Half-decent diy RF generator utilizing AD9951 - an idea

[Yansi]

Hello,

currently lacking a RF generator in my "lab". As there is not enough funds to buy anything half-decent and components, PCBs or time investment is not a problem, I have jumped to the conclusion I should build one myself and get some new experience from that.

I have a bunch of AD9951 available. I know that there are some chinese modules with them, but they are not pretty. By half decent I mean:
A proper balun for the DDS, ALC and attenuator.  Up to 150MHz, starting at 100kHz (preffered). Up to 20dBm output power (at least 15dBm). Maybe AM/FM mod.

Here is the block diagram I have cobbled together. Nothing too complicated, but still a lot of work I think:



Description of the blocks:
DDS: AD9951 (datasheet)(400MSps with 14bit DAC - reasonable SFDR may be achieved).
Balun: I have a bunch of MiniCircuits TC1-1T+ transformers here.  (However not the best choice for the low end - requires 400kHz or more)
LPF: Some kind of elliptical filter - yet to find what the manufacturer recommending is.
AMP1: Whatever general purpose MMIC amplifier.
PIN attenuator: This is the part of the ALC block. I have mady my mind I could use BAP64Q PIN Quad to do this. May work still respectable at 150MHz.
LOG Detector: AD8307 (datasheet). I have bunch of those too. Has enough dynamic range to fully utilize the PIN Quad and is cheap.
AMP2: MMG3H21NT1, HMC482 or whatever I will be able to get or find.
Two output resistor pads with Omron G6K-2F-Y relays.

I am yet to calculate thoroughly the dBm in every part of the circuit, if I will be able to do it like so.  But it seems I can get about -5dBm (10mA DAC current, 1:1 balun at 50ohm) from the DDS itself, loose some dB more in the LPF. Then use the AMP1 to compensate the loss and to drive the "PA" through the PIN ATT, which sounds about right, that the AMP1 will need about 10dB of gain (cruel estimation).  The two output ATT stages will be done using relays and resistor pads.

The ALC loop utilizing the AD8307 and the PIN quad BAP64Q seems to be able to achieve about 50dB dynamic range at 150MHz. Which if the requirement was +20dBm output maximum, means the minimum with this configuration will be somewhere about -70dBm. Which sounds about halfway decent. The ALC servo amplifier will get a setpoint from a DAC to set the output level anywhere in its range.

As the AD9951 has an integrated multiplier before the DAC, I was thinking I could utilize that to to implement a simple AM modulation using just that, if one makes sure the ALC servo loop bandwidth will be low enough (<20Hz) to pass the modulation. The modulation signal (say audio bandwidth would suffice) would get sampled by an ADC, and converted into the amplitude control word for the DDS.
FM modulation of course also possible, by directly manipulating the FTW. This may be to cruel, but will do the job - so I am thinking.

A nice feature would also be to sync the generator onto a 10MHz external standard. What is not so nice is that the AD9951 requires at least 20MHz input. One would need an external PLL (and probably bypass the internal one in the AD9951 in that case to achieve better phase noise).

Can you please apdisprove my ideas are correct?

Thanks, Y.

[DC1MC]

I'm a bit of an older guy and form experience I was thinking that best is to define the the project goals:

 - Frequency range ?
 - Output range, impedance ?
 - Will it be modulated, and if yes, what kind of modulation will be ?
 - Will it have some kind of external reference (10MHz GPS source) ?
 - Will it be syncronizable with an external signal ?
 - What resolution and stability are targeted ?
 - Any other waveforms besides sine ?

 Once you defined your project goals, is much easier to do the architecture and practical design, of course IMHO and IMMV.

 Cheers,
 DC1MC

[Yansi]

I think all is defined above.

To sum up:

- Frequency range: 400k - 150M
 - Output range, impedance: +20dBm (or at least +15), down to at least -50dBm. 50ohm standard.
 - Will it be modulated, and if yes, what kind of modulation will be: AM/FM, audio BW.
 - Will it have some kind of external reference (10MHz GPS source): Would be nice to support one
 - Any other waveforms besides sine: Only "pure" sine.

Please bear in mind, I am doing this mainly to gather more experience in the RF field, so I will prefer to work with what I have already available and make some useful piece of equipment at the same time. 
I am not designing based on spec, I will be designing mainly based on what is available.

[rhb]

Looks damn good to me.  Build it.  I'll gladly buy a couple of your excess boards. 

