Wideband (<30MHz) AM modulator circuit, how to make one?

[Yansi]

Hello,
my curious me again, investigating topics I probably shouldn't: This time AM modulator.   

My friend, who likes to dig deep into old tube receivers, came to me with a question whether I knew how to generate a signal in between 100kHz up to some 20MHz or so, to test and tune those old beauties.

The obvious answer was to use for example a DDS for this, such as AD9834 (cheap 75MHz 10bit DDS). But then a question came, how to amplitude modulate the output?

Suppose I want to make such amplitude modulator, capable of working from 100kHz up to 30MHz (to cover LW up to the end of SW band). After spending some time with google and discarding all obviously foolish circuits, I have found this one, looking rather promising.


The differential approach through a transformer should have large rejection of the baseband signal at the output terminal. Well I, thought, let's have some fun building it. After messing a bit with the component values, it resulted in this:



The transformer is tri-filar wound 5mm toroidal core from N1 ferrite mix, 12 turns (a random number, that happened to fill the whole core circumference with those three 0.2mm wires).  ADT1-6T from Minicircuits would be a rather nice fit here, however the price is just nuts (and I do not have any, after all).

So after building the circuit, I tested its performance.  And become quickly disappointed.

First, the gain seemed lower than expected, well until I have found it is driven well in its saturation region, also producing nasty distorted output. I needed to keep input signal like 50dBm or below to have reasonably clean output.  Then it was able to produce gain of about 16-17dB, which seems good, but the P1dB point just absolutely horribly low.

I then thought, that this is just the property of the differential pair - it has to be driven with very low signal level, large distortion is produced otherwise. So okay, lesson learned there.

Second, the bandwidth. The circuit, as built, seemed it would not have much trouble working at up to some good tens MHz, even 100MHz - even with those plain simple BC847B I happen to have on hand. The gain decayed quite slowly, so these are probably good enough for those 30MHz. However at the low end of the band, say below 2MHz, a second and a third harmonic quickly appears at the output. I am not sure, but the transformer may be at fault? Probably core saturating? Uh, huh? Otherwise I can not explain this.

Third - the modulation. I tried modulating the output, however guessing above 30% of modulation depth, distortion is starting to come up quickly and I saw a lot of harmonics of the modulation signal.  I could not get any decent output above like 50% or so of the modulation depth. I haven't bothered to measure that accurately, it was obvious that is not enough.

So now the question is, how can the circuit be made better? Or should I use a completely different approach to it?
I would like to achieve at least 0 dBm P1dB, 0.1 - 30 MHz operation and say up to 90% modulation depth. What can be done about it?

A better transformer would probably get rid of distortion at the low end.
Using larger bias current (the circuit was originally biased at 2mA, I then upped the supply voltage from 5V to 10V and modified the bias to 12mA) helped a bit to make the P1dB higher, but by not much.
Would using emitter degeneration of the differential pair reduce distortion (increase P1dB)?
What other modifications or circuit solutions would get me to my goal?

[CJay]

https://www.analog.com/media/en/technical-documentation/application-notes/AN-423.pdf

[dzseki]

Or, you can simply use analog multiplier. Depending on your frequency needs you can find several models. AD835 for example goes up to 250MHz, with 2Vpp output.
Also you can read up on "Gilbert-cell" that is supposed to be the holy grail of the multiplier circuits regarding low distortion.

[David Hess]

A Gilbert-cell multiplier is one option and this used to be commonly done with the 1496 balanced modulator/demodulator which was sourced by practically everybody at one time or another.  Another way is to use a wideband operational transconductance amplifier.

[Yansi]

https://www.analog.com/media/en/technical-documentation/application-notes/AN-423.pdf

I know about this application note. But it is a rather nasty circuit of "we can do that too" type.  Parameters are strongly dependent on mosfet transconductance and gate threshold voltage. Due to manufacturing parameter spread of the mosfet, I consider this circuit unusable in practice. Linearity of such circuit is also very questionable.  I find absolutely unacceptable to have (at least) two values to trim for the modulator (offset and amplitude of the driving signal). Temperature drift would be a major one here probably too.

