measuring FM output frequency - Page 1
Forum section: electronics / beginners

author: Jbates58
author_rank: Regular Contributor
timestamp: December 04, 2016, 10:10:27 am
content: hi alla little while ago i built an fm transmittor, based on the BA1404 IChttp://electronics-diy.com/BA1404_Stereo_FM_Transmitter.phpbut it never worked (well i couldn't get my radio to receive any signal). but now i have my scope, im hoping that i can hook it to the ANT. out, and see the frequency that its running at.my plan is to feed it a constant 1HZ signal, so that i can also try to debug the circuit to see if there is any other issues with it. but im hoping that if i feed in a constant 1hz signal, then i should see it on the ANT. out aswell.will that work?Jason

author: VK5RC
author_rank: Unknown
timestamp: December 04, 2016, 10:20:57 am
content: A 1Hz signal I doubt will work, it is based around an audio input so 1kHz I think would more likely to be useful.If you can't hear it (or even receive the carrier) with a known good FM receiver - they should be pretty sensitive - often in uV sensitivity - there are other problems.Does your scope go to 150MHz?Dtat Sheethttp://electronics-diy.com/pdf/ba1404.pdf

author: Jbates58
author_rank: Regular Contributor
timestamp: December 04, 2016, 10:33:10 am
content: my scope is a DS1054. i know that using more chanells lowers they input specs 1 vs 2 vs 3/4 but im hoping that as it is 100MHZ, if i can tune it down to 88 or so MHZ then see the signal.if a higher frequency would be better than i will use that. in trying to learn more about electronics and am getting there slowly.cheersJason

author: Don Hills
author_rank: Regular Contributor
timestamp: December 04, 2016, 10:50:46 am
content: If you're running the circuit off 1.5 volts, try 3 volts. I've built a couple of these BA1404 circuits, and they are somewhat unstable at 1.5 volts and tend to cut out as soon as the cell voltage drops a bit. I went to 3 volts and had no further trouble.

author: Jbates58
author_rank: Regular Contributor
timestamp: December 04, 2016, 10:52:26 am
content: i did have it running @3v.i have a small adjustable regulator kit thing i got from jaycar years ago for something else, and i no longer use it, so i thought that its a good stable supply for this purpose.Jason

author: Simon
author_rank: Unknown
timestamp: December 04, 2016, 11:37:05 am
content: Your scope is rated at 100MHz, regardless of how many channels you use this means that you will see a 100MHz sine wave with an amplitude of 0.7 what is really is (-3dB) and nothing but a sine wave as anything else will be beyond 100MHz (say a 100MHz sqaure wave is superimposed you would need 500+ MHz to see it properly otherwise all you will see if the fundamental 100MHz sine not the harmonics that add to it making is a square) so won't show up or will not be shown as it is in real life. You will probably be able to see "an output" which will at least verify that your circuit is working.The scope input has a 100MHz low pass filter, it's nothing to do with sample rates etc. The first hack to a rigol scope involved physically altering the input filter only so that it would let up to 100Mhz in rather than 50MHz, the software hack was just the icing on the cake allowing the scope to see itself as the 100MHz model and have some extra software settings plus risking no damage to the scope from a soldering iron.

author: Jbates58
author_rank: Regular Contributor
timestamp: December 04, 2016, 11:58:43 am
content: well in aus, on the FM radio band, we start at 88mhz. so if i put in a pure sinewave of any frequency below 100MHZ then shouldnt i be able to see almost the same sinewave output if i attach the probe to the antenna? i figure that if its not at 88mhzFM then i wont see the sinewave at all, or it will be of a very low amplitude.or am i in a totaly wrong ball park?i hope that make sense.Jason

author: DimitriP
author_rank: Super Contributor
timestamp: December 04, 2016, 12:29:21 pm
content: First double check you have installed all the components correctly and you have no cold/bad/missing solder joints.Then set an FM receiver next to it and s l o w l y turn the coil core from one end to the other.If it's working you'll hear it.Oh back to the original question. FM being what it is, the carrier is always on whether it's modulated or not.As for probing the antenna and seeing the "pure sinewave" of the audio input, no. It doesn't work that way.

