It’s been a while since my last post in this thread.
I started this thread because I had a great opportunity to review two RF Signal Generators – the Rigol DSG815 and the Siglent SSG3021X – before making a decision on which one I should buy.
It’s not often that a test equipment distributor – who supplies BOTH brands of signal generators is welcoming in allowing their customer to decide which of the two signal generators best suits their needs.
With an opportunity like this you simply do need to take advantage of this offer and make the most of it, not only for myself to find out which Signal generator best suits MY needs, but at the same time take all of the interested forum members for a ‘informative ride’ during this evaluation / test period.
Well the evaluation time has come to an end and I do need to make my decision as to which of the two signal generators I will purchase.
So here is MY rationale and findings during this evaluation period, what I discovered, what was important to me, and how the DSG815 performed – most importantly – did I decide to buy this unit and if so, why?
OK, let’s start at the beginning.
I have an interest in RF circuit design – in particular microwave frequencies and higher.
Unfortunately it was not until now with covid-19 that I have had an opportunity to ‘learn’ as much as possible about this very interesting area RF circuit design.
For those of you already working or exposed to this area of electronics, you will no doubt know that when your test equipment needs move into frequencies above 10GHz – the cost of this ‘kit’ becomes very serious $’s
For someone who currently does not have a ‘paying customer’ to fund their test equipment acquisition, my choices in selecting and purchasing test equipment (TE) were guided by various factors of which cost was one of the most critical.
So exactly what TE do you need when you want to do RF circuit design?
Since with RF we usually want to know about things in the frequency domain, first on the list is a Spectrum Analyser.
At the time and after quite a deal of research, I decided to purchase the Siglent SSA3021X Plus.
Sure it’s not a >10GHz Spectrum Analyser, but as a starting point it’s a good ‘low end’ device which with the help of the forum members here can be ‘enhanced’ to a good bang for buck device.
To get the most out of any TE, I strongly believe that you must be familiar with each instrument’s limitations and theory of operation. It is only then that you can truly appreciate any measurements you make and more importantly how to meaningfully interpret them.
The Siglent SA was my starting point.
The second and very important TE for a RF circuit design lab is the Oscilloscope.
Once again, keeping it ‘real’ I decided to get the best value your $ can buy – the Rigol 5074 MSO with the associated Logic probes. This is a fantastic bit of ‘kit’ – especially after it is ‘enhanced’ – it’s a device that cannot be beaten for price performance.
We finally are now approaching to a VERY critical piece of TE which I believe is crucial to any RF Circuit design lab – the RF Signal Generator.
Please note – I specifically am talking about an RF signal Generator – not a AWG or function generator – that can produce Sine and Square waves.
What’s the difference?
Big difference (outside the scope of this post for a detailed explanation) and you will note that for a high end AWG you can pay $1500, but for a high end RF Signal generator you will pay over $15,000 – a very big difference.
In simple terms – it’s because there is a great deal of advanced RF circuit design ‘inside’ each and every good RF Signal generator – which is hard to engineer especially at a very low cost.
In fact there is a little paradox here, in order for me to learn about RF Circuit design, I need an instrument – the RF Signal generator – which itself needs instruments like itself in order for the design engineer involved in its design can design a ‘clever’ and very functional instrument for a reasonable cost.
When you are dealing with high frequencies – you are moving into a fascinating area where you are now faced with common ‘electrical’ concepts that no longer behave as one would expect – and more importantly there are now new areas of physics and quantum mechanics which govern the design and behaviour of these circuits and devices you are designing and manufacturing.
If you have seen some of the teardown videos of RF Signal generators Dave made – you will see why he gets ‘excited’ when he peeks inside one of these units.
In fact it was the teardown of the Rigol DSG815 (thank you Rigol for providing this to Dave all the way back in 2015 – a pity that Siglent did not do same when they released the SSG3000X range) that drew my attention to the DSG800 series.
It is no doubt a VERY clever ‘clean’ and functional compact design – especially way back in 2015!
It’s interesting, that in RF design – by simply looking at the circuit board and the remaining mechanical design – you can immediately ‘tell’ if the device has any merit and is likely to have a reasonable performance.
