EEVBlog #823 - Rigol DSG815 RF Signal Generator Teardown

[EEVblog]

A look inside the Rigol DSG815 1.5GHz RF Signal Generator
https://www.emona.com.au/products/electronic-test-measure/rf-test-equipment/dsg-815.html#.VlVk43YrKUk

[Hydrawerk]

Thank you for the review. I wonder what the CapXon capacitors are like.
http://www.s-config.com/bad-capacitors-thanks-capxon-caps-and-samsung/
http://www.tomshardware.co.uk/answers/id-2279711/capxon-capacitors-bad.html

[rdl]

Another decent quality piece of equipment marred by an offbeat, gimmicky, and in some ways childish, front panel design. This one is nowhere near as bad as the DS1054Z or the ridiculous DP832, but I still think they need a change of personnel in that department.

[Wisnaeme]

Looks like the video got pulled by Youtube.
Hope it's back soon.

[janekivi]

Fortunately, I saw all that "spam, scams, and commercially deceptive content" and after page reload  it was gone.
How to do politically correct teardown... : )))

[Warhawk]

Looks like the video got pulled by Youtube.
Hope it's back soon.

+1

[Towger]

Mojo at work again? I managed to watch it during lunch without any problems.
I have to agree about those sideways push buttons. Otherwise looks good.

[bktemp]

Is it only me or do the 3 buttons on the left side of the display look weird?
Initially I thought Dave is using some kind of fish eye lens or something is wrong with the video because the buttons looked totally distorted. But then I realized the buttons really have the odd shape and all other buttons are also unusual.

[Bud]

Yes rigol must stop commercially deceive people with their garbage products.

[homebrew]

No, I'm still not able to view the video...

[PA0PBZ]

Not fair, here I am sitting with a drink and  ready to watch and now it's gone 

[kc0ngu]

Glad I caught it during breakfast (USA Central Time) before UTube idiots got their hands on it.
Hopefully it will be back soon, maybe the Batterizer team complained 

[Mashpriborintorg]

Yeah, that" scam deceptive click stealing"  teardown was removed by Youtube... or after a Rigol claim ? 
Dave, what about the progress of the big teadown you were speaking about  ? We will not be mad at you it requires a multi-part video, if it's easier to produce and manage than a freakin' long video   

[rdl]

Is it only me or do the 3 buttons on the left side of the display look weird?
...

That's the part I consider "childish".
I also wonder if CBS approved the use of their logo?

[Macbeth]

Bah. I was all set to watch this on my Raspberry Pi OSMC media player on my 37" and got the "You tube removed due to spam, scams, and deceptive content" nonsense. I figured surely this is just something wrong with the EEVBlog KODI plugin, only to find it doesn't work on my PC either 

This is either the Batteriser fan club (1 member), the SJW white knight eunuch who has lost his mojo, or the german guy who made that fantastic (nearly) double sided PCB for the "medical tablet PC", the one with the Manhattan Beirut style construction.

[Macbeth]

...or Rigol themselves... but surely not, not after the Riglent fiasco? 

[ElectricGuy]

I want the video back....... Who the hell i need to phone? 

[continuo]

Why was it pulled? 

[ElectricGuy]

Why was it pulled? 

Probably someone reported!!!!

[rdl]

I'd be surprised if it was pulled because of Rigol. I was busy with other things so I didn't see every minute, but it seemed to be overall a very favorable review.

[G0HZU]

I fast forwarded to the RF bit and I can add a couple of clarifications if that helps?

I saw the section covering the output stage and the part that Dave referred to as a possible attenuator is an amplifier. It looks to me like it's a wideband GaN part that uses a lot of D-G negative feedback to flatten the gain over the required bandwidth.

So I suspect that this amp gets switched in for high signal levels. In this application it's probably run at a lower drain voltage to make it more robust but I'd question the use of the feedback network in terms of its impact on stability. If I had one of these generators here, the first thing I'd be doing is to see if this amplifier can be made to go unstable into certain loads on the sig gen output.
The row of chips that have the tops ground off will make up the output step attenuator.

I saw the whole PCB and based on this initial glance I suspect that there may be little difference between the 3GHz version and the 1.5GHz version. Maybe just a doubler or an extra VCO range gets used here. Maybe this section is missing and/or crippled in firmware on the 1.5GHz version?

[number33]

Why was it pulled? 

Perhaps calling someone a mongrel is considered slanderous in Rigoland.

