Do you need more than one RF generator ?

[electronic_guy]

Hi,

I have a CMU 200 and this gives a RF generator with it. But I'm considering if I want to buy another dedicated RF generator, may be a vector signal generator. My work is mainly focused on introductory level RF work. I do not have a definite focused area, just trying to learn high frequency electronics.

Could you please give me an idea on your experience. This might sound silly but, did you ever needed two or more RF generators when you dive into more RF stuff ? Could you give me some examples.

[bob91343]

I don't see a need if all you are doing is simple stuff.  If you are fussing with synthesizers or heterodynes, maybe yes.  Or use one generator to modulate the other, or with a greatly different frequency range.

I use audio oscillators and rf generators but seldom do i need two rf generators.

[james_s]

As with anything like this, it depends on what you're doing. I would suggest finding a project and then acquiring the tools you need to complete that project rather than acquiring a bunch of tools and then looking for something to do with them. If you don't know whether you need something or not, in most cases you don't need it.

[xmo]

Unlike some other surplus mobile phone testers, one of the attractions of the CMU200 is that the spectrum analyzer and RF generator functions of the base unit are available regardless of options and they function across the entire frequency range from 10 MHz to 2700 MHz for the analyzer and from 100 kHz to 2700 MHz for the generator.

The generator has a very wide range of calibrated RF output and supports FM, AM, and SSB modulation.  Changes in the wireless industry caused lots of CMU200s to be on the surplus market at attractive prices.  I couldn't resist buying one.

One thing I learned after playing with it - the AF input and output connectors and the aux connectors on the front panel do nothing without the right options.  In my unit the cables from those connectors were simply tied back and not connected to anything.

There is an audio option that uses them but it is independent of the generator and analyzer.  If you want demodulated audio output and external audio input to modulate the RF generator - you're out of luck unless you have the AMPS option.  That option will give you some capability with limitations.

If you ever want to test FM two-way radios - CTCSS, CDCSS, frequency response, pre-emphasis, de-emphasis and distortion - you might need another instrument.

Likewise, if you want to service, align, and evaluate broadcast FM receivers, the CMU probably won't do the job as well as a stand-alone generator.

[fourfathom]

Two signal sources are needed if you want to perform IMD (Intermodulation Distortion) testing on amplifiers and mixers.  For these tests you will also need a combiner with good inter-port isolation.  But you can do a whole lot of RF design and test before you get to the point where you are making IMD measurements.

[srb1954]

Hi,

I have a CMU 200 and this gives a RF generator with it. But I'm considering if I want to buy another dedicated RF generator, may be a vector signal generator. My work is mainly focused on introductory level RF work. I do not have a definite focused area, just trying to learn high frequency electronics.

Could you please give me an idea on your experience. This might sound silly but, did you ever needed two or more RF generators when you dive into more RF stuff ? Could you give me some examples.

You will need 2 generators if you are doing more advanced receiver testing like blocking tests.

[coppercone2]

if you have 2 generators
you want to link them to one oscillator that is stable if you want to do the interesting stuff, you need low drift. If you have stuff that dances around it won't do much good. you will be more interested in things like phase offset control and stuff then just generating fast signals.

for modulation there is usually a severe bandwidth limit. for yig you have slew rate limits

for those you need different equipment (slower, with modulation), that will be your most likely application.

keep costs in mind. keep the FCC in mind, most of the time you will not be allowed to sweep a wide 1GHz band, the bandwidth you are given is low.

[opabob]

YOU NEED MORE GEAR!!!

[xmo]

It's really easy.  You have taken the first step.  Of course you need another signal generator.  A nice one with low phase noise and I/Q modulation.

Then you will need an ARB generator to create the digital I/Q modulation and then a vector signal analyzer to view the waveforms and a network analyzer to align things and study RF intricacies & the Smith chart and then another spectrum analyzer and oh, why not another network analyzer and... and..

[coppercone2]

this turns into an RF parameter analyzer, these systems exist integrated for $ $ $ $ $

[tkamiya]

It really depends on your goal.