The DSG800 series ‘layout’ just tells you this straight away.
Was this because Rigol has very clever RF engineers?
OR
Is it more likely because R&S has some very clever RF engineers?
I personally think it’s the latter.
Why?
Because it takes quite a few ‘iterations’ not to mention innovations in the circuit design to achieve a ‘ready to manufacture’ product.
Since the DSG800 series was Rigol’s first entry in the RF Signal Generator marketplace (IMHO) – they entered the market at a very high level ‘good performance’ device – something virtually impossible to do in a short period of time without ‘significant help’ from looking at R&S and HP / Agilent offerings.
As the saying goes – ‘don’t do the crime if you are not prepared to do the time’
With RF design – it’s almost identical you can’t make a decent product – if you have not put a massive amount of engineering time to design / test and develop.
Any company who introduces a complex high performance RF test equipment device - cannot do this as a single iteration.
I guess this is further supported by the fact that Rigol has not ‘updated’ the DSG800 series for over 6 years – probably because in this specific market sector (cost vs performance of the DSG800 series) there are no ‘new’ R&S or HP / Agilent designs which could be used to ‘inspire’ Rigol with a revised, higher performance, release of a new RF Signal Generator.
Enough of the ‘politics’ of product development.
The main point I wanted to make is that RF signal generators are a ‘special breed’ of signal generators and should not be confused with AWG / function signal generators which have mainly DAC’s which ultimately produce the waveforms.
So why did I choose the Rigol DSG815 as a contender to purchase?
1. Price
2. Size (it’s very small)
3. Performance (yes it’s the 3rd choice – because if the other two fail – then performance will not be an issue)
4. Future ‘enhancement’
I did not want to pay more than GBP1500 + VAT
With a reasonable discount from my distributor – this was possible.
Before covid-19 I did considerable traveling – so being able to take the RF Signal Generator with me as cabin luggage on a flight is important.
Since I have already invested in a Spectrum Analyser – which could be capable of 3.2GHz response , an RF Signal generator of AT LEAST the possibility of 3.2GHz Frequency response would be most welcome.
The performance of the DSG815 is very good – it simply ‘does what’s written on the tin’ and then some – it does it extremely well!
From all of the impromptu testing I did, the DSG815 operates well within its published specifications.
One of the features which I really need for my design work and testing is the ability to have external modulation input.
Both the Rigol DSG800 range and the Siglent SSG3000X range supports the external modulation input via a BNC connector.
Having said that you should be
BEWARE
That not all inputs are the same!
I found out that the Siglent SSG range – including their more expensive 6000 range (in fact currently ALL of their RF signal generators currently sold, have a 50 Ohm – low impedance eternal modulation input.
This is a terrible oversight by Siglent – who have simply removed themselves from my shortlist and possibly many others who need to use external signals which are ‘high impedance’ usually at least 600 ohms or more.
On the other hand Rigol have included three options with regard to external modulation inputs on their RF signal generators.
1. 50 Ohm
2. 600 Ohm
3. High Impedance (>600 Ohm)
I guess when Rigol ‘designed’ their DSG800 series – they had not forgotten what R&S and HP / Agilent have used on their external modulation inputs.
Sometimes trying to be ‘creative’ – especially when you are a young engineer and think you are doing something clever – you are in fact killing the product.
Remember the R&S and HP / Agilent engineers took many years to design and ‘prove’ their products in the industry. If they support 600 Ohm and high impedance inputs – not just 50 ohm – then there is a VERY GOOD REASON for doing this.
So if you are being influenced by these old masters when you design your equipment – don’t leave things out because they don’t make sense to you (the young engineer) – if they have included an interface with 600 Ohm impedance – then this is a VERY GOOD REASON for you to also make a 600 Ohm impedance interface.
Well done to Rigol – for providing this very important feature!
Siglent – you failed here – learn you lesson.
Despite this I came up with a way for Siglent to recover from this design error.
The suggestion was to make an external ‘interface box’ which will provide a 600 ohm or higher impedance input and simply convert it to a 50 ohm impedance. For this to work effectively – you will need an active circuit and not a balun alone which will be lossy.
