Half-decent diy RF generator utilizing AD9951 - an idea

[Yansi]

Maybe I did not understand you well - but what is then the problem of having a custom coupler made?  As far as I can see (or googled respecively) there are none couplers made for this frequency range, or these low frequencies. Even those that are close either are unobtanium or expensive, likely combination of both.

Therefore I have proposed to use a pair of TC4-1T transformers to make a custom 12dB coupler.  Characterization of such beast is another story, as my lab is well under-equipped, regarding RF test equipment. But I can make it happen, including S parameters on a commercial VNA - just to see how bad it will be.

[RadioNerd]

All I want to say is that both approaches have their strengths and weaknesses.
Unfortunately, your desired frequency band lies between the optimum range of the two. Therefore some compromises must be made. You have to decide which criteria are most important to you and choose the best approach based on that:


Directional coupler + MMIC:
+ Easier to get high output power above 50 MHz
+ No issues with high frequencies; design could be re-used for a generator based on higher frequency DDS or RF PLL IC
- Lower frequency limit (coupler and bias tee for MMIC, 1/f noise, stability etc.)
- Output matching depends on MMIC characteristics
- harder to achieve flat response (more ripple and thus calibration points required)


Opamp + 50 ohm
+ Works down to audio frequencies & DC
+ Very good stability and linearity in the range DC-20 MHz (opamp feedback)
+ Easier to achieve flat frequency response (only dependent on log detector response)
+ Good 50 ohm output matching
- Limited output swing and slew rate (distortions at high frequencies and power)
- Limited to about 100 - 150 MHz using "realistic" parts
- suitable opamps may be expensive

[Yansi]

The frequency range is mainly defined by the DDS chip chosen, however I mainly want to use the generator in the upper part of its frequency range. Those 400kHz I insist upon are mainly due to being able to work with standard 450kHz IF chains.  I'd prefer a design, which I could reuse for even higher frequencies (400MHz - 1Gsps DDS is waiting in the drawer too). If the characteristics of the generator craps out a bit on the low band, I'd not be so mad if it will crap out at the upper band.  I can use bog standard "1MHz all shape generator" to produce a 400kHz sine too. I would also like to have more output power built in, rather than using external amplifier whenever needed.

Based on this I'd prefer the solution using MMIC + coupler. It could be also made as a standalone ALC unit, maybe useful for some experiments in the future - as the achievable bandwidth of the MMIC + PIN attenuator can be quite substatial.

Now the question remains: What part to use for the coupler, or how to make one? Any suggestion is welcomed! Currently I have only two solutions:

1) Use two pieces of MiniCircuits TC4-1T to make the coupler. Can be obtained for $2 a pop. But it leads to a rather difficult pcb layout.
2) Wind myself one using either two separate  toroidal cores, or better use single dual-aperture core - I think that should behave better. Question is, what dual-aperture core to choose?

I can then characterize both of these on a VNA, or probably I can - if they let me use it. 

[rhb]

A daft idea to consider.  Use something like a pair of ADF4351s running in the 2-4 GHz range, filter the harmonics and mix.  It is my understanding from various forum posts that this is what some high end HP synthesizers do. It won't give you the frequency agility of a DDS, but it will give you low THD.

I've got an AD9851 and an ADF4351 awaiting application.  So I'm watching closely.

[Yansi]

Well..  that might work, but isn't that a freaking overkill? I just wanted to make some use of the AD9951 I have, by adding output level control to it. Not to build the worlds best and cleanest RF generator   But I have also a bunch of ADF4350s here, just in case 

BTw, just out of pure fun, I have made myself a 15dB coupler out of completely unknown 2-hole ferrite core. I will measure it's performance later. Currently making a connection kit for it with SMAs.

Does anybody have any tip, where to source suitable dual hole ferrite cores for such an application? (up to 150MHz) Where to source these, with a reasonable price?  Maybe it's time to make my own coupler, as there is really nothing useful manufactured for this kind of application - except for those things that begin at $20 a piece.

[rhb]

Hell, if you've got one just build it, measure its performance and iterate!

I'm also  quite interested in the problem posed by unknown magnetic cores.  And *really* annoyed that my $1800 Instek MSO-2204EA won't let me make an X-Y plot of ch1 and integral(ch2)!

That said, if you've got a signal generator and a DSO, you can dump the data to a PC and use Gnu Octave to plot the hysteresis curve and make other measurements.

[Yansi]

You could always integrate in post-processing.  In this case I am a bit afraid the core will be tooooo much high frequency one, with a low mu.  It is from an oooooold analog SAT TV tuner. It was a part of some diode ring mixer in there. Haven't bothered to investigate further.  I have only four of these cores. Need to source other ones! If I would know where...

[Yansi]

So now some fun is ready to be executed. 

//Sry, pcb not yet cleaned.

[Yansi]

 A quick dirty test  of that little bastard, using the very limited equipment I have at home:

Used a fixed frequency 118MHz source of +7dBm and a SA as the power meter:
Connected to the IN port, CPL is terminated -> measured OUT is 0.75dB lower than IN (about 6dBm).
OUT terminated instead, CPL is measuring about 15.6dB below OUT, at about -9.6dBm.
Now reversing the ports, OUT is being powered, IN terminated, CPL measures -34dBm.