[Hydrawerk]

Was there something offensive in the video?? Sorry, I did not watch it. In the meantime you can see these photos made by Dave. https://www.flickr.com/photos/eevblog/albums/72157661062178650/with/22925949389/

[zze110]

Watched it, didn't see anything offensive.

If rigol pulled the video...

Rigol just pulled a Siglent?  or wait, Siglent pulled a Rigol?

Either way. 

Can hit the 'Download' button and get the video directly.  No RigTube needed.

[dr.diesel]

If rigol pulled the video...

I doubt Rigol had anything to do with it.  He's fondled many of their instruments and it was mostly a thumbs up video.

[jancumps]

If rigol pulled the video...

I doubt Rigol had anything to do with it.  He's fondled many of their instruments and it was mostly a thumbs up video.

They retweeted the video earlier today. They didn't take it down. It's going to be our weird friend

[donmr]

How can they call it SPAM?  Spam refers to unsolicited electronic messages.  If I ask to see it its not unsolicited.

[Royce]

CapXon? Surely they are used to it by now though.

The mongrel hypothesis is a good one too.

[boffin]

I'll get the popcorn. 

This is likely to be as much fun to hear, as the story about the guy who had his Siglent scope pulled off ebay.

[AF6LJ]

Looks like the video got pulled by Youtube.
Hope it's back soon.

Well That was BULLCRAP.
Why the Hell did they do that.
Oh Well, another company I will refuse to buy gear from be it used or new.
Go to Hell Rigol.

[jancumps]

Looks like the video got pulled by Youtube.
Hope it's back soon.

Well That was BULLCRAP.
Why the Hell did they do that.
Oh Well, another company I will refuse to buy gear from be it used or new.
Go to Hell Rigol.

I bet they didn't do it.

[c4757p]

Of course Rigol didn't pull a favorable teardown. Why the hell would they do that? Probably a mistake somehow.

[c4757p]

Another decent quality piece of equipment marred by an offbeat, gimmicky, and in some ways childish, front panel design. This one is nowhere near as bad as the DS1054Z or the ridiculous DP832, but I still think they need a change of personnel in that department.

Just opinions. I like their design

[Macbeth]

Another decent quality piece of equipment marred by an offbeat, gimmicky, and in some ways childish, front panel design. This one is nowhere near as bad as the DS1054Z or the ridiculous DP832, but I still think they need a change of personnel in that department.

Just opinions. I like their design

Me too. The DP832 is a great design. The DP832A was pretty awful - but they did concede and implement a "classic" UI. It's still not as good as the simple DP832 version though 

[eeadata]

i think CAPXON reports the video..

[Hydrawerk]

This download link does not work for me. http://www.eevblog.org/video/EEVBlog823-RigolDSG815teardown-640x360.m4v
Is there another link available?

[Macbeth]

i think CAPXON reports the video..

Oh come on CrapXCon are extremely well known by just about every poor bastard who has had to replace shitty caps on the planet. In fact every one of my fecked up PSU repairs has involved CrapXCon alone. I can only think they haven't rebranded because manufacturers are impressed with their ability to fail immediately outside the 12 month warranty period.

[dexters_lab]

Looks like the video got pulled by Youtube.
Hope it's back soon.

Well That was BULLCRAP.
Why the Hell did they do that.
Oh Well, another company I will refuse to buy gear from be it used or new.
Go to Hell Rigol.

sorry but there is no evidence yet about who took it down or caused it to be taken down. Waving the finger at Rigol is making an assumption at best 

[eeadata]

i think CAPXON reports the video..

Oh come on CrapXCon are extremely well known by just about every poor bastard who has had to replace shitty caps on the planet. In fact every one of my fecked up PSU repairs has involved CrapXCon alone. I can only think they haven't rebranded because manufacturers are impressed with their ability to fail immediately outside the 12 month warranty period.

You (and i too) think CAPXON is a Bullshit brand but may be they think they are better than nichicon or nippon or panasonic ...  And dave truely kill the brand in that video ..

[G0HZU]

I have seen enough in the video to work out an approximate frequency plan for how this sig gen works. Obviously I could do a better guess if I had access to the hardware but this looks like it is heavily based on a Hybrid DDS PLL.
i.e. there is a bank of 3 VCOs that feed a PLL chip and the reference for this PLL chip appears to be a DDS. The big square chip that Dave thought was a DAC is more likely to be a fast DDS. This looks like it gets clocked at about 650MHz by the 2600MHz Zycomm VCO plus a divide by  4 prescaler.

There is also a BFO downconverter that gets its main LO from this fixed /4 signal at approx 650MHz. This BFO presumably provides the lowest range on the generator across 9kHz to the lowish VHF region.