This ‘interface box’ will be put into circuit between your modulation source and the existing 50 Ohm external modulation BNC connector on the SSG range of devices.
So far no official response from Siglent.
Hopefully someone here on EEVblog forum who officially has links to Siglent Factory, can make this same suggestion.
The DSG800 series has a relatively ‘old’ user interface and a SMALL LCD screen – not touch screen – like the SSG range from Siglent.
Despite this being a letdown (who does not like a nice touch screen) from Rigol – its nevertheless very functional.
It take a while to get used to the menu structure on the DSG815, but when you do – its simply a few button presses and you are good to go.
Initially this was a negative for me, but after Siglent disqualified themselves from this head-to-head because of their total failure with regard to the 600 ohm external modulation input, I was forced to endure the ‘older’ Rigol UI until I finally realized it’s not that bad after all.
So here we are, the DSG815 passes the first 3 main points important to me in my purchase decision.
Now what happens with the final point – future ‘enhancement’
As most of us in EEVblog forum – we all appreciate including Dave (with open support from Rigol and Siglent) – the ability that BOTH Rigol and Siglent leave in their instruments the ability for potential software ‘enhancement’.
Basically, the ‘door is open’ to enhance your product to a higher specified and more expensive model.
This is why I have chosen the ‘base model’ DSG815
Having said this – currently there is no ‘official’ (publicly available) method of ‘enhancing’ the DSG800 series.
Despite this – by understanding the architecture of previous Rigol products and their design methodology with regard to software functionality – there are some very clever people here on this forum who love to ‘tinker’ at finding ways to ‘enhance’ products via software – via official but undocumented techniques, that they happen to work out how to do it exactly – because of the time and effort and ‘brain power’ they put into this challenge.
We, who are less fortunate wizards at software reverse engineering, simply benefit from the work these guys put into discovering ways to ‘enhance’ products.
At first, I got alarmed at even contemplating a possible ‘enhancement’ of a product using these techniques. But after a while and especially after reading lots of various posts here in EEVblog and seeing videos Dave made with the blessing of BOTH Riglo and Siglent – I came to the conclusion that BOTH manufacturers are very aware of the ‘enhancement’ of their products.
Do they care?
Probably not – because we are not the ‘regular customer’ from who they make the majority of their $’s
I guess members of EEVblog are a select group and significant enough to possibly increase BOTH of the manufacturers sales well in the 100’s possibly 1000’s of units – BUT - we are no means their ‘bread and butter’ business.
So any incremental sales to me and you – with the possibility of a future enhancement – is no big deal to Rigol and Siglent (IMHO)
So despite the fact that currently the DSG800 series has no official ‘enhancement’ and the Siglent SSG3000X series does have a way which was made public, I nevertheless have faith that very soon, the DSG800 series ‘enhancement’ procedure will be made public as well – allowing both of the competing RF Signal Generators to be enhanced in the future if so desired by their respective owners.
If you are still reading this post and reached this point – the next question is where do we go from here?
After reviewing the DSG815 and coming to the conclusion that it’s a capable RF Signal generator which clearly meets its published specifications – do you still want to ‘buy’ this device?
In the context of the review – in essence there were two contenders – the DSG815 and the SSG3021X, for my immediate needs – I believe that the DSG815 is a better device and has a better potential if ‘enhanced’ in the future. The HW of the DSG800 series is capable of a 3.6GHz frequency at the top end – significantly higher than the SSG3000’s top end of 3.2GHz.
The DSG815 is also a very compact unit – at least 30% smaller than the SSG3000 form factor.
The DSG815 also has a slightly better phase noise than the SSG3000 range.
The DSG815 does have a ‘functional’ External modulation input unlike ALL of the SSG’s range which only supports 50 Ohm impedance External Modulation Input (BEWARE of this if you will be using the External modulation input in your work).
The DSG815 is slightly less cost than the SSG3021X
So from a direct comparison point of view – the DSG815 ‘wins’ for my immediate needs.
But, is this it?
Or are there some ‘other’ alternatives which I (and possibly some of you reading this post) could explore before buying the Rigol or Siglent RF Signal generator offerings?