Or, you can simply use analog multiplier. Depending on your frequency needs you can find several models. AD835 for example goes up to 250MHz, with 2Vpp output.
Also you can read up on "Gilbert-cell" that is supposed to be the holy grail of the multiplier circuits regarding low distortion.

To use a $25+ four quadrant multiplier IC is not the suggestion I would expect, regarding the circuit in question above. Otherwise I could also generate the amplitude modulated signal digitally, using an FPGA and a DAC, or AD9957 digital IQ modulator or similar highly specialized IC. But this is not the goal here, but I appreciate your suggestion, especially of the gilbert-cell. I almost forgot about it.
 
So back to discrete semiconductors and my circuit. What could be done to tame it to perfection?

Would adding emitter degeneration increase the linearity?

What other modification could be done to increase its linearity, apart from completely changing the topology to such as the Gilbert cell?

Thank you David for that app-note, I am going to read through.

Thank you, Y.

[CJay]

https://www.analog.com/media/en/technical-documentation/application-notes/AN-423.pdf

I know about this application note. But it is a rather nasty circuit of "we can do that too" type.  Parameters are strongly dependent on mosfet transconductance and gate threshold voltage. Due to manufacturing parameter spread of the mosfet, I consider this circuit unusable in practice. Linearity of such circuit is also very questionable.  I find absolutely unacceptable to have (at least) two values to trim for the modulator (offset and amplitude of the driving signal). Temperature drift would be a major one here probably too.

It's crude but it works and actually sounds OK (I've tried it for a simple lashup, I also wanted to FM the DDS but that took more processor 'grunt' than I had intended to use)

YMMV of course but it seems to me it would be a lot simpler to design a suitable circuit to drive that pin at audio frequencies with the relevant de-emphasis rather than have to design something with flat gain and good linearity across 30MHz which seems to be your current problem.

[vk6zgo]

The best way to do this would to be to ditch your "wideband modulator", do your modulating at a fixed IF
frequency, then up or down convert the resultant AM signal as required.

The ubiquity of the "Superheterodyne" method bears witness to its advantages over direct modulation.
The first generations of analog TV transmitters had direct modulation at Channel frequencies ("wideband modulators" in the other sense.)
These were quite quickly replaced with "IF modulation" which gives you repeatable performance for any radiated frequency.

That said, old tube based signal generators managed reasonable modulation over a similar range to that which you require, with quite simple modulators.

[David Hess]

An MC1496 is less then $1.

[Yansi]

The best way to do this would to be to ditch your "wideband modulator", do your modulating at a fixed IF
frequency, then up or down convert the resultant AM signal as required.

The ubiquity of the "Superheterodyne" method bears witness to its advantages over direct modulation.
The first generations of analog TV transmitters had direct modulation at Channel frequencies ("wideband modulators" in the other sense.)
These were quite quickly replaced with "IF modulation" which gives you repeatable performance for any radiated frequency.

That said, old tube based signal generators managed reasonable modulation over a similar range to that which you require, with quite simple modulators.

And where is the advantage of that? I would still need to AM modulate a carrier signal, i.e. being able to multiply two signals. But now at least two times!

If one wants to make a synthesizer "the old way", I would need two rather high frequency oscillators, one fixed (your "IF") and one variable, to be able to synthesize 0.1 to 30MHz. Both oscillators would need be quite deep in the VHF region, to be able to produce a clean* frequency result in the range of 0.1 to 30MHz, after the final mixing.

*Clean means no significant harmonic content at the output. Doing this for a fix frequency TV channel, fine, but I think definitely not for a 300:1 range frequency generator.

An MC1496 is less then $1.

That rather comes as a surprise. O_o  Considering SA612 costs at least few times that and has almost the same guts inside.
Which comes to a nice idea: I do not have any MC1496, but plenty of SA612.  I could probably try a modulator using that one?