I have 3 dedicated RF signal generators, 3 sweepers with many plugins, 3 tracking generators, etc, etc, etc.  If you'd want to play with RF, I'd think you'll want two.  Experimenting with hetrodyining and IMD requires at least two.  But other than that, this is a question only you can answer.  How about wait until you actually feel a need (ie. can't do whatever without another) and get one if you do?  There are plenty of them available on used market.

[coppercone2]

do you have all those linked up on an atomic clock? I never experimented with mixers but I figured for radio applications frequency stability would either be expensive (leave equipment) or drifty without syncing clocks

[rfclown]

For many applications if you want synchronization it's sufficent to just tie all the 10 MHz references together. There is usually a Reference In and a Reference Out, so you can just daisy chain the instruments. I went many years before I bought a good reference (because I didn't need it).

[coppercone2]

I got lucky with OXO in most of the stuff but I remember one time I got a power bill and I shit my pants, with a bunch of stuff running it was like 225% more then usual, networking clocks is worth your time and money.

If I decided to weld, bread machine and cook ribs every day, it would have been a kick in the nuts. Thankfully BBQ and acetylene hid that part of the cost 

[RoGeorge]

I do not have a definite focused area, just trying to learn high frequency electronics.

Could you please give me an idea on your experience. This might sound silly but, did you ever needed two or more RF generators when you dive into more RF stuff ?

A simulator like LTspice or QUCS, a few lines of code in your preferred programming language, or a math tool like Octave might be way more useful than a generator when it's for learning purposes.

For me, a dual output DDS generator (Rigol DG4102) was enough.  It is not exactly an RF generator, but it can generate up to 200 MHz, has many type of modulation, many predefined waveforms, etc.  My pretext to buy that was because it has 2 outputs that can be controlled independently, and the 2 outputs can also be synchronized if needed, for example to play with IQ signals, like in this video:



However, for learning only, the above experiment can be made as well with the two channels of a sound card.   

A DDS is a good investment for other applications, too.  For example to generate a certain number of pulses, or as a dual DC voltage source that is adjustable from the front panel knob (has a waveform called DC, very useful because it can output +/-10V through a 50 ohms resistor).  Even tested stepper motors with it.

So far, I never needed more than that DDS with two channels, but I'm not really into RF.

[rfclown]

I completely agree with RoGeorge. I'm an RF guy, and I have several RF generators... but only one regularly sits on my bench. The others I haul out if I need to do IM or blocking measurements or work with mixers (which isn't what I do on a normal day). But the sig gen I use has dual arbs for I/Q modulation.  I use LTspice, QUCS and my prefered programming languages ALL THE TIME. In my prefered languages I deal with IQ signals.

A fantastic gizmo that I'm finally getting a handle on is the ADALM Pluto. For $150 it is an IQ RF signal generator and demodulator. I use Agilent ESG signal generators at home and at work (also MXG at work). This thing can replace those in many situations. For RF signal analysis I use Agilent VSAs and MXAs. I'm working on replacing those in many situations with the Pluto also. It's work to write the code, but the Pluto can get you the baseband IQ. I've use the RTL-SDR in the past which is also fantastic for what it can do (receive only, 8 bit). I wrote an 915 MHz OFDM vector signal analyzer for the RTL-SDR that ran in real time for a 1.625 MHz signal. I've not used GNU radio, but I believe that is something to look into.

[vinlove]

I have a few Heathkit RF generators, and also several different DMM too.
The reason I have a few of these devices is that, I want to re-check whether the 1st set was working and reading correct.
I always recheck with the 2nd and 3rd set for the readings. If they are all reading the same, then I accept it as correct reading.
Because 1x set can be faulty and might be giving wrong readings.

[xmo]

vinlove wrote: "I always recheck with the 2nd and 3rd set for the readings. If they are all reading the same, then I accept it as correct reading."

I call that consensus calibration.  Even if you send all your gear out for scheduled, traceable calibration, how do you know that something didn't change when it got dropped in shipping?

If you set your signal generator to output a -30 dBm signal and your spectrum analyzer shows a -30 dBm signal - then you feel better about both of them.  If a power meter also shows -30 dBm, so much the better.  The more 'opinions' you have that reach consensus - the higher your confidence.

I think a careful lab worker should periodically do these types of tests for confidence in both the instruments and the measurement technique.