Half-decent diy RF generator utilizing AD9951 - an idea

[Yansi]

I understand, why it outputs twice as many volts unloaded. Thats because Zs = ZL.

If I place ALC, a voltage regulator, at the point the ALC senses the voltage, will be a node with a zero impedance, not 50ohm.  And that is the clue how to do it, isn't it?

The biggest help was the Marconi manual phenol has posted, so thanks for that! From observing the schematic and reading a bit of  the text above, I now understand the problematic a lot more.

Great help guys, thanks for helping me to see the truth.

The reason why the Marconi RF generator has only 10dB of ALC range is obvious - it uses a linear detector!  I intend to use a log detector for the output level, so I can leave out some of the attenuator switches - I won't need that fine attenuator step.  Maybe I should reconsider what the ALC range will be though, as 50dB is kind of many.

[ogden]

I understand, why it outputs twice as many volts unloaded. Thats because Zs = ZL.

If you understand that then why you still believe that signal generators regulate power, not voltage?

[Yansi]

Many reason which lead me to believe many things.

I have even thanked you guys for helping me understand some facts and better apply them, are you going to offend me for that now or what? 

[ogden]

Many reason which lead me to believe many things.

I have even thanked you guys for helping me understand some facts and better apply them, are you going to offend me for that now or what?

No offense indeed (because it's obvious that you don't have experience regarding subject), but you offended me by implying that I do not know how generators regulate: "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."

[Yansi]

But that is true. It will not work if the ALC detection point will be at the load directly. However I could not come up with  the idea to use the detection point of the ALC as a zero impedance voltage source point and then connect the load to that point using a convenient 50ohm series resistor. 

In this case it makes sense to measure just the voltage before the 50ohm output resistor.

But it also kind of screws my plan of +20dBm output. If I would use +20dBm P1dB amplifier (I planned to use OnSemi MMG3H21NT1 MMIC amp), then I would get only +14dBm out, otherwise the amp would become grossly overdriven.

So:
1) either I will find a suitable MMIC amp that would do a +26dBm or more on the output at the whole range of 400k to 150MHz. (Can anybody recommend a specific one?)
2) or I will have to settle with a 13dBm output only which I find not enough.

There is also always other possibility, for example to make a discrete amplifier... but that is a task for an experience person, certainly not me. So back to looking for a 0.4W+  MMIC that can be run from DC upwards (150MHz). 

[Kleinstein]

The ALC loop is relatively slow. So the point where the ALC is measuring is not really 0 Ohms.

For the relevant high frequency the impedance is still unchanged it is only the added load to the ALC sense circuit that lowers the impedance a little. A suitable configuration is more like an asymmetric power splitter from the output amplifier with some loss (e.g. 2-3 dB) for the output and quite some damping (e.g. 12 dB) towards the ALC circuit.  So the loss in output power can be smaller than 6 dB. Some damping from the power splitter can actually be a good idea, to isolate the amplifier from possible load mismatch - one usually wants a generator to survive an open output and also a open end cable.

The main thing that changes with the ALC is the amplitude with a non matched output. It would reduce the amplitude with an open output and increase it with too low an load impedance.

[Yansi]

Okay, for small signal analysis it is not 0 ohm, however I think for slow load changes, the node really behaves as an AC voltage source with zero internal resistance.

Or how would you for example explain the Marconi output stage, where I can clearly see a 50ohm resistor in the output path - meaning an assumption was made, that the sensed node at the output of TR10 has 0ohm impedance? The sensing diode matched pair is visible on the screen capture too.

//Btw, what is the purpose of D8 to D10? Kind of interesting construction considering also the two RFCs - one being 2.4uH and the other 1mH. The amp seems to be a wide band from 80kHz up to a GHz.

[yl3akb]

But that is true. It will not work if the ALC detection point will be at the load directly. However I could not come up with  the idea to use the detection point of the ALC as a zero impedance voltage source point and then connect the load to that point using a convenient 50ohm series resistor. 

In this case it makes sense to measure just the voltage before the 50ohm output resistor.

But it also kind of screws my plan of +20dBm output. If I would use +20dBm P1dB amplifier (I planned to use OnSemi MMG3H21NT1 MMIC amp), then I would get only +14dBm out, otherwise the amp would become grossly overdriven.

