Author Topic: why does this FM transmitter perform so poorley?  (Read 459 times)

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Offline yada

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why does this FM transmitter perform so poorley?
« on: April 21, 2017, 07:54:46 AM »
So I built this circuit:


Using a SS9018FBU NPN RF transistor, exactly like the schematic but with a few modifications: Instead of the 4.7pf there are three 10pf in parallel. Instead of a microphone I have a 1k resistor and the inductor is 7 turns of 22ga magnet wire 5mm dia X 7mm long (have no idea what the value is but this is critical to the circuit. Putting metal pin in it can double the signal and completely stop it just by moving it 1/2 in to 34 way in.

As you can see from the water fall it makes five signals and they move back and fourth. The bandwidth is huge and the power supply says it pulls 14ma. Trying different transistors that I listed in the other thread makes no difference. I built it using as few wires as possible. I know a ground plane helps but all I have is perf board, so I was going to try building on that and see if this thing "calms down". Is the microphone substitute resistor the wrong size? I have no idea why this thing acts like this.
« Last Edit: April 21, 2017, 07:57:41 AM by yada »
 

Offline Earendil

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Re: why does this FM transmitter perform so poorley?
« Reply #1 on: April 21, 2017, 08:37:07 AM »
Could you describe the power supply too? Do you use batteries? Have you tried with batteries?
« Last Edit: April 21, 2017, 08:39:06 AM by Earendil »
 

Online hamster_nz

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Re: why does this FM transmitter perform so poorley?
« Reply #2 on: April 21, 2017, 09:20:03 AM »
As a first guess, the transistor's base will be at about 5V, so the emitter will be at about 4.5V, so current through R4 will be about 10mA. So 14 mA total power seems a little higher than expected at first glance. Double-check R2 and R3?

Also, without a MIC, you can just remove R1&C1 - they are only there to bias the microphone, and couple the signal from the microphone to the transistor's base.
 

Online hamster_nz

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Re: why does this FM transmitter perform so poorley?
« Reply #3 on: April 21, 2017, 09:21:29 AM »
Also, the 10pF caps are in parallel. Doesn't that give 30pF rather than 3.3pF?  Try two in series. Should give about 5.0pF.
 

Online hamster_nz

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Re: why does this FM transmitter perform so poorley?
« Reply #4 on: April 21, 2017, 09:27:06 AM »
Is the leftmost resistor on the picture R3? It looks to be 1200 Ohm, 5600 Ohm.
 

Offline kony

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Re: why does this FM transmitter perform so poorley?
« Reply #5 on: April 21, 2017, 09:49:41 AM »
Breadboarding this is not exactly the brightest idea. Get some scrap piece of singlesided copper clad and try again wire-nest style.
 
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Offline DimitriP

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Re: why does this FM transmitter perform so poorley?
« Reply #6 on: April 21, 2017, 10:43:43 AM »
Breadboarding this is not exactly the brightest idea. Get some scrap piece of singlesided copper clad and try again wire-nest style.
It's about time someone said this !!!
   Don't be limited by your own imagination, because sometimes "it" means what you think it means. 
 

Offline yada

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Re: why does this FM transmitter perform so poorley?
« Reply #7 on: April 21, 2017, 05:23:29 PM »
I'm perf boarding it now. First thing I just fixed was that I had the resistor (in place of microphone) connected to wrong side of the 4.7nf cap. Now it pulls 30ma and the frequency moves all over the place. Going to try and fix the parallel caps. I always get that mixed up, series lowers value.

This is my first RF circuit so I'm learning the hard way (bread board to see power consumption first)! 
 

Offline nev23

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Re: why does this FM transmitter perform so poorley?
« Reply #8 on: April 21, 2017, 07:46:55 PM »
7 turns is way too much; try 3.
this circuit performs poorly because it is very simple. Moving or touching the antenna will severely de-tune or possibly stop the oscillator. even moving the PSU wires will affect it.
You need copper clad board and christmas tree construction. Stick tiny scraps of board on top of the main board to make circuit nodes. keep all wires short.
It will still be unstable; there is no buffering from the effects of the antenna, and no temperature compensation.
The only way to remove the effect of the antenna is to set the oscillator at half frequency and use a doubler. Or use a PLL. In either case, a buffer amplifier will also be needed.
Output power from your present circuit will be at most about a milliwatt, so the range will only be a few metres.
 

Online bobaruni

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Re: why does this FM transmitter perform so poorley?
« Reply #9 on: April 22, 2017, 01:52:49 AM »
When biased correctly and constructed well, I have seen this type of circuit go a few hundred metres with decent FM quieting.
Try changing R4 to a higher value  (47R is way too low) and add a cap in series with the antenna so as not to load the LC tank too much or tap the coil about half way for connection to the antenna.
See below a very successful tried and tested version of the circuit you presented, just ignore the mic pre-amp circuit as it's not required unless you need extreme mic sensitivity.
http://www.talkingelectronics.com/projects/FM-Bug/FM-Bug.html
« Last Edit: April 22, 2017, 01:55:09 AM by bobaruni »
 

Offline mmagin

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Re: why does this FM transmitter perform so poorley?
« Reply #10 on: April 22, 2017, 06:51:36 AM »
I built something similar to that from a kit when I was a kid.  Something I discovered later on when I had access to a sufficiently fast scope was that it exhibits AM modulation in addition to FM modulation, which is not too surprising for such a simple circuit.

Now that I think of it, how is it frequency modulating, anyway?  Is it relying on varactor-like effects in the transistor?
 

Offline voltz

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Re: why does this FM transmitter perform so poorley?
« Reply #11 on: April 22, 2017, 07:48:16 AM »
As stated, the construction is not good enough for a VHF transmitter. Breadboard is out. It needs to be on a better base, like copper clad.

Its such a simple circuit with no buffering, any movement close to the antenna will effect the frequency of the transmitter. Any Power supply variance will also effect the frequency and any mechanical movement / temperature changes of the components will also effect the frequency. Hence the better construction required to help stability and for other reasons.

Your waterfall SDR display is receiving way too much signal from the transmitter. Did you connect it directly? If so, dont. just let the SDR pick it up with its own antenna or use an attenuator. Reduce the signal to the SDR. You should end up with a single line (not a broad bright white band as in the pic).

« Last Edit: April 22, 2017, 07:52:36 AM by voltz »
 

Online bobaruni

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Re: why does this FM transmitter perform so poorley?
« Reply #12 on: Yesterday at 01:25:28 AM »
Is it relying on varactor-like effects in the transistor?
Yes, I think so, and definitely there will be some AM but this will be limited out by a good FM tuner, it's not ideal but all you should expect from a single transistor.

 


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