Electronics Forum

by **Jmsheehan86** » Thu Jun 20, 2013 5:49 pm

Hello, I am trying to build an RF FM Transmitter that transmits from 88 Mhz- 108 Mhz but I am having a very difficult time doing so. The transmitter I have tested works on a breadboard, which I read is not suppose to due to high capacitance, but for some reason and luck it does work. The problem is when I try and prototype it on a perfboard or anything else the damn thing will not work! The digital tuner part does but the oscillator does not. I have tried the "Manhattan" and "dead bug" construction styles on a PCB with the use of a ground plane and so far it will not work. It will oscillate, as I can sometimes hear on a receiver, but when I hook the PLL up it does not tune or do anything. I am at a loss as to what to do. Is there something I am missing, maybe shorter leads, longer leads, voodoo rituals and animal sacrifices? Or perhaps the oscillator needs to be designed better. I could use some help please. Attached is the schematic. For VT1 I am not using a BF199 I am using SS9018HBU RF transistor and I am aware that the 220k and 470k resistors in the oscillators should be in the ohms and not the kohm range, also thinking that the BFR91 transistor is backwards, corrected on breadboarded circuit and seems to work and for the varicaps I am using MV2109's as that is all I have. If anyone can help that would be awesome.

It should be noted that I didn't design this circuit but found it on some website and gave it a try. So far, very frustrating.

by **pebe** » Thu Jun 20, 2013 6:14 pm

Can you reduce the circuit so it can all be seen on the page without scrolling?

by **Jmsheehan86** » Thu Jun 20, 2013 6:25 pm

I'll try but I don't know how to do so, I posted this through my iPhone.

by **pebe** » Thu Jun 20, 2013 7:16 pm

At the moment the circuit is spread over 9 screens - too many to comprehend. You could change the format from .jpg to .gif That would give a smaller file.
.jpg is for full colour pictures, .gif is for graphics.

You could also try a program like Shrink_Pic (free download) to reduce its size.

by **Jmsheehan86** » Thu Jun 20, 2013 9:27 pm

Hopefully this works as it is a resized gif file. Note that I am only having problems with the oscillator part and not the tuner part. That works pretty good actually. I think it is how I am building it but maybe it is the way that the oscillator is designed.

by **pebe** » Fri Jun 21, 2013 10:52 am

That’s a bit better but the circuit is still difficult to read because of the bright blue watermark. However, if you have got the circuit to work as a breadboard then either you have made a mistake in the wiring when changing to perfboard, or stray capacities/inductances have affected the oscillator performance. Assuming the latter, then try these:

1. The natural frequency of the oscillator may be outside the capture range of the PLL. You say you can hear the transmission sometimes. Why can you only hear it ‘sometimes’? Is its frequency within the FM band?

2. Monitor the voltage on pin6 of the SSA1057 (PLL voltage output) while you move your hand near L1. What is the voltage and does it change?

3. The Hfe of a BF199 is 38min, whereas the Hfe of the SS9018HBU you have used is 97 - 146. The circuit uses the very basic method of biasing, so you may have excess collector current. Try increasing the value of the bias resistor, R11.

I hope that may help.

by **Jmsheehan86** » Fri Jun 21, 2013 7:40 pm

The big blue watermark does make it a little hard to read and I know I don't like the way that the schematic is drawn, to me it is a little confusing. I did change the biasing resistor R11 from 10k to 47k as told to increase to that on another forum I posted on. The voltage does change on the SAA1057 when I mess with the oscillators inductor, the voltage is fluctuating from 2.410v to 2.450v @90.3Mhz and the oscillation is strong and clear and when messing with the inductor it does change anywhere from 1v to 3v. All this is done on the breadboarded version, I have not tried to remake on a PCB. But I think I am getting closer. I am going to try and build it into sections starting with the oscillator first and tweak it. I was also told on another site that the PNP in the final stage of the oscillator should be made into an NPN. It works with PNP but is it better the the NPN? Also, thanks for the help! I am still learning all this stuff and for the most part I learn through trial and error and most stuff I create does work but this one... starting to make me upset. Still though, all a learning experience.

by **Jmsheehan86** » Fri Jun 21, 2013 10:29 pm

Also, could the 20pf C19 be causing problems? Like putting to much load on the tank circuit? It works on the breadboard but maybe when built onto a PCB it causes the oscillator to fail to oscillate..

by **pebe** » Sat Jun 22, 2013 5:36 am

[quote] I am going to try and build it into sections starting with the oscillator first and tweak it. I was also told on another site that the PNP in the final stage of the oscillator should be made into an NPN. It works with PNP but is it better the the NPN?[/quote]
But the BFR91 specified [u]is[/u] an NPN. I thought you were using that. If you are using a PNP then that could be your problem – but why did it work on the breadboard?