Rather than jump through hoops over the reference frequency, just use what is convenient.  While 10 MHz is the most popular, there are lots of things that require different frequencies.  A number of QRP-Labs devices and the SDRplay RSP2 come immediately to mind. 

Leo Bodnar had designed a GPSDO with two selectable output frequencies which is sold by SDR-Kits.  As Leo's unit is at the same price as a BG7TBL 10 MHz GPSDO (~$200 US), I can't see any reason to complicate your design. Just use something other gear uses. 

However, the RSP2 uses a 24 MHz clock.  So with an RSP2 and your SG phase locked together you've got a cheap VNA which can be upgraded by adding one of Leo's GPSDOs.  I've urged SDRplay to produce something like what you're describing, so maybe they would be interested in producing your design. 

I should like to suggest you look at the FeelTtech FY-6600 thread especially around pages 14-15.  There are spectrum analyzer plots comparing it to an R&S AWG.  I do *not* recommend buying a FeelTech unit until they address the UI FW problem discussed in the reflashing thread.  Mine is a V 3.0 unit which borked itself just after I went through all the work of adding a grounded power cord.

A clean, low cost ($100-150), 14 bit sine wave from 0.4 to 150 MHz in combination with a 12 bit RSP2 would be an awesome piece of kit for HF design and test. 

[Yansi]

Well I will certainly build it simple parts first. Especially I would like to tinker a bit with the PIN quad. Never used any PIN diode attenuators before. Some testing boards are being prepared for it already.

I will take a look at the FY-6600 thread. Not going to buy that - at least not now.

[RadioNerd]

How do you plan to realize the 0.4 - 150MHz directional coupler for the ALC?

[Yansi]

It does not necessarily have to be directional.  I think a resistive splitter would work fine too in there.

[rhb]

Consider using FET muxes for the attentuators.  It might lead to a nice design.  Though perhaps at the price of having to select parts for each step in the attenuator to match the Ron of the FETs.

[Yansi]

What FET MUXing do you mean exactly? Can you give example please?

I do only know about off the shelf RF switches, but their isolation usually very much suck balls and the realys are still superior, at least to those cheap RF switches.

The Omron G6K-2F-Y relays are rather cheap - so I do not see an issue.  At 100MHz they are supposed to offer >45dB isolation.  An equivalent DPDT RF switch would cost a shitton more I guess.

[rhb]

Look at the TS5N412 SPDT for $2.96 quantity 1 from Digikey.  -50 dB isolation.  But if you use two, one at the attenuator input and one at the output I think the isolation would be better. Two devices would give you 5 attenuator steps.

I do *not* know if these would be better for your project.  I'm merely suggesting looking at them. 

Another possibility is the HMC349 for $6-7.  Great specs, but probably overkill in this case.  Though it does look attractive for the switching of an S parameter test set for my new VNWA 3EC.

When I was looking for such things a few years back there were a lot more options.  I don't know if Digikey doesn't list everything or if the higher pole count switches were all discontinued.

[Yansi]

Those TS5N412 does not look right as an RF switch. The HMC349 is nice otherwise.  Mouser does have a lot of interesting RF switches available, including multipole ones.

Mouser has this one for similar price as the HMC349, but seems with better isolation, 74dB at 2Ghz is awesome: http://www.mouser.com/ds/2/464/IDT_F2923_DST_20151124-883678.pdf 

But for now I will stick with the relays. I have those available, can buy more for about $1.5 a pop and this part of circuit can easily be replaced by an electronic switch later.


Currently I am puzzled a bit how to design the ALC loop. (the output atten. seems to be the easiest part) I'd like to test this part of circuit first. And then the DDS implementation with AD9951.

[rhb]

Where can you get RF relays for $1.50?  Please do tell.  I need 4 SPDT relays good to 1.5 GHz.

[RadioNerd]

I would also recommend relays for this application.
Electronic switches as nice but with most of them performance (especially power handling and linearity) drops considerably at frequencies below approx. 10 MHz. An average SP2T may handle 30dBm at 1 GHz but can be limited to less than 10 dBm at 1 MHz... And only very few (expensive) parts can cope with DC. These are all non-issues for traditional relays.

[RadioNerd]

Another comment regarding the leveling circuit:

In the current block diagram, the power sampling is done after the AMP2 block. If you use a resistive splitter (this means poor isolation) at this point, the output power will depend quite a lot on the load attached to the output. This is because you are implicitly leveling the output voltage rather than output power.

A possible way to alleviate this issue is to realize AMP2 with an opamp (or 2-3 paralleled for minimum impedance) with a 50 Ohm series resistor to the output. If you now sample the voltage between the opamp and the 50R output resistor you should achieve both a constant impedance at the signal generator output and a considerably more accurate power leveling without having to use a directional coupler.