Also after a bit of digging, this came up: https://www.eevblog.com/forum/projects/ne602-ne612-double-balanced-mixer-amplitude-modulation-(am)-generation/ 

Questions about my original circuit still remain. I am not satisfied easily 

[Yansi]

https://www.analog.com/media/en/technical-documentation/application-notes/AN-423.pdf

I know about this application note. But it is a rather nasty circuit of "we can do that too" type.  Parameters are strongly dependent on mosfet transconductance and gate threshold voltage. Due to manufacturing parameter spread of the mosfet, I consider this circuit unusable in practice. Linearity of such circuit is also very questionable.  I find absolutely unacceptable to have (at least) two values to trim for the modulator (offset and amplitude of the driving signal). Temperature drift would be a major one here probably too.

It's crude but it works and actually sounds OK (I've tried it for a simple lashup, I also wanted to FM the DDS but that took more processor 'grunt' than I had intended to use)

YMMV of course but it seems to me it would be a lot simpler to design a suitable circuit to drive that pin at audio frequencies with the relevant de-emphasis rather than have to design something with flat gain and good linearity across 30MHz which seems to be your current problem.

Yes, crude and unusable. Almost zero repeatability. I do not doubt it working, but the parameter spread is what prevents one to use the circuit reliably. Also, decreasing the output amplitude of the DDS will I think worsen the SNR (and probably other properties too). It is I think always good to keep DAC output levels maximum possible, (not only) to reduce cross-talk from the digital circuitry. Not that it would matter so much with such cheap 10bit DDS, but as a matter of principle.

Gain (flatness) is non-issue here. Linearity is on the other hand a big one. 

[vk6zgo]

The best way to do this would to be to ditch your "wideband modulator", do your modulating at a fixed IF
frequency, then up or down convert the resultant AM signal as required.

The ubiquity of the "Superheterodyne" method bears witness to its advantages over direct modulation.
The first generations of analog TV transmitters had direct modulation at Channel frequencies ("wideband modulators" in the other sense.)
These were quite quickly replaced with "IF modulation" which gives you repeatable performance for any radiated frequency.

That said, old tube based signal generators managed reasonable modulation over a similar range to that which you require, with quite simple modulators.

And where is the advantage of that? I would still need to AM modulate a carrier signal, i.e. being able to multiply two signals. But now at least two times!

If one wants to make a synthesizer "the old way", I would need two rather high frequency oscillators, one fixed (your "IF") and one variable, to be able to synthesize 0.1 to 30MHz. Both oscillators would need be quite deep in the VHF region, to be able to produce a clean* frequency result in the range of 0.1 to 30MHz, after the final mixing.

*Clean means no significant harmonic content at the output. Doing this for a fix frequency TV channel, fine, but I think definitely not for a 300:1 range frequency generator.

An MC1496 is less then $1.

That rather comes as a surprise. O_o  Considering SA612 costs at least few times that and has almost the same guts inside.
Which comes to a nice idea: I do not have any MC1496, but plenty of SA612.  I could probably try a modulator using that one?

Also after a bit of digging, this came up: https://www.eevblog.com/forum/projects/ne602-ne612-double-balanced-mixer-amplitude-modulation-(am)-generation/ 

Questions about my original circuit still remain. I am not satisfied easily 

The "old way" will work perfectly well if you have several ranges.
No, you don't have to Amplitude Modulate a carrier more than once.

Up & down conversion  of an already modulated carrier is a lot easier.
It is done all the time in Amateur Radio HF Transmitters, which commonly cover a  range of frequencies from 1.8 Mhz to 30 MHz, albeit, in a number of selectable bands ( some can be modified for continuous coverage, although that is illegal)
Not quite your 0.1 to30 MHz but close.

Why do you have to have it continuously tuneable?

Unfortunately, in the real world, the only way you can get rid of harmonics & spurii, is to use selective tuned circuits, or traps, &/or  take smaller "bites" of bandwidth where you can use Band Pass Filters.

