RF circuits and oscilloscope - Page 1
Forum section: electronics / beginners

author: clkaczor
author_rank: Contributor
timestamp: November 25, 2014, 07:50:14 pm
content: I was interested in learning about RF circuits and thought if I wanted to experiment with the FM band 88 to 110 MHZ, what would be the best oscilloscope to experiment and learn with?

author: Zero999
author_rank: Super Contributor
timestamp: November 25, 2014, 08:52:05 pm
content: Perhaps a spectrum analyser will be of more use than a general purpose oscilloscope for RF?

author: Howardlong
author_rank: Super Contributor
timestamp: November 25, 2014, 09:48:31 pm
content: A really useful bit of test kit expecially for RF is a bog standard all-mode receiver. You can pick up things like local oscillators with them when in single sideband mode. That will offer you frequency determination and if it has an option to turn off the AGC you can use them for things like tuning RF filters.Oscilloscopes are of limited value for RF IMHO, but w2aew has some interesting videos on how you can use them for certain limited things with the right adapters, many of them homebrew.Certainly the scope is one of the last bits of test equipment I'd pick for analysing an RF problem, but they are invaluable for most other things, including the non-RF bits of an RF circuit!

author: tggzzz
author_rank: Super Contributor
timestamp: November 25, 2014, 10:54:34 pm
content: If you want to look at frequency domain behaviour, then a spectrum analyser is appropriate.If you want to look at time domain behaviour, then an oscilloscope is appropriate.If you want to look at modulation and demodulated signals, then a modulation domain analyser is appropriate. An MDA is not far off being a combination of spectrum analyser and scope, but is normally regarded as specialised equipment and probably isn't what you want.If you want to look at component behaviour, then a vector network analyser is appropriate.Overall I suspect you need to understand the theory behind modulation/demodulation, the theory and practice of non-linear RF components.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 26, 2014, 12:01:00 am
content: If you don't have any equipment out there, you're better off buying a spec than a scope. Or none at all, and faking it... That's how it was done back in the day.Scopes are better at the time domain, looking at periodic or transient signals and very wide bandwidths. One reason you might use a scope on an RF circuit is to check the harmonic distortion of a signal. Needless to say, you need a bandwidth many times the active frequency to observe the harmonics correctly.Scopes are not good at observing modulation, which is not a periodic phenomenon, but manifests as blurring or jumpiness or instability in an otherwise more-or-less periodic signal. Scopes are good at observing the demodulated signal, of course (assuming a time domain representation is more useful, that is).The bandwidth of a $400 scope is 100-200MHz, but the bandwidth of a spec of the same class is over 500MHz.Tim

author: TSL
author_rank: Regular Contributor
timestamp: November 26, 2014, 12:11:36 am
content: Quote from: clkaczor on November 25, 2014, 07:50:14 pmAnd the question should be asked... Why do you want to experiment in the FM broadcast band ?regardsTim

author: clkaczor
author_rank: Contributor
timestamp: November 26, 2014, 01:06:32 am
content: I would like to learn with FM receiver and demodulation circuits and that radio band is constantly broadcasting so you have a remote signal to work with.

author: KM4FER
author_rank: Regular Contributor
timestamp: November 26, 2014, 02:58:27 pm
content: Let me be a bit contrary here.I am a hobbyist who has had an interest in electronics for over 50 years but after all that time I still do not have a great deal of understanding of the subject as compared to many on this forum. A long time ago I bought a Fluke multimeter to troubleshoot a problem with my car and it has since proven itself to be immensely handy when dealing with house wiring and RV and boat problems and such. I have also used it to verify basic electronic circuits such as power supplies and transistor biasing but except for a few limited exceptions a multimeter only provides a static view of a circuit. Since all the really interesting things in electronics are time variant one needs a tool which will help you understand what is going on. And that tool is an oscilloscope. They are useful for audio, digital and yes even RF if the frequencies are low enough for the scope to handle.As an example, last night I was able to get a direct conversion receiver of my own design(?) working to the point I was able to decode Amateur Morse code signals in the 7MHz range. Using an oscilloscope I was able to see that the oscillator was in fact oscillating and at the right frequency, that the diode ring mixer was actually mixing signals, that the filters were actually filtering and the amplifier actually amplifying.Yes, I would love to have a spectrum analyzer, a frequency counter, a signal generator and a variable power supply but I chose to spend my available money on an oscilloscope and I am not sorry for it. To me the oscilloscope gives you the most value (usefulness) for your money.As for studying RF I would strongly suggest that you start out at lower frequencies, maybe the AM radio band at about 1MHz as opposed to the FM band at 100MHz. At the lower frequencies you will have less problems with stray capacitance and inductance that will confuse you and yet you will still be able to learn a lot.That's my opinion, others may disagree.earl...