[Yansi]

Where can you get RF relays for $1.50?  Please do tell.  I need 4 SPDT relays good to 1.5 GHz.

Those G6K-2F-Y Omrons aren't exactly RF relays, you can buy them cheap from China.  And they work just fine.

RadioNerd: Good point on that one. However... what if I would build a generator up to some hundreds of MHz? I do not think the opamp way is the right one to choose. Also I do not think there are opamp output stages in any of the RF generators. There must be a simpler way to achieve this. But you are absolutely correct, that by sensing just the voltage you won't achieve constant power.  There would be a lot of problems driving nonlinear loads (like diode ring mixers) too.

Directional coupler for the 400k-150M range could be made using RF transformers. (Have bunch of these MiniCircuits ZEDC-15-2B laying around - they use two small transformers inside. It is a 15dB directional coupler 1 to 1000MHz.  I think it could be possible to make a directional coupler on the same principle.

Or is there any other solution to output power leveling, that would level power indeed, instead of just voltage?

I do not like the opamp idea too much, as I am afraid that OPamp capable of 100mW @50ohm at 150MHz would cost quite a lot more.

[DC1MC]

I would agree with the RadioNerd about using the relays instead of any strange solid state devices, EVERYBODY uses them, from FeelTech to R&S and Tek, so it has to be something in it , the dynamic and frequency range it's actually too large for an off the shelf component.

[Yansi]

Well, is it after all possible to use an opamp? I have done a small research and am not sure about this solution would work.

I did find THS3001 to be the No1 for arb generator output stages.  Its availability is bad and it cost like $10 a pop.
A  GBW of 420MHz would allow me only to produce a gain of 2 at 150MHz. That is just poor 6dB.  If I wanted 20dBm output, well... that's a LOT of drive signal on the input, considering even that I would burn half the power on the series resistor to make it behave like 50ohm source. So I would need it to amplify the signal up to 23dBm level, which means 4.47Vrms output swing - well... I doubt this opamp is gonna cut the mustard, as the voltage swing required at 150MHz calls for almost 6kV/us slew rate... Well, no sorry bob. The opamp has only 6.5kV/us SR rating - am afraid of some nasty distortion happening in there.

I could probably find a better opamp, but is it worth it to use even more expensive part in there?    The price of the opamp would become a significant portion of what the AD9951 would cost.  I am not liking that 

So back to the MMIC amps... ?

[Yansi]

Regarding the directional coupler: If this would solve the power leveling issue RadioNerd noted: I'd prefer to use an off the shelf solution for it, rather then trying to make custom transformers (even though it would cost less, as many of us have hoardered a lot of different RF cores) - just to make the result more repeatable.

After a quick search, I could not find any suitable directional coupler for this frequency range. All of them have just too high minimum operating frequency and well, I do not want to give up on those 400kHz (and probably will try to push it as low as it will be possible).

One can get MiniCircuits TC4-1T+ transformers for quite reasonable $, so why not use two of those to make a 12dB directional coupler? It would produce +8dBm maximum for the log detector - which is very reasonable amount and overall solution I think.

Does anybody know of some off the shelf 10-20dB directional coupler for the frequency range of 0.4 to 150MHz?

But still I am not sure, if the directional coupler solves the issue of output power leveling to "strange loads" like short circuit and non-linear loads (mixer diodes). I hope it does. 

//EDIT: Just found this. I am stunned things like this can be patented  http://www.freepatentsonline.com/5337006.pdf

[RadioNerd]

Regarding the opamp approach:

1) Your 20 dBm requirement makes it indeed not easy to find a suitable opamp. 20dBm @50Ohm actually means 12.6Vpp before the series resistor... I would rather suggest to relax this requirement to something like +13 to +15 dBm (like most RF signal generators) and using an external amplifier in the situations where you really need more than that.

2) Many wide bandwidh opamps (like the THS3001) are of the current feedback type. Current feedback opamps do not have the traditional GBW limitation of voltage feedback opamps. Another possible candidate would be the LMH6703. It has a somewhat lower slew rate but it is much cheaper and easy to find (e.g. digikey).

3) some amount of distortion at the high frequency end is not that terrible for a sine wave signal generator. This will mainly result in harmonics that can be filtered with a low pass filter. At lower frequencies the opamp gets increasingly linear anyway as the open-loop gain ant thus the effect of the feedback increases.