[IconicPCB]

As  first step towards a better balance, try a 1K pot in the emitter circuit of Q1 and Q2 connect the wiper to the collector of Q3.
Apply the signals and try adjusting the balance between the two transistors.

Also swap C6 and C5

[Yansi]

The "old way" will work perfectly well if you have several ranges.
No, you don't have to Amplitude Modulate a carrier more than once.

Up & down conversion  of an already modulated carrier is a lot easier.
It is done all the time in Amateur Radio HF Transmitters, which commonly cover a  range of frequencies from 1.8 Mhz to 30 MHz, albeit, in a number of selectable bands ( some can be modified for continuous coverage, although that is illegal)
Not quite your 0.1 to30 MHz but close.

Why do you have to have it continuously tuneable?

Unfortunately, in the real world, the only way you can get rid of harmonics & spurii, is to use selective tuned circuits, or traps, &/or  take smaller "bites" of bandwidth where you can use Band Pass Filters.

Old way will probably work, with MINIMUM number of 2 multipliers (one for modulator), other for mixing the carrier with another LO. I will also need two low pass filters with steep roll-offs for the two, to produce harmonic-free signal from 0.1 to 30MHz.  Generating signal this way is MUCH more complex circuit.

You should perhaps state some facts why it is better, because I do not see anything better in that and already provided facts why it is not.

To restate these facts:

Old school frequency synthesis uses two rather high frequency oscillators, mixed together to produce a signal with low harmonic content. All harmonics of the oscillators must be filtered out, then mixed together with a highly linear mixer (so no new harmonics are created) and then again low pass filtered.

This way one needs a minimum of TWO oscillators: One fixed frequency, one continuously variable one, over the required output range. The fixed LO needs to have the AM modulator (to satisfy your requirement of modulating a fixed IF frequency), in this case the modulator will need to work deep in the VHF region, although fixed frequency.
Then I need minimum two analog multipliers, one for the AM modulator, one for the final mixing.
I also need two VHF low pass filter and one output low pass filter (brobably not that much needed, but better to have one).
Not counting the numerous buffer amplifiers and places requiring tweaking of signal levels and obligatory impedance matching, also the need for using a high frequency DDS or fractional PLL for the variable LO, to be able to obtain stable frequency output and continuous tuning step of say 10Hz across the whole 30MHz output range.

What do you see better in this approach, compared to the one at the bottom of the image?

What is the argument to not have a wide-band modulator? Apart that it could have a tuned LC tank at the output, to suppress a little bit of upper harmonics (still it would have to be low pass filtered). Still you need an analog high frequency multiplier, with good linearity.
You may probably argue with gain variation over frequency, but you will have that one even in your complex synthesizer topology, due to the variable LO.  Output signal level is however fixed very easily using an ALC loop.

I do not see any advantages of the complicated way you are presenting.

Why do have I to have it continuously tuneable?  Because I am trying to design a simple HF generator from 100k to 30MHz. Not an LO source for an amateur transceiver with just few very narrow bands. Originally, as stated in my first post in this thread, this will serve as a signal source for testing and fixing LW/MW/SW tube receivers. The frequency ranges available on these are pretty wide compared to an amateur transceiver.

As  first step towards a better balance, try a 1K pot in the emitter circuit of Q1 and Q2 connect the wiper to the collector of Q3.
Apply the signals and try adjusting the balance between the two transistors.

Also swap C6 and C5

So in fact you are wanting me to add emitter degeneration (that's what I am asking from the beginning). Will try it.

I will however not swap the two capacitors (C5, C6), only increase capacitance of C6. I think I know where you are going with that. However that still won't fix poor P1dB/IP3 of the modulator (emitter degeneration might?), will just reduce cross-modulation of the bias point from the modulating signal input.

[BigMark]

You might be better off researching 'Heising Modulation' (High Level Modulation) which was popular in the 1930's with various tube transmitters. Tended to use a choke or low pass filter to isolate the AF and RF stages. I think your just trying to drive the RF stages of an old AM receiver, so you don't need to run much power and therefore is little need to over complicate the stages with exotic filtering.