The strange little isolated region in the middle of the PCB looks to be the ALC detector that sets the level accuracy. It presumably is also part of the AM system and so it needs to work very precisely across the whole modulation envelope. So maybe Rigol had to crudely ovenise it to get respectable performance from it across various external temperature changes.

There is an Analog Devices DAC near the big DDS chip. I think it was an AD5449. I guess this is used for the modulation and LF generator output?

The block diagram below is just guesswork and it really needs to be refined and corrected by someone with access to the real hardware so they can correct the frequency ranges etc. But it's (probably) worth posting up as it is because it is a useful starting point at least .

[G0HZU]

Note that most lab sig gens that use the BFO system for low frequency generation run the BFO Local Oscillator (LO) at ten times the highest frequency.

So with a BFO LO of maybe 650MHz I expected this bottom range to run from 9kHz up to 65MHz rather than up to 100MHz. But I'm not sure if the VCO and prescaler system can get this low in frequency. Maybe it can? So I took a guess at 100MHz.

[dr.diesel]

And it's back up!

[ElectricGuy]

it's online!!!! 

[tautech]

i think CAPXON reports the video..

Oh come on CrapXCon are extremely well known by just about every poor bastard who has had to replace shitty caps on the planet. In fact every one of my fecked up PSU repairs has involved CrapXCon alone. I can only think they haven't rebranded because manufacturers are impressed with their ability to fail immediately outside the 12 month warranty period.

You (and i too) think CAPXON is a Bullshit brand but may be they think they are better than nichicon or nippon or panasonic ...  And dave truely kill the brand in that video ..

There was a recent thread on CAPXON with no reasonable proof they are any worse than other brands of recent manufacture:
https://www.eevblog.com/forum/chat/crapxon-engineers-morally-bankrupt/

[bitshape]

Not only Crapxon, but on the big board also 7 small SamYoung SMD elek.caps. But this time is GP series (general purpose 1000-2000h):   
SamYoung BDS (MVK) series:   (Recognizable by the 'K' mark on top.)

105C 1,000~2,000Hrs assured.
For CD/DVD-ROM, Navigation, LED MT/TV. 

Would really like to have seen Panasonic FK series here (or other Japanese Nichicon, Rubicon or NipponChemicon low imp. long life & high reliability caps) especially for the starting price-point of this product in EU (€2300,- inclVAT).

[G0HZU]

I marked up one of Dave's images of the RF PCB with a few labels.

Note that this is all just based on guesswork looking at the PCB. But this should agree with my earlier block diagram.

Hope it's (reasonably accurate and) useful

[Someone]

Could we see some more detail on that IEC inlet and ground bonding? There are several special features in the PCB, screws, and soldering required to meet international standards for integrity of the earthing, its usually easier to go with the traditional crimped wire to a eyelet held with a shake proof washer.

[Radio Tech]

Would love to have that on my bench!   

[G0HZU]

I'm not completely sure if the big square chip is 'just' a DDS but I think it needs to be able to tune very rapidly in order to support the FM modulation modes. So I think there will be a fast/efficient interface between itself and the big Spartan chip.
So it will need to have some of the features of a DDS. eg tiny frequency resolution (microHertz?) and very fast frequency tuning.

[bitshape]

BUt that 1000-2000h spec is for caps at 105^oC. Doesn't the lifetime get much longer at lower temperatures? I heard something like it doubles for each 10^oC lower.

Yeah, but for this equipment-price there are better suited (low imp & high reliability) caps on the market from Panasonic, Nichicon et al.
Also, Dave says the thing is eating 50 Watt's for breakfast! -> I bet those small caps get somewhat Hot around the reg.IC's, possibly failing before Crapxon does. 


And I don't like the leftside- & sideway-buttons/layout, also spotted a software bug "The instrument's fan has stoped" [28:01]
But further nice design and lot's of quality parts as my eyes could see if we left the E-LyticCaps out. 

[Ampere]

Great video, Dave. I was actually wondering how the signal was passed between those isolated modules right before you explained the hidden trace between the microvias. It's good to see something that's well-designed right after watching that tablet teardown.

Shame about those weird buttons, though. It seems like it was useless design step that didn't really add any value to the product. We've been using buttons that push straight down for a long time and nobody has ever said "I really wish that I could push my buttons to the side".

This is either the Batteriser fan club (1 member), the SJW white knight eunuch who has lost his mojo, or the german guy who made that fantastic (nearly) double sided PCB for the "medical tablet PC", the one with the Manhattan Beirut style construction.

