Using a function generator as an AM/FM transmitter. - Page 1
Forum section: electronics / testgear

author: JoeN
author_rank: Frequent Contributor
timestamp: November 20, 2015, 06:02:19 am
content: It just occurred to me that my function generator can do AM and FM modulation and can accept an external source to be modulated. I am guessing that with the right BNC antenna, it could be made to be a low-power transmitter. Am I right, or am I missing something? What would be a good antenna to test with, any suggestions?

author: HighVoltage
author_rank: Super Contributor
timestamp: November 20, 2015, 08:09:40 am
content: Sure you can do that

author: ElektronikLabor
author_rank: Regular Contributor
timestamp: November 20, 2015, 08:27:15 am
content: Maybe this puppy could boost the AWG-Transmitter a little bithttp://www.nxp.com/documents/data_sheet/BGD714.pdf

author: JoeN
author_rank: Frequent Contributor
timestamp: November 20, 2015, 08:56:34 am
content: Quote from: medvedev on November 20, 2015, 08:27:15 amI am going to do it as an experiment. I want to be able to receive it across the room. Any further is a liability, not an asset, in my opinion. Any good suggestions for a decent antenna that would work at AM through FM frequencies? I know jack crap about antennas. Might this do the trick? It doesn't have to be efficient or perfect, just be able to radiate out a few mW.http://www.ebay.com/itm/765mm-360-Stretch-Telescopic-Antenna-Q9-BNC-Connector-Aerial-FM-Radio-Scanner-/361323131181

author: JoeN
author_rank: Frequent Contributor
timestamp: November 20, 2015, 09:03:36 am
content: Quote from: HighVoltage on November 20, 2015, 08:09:40 amCool. That is exactly what I was thinking about doing, except just using a radio to check the radiated RF output.Any idea what antenna he is using?

author: PE1RKI
author_rank: Regular Contributor
timestamp: November 20, 2015, 09:21:08 am
content: i used an hp3314a as a 6MHz audio carrier for my television transmitter a few times to chat with others over the atv ham band on 2380mhz. works pretty good. audio was good quality.i tried to make a 1252mhz tv transmitter with an hp8657b but the input filters are to narrow.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 20, 2015, 09:55:43 am
content: There are no AM and FM frequencies, just frequency bands that are usually used with these type of modulations.The antenna for sending can be very similas to the recieving side. The basis one is the quarter wavelength dipole - so just a wire or slap of the about right length.So the one linked from ebay may work for something like 50 MHz to 300 MHz, depending on the lenght used.

author: JoeN
author_rank: Frequent Contributor
timestamp: November 20, 2015, 09:57:52 am
content: Quote from: Kleinstein on November 20, 2015, 09:55:43 amSo am I screwed for AM? What kind of an antenna do I need to transmit AM? It seems to me receive antennas for AM are always a hell of a lot shorter than a quarter of any AM wavelength.

author: PE1RKI
author_rank: Regular Contributor
timestamp: November 20, 2015, 11:21:51 am
content: just a piece of wire will do, but not coax, make sure you dont create static issues and destroy the generator. so don't use a blank wire, keep the plastic insulation on it.

author: retrolefty
author_rank: Super Contributor
timestamp: November 20, 2015, 11:34:03 am
content: Quote from: JoeN on November 20, 2015, 09:57:52 amThe reason that most AM band receiver antennas are physically small is that they are usually wound on material that increases it's effective inductive value. If you check out commercial AM band transmitter you will fine that antenna size is very very large.RF antenna design is independent of type of modulation used. Only the frequency and power of the transmitter determines the antenna type and dimensions to use.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 20, 2015, 01:16:30 pm
content: Quote from: JoeN on November 20, 2015, 09:57:52 amAsKleinsteinpointed out,there arenoAM or FM frequencies.They are types of modulation & can be used at any frequencyIn practice,AM is used in Medium Frequency Broadcasting,in HF Broadcasting,the 27MHz CB Band,& in the aeronautical Band above 108 MHz.FM is commonly used at VHF,UHF & SHF frequencies,& less commonly,by Amateur Radio Operators around 29MHz.The signal strength of MF AM Broadcasters is such that an antenna very much smaller than any resonant length will give satisfactory results.The same applies,to a lesser extent to FM Broadcast Stations between 88MHz & 108MHz.In your case,you don't really need an efficient antenna to hear it across the room-----a metre or so of ordinary "hookup" wire will do.