A quick look I found this website which might be a guide.
https://www.w8ji.com/Heising%20modulation.htm

For the choke you could just use a low pass inductor/capacitor filter. Inductor in series for DC pass. A circuit using FET's can be found in this book at the top - page 652. Also see attached image.
https://books.google.co.uk/books?id=uoj3IWFxbVYC&lpg=PA652&ots=11rUNkQAsS&dq=heising%20modulation&pg=PA652#v=onepage&q=heising%20modulation&f=false

Using a mixer like someone proposed is low level modulation which is popuilar with transceivers and TV transmitters. Probably over complicated for your needs. TV transmiters had major lineartiy requirements which required video level and IF pre-correction and transceivers tended to use many common stages to reduce the size/scale of the equipment. So AM and FM shared many stages.

[Yansi]

This exactly would not work as I intend it to. This will produce significant harmonic content and low pass filter will not correct that - there is no way a single low pass filter would get rid of higher harmonics, considering the amount of tuning range (300:1) I need.  It would need multiple (not two, or three, but likely tens of them) to be able to get to the level of signal clarity a DDS produces.

I think the wide-band Gilbert cell approach is so far the easiest and best one. I just need to etch a board for the NE612 to have a go with it, until the "proper" MC1496 will arrive.

I just need to make that analog multiplier to perfection, that's all what's needed.  I have added the emitter degeneration resistors and it did quite the expected: Significantly decreased gain (up to the point it has only attenuation, like about 15dB or so with 2x 470R) and increased its capability to deal with more dynamic signals - which was also expected.

However the modulation characteristic of the single diff-pair circuit is still very poor. I can't get over few % modulation depth without introducing significant distortion in the modulated output.  I do not know what's wrong, probably circuit biasing? Or it is just the limit of this simple circuit, that could be fixed with the Gilbert cell?

[Wolfgang]

Hi Yansi,

another chip you may try is the good old NE602/612, also a Gilbert cell multiplier up to more than 100MHz.

[Yansi]

Hi Wolfgang, have you not probably read the thread, I am already etching a test board for it. But thanks for the tip 

[Wolfgang]

Sorry, your last post slipped my eyes. Much success ! 

[David Hess]

An MC1496 is less then $1.

That rather comes as a surprise. O_o  Considering SA612 costs at least few times that and has almost the same guts inside.

The 1496 balanced modulator/demodulator is more of a basic building block with many configurations and applications so demand is higher yielding lower costs.  Practically anything else is more specialized.

Another option in the past would be to configure a couple of transistor arrays as essentially a 1496.  You could also build a 1496 or half of a 1496 out of matched discrete transistors.

[Yansi]

I have never heard before and never seen a MC1496. Probably not so popular IC around here. But SA6x2 is a pretty common one.

So, I have built a simply stupid circuit using the SA602 to make AM modulator out of it.  I am feeding a little offset current to one of the inputs, to have a bit of carrier leak through (otherwise I would get a pretty good DSB modulation).

It works very good, however only with very low input signals. The output is also extremely weak. Can't get more than -42dBm carrier level output, otherwise significant distortion occurs at the output.

The input carrier level (into the OSC-B pin) is equivalent to -15dBm,  which is around 40mVrms
the modulation input level is 30mVp (21mVrms).
And I am getting output carrier level of -42dBm, the overall output level is 3.2mVrms.  Jeeez, that's fu*ng low!

If I try to push the levels, unacceptable distortion occurs.  To be honest, I do not have much like for such low signal levels

First I think the ultra-weak output is caused by my non-ideal output circuit configuration. I have connected a 1:1 RF transformer directly at the SA602 output pins.