So: We have 0.75dB insertion loss.  Fair enough.
We have a 15.6dB coupling, which is nice, as 1:6 ratio coupler should theoretically be exactly 15.56dB.
Directivity - am not sure how to measure one, but if it is the diference between  CPL levels for the same FWD / REV power, than we have 34-9.6 = 24.4dB directivity. Thaaaats very disappointing unfortunately.

Please bear in mind, the measurement is very dirty and the dBm values can not be taken that seriously, there might be about 0.5dB (or even more) inaccuracy of the reading.

//EDIT: As a reference, I took a measurement on a MiniCircuits ZEDC-15-2B. Loss: About a dB or so.  Coupling 15.2dB. Directivity 45dB. Which is about right on the money with the datasheet.

Why is my coupler so bad? Is that due to the single core construction? (The ZEDC-15-2B uses a pair of toroids).

[DC1MC]

When I need magnetic cores; I always buy one of these and dismantle:
https://www.ebay.de/itm/RF-SWR-Reflection-Bridge-0-1-3000-MHZ-Antenna-Analyzer-HF-VHF-UHF-return-loss/222740607922

[ogden]

Mouser has this one for similar price as the HMC349, but seems with better isolation, 74dB at 2Ghz is awesome:

That's huge overkill. 20dB or better per switch will be good enough - considering two switches per 20dB attenuator.

Coupler will only screw-up linearity of your gen w/o adding any useful function. Better just use simple resistor power splitter (1:20 dB or so).

[yl3akb]

Exactly. You are not measuring reflected signal, so no need for good directivity. Leveling calibration would only be correct when loaded with 50 ohm loads anyway. Check this about unequal resistive power splitters: https://www.highfrequencyelectronics.com/Mar07/HFE0307_Adams.pdf

[Yansi]

I know, those unequal resistor splitters aren't exactly fun to be calculated. 

Well, I'd be glad if I would not need a coupler. But that what I am trying to figure out the whole time. The fact that the levelling will only work at 50ohm load may be a problem!

Imagine I have a highly non-linear load: A diode mixer.  The mixer will be a level 7 type, i.e. requiring the LO source to deliver +7dBm into 50ohm load. If I set now the generator to 7dBm and load it with the mixer, the voltage will get clamped and highly distorted, by the diode load in the mixer. Due to this fact, the ALC in my generator will go nuts, as it will see the output having lower voltage, than expected. Due to this, the servo amp will saturate, making the PA in the generator to go full power, trying to force the required voltage over the nonlinear load.

I do not think that this is how the ALC is supposed to work. (It'll not work for this case at all!)

[RadioNerd]

You are not building a VNA. For ALC applications a directivity of 15-20 dB should be OK.

[Yansi]

Okay, I do not have any kind of reference as what directivity may be required for what application.  Still I do not know how the ALC using a coupler will respond to open/short/nonlinear loads.

Guessing at the open condition, no forward power will be seen, therefore one would expect silly ramp up of the output level to the max. Guessing the same will happen with the output shorted.
But I understand it as it may work with the nonlinear load - somehow.
/\ really not so sure about these


But what about this one?  Using an asymmetric power divider for the ALC followed by an attenuator?  The attenuator will definitely have two advantages:
1) Makes the output match better
2) Load isolation. Seems it is just what we want. The output will behave more in a "resistive" fashion and will tolerate non-linear loads much better.

The main drawback being it is deliberately lossy. I would need either to pull back the requirement of +20dBm output, or amplify it even more (I'd rather not do that).

Supposed I could live with only +13dBm output, the following could be done using a 20dBm P1dB amplifier:



What I find also interesting about this solution is that it will be a lot more linear. Definitely more than a coupler, where the loss, coupling and even directivity change significantly across the frequency span.

It is a kind of win-win solution, it seems.  Or does anyone have any better solution than this? (I am still not liking the coupler approach due to it being almost unobtanium component and also complicated to make and even more complicated to make behaving well over 400k-150MHz)

[yl3akb]

Good point regarding to nonlinear loads. It is an interesting topic, so I did a quick simulation of Your coupler schematic with different extreme loads (open, short and single, parallel diode), just to get the idea. By increasing transformer inter-winding capacitance, I intentionally corrupted directivity to just ~10dB. Here are few waveforms of signal at coupled port with different loads. In all graphs, gray is case with ideal 50 ohm load (for easier comparison). With directivity of ~20 dB the load impact is even smaller of course. If it is helpful, I can try different quick things also, like S-params, etc (Its AWR microwave office)

[ogden]

Okay, I do not have any kind of reference as what directivity may be required for what application.  Still I do not know how the ALC using a coupler will respond to open/short/nonlinear loads.

You can check schematics of signal generators to see how ALC is usually done. Keysight (Agilent/HP) provides service guides for obsolete instruments, with schematics. Anritsu as well. For instance HP8648 uses diode detectors, obviously w/o coupler (attach).

