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R&S SMIQ as a replacement for a general signal generator?

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KJDS:

--- Quote from: ivaylo on October 22, 2015, 05:45:45 am ---I replied to your other thread. E4433 with UND can be found cheaper in the states and many sellers ship internationally.

Why are those E4406 so cheap (well, so to speak)? Limited spec or what?

--- End quote ---

E4406A are cheap because they're no longer wanted my modern businesses and Nokia put about a thousand onto the second user market a few years ago. I've never seen more than ten signal generators for sale at the same time.

trukresom:

Since the OP mentions the possible use of an SMIQ with AM and FM I would like to underline
that the basic SMIQ is not equipped to do FM. For FM and PM the SMIQ has to be fitted
with the Option B5.
A characteristic of the SMIQ which I find is very pleasant is the fact that when making small
changes in frequency with the knob, the output amplitude remains constant. This is not the
case for example with my HP8644B; even the smallest frequency change (0.01 Hz) causes
a drop of the output amplitude of almost 100% for a few milliseconds.

G0HZU:

--- Quote ---I'm still not sure which model I should choose...Agilent or R&S....I like both brands...
--- End quote ---

Looking at your wanted ad in the Buy/Sell section are you sure you really need to purchase a generator like this?

If you just want to learn stuff about vector modulation I'd recommend either making your own basic vector modulator or maybe just buy a basic vector modulator chip (or eval PCB) from one of the major chip manufacturers. eg Hittite or RFMD etc etc.

Build it and play with it and maybe use your C# skills to use a PC soundcard to drive it?

I really do think you will learn a lot more and learn it faster if you do it this way?

But if you really do want to buy a vector sig gen then either the Agilent or SMIQ will be fine for general research or learning.
However, you are going to end up with your head in their manuals a lot of the time doing stuff their way in terms of programming etc.

I would still repeat the warning about LF >> VHF phase noise performance though. Especially for the ESGD series from Agilent. If you expect this generator to 'replace' a decent shortwave band RF generator in terms of noise performance then you could hit problems if you wanted to do any critical receiver testing of something like a decent HF ham radio. The same applies if you wanted to use it as a temporary LO in a shortwave converter. The relatively high phase noise will markedly limit the performance of the converter in terms of reciprocal mixing compared to even a simple homebrew oscillator.

G0HZU:

--- Quote ---I disagree with the notion that ESG and SMIQ are not "lab grade" (which is a silly term anyways), unless you think "lab grade" means "best specs ever in all areas" (which it doesn't).
--- End quote ---

Generally speaking, a decent lab sig gen should produce phase noise performance in the same ballpark (or better) as a fairly basic oscillator designed for that frequency.

HP set the bar for this way back in the early 1970s with the HP8640. AFAIK HP designed this solid state generator to try to emulate the excellent phase noise performance of the vacuum tube based sig gens of the 1960s. They did it using a mechanically tuned octave bandwidth cavity oscillator (256-512MHz?) followed by a chain of dividers and roofing filters and a classic ALC levelling circuit.

In doing so they created the industry benchmark for phase noise (and spurious) performance for many years.

This old generator will produce very low close in phase noise when used in the MW and SW bands. The problem is that this type of generator is expensive and it also needs lots of switched LPFs to cover all the divider ranges. Also, using division means that accurate calibration of FM deviation has to be achieved at the cavity oscillator for every divider range.

Lots of other makes of sig gen have since adopted a similar oscillator + divider approach and a decent lab generator should at least try and get close to the phase noise performance of the old HP8640 across 'most' of its range.

(Modern generators often use several VCOs to replace the big old cavity oscillator in the HP8640 and this usually compromises performance. However modern generators can use modern synthesis techniques to clean up very close in phase noise so they can outperform the HP8640 in some areas)

Because it uses a cheap mixer based frequency plan to generate low frequencies (rather than using dividers and filters) the ESGD (AKA Economy Signal Generator Digital) generator fails this benchmark by about 40dB down in the lower parts of the SW band. eg for phase noise offsets of a few kHz at a CF around 10MHz or so. That's a pretty significant fail in terms of phase noise ;)

G0HZU:
In terms of vector modulator performance I can comment on the E4433 ESGD as I have a couple of them here. Both give very similar modulator performance but there are a few niggles...

The basic calibration of the IQ modulator is fairly good. I can run a full modulator calibration across the full range of the generator (takes a while to do this)  and then not bother to recalibrate it for quite a while and still get pretty decent carrier and image rejection on something like a multi carrier SSB plot. I generally don't bother to calibrate it for most tasks so it can go for weeks without a calibration.

However, for critical applications it does allow a User Cal that lets you calibrate the modulator across a narrow band of interest and this takes a few seconds. The performance of the IQ internal cal is quite good in terms of image rejection but if I had to fault it, the resolution of the manual trim is too coarse.

With a raw auto calibration it can deliver about 50dB carrier suppression and over 60dB image suppression for simple narrowband waveforms but the manual trim step size is too coarse to let you 'improve' the carrier null much beyond the auto calibration. The image suppression obviously degrades a bit for wider bandwidth signals that get close to the full modulation bandwidth of the modulator.

Overall, it's pretty good but I think it could be better.

Note: One of my E4433 generators is an older A model and it has a scary bug/feature during IQ calibraion in that it does the calibration with the generator set at full level with the output enabled!
This is not good if you have a circuit connected that could be damaged by a +20dBm signal!

The B version (sensibly) sets the attenuators to max during IQ modulator calibration so no signal leaks from the generator during IQ calibration.

It could be that my generator is faulty in this respect but I suspect that this is a firmware bug in this early model.

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