So:
1) either I will find a suitable MMIC amp that would do a +26dBm or more on the output at the whole range of 400k to 150MHz. (Can anybody recommend a specific one?)
2) or I will have to settle with a 13dBm output only which I find not enough.

There is also always other possibility, for example to make a discrete amplifier... but that is a task for an experience person, certainly not me. So back to looking for a 0.4W+  MMIC that can be run from DC upwards (150MHz). 

Here is nice summary about signal generator output stage, which also describes voltage source+50ohm resistor concept. Why dont You just use switchable PA (with good gain-temp. stability) at the output for high levels, like shown in the document?
http://aeroflex-cdip.ru/eng/pdf/nav-750-an2.pdf

[Yansi]

Well that looks as a viable idea! ... however requiring additional relay to the path.  But would help with a lot of things. As it seems, DC-to-Daylight MMIC amps with more like 20dBm P1dB are unobtainium or only for those with higher income.

The other solution left is to try building a discrete amp. Replicating the output stage from the Marconi does not seem to bu that much of an problem, at least it would be a nice experiment. However would must first have equipment to be able to fully characterize it. Sigh.

[rhb]

This and related threads have got me to looking at eBay and AliExpress offerings.  Which leads in turn to the question, how good an instrument can one build using cheap Chinese modules based on chip vendor reference designs?  This seems to me particularly useful in light of the fact that many instruments incorporate the same functional elements.  So a bunch of modules and SMA jumpers would allow someone to set up an instrument for a single test and then reuse the parts for something different the next time.

I'm old now and can finally square buying toys with my conscience.  But I have painful memories of trying to build a transceiver from scratch with nothing but a 5 MHz recurrent sweep scope and a VOM.  Oh, how I envied the EEs who could use the class lab gear.

[DC1MC]

This is my plan as well, get modules implementing reference designs on well known signal generator chips and build around them.
I pray that our Chinese friends didn't apply too many of their "optimize for cost" rules to them or at least let the pads to solder your own quality stuff there.

[Yansi]

Not only optimized for cost, but usually very bad in terms of PCB layout.  I have some nasty examples of chinese RF pcb routings, but I'd leave that for a separate thread.

If we talk about the chinese modules - does anybody know what MMICs they use on these 0.5W RF amplifier modules? I couldn't figure that out.

https://www.ebay.com/itm/1PC-0-5W-40-1500MHz-RF-amplifier-40-1500MHz-0-5W-13dB-gain/141981673312?hash=item210ec46760:g:OdoAAOSwiYFXKfyJ
https://www.ebay.com/itm/50MHz-1GHz-0-5W-Broadband-RF-Power-Amplifier-Radio-Signal-Amplifier/181797531492?hash=item2a53fa2f64:g:losAAOSwyQtVnzfc

and then those modules (they seem to have a discrete transistor output stage, not a MMIC in there):
https://www.ebay.com/itm/1PC-2W-RF-wideband-power-amplifier-1-930MHz-2-0W/122061596083?epid=881797178&hash=item1c6b7025b3:g:jEUAAOSwIgNXmLZF

[Yansi]

I may have made a rather stupid thing. I have replicated the RF output stage from the Marconi generator.  But used a bit different components.  The RF transistor is BFG193 (the beefiest SOT223 one I could find in my garbage pile).  Biased Ic 45mA , Uce 8V.  I think that is the reasonable maximum for him.

I would like to test this using for example a BFG253 - much beefier one, but I don't have any yet.

The question about the three series connected diodes still remains. Does anybody know their purpose? The schematic of my test gizmo is almost the same, as the Marconi.

I will test the circuit in a moment. Unfortunately, my  lab is still lacking any kind of RF generator, so for further proper testing it will have to wait till I get some proper tools for that. At least I will try to test it at multiple frequencies.

[Yansi]

To my biggest surprise, that damned thing works 

Testing it so far using only my 118MHz LC oscillator, I am able to get about 17.5dB of gain and almost 20dBm output power.

I have found a bunch of BFG541 - a little bit more beefy transistor. So I have beefed up the supply voltage to 12V (becoming close to the BFG541 limit... oops) so that the Ic bias goes up to 70mA.   Now the beefed up amplifier is giving 23dBm. 200mW! Nice!

