Need help in selecting crystal for VHF front end

[mbarazeen]

I have a wireless microphone system with dual mic with 259 and 234 MHz. but the wirless microphone for 234 MHz is missing.
i have another 200.3 MHz microphone that i wish to tune it to the exisiting 234 MHz tuner.
so i opened the receiver and checked, the front end FM demodulator is BA4424 with crystal oscilator.

i found the following crytels are in use for both channels.

259MHz - 62.100
234MHz - 52.975

to confirm i can tune this one, i interchanged both crystal and by the help of a ossciloscope (IF out) i coudl tune the other tuner to 259MHz.

now what i want to know is how to select a right crystal for 200.3 transmitter? i tried to figure out from above two values, but confusing how it comes.

Thanks

 

[Nigel Goodwin]

You can't make any sense of it because the output frequencies aren't correct - they have been truncated to whole numbers.

 

[ronsimpson]

259MHz - 62.100
234MHz - 52.975

I do not understand your numbers.

A crystal can run in over tone mode where, in this case, you get 4x the frequency marked on the part.
62.100x4=248.4
Add the IF frequency of 10.7mhz.
248.4+10.7=259.1

Your second set of numbers do not work out right so I don't know.

 

[mbarazeen]

I do not understand your numbers.

A crystal can run in over tone mode where, in this case, you get 4x the frequency marked on the part.
62.100x4=248.4
Add the IF frequency of 10.7mhz.
248.4+10.7=259.1

Your second set of numbers do not work out right so I don't know.

Yes you are right about Fx4 mode and adding IF gives nearly 259. as you said 2nd set seems not correct and this is where i confused. the frequency is marked on the unit to select wireless transmitters as required. so frequencies that are marked is 259 and 234, when i checked the crystals it has markings as i mentioned.

so may it work if i select a crystal of (200.3-10.7)/4 = 47.4 MHz?

Thanks

 

[languer]

The way I read it; you want to use the 200.3MHz microphone with the 234MHz receiver - by changing the 234MHz receiver. If this is the case; the only way I see this working out with the numbers you provided is if the 234MHz uses a 21.4MHz IF and not a 10.7MHz IF. The 21.4MHz IF, although not as common as the 10.7MHz IF is still pretty common. To check this you have to look at the markings of the IF filter on the 234MHz receiver - it should be either a ceramic filter or crystal one.

 

[mbarazeen]

The way I read it; you want to use the 200.3MHz microphone with the 234MHz receiver - by changing the 234MHz receiver. If this is the case; the only way I see this working out with the numbers you provided is if the 234MHz uses a 21.4MHz IF and not a 10.7MHz IF. The 21.4MHz IF, although not as common as the 10.7MHz IF is still pretty common. To check this you have to look at the markings of the IF filter on the 234MHz receiver - it should be either a ceramic filter or crystal one.

Both use same IF 10.7MHz, as i mentioned i had a trial to tune 259 MHz by ONLY changing the crystal to 234 receiver. it worked fine.
may be the marking of chanel frequency is wrong, from the time i got it, the transmitter was missing to that frequency and not tested.

Thanks