Did I miss something? What does the SJW crowd have against Dave? Did he offend someone who identifies as a CapXon capacitor?

[c4757p]

Damn, a lot people don't like the buttons.

I'm curious about the output frontend. Looks like they have the attenuators before the output amplifier - is that normal? I'm no RF guy, but my intuition is that the SNR will be crap at low output amplitudes, with the signal almost swamped by amplifier noise. Shouldn't the attenuators come after the amplifier?

[retrolefty]

Looks like the video got pulled by Youtube.
Hope it's back soon.

Well That was BULLCRAP.
Why the Hell did they do that.
Oh Well, another company I will refuse to buy gear from be it used or new.
Go to Hell Rigol.

 Down girl, down. That some poster here speculated that Rigol had the video pulled is no reason for you to swear off Rigol just yet. What if they didn't do it all, which is more probable (in my speculation)?

 Facts can take time to appear but worth it in the long run. Hate can spread around the world before the truth can get it's pants on.

[G0HZU]

I had a closer look at some of the printed elliptic filters and I think the hardware may well support 3GHz operation?

Could the block diagram be closer to this one below?

If I assume the outer RF layer is 0.02" thick with a dielectric constant of about 4 then I think the first roofing lowpass filter after the bank of VCOs will cut off at about 3GHz and not 1.5GHz.

So could the 1.5GHz sig gen be crippled in firmware? Could the frequency plan for the block diagram be as below?
How thick are the outer layers of the PCB? Any images of the side view of the PCB?

All of the above is guesswork but the first ELP filter does look like a 3GHz design to me...

[EEVblog]

Well That was BULLCRAP.
Why the Hell did they do that.
Oh Well, another company I will refuse to buy gear from be it used or new.
Go to Hell Rigol.

Down girl, down. That some poster here speculated that Rigol had the video pulled is no reason for you to swear off Rigol just yet. What if they didn't do it all, which is more probable (in my speculation)?
 Facts can take time to appear but worth it in the long run. Hate can spread around the world before the truth can get it's pants on.

Yes, relax, it almost certainly wasn't Rigol.

[dr.diesel]

I'm curious about the output frontend.

Yeah, quick look only, but I suspect a similar market as the DSA815, mediocre performance, meh really.

[EEVblog]

Did I miss something? What does the SJW crowd have against Dave? Did he offend someone who identifies as a CapXon capacitor?

We had one SJW nutcase on this forum who has caused no end of problems and continues to stalk me and make my life hard (he's failed). Don't know if he's behind this, but he has threatened doing just this in the past, but it's the wrong flag if that's the case, this video was flagged as spam for some reason.

[c4757p]

Well, if G0HZU's diagram is accurate, the amplifier can be bypassed, which could give the best of two worlds: high amplitude output, and better linearity at lower amplitudes. If that's what they're doing, I totally missed that possibility, and totally approve.

[G0HZU]

I'm curious about the output frontend. Looks like they have the attenuators before the output amplifier - is that normal? I'm no RF guy, but my intuition is that the SNR will be crap at low output amplitudes, with the signal almost swamped by amplifier noise. Shouldn't the attenuators come after the amplifier?

I think the (GaN?) amplifier at the output is probably only there as a switchable booster amplifier when a high signal level is needed up at the top end of the sig gen's frequency range.

The loss in that suite of step attenuators up at a few GHz is probably enough to require a boost at high power levels at high frequencies?

It looks like there is another one of these GaN amps just before the suite of attenuator chips. But this one has the logo ground off it. Did someone forget to remove the logo from the final amplifier? Or maybe it is a replaced part (repair) on a demo unit and the repaired part hasn't had the logo removed?

Of all the parts on the signal path this output GaN amplifier is the one I would have expected to have the logo removed

It 'looks' like a Nitronex GaN part but the text/logo are not what I'm used to seeing. Not sure what it is unless it's some kind of lower spec clone of a Nitronex part?

[G0HZU]

Well, if G0HZU's diagram is accurate, the amplifier can be bypassed, which could give the best of two worlds: high amplitude output, and better linearity at lower amplitudes. If that's what they're doing, I totally missed that possibility, and totally approve.

Putting 'anything' after a decent ALC detector is a calibration risk on a wideband sig gen. In this case Rigol have fitted a string of attenuator chips (rather than a classic high performance mechanical attenuator with a flat frequency response) and also a switchable booster amplifier.

This introduces a lot of baggage wrt calibration so I suspect that this sig gen will have quite a complex calibration system. It will have to provide ALC correction factors across frequency that take into account the ripple/droop vs frequency in the attenuators and also the frequency response/ripple of the booster amplifier for when it is selected or deselected.