[Yansi]

Well.. that LMH6703 is interesting and even seems to be cheaper than the THS3001.  But still - not enough swing can be done at such frequency to achieve the desired power level.

What are otherwise the drawbacks of the solution using the +20dBm MMIC and a directional coupler? Pricewise it seems it will end up similar, may be a little cheaper ($2 MMIC, $4 coupler). MMIC + coupler can provide more power, but seems to be a more complicated solution.

Still I do not know how the  solution with a dir. coupler will behave when the output will be short, open or put across a load that will limit voltage (diodes).

Maybe I could relax the output power requirement to 15dBm (32mW) or 13dBm (20mW)... but I did not think of 20dBm being such big problem at 150MHz. Be it a GHz or two, I'd expect a problem, but certainly not at 150MHz. I wanted the larger power to be able to characterize stuff with it (being able to overdrive stuff a bit) without the use of additional amplifier.

So what do you recommend, how would you design an ALC circuit, that could work up to 13/15/20dBm? I do not really know how is it supposed to be done to work properly under different loading conditions.

I already know that simple voltage sensing will not work, as it will regulate voltage (obviously), but not the output power. (In a short circuit condition, it will maybe even try to blow itself up. With a voltage limiting load, it won't work either, as it will try to push the voltage up resulting in more power delivered, than expected).

So how it is supposed to be done?  Without opamps... you can't use those very much at the higher frequencies either. I'd rather leave those for arbitrary waveforms. (1Gsps DDS is also waiting here, but I wanted to start with something more easy first - at just 400Msps / 150MHz)

[RadioNerd]

The frequency range is the difficult part in my opinion... for 10-150 MHz I would certainly take the MMIC+directional coupler approach, which should also allow to reach the desired output power goal. For the range below 10 MHz however I would rather use the series resistor approach with an opamp or discrete transistors. This would make it also much easier to add DC offset to the output if desired.

Minicircuits has directional couplers with good directivity in the frequency range from 0.2-250 MHz. I newer used one of these however and they are also quite expensive. I assume they would work, though...

[Yansi]

So the directional coupler approach (my first thought as per the diagram) would actually work? Even in case of a non-linear load? How would it behave in such case?

I have some gaps in knowledge - what forward power will the directional coupler give to the detector, with the output shorted / open? Just to be sure I understand that correctly.

(I have browsed through the MiniCircuits web and did not found any suitable couplers. All of the couplers have too high minimum frequency, or their cost speaks directly against using them. Hence why I have proposed to make a custom one, using off the shelf transformers.)

[RadioNerd]

coupling factor flatness and directivity are the most important parameters of the coupler for leveling applications.

The coupling factor flatness (i.e. frequency response) indirectly determines the number of frequency points at which you have to calibrate the amplitude of the signal generator to acheve a constant output power over frequency (toghether with the response of the detector of course).

The directivity defines how well the coupler isolates the forward and backward travelling energy on the coupled port. The extremes are short/open (100% reflected) and perfect load (0% reflected). The better the directivity the less the coupled power is altered by the reflected power.

[Yansi]

To sum that up, you are telling me what? That it will not work good enough using directional coupler? What else do you recommend then to make output leveling?

If multipoint calibration is so much of an issue in a RF generator, what is used in commercial equipment to do so, that it does not need so much calibration?  Based on what principles are the ALC circuits made in such units?

And don't tell me they use OPAMPs,  please    I imagine they use a different synthesizers for different bands - at least for the low band. - that may utilize the opamp, sure. But the rest?

[RadioNerd]

Sorry if I was not clear. The coupler approach will definitely work. However, its performance directly determines the performance of the leveling circuit (frequency response and accuracy).
Whereas commercial parts come with a nice datasheet and sometimes S-parameters, the custom coupler must be tested first to see whether its performance is OK.

regarding your other points:

1) Wideband RF signal generators usually use a separate DDS based signal path for the low frequency end. Its quite possible that opamps (e.g. like the ARB output stage amp you mentioned) are used for this range... The high end (from about 100 MHz) is of course leveled using a directional coupler.

2) Commercial RF generators do use multiple calibration points (who said that this is not an issue in commercial equipment?). And a lot of them. For example, in the description of the TSG4100A series signal generator, Tektronix states that its power leveling is based on a calibration matrix with 40000 entries... (https://www.tek.com/datasheet/tsg4100a-rf-vector-signal-generator-datasheet). The only difference is that they put a lot of effort in the efficiency and automation of the calibration routines, which may not be the most exciting part for a homebrew project.