[edit] Note that amp output is 4db above set level which means they use ~3..4dB "impedance matching attenuator".

One more thing: AD9951 output level can be controlled using DAC_Rset pin. You can consider to drop pin diode attenuator and use "digital ALC".

[Yansi]

I do not think the schematic capture you have posted is definitive enough one could guess how they do it.

If you think of using only a diode detector, i.e. voltage measurement, then it just won't work with non-linear loads that tend to clamp voltage to a certain level. That is what I am trying to solve here the whole time. Putting a non-linear load, in the simplest form a diode ring mixer, which definitely can be connected to a RF generator, will cause the generator to ramp up the output power to silly level, because it only measures the output voltage, not power. What you set with the dBm is POWER not voltage.

Also note that the LO input of a diode ring mixer presents a load consisting of two diode junctions in series plus the leakage inductance of the LO transformer. Usually these mixers are specified in the equivalent dBm level that would the Zo=50ohm  LO source give into ZL=50ohm load.  How is then one supposed to use an RF generator to feed such load?

[ogden]

If you think of using only a diode detector, i.e. voltage measurement

When you measure voltage on 50ohm load, it's just math "problem" to make it power, not voltage measurement.

I do not think the schematic capture you have posted is definitive enough one could guess how they do it.

For sure they do not use couplers

One more: http://www.dennlec.com/images/manuals/hp-8647a-op-service-manual.pdf

[edit] You regulate power level of attenuator input, not generator output. Think what will happen with your diode mixer argument if you switch 20dB output attenuator on.

[Yansi]

You set the POWER, not voltage.

The load is not always 50 ohms and not always linear. How is your mathematics gonna deal with that one?

If I set +13dBm on an RF generator and give it a two antiparallel diodes as a load, what will be the output voltage? Will the generator say "OUTPUT UNLEVEL"? I don't think so. It should deliver 13dBm  (20mW) to the load, not 1V RMS - which indeed is not possible with a nonlinear diode load.

I still do not know how this is supposed to work in an RF generator. I am still trying to figure out. I am not liking the fact, that a voltage self limiting load will make the ALC go nuts. Just trying to find a solution for that.

[ogden]

If I set +13dBm on an RF generator and give it a two antiparallel diodes as a load, what will be the output voltage, huh?

How regulation of attenuator input will help in case of nonlinear load? Huh? What I am telling - generators do not deal with problems you are trying to solve They regulate output *voltage* for 50ohm load. If your load is not 50 Ohms, then output voltage will not match. In case you set output POWER, then your load *must* be 50ohms.

[phenol]

check out the output section of an old Marconi 2019 sig gen. It uses matched pairs of detector diodes and no directional couplers. The low frequency portion of the output stage has jfets acting as voltage-controlled emitter feedback resistors.
https://doc.xdevs.com/doc/Marconi/MARCONI%202018A_2019A%20Service.pdf
page 222

[Yansi]

Okay, supposed it is as you present it: There is always an attenuator in line.  That is a solution I have already presented a few posts back. (see image below).

But what about the case, where the attenuator is set to zero and ALC connected right to the output? That is the problem I am trying to solve.

Maybe it got already lost in the thread becoming lengthy, but the original goal for my AD9951 gizmo was to use the ALC to directly set output level in the range of +20 downto -30dBm and then use two* additional 20dB attenuator stages to widen the range downto -70dBm.

*I do not think that a single 40dB attenuator stage is easily diy-able, as it would require switches with rather high isolation .. ? But maybe it is still possible, who knows.

[ogden]

But what about the case, where the attenuator is set to zero and ALC connected right to the output? That is the problem I am trying to solve.

Maybe it got already lost in the thread becoming lengthy, but the original goal for my AD9951 gizmo was to use the ALC to directly set output level in the range of +20 downto -30dBm and then use two* additional 20dB attenuator stages to widen the range downto -70dBm.

Well, all generators (I know) outputs twice voltage when unloaded. Think about that for a moment.

*I do not think that a single 40dB attenuator stage is easily diy-able, as it would require switches with rather high isolation .. ? But maybe it is still possible, who knows.

As I already said - you can use quite crappy switches for attenuator switching at given frequencies. 20dB leakage per switch can be considered good for 20dB attenuator - because two switches will leak -40dB of incoming 0dB signal, -40dB will be added to -20dB of attenuator output. Anyway nonlinearity of your whole thing will be worse than that.

[ogden]

*I do not think that a single 40dB attenuator stage is easily diy-able, as it would require switches with rather high isolation .. ? But maybe it is still possible, who knows.

As I already said - you can use quite crappy switches for attenuator switching at given frequencies. 20dB leakage per switch can be considered good for 20dB attenuator - because two switches will leak -40dB of incoming 0dB signal, -40dB will be added to -20dB of attenuator output. Anyway nonlinearity of your whole thing will be worse than that.

Anyway single 40dB output switch is not that good idea. Better have narrow ALC regulation range (like max 10dB) and more attenuator steps on the output.