If I now think about it (apart from I should stop copying stupid designs from the web), the differential output resistance of the SA602 is 3kohm, as each collector is internally loaded with 1.5k resistor to V+, and the circuit is working in constant current mode - so the output resistance is defined mainly by the internal load resistor of 1.5k for each output, making 3k output resistance differentially.  (Yes, I mean resistance, ignoring the parasitic capacitances, which at my low frequency of few MHz are probably insignificant to the problem)

So this output resistance (or impedance, whatever you call it) makes a nasty voltage divider, that eats my signal level - as of course, as a load sits my spectrum analyzer with 50 ohm input.

If I think about it, using a center tapped transformer with the center tap connected to V+, as the one in my previous circuit, the internal 1.5k resistors become parallel to the transformer, not in series with it, which should increase the signal level substantially, if I am correct.

//EDIT: Using center tapped transformer made the level only few dB higher, like a 3dB or so (now overall output level measured is 5.6mVrms). Still too low.

[Yansi]

Well silly me, I could just use the transformer to match the output impedance.  But the ratio is rather nasty 7.75:1.

Still, using such transformer (31 to 4 turns) did not improve it by much, output level still only circa 11mVrms (carrier at -33dBm or around that)

So very very small levels everywhere, I do not like that really.

[Wolfgang]

Yansi, did you have alook at the Philips/NXP app notes about the NE612 ? I think they have several input/output matching circuits there, IMHO.

[nugglix]

"If the required LO is beyond oscillation limits, or the system calls for an external LO, the external signal can be injected at Pin 6 through a DC blocking capacitor.
External LO should be at least 200mVP-P."

From page 4 of the attached data sheet.
40mVrms seems to be a bit below the required voltage.

[Yansi]

I call NO on that required datasheet voltage, as at such level, I get significant second and third harmonic of LO appear at the output. 3rd harmonic is only like 30dB bellow the fundamental.  That is unacceptable.  Conversion gain may be higher, though, but linearity suffers significantly.

At the level of -20 to 15dBm of LO level, the second and third harmonics at the output are negligible, at least for my spectrum analyzer and the level of fundamental (+ the 2 side-bands of modulation) does not have that much lower level.   

[Wolfgang]

What you could do to keep the NE612 in the "linear" range and still have a useful output amplitude is to add a differential video amp at the NE612s output.
How many dBms at the output do you really need ?

[dzseki]

If you'd end up desperate, you could be my guest for a "pulled" AD835 in SO-8 package, just tell me your address 

[IconicPCB]

The purpose of the pot was not to reduce the gain but to provide a method of better balancing of the discrete pair.
You are working with a couple of unmatched transistors.

[BrianHG]

Have you considered discrete UHF TV modulator circuits, ignoring the audio input.  The simpler ones AM modulation usually goes way beyond the 6 MHz bandwidth and the circuits bandwidth may be easily increased.

NXP have some old dirt cheap all in one ICs with true proper linear 4 quadrant mixers with a simple oscillator tank which go from VHF through UHF, with internal limiting and signal leveling/clipping.

The last dirty trick which will be linear, cheap, and use available parts.  I can recommend is using a high speed 74HC4053 analog switch.  Clock the A/B mux input at the MHZ you want your output oscillation broadcast band to be and feed the A/B inputs a differential signal of your base-band source signal.  (A input = + signal, B input = - signal.  Just don't cross the mid 2.5v unless you want the modulated output to invert...)  The mux output will have your AM modulated signal.  The output will be square wave, but, a simple RLC bandpass filter will clean that up.

Don't go beyond 60Mhz modulation with a 74HC4053.  Faster analog muxes exist if you want an above 100MHz modulated output.  In fact, simple VGA analog muxes for superimposing graphics on video will have amplified inputs with a switched amplified output where you can modulate up above 100Mhz available at analog devices.

[Yansi]

If you'd end up desperate, you could be my guest for a "pulled" AD835 in SO-8 package, just tell me your address 

What you could do to keep the NE612 in the "linear" range and still have a useful output amplitude is to add a differential video amp at the NE612s output.
How many dBms at the output do you really need ?