Also the quality of the AM can suffer because a classic ALC system on a sig gen will have feedback that can help to maintain a faithful AM modulation envelope even with a less than perfect amplifier. But that final GaN amplifier is outside the correction loop. So it will need to be quite well rated in terms of linearity if decent AM quality is desired

Also the output source impedance will probably not be that close to 50R across the range of that GaN amp. So the signal level (mismatch) uncertainty into less than ideal load impedances will probably be quite scary when the sig gen is in that mode with the booster amp selected.

[Monkeh]

BUt that 1000-2000h spec is for caps at 105^oC. Doesn't the lifetime get much longer at lower temperatures? I heard something like it doubles for each 10^oC lower.

Yeah, but for this equipment-price there are better suited (low imp & high reliability) caps on the market from Panasonic, Nichicon et al.

They are substantially more expensive.

Why must people get their panties in a twist over general purpose caps in applications where they are not stressed?

[rfeecs]

It 'looks' like a Nitronex GaN part but the text/logo are not what I'm used to seeing. Not sure what it is unless it's some kind of lower spec clone of a Nitronex part?

Wow.  I wouldn't have guessed they would use a GaN part for just 20dBm output power.  I suppose they are way backed off for decent harmonic performance.

It looks like it is this part based on the part number.  Seems like total overkill:
http://cdn.macom.com/datasheets/NDS-036r2%20NPTB00004A%20datasheet.pdf

[EEVblog]

Well, if G0HZU's diagram is accurate, the amplifier can be bypassed, which could give the best of two worlds: high amplitude output, and better linearity at lower amplitudes. If that's what they're doing, I totally missed that possibility, and totally approve.

Yes, that's likely what's happening. You wouldn't have a permanent amplifier after and attenuator in a performance instrument, that's dumb.

[G0HZU]

If that is the actual part they are using then they will be using it at a much lower drain voltage than 28V. I would question if it really is a Nitronex part. Could it be some kind of cheaper/derated clone with a similar part number?

[EEVblog]

Why must people get their panties in a twist over general purpose caps in applications where they are not stressed?

It's about best practice.
In a penny pinching low end scope, understandable. In a $2k-$3.6k instrument, not a good look.

[dr.diesel]

Why must people get their panties in a twist over general purpose caps in applications where they are not stressed?

I've replaced CapXon caps in stuff less than 5 years old, at "normal" temp conditions.

For $3600 (3Ghz model) using shit caps is inexcusable.   

[EEVblog]

Why is a 1000hr Nichicon more reliable than a 1000hr Capxon?

Better design and quality control.

[rfeecs]

If that is the actual part they are using then they will be using it at a much lower drain voltage than 28V. I would question if it really is a Nitronex part. Could it be some kind of cheaper/derated clone with a similar part number?

Doubt if it is a clone.
This thing is unmatched, with feedback only.  If they bias at 100mA, they would get a 5V peak swing into 50 ohms, giving 24dBm.  Minus losses.  So it's in the ball park after all.

[Monkeh]

Why must people get their panties in a twist over general purpose caps in applications where they are not stressed?

I've replaced CapXon caps in stuff less than 5 years old, at "normal" temp conditions.

For $3600 (3Ghz model) using shit caps is inexcusable.   

I wasn't talking about the CapXon caps.

[Alex Eisenhut]

Another decent quality piece of equipment marred by an offbeat, gimmicky, and in some ways childish, front panel design. This one is nowhere near as bad as the DS1054Z or the ridiculous DP832, but I still think they need a change of personnel in that department.

They're stylistically meant to be that way. So that it rhymes.

[crispy_tofu]

Damn, a lot people don't like the buttons.

I don't know about you, but those buttons on the left of the unit really irk me. Why are they italicised? I think they went a bit too far on the design... 

[halexa]

Hi,

I notice this RF-Voodoo thingy in the video, firts i thought it was the Open source hardware loggo. Just wondering if any one knows what it does?

[f4eru]

I notice this RF-Voodoo thingy in the video, firts i thought it was the Open source hardware loggo. Just wondering if any one knows what it does?

That's a coaxial test pad.
see page 9: http://www.ingun.de/media/pdf/Werbemittel/INGUN_RF_Catalogue_EN.pdf

I wonder what U801 is (the temperature stabilized part marked TKK)
Perhaps it's a RSSI meter so the power can be regulated to a precise value...