For the RF generator using the AD9951, I would need it to deliver 200mW into 100ohm load in the whole range of 400kHz up to 150MHz. Which may seem very possible, using something as BFG235.

I would be also interested to know, how to estimate what kind of bias do I need to achieve a specific amount of output power. 

I will put the amplifier through a more thorough test later - to measure the gain dependence on frequency and then the P1dB over tthe frequency range. With my current home rf test equipment setup, I am not able to do it unfortunately. 

[rhb]

The question about the three series connected diodes still remains. Does anybody know their purpose? The schematic of my test gizmo is almost the same, as the Marconi.

It took a bit of time for me to locate the schematic in the Marconi manual.  Ahh, the days when they told you everything.  Though it's a lot easier to flip through paper than a PDF.

The function of those is to hold the collector of the Darlington pair at 12.9 V or higher.  Consider the case of TR9 at Vce of 0.2 V.  They do a similar setup in the PS for the HP 8601A voltage regulators using 2 diodes.  The voltage drop is tightly controlled, but without the noise of a zener in reverse breakdown.

That I happen to know this is a strong argument for enduring the aggravation of fixing old gear instead of buying new.  It *is* educational.

[phenol]

if you are talking about the string of 3 diodes (d8-d10), those are clamping the output and act as protection against transients. There’s another clamp (d21) placed after c30.

[Yansi]

The function of those is to hold the collector of the Darlington pair at 12.9 V or higher.  Consider the case of TR9 at Vce of 0.2 V.  They do a similar setup in the PS for the HP 8601A voltage regulators using 2 diodes.  The voltage drop is tightly controlled, but without the noise of a zener in reverse breakdown.

Errr... what darlington pair? Maybe you look at a different diodes.

I have already posted the part of the schematic in the thread, you might have overlooked And also the diodes are in the schematic of my test gizmo.

Yes, they are obviously clamping the voltage to Vcc + 1.8V or so. But that does not make much sense for me, as to why the clamp is connected into the rf choke split point.  That means that the clamp will behave non-linear with frequency. The higher the frequency, the larger the clamping voltage.  At low frequency (the amp in the Marconi does the whole range from 80kHz up) the larger valued 1mH choke will act as the main one. However the swing will become very limited by the diode string.
If I remember correctly, the Marconi is also 20dBm output, so it needs 4,5Vrms  voltage at the collector. That kind of contradicts what the clamp does there.   

How is one supposed to estimate what kind of bias (Uce x Ic)  is needed for a transistor to achieve a given amount of power into a load?

[rhb]

Sorry, it's not a Darlington.   I just assumed that from the double emitter symbol which I had not seen before.  It's a 4 GHz transistor with 2 emitter leads.  I'd never seen that before.

I did not notice any schematic other than @phenol's link to the manual.

The function of the diodes is to limit the current through R39 to ~210 mA ((3* 0.7)/10).  If the voltage drop across R39 exceeds ~2.0 volts the diodes conduct.

[DC1MC]

I'll take a shot on the bias vs. delivered power.

I assume that we're talking here about class A amplifiers.

So assuming the transistor it's fully linear (it isn't) you set the biasing point to burn without signal exactly 50% of the power needed. The input signals should swing the Ic between 0 - 2xIcIdle.

But of course, all perfect transistors are sold-out and you have to do with you have in the drawer. And then discover that on very low and and very high currents the guys have a lot of nonlinearity, so you have to burn much more energy in idle mode to move the low further away from 0 and also select a transistor with a high enough dissipated power and beta to not "suffocate" at high currents.
That means no hard rules, a lot of experiments and lot sorting is needed, that somehow explains the prices of good signal generators.
 

[Yansi]

Sorry, it's not a Darlington.   I just assumed that from the double emitter symbol which I had not seen before.  It's a 4 GHz transistor with 2 emitter leads.  I'd never seen that before.

I did not notice any schematic other than @phenol's link to the manual.

The function of the diodes is to limit the current through R39 to ~210 mA ((3* 0.7)/10).  If the voltage drop across R39 exceeds ~2.0 volts the diodes conduct.

Still do not understand your explanation. The diodes simply can not conduct when the R39 voltage drop rises. They are reverse biased all the time. Except when the node between the RF chokes swings about 1.8V above supply voltage (+15V).

