Frequency measuring on FM transmitter

Thread Starter

Bobes

Joined Mar 19, 2020
51
Hello. I recently bought this frequency meter: https://www.ebay.com/itm/NEW-1MHz-1-1-GHz-Frequency-Counter-Tester-Digital-LED-Meter-For-Ham-Radio/254274513017?hash=item3b33f12879:g:DVMAAOSwP2ddEODw. But it looks like, that I cannot use it . For testing it, I built an LC oscillator circuit - a simple FM transmitter (picture in the attachment). But I connected testing probes everywhere, where I thought, that the frequency of around 100mhz could be, e.g. ground to collector, to emitter... But I couldn't get the frequency displayed. So I wanted to ask you for a help, how to measure the frequency of this circuit with this frequency meter (or if is it possible). Thank you for your answers!
 

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Papabravo

Joined Feb 24, 2006
13,741
Aside from the inability to measure anything with the frequency meter does the transmitter work? Where did you get the circuit? Can you tell us anything about how you think it might work? The reason I ask these questions is that the circuit looks more than a little bit strange.
 

seanstevens

Joined Sep 22, 2009
71
Papabravo is right, these types of basic unbuffered oscillators generally stop when you connect a load to it, in this case your frequency meter probe. As asked before, does the circuit operate i.e. you can monitor it on an FM radio? If it does but when connected to freq meter it stops then I would suggest connecting a low PF capacitor - maybe one of the following caps 22/47/56/100PF connected to emitter side and connect the freq mete probe to the other side of this cap. If the signal is too weak or higher input is needed to the freq meter and only if you can monitor on FM radio, then try connecting this cap to the collector of the transistor.
Whatever happens, when you connect a freq meter to this kind of unbuffered circuit, the frequency will change for the period of probing but at least it gives you an indication.
 

Audioguru again

Joined Oct 21, 2019
1,503
The PNP transistor has no part number.
The electret mic probably does not draw enough current to turn on the transistor. My FM transmitter works fine and has a separate resistor for the mic and another resistor to bias the transistor with a coupling capacitor in between.
C2 in my FM transmitter is only 4.7pF.
Your inductor has no parallel capacitor (mine uses a 5pF to 35pF trimmer).
My FM transmitter has an RF amplifier buffer between the oscillator and the antenna.
my FM transmitter has a 5V low dropout voltage regulator for the mic preamp and the oscillator.
My FM transmitter does not have a capacitor to ground at its antenna.
It will not work if you build it on a solderless breadboard. My FM transmitter is soldered on a compact stripboard layout.

Can you hear your FM transmitter on an FM radio? Mine sounds great because it has the same pre-emphasis circuit used by FM radio stations. Yours will sound muffled with no high audio frequencies.
 

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Thread Starter

Bobes

Joined Mar 19, 2020
51
Rh
The PNP transistor has no part number.
The electret mic probably does not draw enough current to turn on the transistor. My FM transmitter works fine and has a separate resistor for the mic and another resistor to bias the transistor with a coupling capacitor in between.
C2 in my FM transmitter is only 4.7pF.
Your inductor has no parallel capacitor (mine uses a 5pF to 35pF trimmer).
My FM transmitter has an RF amplifier buffer between the oscillator and the antenna.
my FM transmitter has a 5V low dropout voltage regulator for the mic preamp and the oscillator.
My FM transmitter does not have a capacitor to ground at its antenna.
It will not work if you build it on a solderless breadboard. My FM transmitter is soldered on a compact stripboard layout.

Can you hear your FM transmitter on an FM radio? Mine sounds great because it has the same pre-emphasis circuit used by FM radio stations. Yours will sound muffled with no high audio frequencies.
Thank you for your circuit, I think, your is better. Npn transistor are generally better in the transmitters.
 

Audioguru again

Joined Oct 21, 2019
1,503
Years ago, a person posted your second circuit with the same NPN transistor and complained that it did not work. So I built it and fixed every thing that was wrong with it.
 

seanstevens

Joined Sep 22, 2009
71
Audioguru again - slightly off subject, could you tell me what kind of ranges do you get with your buffered TX. Obviously depends on the antenna but I am curious how far you get with buffered and a 9v and a 1/4 wave ant.
 

Thread Starter

Bobes

Joined Mar 19, 2020
51
Audioguru again - slightly off subject, could you tell me what kind of ranges do you get with your buffered TX. Obviously depends on the antenna but I am curious how far you get with buffered and a 9v and a 1/4 wave ant.
Normally, with nothing connected, it's around 8MHz, when connect some small signal it shows 0Hz, with RC transistor oscillator it show good frequencies, up to 20MHz, then the oscillator stops oscillating, so it certainly works, but when I measure LC circuits, I got 0Hz (probably too small signal).
 

Audioguru again

Joined Oct 21, 2019
1,503
Audioguru again - slightly off subject, could you tell me what kind of ranges do you get with your buffered TX. Obviously depends on the antenna but I am curious how far you get with buffered and a 9v and a 1/4 wave ant.
The range of my FM transmitter is affected immensely by the sensitivity of the FM radio. Its range is across the street to a very cheap one-IC FM radio from the Dollar Store. Down the street to a cheap clock radio or 2km to a hifi FM tuner or a good quality car radio.
I live near a huge river. Its valley is about 2km across. I had my FM transmitter on one side of the valley playing a recording and my hifi and car radio picked it up on the other side.

WARNING: My FM transmitter is illegal because it can over-ride radio stations. A guy who built it saw the radio cops with an antenna on their van driving down his street looking for who caused the interference. But I think they were looking for me.
My simulated output power is 96mW. Tweaked, it produces 280mW. 10mw is legal if you do not produce interference to a radio station.
The Micromitter kit has an RF attenuator on its output so that its range is limited to about 10m.
 

seanstevens

Joined Sep 22, 2009
71
The range of my FM transmitter is affected immensely by the sensitivity of the FM radio. Its range is across the street to a very cheap one-IC FM radio from the Dollar Store. Down the street to a cheap clock radio or 2km to a hifi FM tuner or a good quality car radio.
I live near a huge river. Its valley is about 2km across. I had my FM transmitter on one side of the valley playing a recording and my hifi and car radio picked it up on the other side.

WARNING: My FM transmitter is illegal because it can over-ride radio stations. A guy who built it saw the radio cops with an antenna on their van driving down his street looking for who caused the interference. But I think they were looking for me.
My simulated output power is 96mW. Tweaked, it produces 280mW. 10mw is legal if you do not produce interference to a radio station.
The Micromitter kit has an RF attenuator on its output so that its range is limited to about 10m.
Excellent. Can get better results from rf transistors but as you say its illegal.
 
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