[G0HZU]

I wonder what U801 is (the temperature stabilized part marked TKK)
Perhaps it's a RSSI meter so the power can be regulated to a precise value...

My guess is that the 6 pin package just contains a couple of matched detector diodes.

One will be used as the RF envelope detector diode for the ALC and the other will be used as part of the detection compensation. i.e. it will be used to cancel out the Vdrop in the RF diode over temperature. The oven will presumably be used to make this diode compensation scheme even more stable.

[G0HZU]

I had a look through this Rigol datasheet and there are a few clues wrt the frequency plan there.

http://beyondmeasure.rigoltech.com/acton/attachment/1579/f-0636/1/-/-/-/-/DSG800_DataSheet_EN.pdf

eg it lists 5 bands on the second page and this gives a clue as to the division ratio associated with each range.

So I tried to amend my block diagram to include this info. See below for another attempt to produce a frequency plan for the 3GHz version. I still think the same hardware may be used for the 1.5GHz version. However, one thing I can't fully explain yet is the phase noise plot they provide on page 3.

The traces for 3GHz and 1GHz make sense because there is a divide by two on the 1GHz range plus it will use a VCO running at 2GHz. So you can expect the phase noise to be 6dB lower because of the divide by two and maybe another 4 dB lower because of the 2GHz VCO. So the phase noise at 1GHz is about 10dB better than the phase noise at 3GHz.

But I can't explain the phase noise plot at 100MHz.It seems lower than I expected and this implies there are some more stages of division somewhere. Otherwise, I'd expect the phase noise profile at 100MHz to only be a couple of dB lower than the phase noise profile at 1GHz if my block diagram is correct.

The table on page 2 only quotes a divide by 4 for 100MHz so this agrees with my block doagram but this means a cleanup of (only) about 12dB wrt the phase noise at 3GHz and then the BFO is used to downconvert to 100MHz.

So something doesn't add up here because the cleanup at 100MHz is more like 24dB. I'll try and spend some time looking at the other side of the RF board to try and spot an extra divider (divide by 16?) and an associated lowpass filter. The phase noise plot suggests that 100MHz is achieved with a simple divide by 16 rather than the  'divide by 4 plus BFO' shown in my block diagram.

[f4eru]

I wonder what U801 is (the temperature stabilized part marked TKK)
Perhaps it's a RSSI meter so the power can be regulated to a precise value...

My guess is that the 6 pin package just contains a couple of matched detector diodes.

One will be used as the RF envelope detector diode for the ALC and the other will be used as part of the detection compensation. i.e. it will be used to cancel out the Vdrop in the RF diode over temperature. The oven will presumably be used to make this diode compensation scheme even more stable.

Could be, but there are also nicely integrated log amplifiers/detectors, with 70dB dynamic range.
for example : http://www.analog.com/en/products/rf-microwave/rf-power-detectors.html#envelope-peak-detectors
Also, it's referenced "U" for IC, not "D" as a diode pack, but that's just supposition.

What's the dynamic range to be expected from a nicely compensated diode detector ?

[G0HZU]

It's not going to be a logamp or one of the true rms detectors because it will almost certainly need to be used for the AM modulation as well as the ALC. So it will need to be able to track the modulation envelope quite accurately and produce an accurate demodulation waveform to feed back to the error amplifier in the ALC system. So it needs to be very, very fast and it needs to track the outer envelope of the AM waveform very accurately

In AM mode the wanted baseband AM modulation waveform is typically fed to the reference pin of an error amplifier in the ALC feedback loop. So the ALC detector diode will try and track the RF envelope and ensure its detector output is fast enough to track the baseband waveform. This is exactly what you want for generating good quality AM modulation

I think it will be a classic dual diode from someone like Agilent/Avago. eg one where the two diodes are matched but not connected inside the 6 pin package. eg something like the classic HSMS-28xx series?

 

[G0HZU]

What's the dynamic range to be expected from a nicely compensated diode detector ?

I guess it depends on the complexity of the design but the classic way to provide AM and ALC in a lab sig gen is to use a decent diode based envelope detector. This method dates back many decades and a classic example is the HP8640 lab sig gen from the early 1970s. I think that all of my various sig gens I have here use this method. I expect Rigol will adopt a similar system.

[G0HZU]

I had a rummage through a few diode detector datasheets and I think the ALC detector (marked as TKK) could be a HSMS-286K dual diode from Agilent/Avago?

[G0HZU]

I also had a closer look at the big square chip (DDS?) and this looks like it may well be a dual DAC rather than a DDS.