author: Simon
author_rank: Unknown
timestamp: December 04, 2016, 12:49:13 pm
content: Yes higher frequencies will be visible but ever more attenuated. But you need to remember that for example a square wave is made up of the fundamental sine and at least 5 harmonics that end up 11x the fundamental frequency, these will be disproportionately attenuated distorting your signal.If you have a crappy cheap FM radio and put it near your transmistter it will pick it up even if wildly off frequency as it should be so much more powerful than any other distant transmitter that it will overpower anything (experience learnt from walking past a local church when I lived in italy and coincidentally all I could pick up was radio maria as any other channel dissapered when i went between the church and other buildings - no doubt the poky iuppy bits of metal on the church tower doubled as antennas)

author: Jbates58
author_rank: Regular Contributor
timestamp: December 04, 2016, 09:43:25 pm
content: Quote from: DimitriP on December 04, 2016, 12:29:21 pmok. cheers for that.however, will i be able to see the carrier wave that is been transmitted? and in that, wont i be able to see the aprox frequency?in the circuit, there is an inductor. i coldnt find an inductor that was able to match the specs needed, so i tried to roll my own using a calculator on my phone. so that is possibly even not correct and i dont have a tool to test it with. so if it is out by a mile then i have no way to know. hence my question can i connect my scopt to the antenna to even get an aprox. idea if its sorta working.Jason

author: DimitriP
author_rank: Super Contributor
timestamp: December 04, 2016, 11:53:11 pm
content: QuoteThis changes everything.The link you posted misled me into thinking that you build it from a kit with all the right parts in all the right places. Thi increases the possibility of everything having gone wrong.Most bets are off.Is it on a PCB or a proto board ?Well....use two 1. 5 V battries to power it up. Connect the probe to the antenna output and see what you get.If nothing else you should be able to see the 38KHz oscillator working by probing pin 5 or pin 6 or at least 19KHz on pin 13.See what you see.....

author: VK5RC
author_rank: Unknown
timestamp: December 05, 2016, 03:59:00 am
content: If a kit works first time, you learn how to solder, if it doesn't work, you learn electronics! (Don't ask me how!!)I would try to get a "made" part to start with, then maybe an ability to test L, e.g. LCR meter, there are quite few ~$20 eBay types that aren't too bad.

author: Jbates58
author_rank: Regular Contributor
timestamp: December 11, 2016, 10:30:26 am
content: ok, so i did some probing today,i used a function generator app to get a constant stable frequencyand following the audio input path, i get the wave on both sides of the 1N Cap and 47K resistor and the input leg to the 10Uf cap, but nothing on the output of it.i checked the polarity and its all good, and i also checked the cap with my meter and it seems all good aswell.what would cause there to be noting on the output?to my understanding of caps, as its directly on the input path, and there is no way for the audio to get around it, wont it fully suppress the audio waveform? in the first stage filtering part, i think that is a R/C filter? is that correct? In which case i can understand that the cap will filter some of the waveform, but the entire part of it can flow through the resistor.cheersJason

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 11, 2016, 10:36:30 am
content: Can you see any signal without the ic connected? (pins 1 and 18)Can you post a picture of your setup?

author: Zero999
author_rank: Super Contributor
timestamp: December 11, 2016, 11:24:35 am
content: How are you powering the IC?The schematic you posted says Vcc should be between 1.5V and 3V but the datasheet recommends 1V to 2V.www.littlediode.com/datasheets/pdf/Datasheets-BA1/BA1404.PDF

author: Jbates58
author_rank: Regular Contributor
timestamp: December 11, 2016, 09:04:19 pm
content: Quote from: Hero999 on December 11, 2016, 11:24:35 amim using a adjustable voltage reg. voltage is stable. i am using 3v as stated in the site. but did notice that the data sheet mentioned 2V max.

author: Audioguru
author_rank: Super Contributor
timestamp: December 11, 2016, 11:31:21 pm
content: Why do you think your 'scope will show audio on the antenna of the FM transmitter? The antenna will have only continuous 88MHz to 108MHz if the coil and its core are correct. An FM radio tuned to the radio frequency of the transmitter will produce the audio.

author: DimitriP
author_rank: Super Contributor
timestamp: December 12, 2016, 04:02:11 am
content: Quote from: Audioguru on December 11, 2016, 11:31:21 pm

author: vk6zgo
author_rank: Super Contributor
timestamp: December 12, 2016, 05:30:29 am
content: Quote from: Simon on December 04, 2016, 12:49:13 pmSimon,you have gone deep into the "rabbit hole" on this one.FM Broadcast transmitters by law are required to have very low harmonic content,so your comments about a 100MHz square wave are not germane to the discussion,To the OP-----Yes,your DSO will be able to see the 100MHz sinewave carrier,but it will be 3dB down in level with respect to the 'scope's mid-band response.A standard probe won't be the best thing to look at it with,but it should be discernibleQuoteThisisgood advice,as if the thing is oscillating somewhere near the right frequeny,you should be able to tune it in by squashing or stretching the inductor..

author: vk6zgo
author_rank: Super Contributor
timestamp: December 12, 2016, 05:34:41 am
content: Quote from: Jbates58 on December 11, 2016, 10:30:26 amIf you are still messing around with a 1Hz signal,that probably won't make it through the coupling cap.Use 1kHz.In any case,you don't needanymodulation to determine if your 100MHz carrier is present.