The types, sorted by what they are for
There are really four things people call a signal generator, and they are not interchangeable.
Function generators
The workhorse of a general electronics bench. Sine, square, triangle, sawtooth, ramp, usually up to a few tens of MHz. You use one to test an audio circuit, clock a digital board, or throw a known waveform at something and see what comes back. Cheap, simple, and if this is all you need, do not let anyone upsell you into RF gear. The bench standards here are things like the Keysight 33500 and 33600 series and the older 33220A and 33250A.
RF signal generators, analog
These make a clean carrier from hundreds of kHz up into the GHz, with analog modulation — AM, FM, phase, and pulse. You use one to test a receiver, characterize a filter, or drive a mixer. The thing you are paying for is spectral purity, low phase noise and low spurious, because a receiver test is only as good as the cleanliness of the source. The HP and Agilent 8648 series, the 8644A, the ESG analog units, and the Keysight N5181 MXG live here.
Vector signal generators
Everything an analog RF generator does, plus digital modulation. I/Q, real modulation schemes, the actual waveforms of 5G, LTE, Wi-Fi, radar, GNSS. If you are testing anything that carries digital data over RF, this is the tool, and nothing cheaper will do the job. The Agilent E4438C ESG, the Keysight N5182 MXG, the N5166B CXG, and the Rohde and Schwarz SMW200A and SMB100A are the names you will run into.
Arbitrary waveform generators
The wildcard. An AWG plays back any waveform you can define point by point — complex modulation, replayed real-world signals, distorted or impaired waveforms for stress testing, radar scenarios. When you need a signal that does not come out of a standard modulation menu, this is what generates it. They cost more and you buy one when a vector generator's built-in formats are not enough.
Browse what we have in stock: RF signal generators, vector signal generators, function generators, and arbitrary waveform generators.
The specs that actually decide the purchase
Datasheets list thirty numbers. Four of them usually decide whether the instrument fits.
Frequency range
Obvious, and still the thing people get wrong, usually by not leaving headroom. If your highest signal of interest is 6 GHz, do not buy a 6 GHz generator, because you will want to look at harmonics and you will want margin. Buy past your top frequency, not to it.
Phase noise
This is the one that separates a good RF source from a mediocre one, and it is the spec people ignore until it ruins a measurement. Phase noise is how clean the carrier is close in. For receiver testing, blocking tests, or anything where a nearby strong signal can mask a weak one, phase noise is the whole game. It is quoted as dBc/Hz at an offset, like −137 dBc/Hz at 20 kHz offset on a 1 GHz carrier — more negative is better. If your source's phase noise is worse than the thing you are testing, you are measuring your generator, not your device.
Output power and level accuracy
How much power it can put out and how accurately it hits the level you set. Matters enormously for calibrated receiver sensitivity work, where you are setting a level near the noise floor and you need to trust it.
Modulation capability
Analog only, or vector, or arbitrary. This maps back to the category question and it is where the options pricing lives. On a used vector generator especially, the installed options determine what it can actually do, and two units with the same model number can be very different instruments depending on which licenses are loaded.
That last point is the one that costs people money on the used market, so let's stay on it.
Options are the whole story on used vector generators
An E4438C or an N5182A is not one instrument, it is a platform. The digital modulation formats, the baseband generator, the extended frequency, the low phase noise option — these are all separately licensed. A bare unit and a fully loaded one look identical and behave completely differently.
So when you buy a used vector signal generator, the model number tells you almost nothing. The option string tells you everything. Before you buy, get the full option list and confirm it includes the formats and the frequency and the baseband you actually need. We have watched people buy an E4438C thinking they were getting 5G-capable digital modulation and receive a nearly bare analog-ish unit that technically shares the model number. Read the options. On these instruments, the options are the product.
The failure that is not a failure
Here is a detail worth the whole article, because it saves you a repair bill.
The HP and Agilent 8648 with the pulse modulation option throws "NO RF OUTPUT" and people ship it back for repair constantly. In a large number of those cases, nothing is wrong with the instrument. The pulse modulation is simply switched on, so the carrier is being gated off, and cycling the power does not reset it because pulse modulation does not return to a default state on power-up. Turn pulse modulation off and the output comes back. Keysight has this documented in their own knowledge base because so many good units came back for no reason.
Real faults, when it actually is broken
When it is genuinely a hardware problem, these are the ones we see most.
