have ever seen!  Things like grid dip meter or absorption wave meter.  Nowadays gate dip meter if they still make them....   Very much enjoyed your posts.  It's interesting to note that some of the old VHF/UHF technology (like parametric amplifiers) whose principles are now be used in fiber optic amplifiers.

Best regards HLD