I also missed a divide by two prescaler at its clock input so could it be an AD9747 Dual 16 bit DAC from Analog Devices? The pinouts look similar at a glance and it appears to have dual outputs so maybe one output feeds the main PLL as an agile reference signal and the other gets used for LF signals?

I wonder if Rigol are overclocking it a bit because it looks like it is getting clocked at something like 2600/8 = 325MHz.

It's providing the same functionality as a DDS in terms of producing an agile (small stepping) reference for a hybrid PLL but it looks like the Spartan chip can control this DAC fast enough to provide an HF reference signal with fast modulation rates etc. Impressive stuff indeed...

[rfeecs]

The video clearly shows the VCO is a CRO3640B.  Dave even reads out the part number.  Not sure why he pulled up the wrong data sheet, except that there is none on the web site for the right part number.  There is a CRO3640A data sheet.  The frequency is 3640MHz:
https://www.zcomm.com/pdfs/datasheets/CRO3640A-LF.pdf

[f4eru]

I guess it depends on the complexity of the design but the classic way to provide AM and ALC in a lab sig gen is to use a decent diode based envelope detector.

I didn't know that the proper way to generate quality AM is to implement an AF closed loop through a detector. Nice to know

[G0HZU]

The video clearly shows the VCO is a CRO3640B.  Dave even reads out the part number.  Not sure why he pulled up the wrong data sheet, except that there is none on the web site for the right part number.  There is a CRO3640A data sheet.  The frequency is 3640MHz:
https://www.zcomm.com/pdfs/datasheets/CRO3640A-LF.pdf

Good spot...

I lazily went with Dave's datasheet that showed ~ 2600MHz. This solves one issue I had with the 3GHz variant. It didn't seem to be a good idea to have a permanent VCO that sat inside the 3GHz tuning range of the sig gen because this can lead to beating effects if the sig gen is tuned to a nearby frequency.

So with this VCO up at about 3640MHz I also had to revisit what was fitted as the big mystery DAC. This is because I now think it will be clocked at 3640/8 = 455MHz.

Maybe it is something like an Analog Devices AD9122 DAC?

[GK]

At a rough guess how close would the BOM for all PCB parts including all specialist RF ICs bought in one off/single-figure-small quantities compare to RIGIOLs retail price for the complete instrument?

[AF6LJ]

The video clearly shows the VCO is a CRO3640B.  Dave even reads out the part number.  Not sure why he pulled up the wrong data sheet, except that there is none on the web site for the right part number.  There is a CRO3640A data sheet.  The frequency is 3640MHz:
https://www.zcomm.com/pdfs/datasheets/CRO3640A-LF.pdf

Good spot...

I lazily went with Dave's datasheet that showed ~ 2600MHz. This solves one issue I had with the 3GHz variant. It didn't seem to be a good idea to have a permanent VCO that sat inside the 3GHz tuning range of the sig gen because this can lead to beating effects if the sig gen is tuned to a nearby frequency.

So with this VCO up at about 3640MHz I also had to revisit what was fitted as the big mystery DAC. This is because I now think it will be clocked at 3640/8 = 455MHz.

Maybe it is something like an Analog Devices AD9122 DAC?

Nice job on reverse engineering this.

[ktulu]

Did anyone noticed the 3 LED looking diodes in the VCO section? (D200, D205, D210)

[G0HZU]

Nice job on reverse engineering this.

Thanks   I can only guess at all of this though because I don't have access to the hardware. So there are bound to be mistakes.

Here's an updated block diagram although this block diagram still doesn't explain the very low phase noise on the 100MHz trace on Rigol's datasheet. So something still isn't right somewhere.

But the 5 bands shown in my block diagram agree with the 5 frequency bands shown in their datasheet in terms of frequency range and divsion ratio.

Did anyone noticed the 3 LED looking diodes in the VCO section? (D200, D205, D210)

Presumably these wink on and off as each VCO gets selected. This would help an engineer during the dev phase.

The transistor used for the VCO appears to be an Infineon part = BFP450. I have a non linear model for this part here and I simulated the VCO layout for each range using this part. I adjusted the C-E capacitor for best negative resistance and the little printed resonator and the two series varactor diodes do seem to deliver the required oscillation frequency to cover about 1800MHz to 3600MHz in three ranges if I use typical capacitance values for this type of VCO tuning diode.

I'm estimating that the 50 ohm traces are about 0.04" wide and the outer RF layer is somewhere around 0.02" thick with a dielectric constant around 4 to 4.3.

The elliptic lowpass filter marked as 3000MHz in my block diagram does seem to cut off at just over 3GHz if I simulate it in Sonnet using the PCB data above.