author: XFDDesign
author_rank: Frequent Contributor
timestamp: November 27, 2014, 01:39:27 am
content: If it's just a receiver, the 40m band is usually hopping anyway...The MicroR1 - a Simple 40m / 7 MHz Direct Conversion ReceiverSensitivity of component accuracy isn't anywhere near as ape-crazy as 2meters, component cost is much lower, and the grade of tools required to pull things off is a very low barrier of entry (technically for 40m you could pull it all off with a multimeter if you knew what you were doing).

author: TSL
author_rank: Regular Contributor
timestamp: November 27, 2014, 04:27:12 am
content: I have to agree about starting at lower frequencies, things are far more forgiving and home brew test gear easier to make.Get yourself a copy of the ARRL Handbook - that will get you a long way in learning about RF.http://www.arrl.org/shop/ARRL-Handbook-2013-Softcover-Edition/If you have the budget, get a used Communications Test Set aka Service Monitor, such as this...http://www.ebay.com.au/itm/HP-Agilent-E6380A-8935-CDMA-Base-Station-Test-Set-Service-Monitor-/390973393032While they were originally made for CDMA phone testing they are also extremely capable in the analog domain and you have a single instrument that contains an Oscilloscope, Spectrum Analyzer,Sig gen, tracking gen, DTMF encoder/decoder, am/fm modulation meter, power meter and more.This gives you a single integrated instrument that wont take up acres of bench space like individual instruments will.The 8935/E6380 is a popular instrument with HAM's because of this. It is one of many RF test sets that can be bought for reasonable $ ( I have one)You can compare a number of them here...http://www.amtronix.com/diff.htmAmtronix sell them calibrated and have "how tos" on their web site.The are others from IFR, Motorola, Cushman, Rohde & Schwarz, all at varying $And you can start with just a good multimeter - get one that does L & C to really make things easier.regardsTim

author: Dave Turner
author_rank: Frequent Contributor
timestamp: November 27, 2014, 07:28:42 pm
content: You might consider a Grid Dip Oscillator. The one I had only went up to 30MHz, built from a design in practical wireless (think it was) back in the 60's. I learnt a lot from just building it.

author: Howardlong
author_rank: Super Contributor
timestamp: November 27, 2014, 07:59:20 pm
content: Agreed with others, lower frequencies are a lot easier.I remember breadboarding a 6m (50MHz) transverter in the mid 80s with a 1k ohm per volt multimeter, an LED and a wavemeter. It was deaf as a doorpost, but it worked. The problem was that it was very hard work to get it going, enough to really put me off doing much homebrew RF for some years.One problem with RF is that if you are committed you can get stuff to work with rudimentary kit, but getting it to work well takes quite a bit more effort, and that's where the right test equipment helps an awful lot.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 28, 2014, 10:07:16 am
content: Quote from: T3sl4co1l on November 26, 2014, 12:01:00 amYes,& no.If I was going to poke around inside an FM Broadcast receiver,a Spectrum Analyser would not be my instrument of choice.The various places you will want to look for signals inside such a receiver will not be nice AC coupled 50 Ohm output impedance.If you are unlucky you may place a DC supply point across the input of your SA,cooking the input stage.An Oscilloscope cannot provide the depth of information that an SA can,but you can look at the local oscillator,(this level will be a fair way down with a "100MHz"scope,but visible),then the 10.7MHz IF,examine the output of the FM detector,look at the receiver audio output,& so on.Modulation? I would agree about FM,but AM & SSB envelopes are quite easy to display with an analog 'scope,especially if you can modulate the signal with fixed tones.Spectrum Analysers are very useful if you can look at a device made up of modules with standard input & output impedances,but that is seldom the case with domestic equipment.