author: JoeN
author_rank: Frequent Contributor
timestamp: November 20, 2015, 01:39:56 pm
content: Quote from: vk6zgo on November 20, 2015, 01:16:30 pmThanks for pointing that out again. I knew that yesterday, and thanks to you, or not, I still know it today. I wasn't trying to imply that some type of modulation could only be used at certain frequencies any more than I would say that certain voltages can be used with certain waveforms - neither of those make sense. What I was trying to imply, and maybe I should have been more obvious about this, is that the AM I want to broadcast needs to fall into the band 540 kHz to 1,700 kHz and the FM I want to transmit needs to fall into the band 88 MHz to 108 MHz for a very simple reason - I want to use a standard radio to receive it. I completely understand that I can do AM at 1GHZ if I wanted to, but it would make it a lot harder to receive.What I was getting at was that I keep hearing that an antenna needs to be about 1/4 wavelength. Well, if I was putting out a signal (it would be AM so I could actually receive it with my radio) at 1000Khz which I might want to do, the wavelength is 300 meters, the quarter wavelength is 75 meters, and I rarely see anyone with an antenna that long just to receive AM.However, the answers which have ranged from "use a wound antenna" (which I see now that I look for them) to "don't worry about it" makes me happy. I will get a crappy antenna and see how it works.

author: HighVoltage
author_rank: Super Contributor
timestamp: November 20, 2015, 03:29:15 pm
content: You can just use a short 1 meter long antenna with a BNC connector or even simpler, use a hookup wire. Just try it out, it will work very well, even if your antenna is not optimized in length. Older car radios also had only one antenna for FM and AM reception (about 1 m long) and it worked reasonable well.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 20, 2015, 03:32:48 pm
content: Quote from: JoeN on November 20, 2015, 01:39:56 pmWell,it might have been nice if you said all that in the beginning!Your use of "AM" & "FM" as shorthand for the MF Broadcast Band & VHF Broadcast Band respectively,is exactly that which a layperson would do,& as we are not mind readers,we only had that to go on!No domestic Medium Frequency Broadcast Bandreceivingantenna will be a resonant length.Even back when people used external antennas,due to fairly insensitive receivers & few local Stations,they were only about as long as an average back yard.Modern receivers are very sensitive----look how short car radio antennas are!Transmitting antennas?------OK if you wanted to be heard 50 miles away,you might need a full length antenna, but,in the same room,a "crappy" antenna will work.In fact,you might not need an antenna at all.It all depends on how well screened your function generator is.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 20, 2015, 08:25:04 pm
content: FYI, you might have modulation capability, but you might not also have the stability and purity to do a good job.New DDS based instruments are probably okay, though they typically suffer from spurs. You'd want to check the SFDR (spur-free dynamic range) spec, or better yet, measure it yourself. Carrier stability should be excellent (crystal controlled, give or take the close-in jitter inherent in the DDS process). Depending on how the modulation is performed, it might not be as clean as desired, but should still be okay for radio I would think. (I don't know -- I wouldn't think DDS type generators would be able to do it by analog hardware, not FM at least. I would imagine they usually have an ADC and handle the modulation digitally. So the bandwidth or update rate of that input will matter. You might not get the full ~100kHz bandwidth necessary for a full commercial FM channel including data streams, but I wouldn't think they'd make something so poor that it couldn't handle < 10kHz AM radio, or narrow band FM.)Older, analog function generators are typically RC oscillators, and suffer from noise and jitter inherent in the circuit. First of all, the tuning circuit is lossy and noisy (there's a resistor!); second, there's a comparator, which must be fast and high gain to get a precise threshold, but high gain and high bandwidth also imply high noise levels, leading to uncertainty both in amplitude (= where the voltage threshold is) and frequency (= when the threshold is crossed). Therefore, jitter and stability aren't usually very good, perhaps on the order of 0.1% of the center frequency. This is pretty troublesome for AM BCB (0.1% of 1MHz is ~1kHz, i.e., expect a loud hiss or rumble behind any modulation you introduce!), and impossible for FM BCB (which has a channel that wide already -- the rumble/hiss will completely dominate!).The stability of RC oscillators is also typically poor, so you might just manage to set the frequency to what your radio is tuned to (assuming you have an adjustable narrowband sort of receiver -- a wideband e.g. SDR might track a wide swath however, and not mind), but it'll drift over time, due to thermal variation, line voltage, just residual stress in the control pots, etc...Reference: my primary function generator is a Wavetek 193. It's pretty good (all the functions you need, including sweep/mod, up to 20MHz), but certainly doesn't have the stability needed for radio work. Tuning over its output, you basically just find a wad of hiss. It's also fairly leaky, due to lack of shielding and filtering (it's inside a plastic case) and poor (RF) grounding of the connectors. (In other words, it might be unplugged from everything but AC power, but still be perceptible at the fundamental or harmonics.)Even a basic LC oscillator has orders of magnitude better stability (I've played with an old Eico 322 RF generator, which has a vacuum tube oscillator, selectable coils for bands, and a variable capacitor for tuning). The AM/FM noise from one is typically imperceptible, and dominated by incidental noise (supply ripple, mechanical resonances -- tap on the enclosure and it goes "BROOongngng"!). Tuning to the fundamental or a harmonic results in strong quieting (i.e., the quiet, stable carrier, unmodulated), just as you should expect.You should expect the same result (or as good as, or better) with a DDS, at least around the fundamental (since spurs are a thing), but also with pretty good modulation as well (at least within whatever the modulation bandwidth/speed/capability of the instrument is).Tim