This (and the VCO resonator dimensions) does suggest that this hardware is designed to cover up to a 3GHz frequency range.

So maybe the 1.5GHz version is just crippled in firmware? So maybe it can be hacked to become the 3GHz version?

[G0HZU]

If Dave still has the sig gen I can suggest some (spurious) tests to see if the block diagram is valid? i.e. places to look for unwanted spurious effects of this frequency plan.
The sig gen doesn't have to be taken apart again to do these tests. Anyone can do them if they have the generator and a spectrum analyser.

[rfeecs]

The transistor used for the VCO appears to be an Infineon part = BFP450. I have a non linear model for this part here and I simulated the VCO layout for each range using this part. I adjusted the C-E capacitor for best negative resistance and the little printed resonator and the two series varactor diodes do seem to deliver the required oscillation frequency to cover about 1800MHz to 3600MHz in three ranges if I use typical capacitance values for this type of VCO tuning diode.

Awesome reverse engineering.  Maybe you're becoming a little obsessed?
I kind of wonder why they bought the Z-Comm VCO instead of making their own.

I'm estimating that the 50 ohm traces are about 0.04" wide and the outer RF layer is somewhere around 0.02" thick with a dielectric constant around 4 to 4.3.

The elliptic lowpass filter marked as 3000MHz in my block diagram does seem to cut off at just over 3GHz if I simulate it in Sonnet using the PCB data above.

This (and the VCO resonator dimensions) does suggest that this hardware is designed to cover up to a 3GHz frequency range.

So maybe the 1.5GHz version is just crippled in firmware? So maybe it can be hacked to become the 3GHz version?

It does look like it.  It seems bizarre that they sell the same unit for 1.8 times the price for 3GHz vs 1.5GHz.  But they do that with oscilloscopes, too.  I find it borderline morally bankrupt.

[G0HZU]

Maybe you're becoming a little obsessed?
I kind of wonder why they bought the Z-Comm VCO instead of making their own.

I've designed a few xGHz VCOs in my time but nothing as good as that Zcomm unit in terms of phase noise. It does produce very low phase noise
It needs to have extremely low phase noise because it is required to provide the low noise clock for the DAC and also the BFO LO. There will be 12dB cleanup in the divide by 4 for the 910MHz BFO and this will mean that the phase noise of the BFO will be insignificant wrt the phase noise from the discrete (~3GHz?) Rigol VCO #3 after it has been divided by 4.

If I chuck some typical design parameters into a basic oscillator design equation then this shows that this is a special design by Zcomm. It helps that it is a narrowband VCO but I suspect that I would struggle to design a VCO that could get within 10dB of that phase noise response. I could easily manage to get within 15dB and I'd expect to get within about 10dB but I've never designed anything that clean up at 3.5GHz before.

It's tempting to try but I think I'd end up using more DC input power and more space even if I got close to the Zcomm spec...

[KE5FX]

Yep, rolling your own VCOs is not a good plan in any commercial design unless your name is Rohde or Leeson or Scherer or something like that.  Companies like ZComm and Synergy Microwave have put decades' worth of R&D into those things. 

Looks like the signal path for 100 MHz output is completely different; they aren't just dividing the output from a UHF synthesizer block.  There's a PLL shoulder at 10 kHz that's completely absent in the 100 MHz trace.

Also, I would expect them to use an offset mixing scheme in their hybrid PLL instead of multiplying a DDS... but then, I'd expect lower PN if they did that.

[G0HZU]

Yes, I can't explain the odd looking phase noise trace at 100MHz.

Maybe there is an extra range squeezed in that covers a segment around 100MHz. Maybe this feeds direct from the DAC and bypasses the system that uses the 3 VCOs and the dividers. So the phase noise would be better in this sweetspot band of frequencies.

The signal path that has the AM/ALC attenuators looks like it has blocking caps designed for VHF and above so I'd expect the BFO circuit will be used for 9kHz through to several tens of MHz at least. But maybe the next range up will be this DAC/DDS driven range.

At work, we stopped designing our own VCOs (that go into saleable products) many years ago. The cost is way too high because of all the NRE time/costs in order to prove the design over temperature and shock and supply/load pull etc etc.  However, you do still need to know a bit about VCOs when selecting a suitable VCO because even the commercial ones will sometimes have 'gotchas' built into the design. With experience it is possible to look through a VCO datasheet and see what is 'missing' or different in order to predict what these gotchas are.

I do enjoy designing oscillators up in the microwave bands although these days I only design them as test oscillators.