A frequency counter

[AmokCrow]

I've been fixing (or trying to get to work in the first place) my transmitter, a Multiplex PicoLine. Fresh out of the box it was way out of tune and had an output of two volts, measured with an oscilloscope. I managed to get more output by tuning it, but I fear it might have affected the FM-coding frequencies. As I don't want to drop planes out of the sky, I'll have to get the frequencies right. BUT, a frequency meter costs more than a new tx.

I finally decided to do the counter myself. In theory it shouldn't be too hard. Some 4026 decade counters (with 7-segment output :) ) in a series after a suitable freq-divider (of at least 1/100), timed with something crystal-based to make sure it's stable.

Just, has anybody done something like this before? I'd like to hear some opinions from those more experienced than me. Especially any common errors that might get costly ICs trashed would be good to know beforehand.

Thanks in advance

-Michael

EDIT: Just to make it clear, I'm asking tips for a freq-counter, not for tweaking the transmitter.

[Acetronics]

Hi, Michael

There's only one solution left for you ... send your set to the after-sales service to be tuned again.

What you have done is only UN tune it ... it is not so simple !!!

Stop the disaster here ... and never re-open the "black-box" !!!

Alain

[AmokCrow]

I wish it was that simple, but I somehow managed to get it out of garantee by losing my receipt.
I'm not doing this completely blindly either. I do have a friend who is a professional radio engineer and he will see to that the tx will comply with the laws and standards when I finally get it working.
The only thing I'm missing is the equipment to measure frequencies and I'm only asking if someone has made such equipment before. Do note that a freq-counter can be used for a lot of other applications also.

I'm not stupid and will do no flying whatsoever with the tx if I have any doupt that it could cause any kind of interference to others. That's why I got the engineer friend in this: he makes sure that I don't mess up.

-Michael

[Daniel7866]

Hi Michael,

Here is a thread for a home made freq. counter http://www.rcgroups.com/forums/showthread.php?t=499807 .

Daniel

[vintage1]

Well years ago I was involved with the design of a VCO frequency locked FM radio..

..the biggest problem was getting it down from 110Mhz to something the Schottky TTL would cope with.


Then it was an easy nmatter of comaring it with a frequency we set up n a crystal osciallatoir..which of course we set up using a frequency counter...:D :D :D

In short mate, no way out..you have to calibrate your own freq meter even if you DO build one. Against a fquency counter of known quality.

Best is to hire one for the duration...or buy one ..unless you think you can set up an oscillator to around 100hz in 72Mhz...about one part per million..

Actually, unless you have tweaked the FM VCO on the transmitter..usually a variable capacitor somehere in the far end of the RF module,tuning the output coils to get a better output will NOT have affected basic frequency of the VCO.

If youi have a reciever of known good operatuoion with a branded Xtal in it, you could hazard sticking an audio input on to the FM demodulated output - pin 9 on most chips - and tuning te transmitter for best output.,..using the old 'its fading there..and there..i'll set it halfway between' technique.

Of course if you HAVE been fiddling with the outout tuned circuit you REALLY need a $5000 spectrum analyser to make sue the sidebands are in spec..

Shouldn't cost you more than $6000 to get your $100 transmitter set up correctly, in all..;)

[AmokCrow]

Thanks for the link and info.

And yes, I read somewhere else too on this site that the variable capacitor is the one you fine-tune the channel with -and so I haven't touched it yet.
Tuning for increased output I actually did with the 'turn till the oscilloscope-curve starts to get narrower' method you described. :) ...If I understood correctly.

But in order to check whether the actual frequency is what it should, I'll need to separate the logic pulse input that's going to the "diode-capacitor -what-ever-its-correct-english-name-was", then tune one state at a time the two states of antenna output to the spec. frequency.

And yes, I know I'll have to calibrate the meter against a good meter OR a known accurate frequency, but that is a small task.

And after all the blood, sweat and tears I'll be left with a good freq-meter. "I know it's good because I built it", like they say.

-Michael

EDIT: And now that I look carefully in the tx I think one transistor is a little too warm. Could the original problems with output be caused by a bad transistor?

[AndyOne]

The radio-ham method of calibrating an RF oscillator is to use a radio receiver tuned to one of the known frequency standards transmitted around the world then derive a harmonic or sub-harmonic of the oscillator to be calibrated at a suitable low level and let the receiver generate a beat frequency. The oscillator is tuned until the beat is as near zero Hz as possible and job done.
You will need a receiver for this one and some know-how but I guess your friend could help here.

All this may be a bit academic if you have just detuned the RF amplifier stage which doesn't effect the frequency generating circuit, it is possible to retune an RF amplifier to get more power out but the power may be on the wrong frequency or frequiencies. The best way to set it up again is to use a spectrum analyser as this will show exactly where in the band all the power is concentrated.

If it was my transmitter I would send it back for servicing guarantee or no.

Andy.

[armindilo]

Does your radio engineer friend not have a f counter? Or possibly his employer?

[dalbert02]

On a side note, I just bought this MFJ model which looks like a BK Precission with a different label. They seem pretty cheap, do you think they are 'good enough' for this type of work? A friend of mine that is an EE says the BK is the same quality as Fluke.
Thanks,
-dave

[Acetronics]

Hi, Dalbert

Prices here ... http://www.mfjenterprises.com/products.php?prodid=MFJ-888

From Amok's posts ... do you think it enters his budget ??? :D

Alain

[dalbert02]

Well, yes, the 888 is a little expensive at nearly $200, but the 886 is just over $100 at http://www.universal-radio.com/catalog/meters/0815.html where I bought mine.
-dave

[Acetronics]

Hi, Dalbert


From Amok's posts ... do you think it enters his budget ??? :D

Alain

Hi, Dalbert

As the servicing bill at MPX After sales is too much for our friend :eek: ... and the freq counter costs more than the R/C set :eek: :eek: ...

I do not see any convenient solution !!! :D

Alain

[AmokCrow]

I never said the after market repair bill would be too big for my budget. There just aren't any certified repair shops in Finland that I know of. :rolleyes: If I did I wouldn't be doing this myself.
(And I'm NOT going to send the thing to Germany either.)

The $100 device I could propably afford, my budget isn't that tight. Only the cheapest I found at the local stores was $300.


-Michael

[Acetronics]

There just aren't any certified repair shops in Finland that I know of. :rolleyes: If I did I wouldn't be doing this myself.

-Michael

A Mail to Multiplex Home about the nearest servicing should be the best starting act ... don't you think ??? ;)

Alain

[Zlatko]

A while back I saw a link to a $60 US frequency counter kit on the Microstar forum.

I've searched for it but can't seem to find it again :(

Perhaps someone else from there can help ? ( ZAGNUT ? )

Cheers

BTW, this had a LCD screen with crystal control. I think the acuracy was under 10ppm up to 100MHz .....