I would like to have about 0dBm at the output, not that critical. However I am afraid that trying to amplify by those 30 or 40dB will have significant impact on output noise level. That is no little amplification! 40dB may even be a huge struggle for something as a video amp. Even if the amp would have a like 300MHz bandwidth, it would barely suffice. (GBW is divided by gain).

If you'd end up desperate, you could be my guest for a "pulled" AD835 in SO-8 package, just tell me your address 

Well, Hungary is not that far, wouldn't cost a metric shit ton, but no thanks, that part is worth more than the DDS itself. 

The purpose of the pot was not to reduce the gain but to provide a method of better balancing of the discrete pair.
You are working with a couple of unmatched transistors.

I understand that, however adding a 1k pot (470R emitter degeneration) decreases gain significantly - that's what it does too.  I won't be afraid of transistor mismatch. These were from a single batch, modern SMT parts. 

Have you considered discrete UHF TV modulator circuits, ignoring the audio input.  The simpler ones AM modulation usually goes way beyond the 6 MHz bandwidth and the circuits bandwidth may be easily increased.

NXP have some old dirt cheap all in one ICs with true proper linear 4 quadrant mixers with a simple oscillator tank which go from VHF through UHF, with internal limiting and signal leveling/clipping.

The last dirty trick which will be linear, cheap, and use available parts.  I can recommend is using a high speed 74HC4053 analog switch.  Clock the A/B mux input at the MHZ you want your output oscillation broadcast band to be and feed the A/B inputs a differential signal of your base-band source signal.  (A input = + signal, B input = - signal.  Just don't cross the mid 2.5v unless you want the modulated output to invert...)  The mux output will have your AM modulated signal.  The output will be square wave, but, a simple RLC bandpass filter will clean that up.

Don't go beyond 60Mhz modulation with a 74HC4053.  Faster analog muxes exist if you want an above 100MHz modulated output.  In fact, simple VGA analog muxes for superimposing graphics on video will have amplified inputs with a switched amplified output where you can modulate up above 100Mhz available at analog devices.

Similar was already advised by BigMark, however the same problem applies - this can only and only be used for fixed frequency, or very small range of frequencies. Otherwise the required set of switchable filters would be enormous. Hence why I may seem to look for an unobtainium - I just want to see, if there's a simple enough circuit, that would work as a wide-band AM modulator.


I also still may turn around and try a different approach:

1) Make the modulation of the DDS bias current to perfection
2) one very neat trick I have almost forgot about, that is used in many off-the-shelf RF generators (seen that at least in a Marconi instrument):

Every decent generator should have amplitude level stabilizing circuitry (ALC).  If the ALC loop is made fast enough with a linear detector, it could be used for AM modulation.  As I am only interested in modulating audo signals, probably less than 9kHz (like AM broadcast is), this should not be an issue!
Also probably this will be even better, as the ALC block (which I would try to add anyway - although not really that precision necessary for checking tube receivers) would get reused twice.  100% modulation not really required either, so the linear detector could probably be made simple enough.

Reusing the ALC for AM would need to master two key circuit blocks: The variable gain stage (probably a handful of PIN diodes, or a FET tetrode like a BF998?) and the detector.

[BrianHG]

A Motorola MC1374 PorD should work for you with the 4.5mhz carrier coil removed.  You can amplify the output as you see fit.  A tuning diode on the AM oscillator coil will give you tuning.

[Wolfgang]

What you could do to keep the NE612 in the "linear" range and still have a useful output amplitude is to add a differential video amp at the NE612s output.
How many dBms at the output do you really need ?

I would like to have about 0dBm at the output, not that critical. However I am afraid that trying to amplify by those 30 or 40dB will have significant impact on output noise level. That is no little amplification! 40dB may even be a huge struggle for something as a video amp. Even if the amp would have a like 300MHz bandwidth, it would barely suffice. (GBW is divided by gain).

You could amplify in two stages, then the gain should not be a big deal. How clean (noise) should your signal be ?

[Yansi]

As clean so the modulator will not significantly worsen the signal clarity from the DDS (AD9834).