What are the alternatives you might ask?
Surely not the R&S and HP / Agilent offerings which are in a totally different price league?
No not at all
The alternatives are some of the ‘oldies’ – yes USED equipment – which is still serviceable today.
From the variety of ‘oldies’ available the most prevalent are the HP devices and more recently the Marconi Instruments (Now IFR) 2025 range.
The HP devices are extremely well engineered – but they are a ‘beast’ of a unit – large and power hungry. You also have to be extremely lucky to be able to pick-up a ‘decent’ fully functional unit.
On the other hand the IFR 2025 devices are currently more prevalent – mainly due to the UK ministry of defence (MOD) taking all of them out of service and dumping them to various surplus TE dealers.
Some of the IFR2025’s have only 100 hours on their ‘clock’ – that’s less than 5 days of continuous use during the last 15 years! Mainly due to the ‘redundancy’ policy of the MOD – to have available TE in stores for critical situations where higher than expected deployment is required.
Basically, if there is a ‘war’ and the UK government is getting hammered and their air force assets are in need of servicing – they will have enough ‘reserve’ capacity to do this – rather than put out an order to purchase another 100 IFR2025’s which will not be delivered until well after the ‘war’.
So, if you live in the UK and know someone who had access to this surplus MOD supply of TE – then you could be lucky and pick-up a 100 hour old IFR2025 for about GBP1500 – a bargain!
Despite the IFR2025 being over 15 years old (they still sell them today new so I am told) – they have a great specification and a much lower phase noise than BOTH the Rigol and Siglent offerings.
You also have FULL access to a detailed service manual – where you can perform your own ‘calibration’ if required – to the specific instructions and specifications as detailed in the factory service manuals.
I guess Marconi Instruments was not afraid to release the FULL schematics and BOM of these devices – simply because they have nothing to hide – the did totally design these units and did an amazingly good job.
I was fortunate enough to ‘sample’ one of these ex MOD units – it had 240 hours on the clock.
It was a IFR2025 with a high power option – this gave it 25dBm output at its maximum frequency of 2.51GHz
It also had a -140dBm attenuation level well beyond the Rigol & Siglents -110dBm levels.
The other most interesting observation – was that this device which was last calibrated in 2013 – was perfectly in spec (at least as could be measured on my Siglent SSA) with respect to the attenuation and power levels across its entire operating frequency range – not bad at all!
Part of me wants to buy one of these ‘100 hour’ IFR2025’s – but they are not easy to find and then the sellers want about same price as a new DSG815
Is the IFR2025 better than the DSG815?
Probably yes without doubt in terms of sheer performance and specifications.
BUT
It’s an old device and not as portable as the DSG800 series.
For me – I love the ‘oldies’ – if you can find a good one – they are a worthy investment.
I wanted to discuss the above – simply to let everyone know that if you are on a budget, have plenty of time (to find a good specimen) and are prepared to ‘tinker’ (possible calibration required) – then don’t rule out getting an ‘oldie’ as an alternative to today’s RF Signal Generator offerings from China.
I hope that this rather long ramble as a conclusion to my impromptu review and findings on the DSG815 have been helpful to anyone interested in purchasing a new RF Signal Generator.
I don’t like to promote and blow ‘sales’ related stuff in my forum posts
However
The DSG815 (and SSG3021X) review would not be possible if my Rigol & Siglent Distributor (telonic.co.uk) was not accommodating to allow me to do this.
For this I must thank them and give them credit in
supporting this forum community by allowing me to do these reviews
If you do decide to purchase either a DSG815 or the SSG3021X – please give them a try and tell them about this forum post – I’m sure you will get a significant discount.
If you found this DSG815 review useful, that’s great.
If you did not, then I'm sorry
Hopefully in the very near future I will publish one of my projects – on how to create a FM radio ‘test box’ which you can add to an existing RF Signal Generator that has an External modulation input (600 Ohms or Higher impedance).
With this device you will be able to ‘tune’ FM receives – for the stereo separation and modulation parameters as well as testing the RDS decoding system of the receiver
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