But then I do not understand how is that power stage able to deliver  4.5Vrms (13Vpp) of swing into the load at lower frequencies, where the bigger 1mH choke is the significant one for the operation. The collector voltage has to swing around the +15V rail, in between +9 to +21V.  (I have omitted the R39 drop, which is by the looks of it 0.9V - the stage is biased at 15V 90mA)

I'll take a shot on the bias vs. delivered power.

I assume that we're talking here about class A amplifiers.

So assuming the transistor it's fully linear (it isn't) you set the biasing point to burn without signal exactly 50% of the power needed. The input signals should swing the Ic between 0 - 2xIcIdle.

But of course, all perfect transistors are sold-out and you have to do with you have in the drawer. And then discover that on very low and and very high currents the guys have a lot of nonlinearity, so you have to burn much more energy in idle mode to move the low further away from 0 and also select a transistor with a high enough dissipated power and beta to not "suffocate" at high currents.
That means no hard rules, a lot of experiments and lot sorting is needed, that somehow explains the prices of good signal generators.

That's what I have thought. In reality, you need to have it operating deeply in class A to obtain the best linearity. But I think there are other factors t consider: For example at 0.2W into 100ohm, I need a peak current of 64mA. The bias therefore has to be larger than 64mA and the supply voltage bigger than the peak voltage value expected at the load. The C of the NPN then sees twice the Udc. That makes indeed sense. (The RF stage in the Marconi is biased 90mA, supply voltage 15V)

[phenol]

iirc, the instrument delivers 13dbm max into 50 ohms

[Yansi]

Yea, I have just noticed that I dunno why I was thinking still about 20dBm.  With 40mW into 100ohms, that diode limiter makes sense and just barely makes the voltage swing available for the 13dBm output. There is not much margin left, if any.

[rhb]

Ooops.  Still not awake then  :-(   Up rather later than normal.

The diode transition capacitance decreases as the reverse voltage increases.  So L8 and L9 form series resonant circuits whose tuning varies with the current through R39. I don't have a clear mental concept of what the ac behavior of such an arrangement looks like.   This rather begs for some SPICE runs.   From when it was designed I'd guess that this was arrived at empirically to suppress oscillation.

[Yansi]

I'd believe it is a clamp.  But with only 40mW output into 100ohms it makes sense. I was thinking the whole time the Marconi 2018A was specced at 20dBm output. Which is not true. My fault there.

So some really nice solutions for the ALC are coming together.  For just 13dBm (20mW) output, one could get out easily with just the 20dBm+ P1dB MMIC from Onsemi (MMG3H21NT1, readily available from Mouser for reasonable price).  (you need 16dBm into 100ohm, but I'd rather let the MMIC work into its rated load of 50ohms, so then you need 19dBm to obtain the same 13dBm at the output. The other 100ohm load will be the ALC detector input)

But still I don't want to give up on those 20dBm. Not that easily. I'd like to at least try making a power stage using a discrete transistor. However it is a bit complicated for me, as I do not have the required RF test equipment in my lab to verify the performance of such an amplifier. Having occasional access to a local RF lab at work (or maybe will try local university?) is a rather lengthy process.

[Yansi]

I could not help myself,  but due to the lack of proper PIN diode quad (BAP64Q), I must have tried something. I took an old analog TV tuner a grabbed the PIN diodes from it: I got three BA283 diodes.

So I breadborded the following circuit:




Where R1 = 2k2, R2 = 1k, R3 = 2k7. All diodes BA283, all capacitors 100nF. L1 some kind of RF choke from the same tuner as the PINs are.

I have yet again used my crusty trusty 118MHz LO source and measured the attenuation as a function of control voltage. The reference voltage V+ was 2.65V. I swept the control voltage from 0 to 5V.



What I quickly found very interesting is the hump on the attenuation plot. The attenuation is not monotonic function of the control voltage. Why is that? This makes it of very limited range usable for any kind of servo loop. What might went wrong that I got such a curve?

The only positive thing about the bread-borded rats nest I can say is it has only about 0.7dB insertion loss minimum and a decent attenuation of 57dB maximum with zero control voltage, both at 118MHz, 0dBm input level.