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reasons why frequency shows difference in three devices

daisyyyyyy

Nov 26, 2016
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Hi, I had build a transmitter. I check its transmission frequency at antenna using my smartphone's radio and it shows 97MHz. However, when I test again using oscilloscope, it shows 50MHz and another 107MHz in spectrum analyser. Using the same song as input, but why 3 different results?
 

davenn

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Sep 5, 2009
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what is the frequency supposed to be ?
is it a crystal or PLL controlled oscillator ?
if it's a free running osc. it could be all over the place

if the oscillator is full of harmonics ... an extremely bad thing for a transmitter ....... then test equip could easily lock onto the strongest harmonic
say a frequency counter will do that

using my smartphone's radio and it shows 97MHz.

by no means reliable

using oscilloscope, it shows 50MHz

what sort of scope ... make / model ?
scopes are not the best at showing frequency

107MHz in spectrum analyser.

Spec analysers are the best way. They will show you the main carrier freq and also any harmonics and their relative strengths


so show us a schematic of this transmitter and a photo or 2 of the construction


Dave
 

daisyyyyyy

Nov 26, 2016
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transmitter v3upload.jpg transmitter v2upload.jpg transmitter v4upload.jpg
The first and second is my transmittor, the last is the result of spectrum analyser. The oscilloscope will be tds-2012b.
 

davenn

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The first and second is my transmitter, the last is the result of spectrum analyser. The oscilloscope will be tds-2012b.

transmitter v3upload1.jpg

OK ..... you cannot build RF circuits like this and expect them to work properly. It will be dreadfully unstable
all those big long loops of the jumper wires will be causing a nightmare of radiated RF that will affect other parts of the circuit
they will also totally mess up all your inductance values and the oscillator will never operate at the freq requires.
Basically ... DONT use breadboards for RF
Component leads should as short as possible. This reduces stray capacitance and inductance

transmitter v2upload2.jpg

have you actually simulated the circuit to see if it operates ? ( Im not sure if it is drawn in a simulation package or not ??)

OK you have a free-running oscillator, it's going to drift all over the place with temperature, waving your hand nearby and all the other things I mentioned about the construction

just a gentle nudge in the right direction when drawing circuits, tray to learn to draw circuits a little tidier ...
that is .... avoid all those multiple connections/lines you have around the collector of Q2

you could have it looking more like this ......

transmitter v2upload1.jpg

see how much cleaner it looks ? ... makes it easier to diagnose too :)


transmitter v4upload1.jpg


nice spec an, wouldn't mind one that went to 6GHz .... my Tek 2715 only goes to around 1800 MHz (1.8GHz)

I would really like one that went to 12GHz as I do a lot of stuff on 2.4, 5.6 and 10.3 GHz ... maybe one day


OK ... are ALL those signal blips to the left and right of the blue vertical "M" line signals from your transmitter ?


Dave
 
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Audioguru

Sep 24, 2016
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I think it is Multisim (<text>, <text>, <text>) that makes wires on a schematic go every which way instead of straight to where they need to go. The schematic is also hard to read because it was saved as a fuzzy JPG instead of as a very clear PNG file type and it has that awful dark background and grid blocking all the text.

The solderless breadboard causes trouble with high frequency circuits because the many strips of contacts and wires all over the place have capacitance between them and are antennas picking up all kinds of interference.
Even the mic preamp picks up the radio waves from the transmitter. Use a compact layout soldered on a pcb or on stripboard instead.

Also it is likely that the smartphone's radio and the spectrum analyser are overloaded by the strong nearby signal.
My similar FM transmitter is smeared all across the dial on a cheap FM radio.
 

shrtrnd

Jan 15, 2010
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And use some shielding when you're trying to make measurements. Besides what you're picking-up from your own
components, you're picking-up every outside radiated signal in the immediate area.
 

daisyyyyyy

Nov 26, 2016
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@davenn @Audioguru @shrtrnd
okay, I will try your recommendation. Maybe my last choice is making a PCB rather than breadbroad...
About the schematic design, I never know I will show it to someone, so I just connect it as long as it works. However, thanks for your advice, I will practice it in my next work.
haha, thanks but the spectrum analyser belongs to my friend... I'm not confirm about the source of the blips but I can heard some dull sound at the range 97~98M and 107~108M using mini radio. Are them the blips? Or the blips are some disturbance from the environment?
 

Audioguru

Sep 24, 2016
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I fixed a circuit that did not work and I soldered it compactly on stripboard. It has a 5V low dropout voltage regulator for the mic preamp and RF oscillator and it has pre-emphasis like all FM radio stations have so it sounds great. Its range to my very sensitive hifi stereo and car radio is very far but is across the street to a cheap radio. Here it is:
 

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daisyyyyyy

Nov 26, 2016
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I fixed a circuit that did not work and I soldered it compactly on stripboard. It has a 5V low dropout voltage regulator for the mic preamp and RF oscillator and it has pre-emphasis like all FM radio stations have so it sounds great. Its range to my very sensitive hifi stereo and car radio is very far but is across the street to a cheap radio. Here it is:

your circuit seem complex but look tidier than mine...nice work!!
honestly, i cant understand the circuit completely except those transistors...
may i know where the voltage regulator is? Are r8 and c12 give the effect of pre-emphasis?
 

Audioguru

Sep 24, 2016
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may i know where the voltage regulator is? Are r8 and c12 give the effect of pre-emphasis?
C4 bypasses R5 at high audio frequencies boosting them for pre-emphasis. C4 changes value in different parts of the world because the pre-emphasis is different.

As Davenn said, R8 biases the RF amplifier transistor Q3 and C12 couples the RF, they have nothing to do with audio.
My assembly is small with low capacitance between its wires, your assembly is huge with inductance and capacitance all over the place.
Here is a photo with my parts marked:
 

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BobK

Jan 5, 2010
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Very nice!

Question for AK:

I have never quite understood how simply coupling audio to the base of the oscillator produces frequency modulation. Is it because the base emitter (or base collector) diode changes capacitance with voltage like a varactor?

Bob
 

Audioguru

Sep 24, 2016
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Yes, the audio modulation changes the transistor's capacitance up and down producing FM. The modulation also causes AM but an FM radio ignores it.
 
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