author: Zero999
author_rank: Super Contributor
timestamp: November 28, 2014, 12:12:01 pm
content: Quote from: vk6zgo on November 28, 2014, 10:07:16 amI agrewe with what you're saying but isn't the input stage of a SA normally AC coupled to prevent this kind of thing?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 28, 2014, 03:56:26 pm
content: Specs often have "High-Z" inputs, just like scopes do. You can put a scope probe on the spec. Or buy/build a buffer circuit that emulates a scope's 1M/20pF (or whatever) input and delivers 50/75 ohms to the instrument.Such a buffer is also required if you need probe extensions: the probe is only capable of compensating its own cable plus scope capacitance.The extreme case, with zero probe cable length and all impedance matched cable, would be a FET probe.But really, if you're talking about probing a radio receiver live, you're still screwed because the scope probe has too much capacitance to connect directly. You can still probe insensitive, low impedance nodes, or you can capacitively couple by hovering the probe tip near a connection (without touching it), which only pulls the frequency response a little.Tim

author: aveekbh
author_rank: Regular Contributor
timestamp: November 28, 2014, 05:23:19 pm
content: I'd recommend both.Over the years, I have used the spectrum analyzer far more than the scope (at work, I work with WiFi and Bluetooth, so 2.4 and 5 GHz). But they are good for different things - there are times when I have coupled the scope to the SA to correlate control signals and RF output. And there is a mode in SAs called zero span, which behaves like an oscilloscope with analog bandwidth = RBW.Of course, I am talking about 2.4 and 5 GHz signals. With low frequency stuff (order of 10 MHz), we can usually do everything with the scope.Quote from: Hero999 on November 28, 2014, 12:12:01 pmThe SAs we have at work are DC coupled. In fact the limits are +30 dBm but 0.2 V DC. We ensure we have DC blocks on them.I guess DC coupling is needed for low frequencies. The spec sheet forAgilentKeysight N9020A MXA states:Quote.

author: Electronics-Repairman
author_rank: Contributor
timestamp: November 28, 2014, 05:40:24 pm
content: Quote from: clkaczor on November 25, 2014, 07:50:14 pmIt maybe in your interest to take a look at W2AEW You-Tube Chanel ,Alan's also a member on here.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 29, 2014, 11:08:29 pm
content: Quote from: T3sl4co1l on November 28, 2014, 03:56:26 pmYou mean that all the people who have been doing this for years haven't been seeing anything?Remember that only the early stages of such a receiver are at VHF---the 10.7MHz IF where most of the selectivity occurs is not anywhere near as "touchy".Perfection isn't always necessary,especially in the context of the OP's original query.You can get useful results without them being exactly right.Many circuitsdohave test points intended for 'scopes.They are not low impedance enough for direct connection of an SA,but they offer enough isolation for a 'scope.The easiest "buffer amp" for an SA is to use an analog 'scope as a variable gain amplifier,& connect the SA to the"vert amp" output on the rear of the 'scope.This is also useful with frequency counters.

author: hans12
author_rank: Newbie
timestamp: November 29, 2014, 11:32:43 pm
content: If you are just starting out and/or are low on budget and you want to do RF buy a cheap TV stick with an R820T tuner. This tuner combined with some software will let you listen in to many radio signals in many types of modulation. Google for RTL-SDR.If you are high on budget, buy a spectrum analys/zer.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 01, 2014, 12:21:51 am
content: The RTLSDR is what I was going to suggest. Just a little slow for scanning a large piece of bandwidth, but if you also have a scope that supports XY mode (even one of those little CRT TVs hacked into a scope), you can then pull a tuner module out of an old TV or similar and use that to make a spectrum analyzer that you can then "calibrate" against the RTLSDR.

author: G0HZU
author_rank: Super Contributor
timestamp: December 01, 2014, 12:40:17 am
content: Quote from: clkaczor on November 26, 2014, 01:06:32 amThe modulation format is usually stereo so this is a multiplexed signal. Maybe not the best format to study for a beginner and also very oldschool in terms of technology.If you want to learn a few things about radio transmission and reception I'd recommend you buy a few old multimode CB radios and study how they work. The Uniden built versions are the best in terms of simple and effective design for AM/FM/SSB. There's a bit of everything in one of these old radios. PLL design, TR switching design, PSU regulation, RF/IF/demodulation in the receiver and various modulators for the transmit section. It's no big deal if you damage the radio. Just buy another...As for test gear, I'd recommend something like an old Tek 465 scope (or a 475 if you want a flatter response to 100MHz). Also a basic signal generator. eg a Marconi 2022 if you can get one. Also, try and make a few bits of RF 'accessory' test gear yourself.eg make a simple RF detector probe to use with a DDM or AVO8 (or the scope). Also make a simple return loss bridge for use with the sig gen and the RF detector. Don't bother with a conventional spectrum analyser yet unless you have lots of spare cash.Also, be wary that all of the above is 30-40 year old technology so you really only want to do all this as a stepping stone to more modern stuff like SDR technology.

author: aveekbh
author_rank: Regular Contributor
timestamp: December 01, 2014, 02:30:44 pm
content: Quote from: G0HZU on December 01, 2014, 12:40:17 amAn SDR is an excellent idea!