It is only a buffer stage and almost any small signal transistor like a 2N2222 should do the job. Turn the arrow of VT2 on the circuit the other way round and the circuit will be correct (the emitter connects to the bottom end of R14). Check that you have about ½ of supply voltage at the collector.

[quote] Also, could the 20pf C19 be causing problems? Like putting to much load on the tank circuit? It works on the breadboard but maybe when built onto a PCB it causes the oscillator to fail to oscillate.[/quote]
You could compare the oscillator with this one and change values accordingly.

http://electronics-diy.com/tx200.php

I hope those suggestions will fix it.

by **Jmsheehan86** » Sat Jun 22, 2013 2:31 pm

I have been using a NPN this whole time. Just looked at the datasheet for the planer transistor I bought off eBay and sure enough, NPN. Not sure where I got confused at. Maybe cause the person that wrote the schematic used the wrong symbol. I am going to take some pictures of the oscillator and upload them. I am sure that I am doing something wrong in the construction so if you could tell me that would be a big help for I think it has something to do with the way I am building it. I'm betting it is something very simple too and I am just overlooking it. Oh, and thanks for all the help. I feel like such an amateur but still...

by **Jmsheehan86** » Sun Jun 23, 2013 5:54 am

I looked into that oscillator, the TX200, I can use that one with the PLL I am using without changing anything around? I might try and build that one then if so.

by **pebe** » Sun Jun 23, 2013 2:19 pm

Looking at your photo, it seems you have left out C19 (20pF) and C16 (1nF). The circuit will not work without them. I would try that first before trying an alternative circuit.

I think if I were making this up I would build the oscillator and buffer on a small piece of padboard because that would be easier to make and/or modify. Then from it just connect wires from ground, +supply voltage, the control voltages to and from PLL, and the output, to pads on the main pcb.

by **Jmsheehan86** » Sun Jun 23, 2013 6:26 pm

Yep yep, forgot C16. I did realize that I had forgot C19 before I applied power and went ahead and soldered that on there. I am only building the oscillator and buffer on one PCB, the PLL is already built on some FR4 Perfboard and seems to work great. I was wondering, though, if the way I am building it good enough for up to at least 100Mhz? I take a PCB and use my dremel tool to etch out pads and solder the components to that and make little tracks of needed. All while connecting everything that is grounded to the copper ground plane. I did build up just the oscillator last night and took a wire from the collector and connected it to the varicaps and buffer stages that are breadboarded. Turned receiver on and I can hear that it was trying to oscillate. Messed with the oscillator and got some noise from the receiver so I am getting closer. Kinda a downer that I do not have an oscilloscope or frequency counter at my disposal. All trial and error on my end till I get the cash to buy such things.

by **pebe** » Mon Jun 24, 2013 9:10 am

Using a Dremel to make pads on the pcb may result in their having too much capacitance to the rest of the copper sheet. I still think it is best to make up oscillator and buffer on a small piece of perfboard. The oscillator and its varicaps should be mounted on the same board – all the RF bits are then in the same place, with no RF leads (except the lead to FFM) going off board. That lead should be as short as possible.

If you use an FM receiver to test it, tune the receiver for a noise-free spot on the dial – which would indicate a carrier is present. Then hold the coil to detune it and see if the noise returns. If so, you have identified the carrier.

Another way of detecting RF is to connect a 100K resistor between C26 output and ground. Then add a small signal diode (like a 1N4148) in parallel with the resistor, with its cathode to C26. Then connect one end of a 1K resistor to the junction of C26 and the diode, and connect a voltmeter from the free end of the 1K and ground. If the oscillator is working you will be able to measure a voltage there.