No RF output (not the pulse-modulation trap)
Usually means an output amplifier, the ALC (automatic level control) loop, or the attenuator. Attenuator problems are common on instruments that have seen heavy production use, because mechanical step attenuators wear out after enough cycles.
Level accuracy drifting out of spec
An ALC or detector calibration issue, and often just a calibration rather than a repair.
Phase noise degraded
Points at the reference or the synthesizer section, and matters because a generator that has quietly lost phase noise performance will fail the very receiver tests you bought it for without obviously looking broken.
Frequency unlocked or off
An OCXO or reference problem. The oven-controlled oscillator ages, and an old reference drifts. On a lot of older units this is repairable and worth it.
Modulation not working
On vector units this can be the baseband generator; on analog units, the modulation section. And plain power supply and display failures, which on decade-old instruments are just age, and are usually the cheapest thing on this list to fix.
If you have a signal generator doing something unexpected, our test equipment repair service handles bench-level diagnosis on these — send us the model number and what you are seeing.
Calibration, and why it matters more here than on most instruments
A signal generator is a source of truth for everything downstream of it. If your generator's level is off, every receiver sensitivity number you take is off. If its phase noise has degraded, every blocking and selectivity measurement is optimistic. If its frequency reference has drifted, everything is shifted. Unlike a lot of instruments where a small error is an annoyance, a signal generator's errors propagate into every measurement that uses it.
So the calibration on these actually matters, and a real calibration verifies the things that decide whether the instrument is trustworthy: frequency accuracy across the range, output level accuracy and flatness versus frequency, harmonic and non-harmonic spurious, and phase noise. A certificate that just says "pass" tells you nothing. You want the measured numbers at each point, because a generator that barely passes today is a generator that fails next quarter, and the numbers tell you where it sits. See our calibration process →
Common questions
What is the difference between a function generator and a signal generator?
A function generator makes basic waveforms at lower frequencies for general bench work. A signal generator, in the RF sense, makes clean high-frequency carriers with modulation for RF and communications testing. Different tools for different jobs. If someone uses the terms interchangeably, ask what frequency and what modulation they actually need.
Do I need a vector signal generator or will analog do?
If you are testing anything that carries digital data — 5G, LTE, Wi-Fi, digital radar — you need vector. If you are doing receiver sensitivity, filter, or basic RF work with analog modulation, an analog RF generator is cheaper and sufficient. The dividing line is digital modulation.
Why are two used units with the same model number priced so differently?
Options. On vector generators especially, the installed licenses and hardware options define what the instrument can do, and they are expensive when bought separately. A loaded unit is worth far more than a bare one with the same model number. Always get the option list.
My signal generator says NO RF OUTPUT. Is it broken?
Often not. Check whether pulse modulation is on, whether RF output is toggled off, and whether the amplitude is set very low. On the 8648 with pulse modulation, this exact message is usually just pulse mod left on. Rule out configuration before assuming hardware.
How often should a signal generator be calibrated?
Annually is the common interval, more often if it is a reference instrument for calibrated measurements or if it sees heavy use. If your downstream measurements depend on trusting the source, keep it in cal.
Can you repair discontinued signal generators?
Many of them, yes. The HP and Agilent 8648, 8644, ESG E4438C and E4428C, and the older synthesized units are all repairable well past the point where the manufacturer stopped supporting them. Attenuators, ALC boards, references, and power supplies are the usual repairs.
What phase noise do I actually need?
Depends on what you are testing. For general work, standard phase noise is fine. For receiver blocking, close-in selectivity, or testing anything with tight adjacent-channel requirements, you want the low phase noise option and you want the source cleaner than your device under test. If you are not sure, tell whoever you are buying from what you are testing and let them work backward to the spec.
What we do with these
We buy, sell, calibrate, and repair RF signal generators, vector signal generators, function generators, and arbitrary waveform generators across Keysight, Agilent, HP, Rohde and Schwarz, and Anritsu. That includes the discontinued units the manufacturer no longer supports — the 8648 and 8644 families, the ESG and MXG lines, and older synthesized sources.
Calibration is in-house and NIST-traceable, with the measured frequency, level, spurious, and phase noise data on the certificate rather than a pass sticker. If you are trying to figure out which generator fits your work, or you have one that is misbehaving, send us the model and what it is doing. Half the time the fix is a setting, and we will tell you that honestly before you ship anything.
Not sure which type fits your test, or trying to figure out if that "NO RF OUTPUT" is a setting or a real fault? Send us the model number and what you are seeing. We will tell you what we think before you spend anything.