author: German_EE
author_rank: Super Contributor
timestamp: November 20, 2015, 08:41:11 pm
content: There is prior art here. In an episode of Stargate SG-1 a partially completed spaceship is taken over with Col. Samantha Carter inside. She finds an HP (I think) signal generator in a store room and connects the door intercom to the modulation input to transmit an emergency message. No mention of what antenna she used inside a metal spaceship though.

author: JoeN
author_rank: Frequent Contributor
timestamp: November 21, 2015, 01:36:59 am
content: Quote from: German_EE on November 20, 2015, 08:41:11 pmMaybe a carbon-fiber hull?Quote from: T3sl4co1l on November 20, 2015, 08:25:04 pmThanks for all the information. I bet it will work out. When I got back into electronics a few years ago I did something that was probably stupid and wasteful. I bought a function generator that was probably more than I will ever need, a DG4162, same model as in the video on the first page, and pretty much based on Dave's recommendation. But it looked a heck of a lot better than the $50-$200 specials on eBay and was a far better value than Agilent, which seems to price function generators as if they are made out of solid gold (they are better on scopes and DMMs, I have a DSOX2012A and a 34410A). So I expect that will do well enough. Will test it out tomorrow I think.Yes, I am one of the guys who bought far too much in the way of instruments relative to personal skill. But that's all, nothing fancier than that stuff plus a Hakko iron and two relatively cheap Agilent power supplies.

author: dom0
author_rank: Super Contributor
timestamp: November 21, 2015, 11:27:40 am
content: Quote from: JoeN on November 21, 2015, 01:36:59 amWait, what, how did you even get adigital function generator permitfrom theElectronics Overwatch Office(EO2) without alevel 4 certificate of skills?!

author: JoeN
author_rank: Frequent Contributor
timestamp: November 21, 2015, 07:30:37 pm
content: Quote from: dom0 on November 21, 2015, 11:27:40 amIn the United States we can have even a high speed high capacity assault function generator without a license, you can get it out of the back an unlicensed dealer's car at a HAM show.

author: SeanB
author_rank: Super Contributor
timestamp: November 22, 2015, 07:27:19 am
content: We used to take a radio, connect to the signal generator modulation input ( older HP one which got to 200MHz) and connect a 2m length of wire ( or whatever was close to hand and which would fit in the BNC centre pin) and use it to either swamp an existing station, or to redistribute another. Hilarity at people in the other work areas furiously tuning up and down with the car radios we used as work radios, trying to get their favourite stations. We got around 200m range with that setup.