[Wolfgang]

... the DDS chip has about 72dB dynamic range. The NE612 is not particularly low-noise, so this could be a bit of a challenge. The critical part is the first amplifier. If you need a premium part to meet specs, its probably better to use a multiplier chip instead.

[Yansi]

Thats a narrowband figure, mind you. I think if the rest of the garbage will be -50 to -60dBc, it will be good enough for the girls I go out with.

However the NE612 modulator isn't able to even come close to that, you are absolutely right. I am getting garbage -40dBc or even less. That's not nice.

[David Hess]

I have never heard before and never seen a MC1496. Probably not so popular IC around here. But SA6x2 is a pretty common one.

I suspect the 1496 is the longest lived and most common RF IC ever but the SA602 is not far behind.

It works very good, however only with very low input signals. The output is also extremely weak. Can't get more than -42dBm carrier level output, otherwise significant distortion occurs at the output.

Gilbert Cells have low IP3.  They are not suited to low distortion at high signal levels and are easily overloaded.

[Wolfgang]

Thats a narrowband figure, mind you. I think if the rest of the garbage will be -50 to -60dBc, it will be good enough for the girls I go out with.

However the NE612 modulator isn't able to even come close to that, you are absolutely right. I am getting garbage -40dBc or even less. That's not nice.

Yeah, the NE612 is cheap, but it was never intended as a front-end low noise mixer. In fact it was planned as a mixer for the IF strip of old mobile phone receivers, around 70MHz and with sufficient signal level so noise was not decisive. Maybe AD835 and successors is the way to go. Not cheap, though.

[Yansi]

If I remember correctly some datasheet/appnote or whatever it was, the NE602 was used directly as an input mixer for those cordless phones, operating at 45MHz.    I remember there was even a figure "how low signal it could still receive". 

[Wolfgang]

Correct, but the 45MHz were the frequency of the first IF strip and not the "direct input" (that was about 900MHz at the time). So amplification, mixing, some AGC and filtering happened *before* the signal reached the NE612. For a front-end mixer the NE612 has another weakness: IP3 is much too low. Still its used much in radio amateur circuits with a low parts count.

[Yansi]

Okay it might be, however I remember an antenna directly drawn at the input of the mixer. 

And I know and have been warned before, that SA6x2 has problems with strong signals. But U haven't expected this be so bad, that I would not get more than like -35dBm out of it without ugly distortion happening.

[Wolfgang]

Okay it might be, however I remember an antenna directly drawn at the input of the mixer. 

And I know and have been warned before, that SA6x2 has problems with strong signals. But U haven't expected this be so bad, that I would not get more than like -35dBm out of it without ugly distortion happening.

One reason why the very low IP3 was not of much concern for the originally intended purpose was that the NE612 had to digest only a narrowband signal and its output went into another narrowband (2.IF strip) filter. A lot of harmonic products fell out of the passbands so it was not all that bad.

I have seen designs with the antenne at the mixer input, but those were strictly hobby circuits and never something professional. For a shortwave beginner receiver with low parts count, why not ?

Another issue to observe is that NE612 IP3 depends on LO amplitude. If we use several 100mV of LO, we get a lot of mixer products, but a better IP3. For multiplying purposes, this is not an option.

[Yansi]

Another issue to observe is that NE612 IP3 depends on LO amplitude. If we use several 100mV of LO, we get a lot of mixer products, but a better IP3. For multiplying purposes, this is not an option.

I can confirm that from my experimenting.

[MT]

https://www.analog.com/media/en/technical-documentation/application-notes/AN-423.pdf

I know about this application note. But it is a rather nasty circuit of "we can do that too" type.  Parameters are strongly dependent on mosfet transconductance and gate threshold voltage.

Us a linear NPN current sink instead of mosfet, or the LT1228, or this circuit might do:

[Yansi]

And how is this circuit supposed to work as a VCA? It is just an extremely non-linear trimpot driven something, which looks like a T-pad.

[Yansi]

Currently I have had no free toy to toy with this idea or to test any circuitry.

But I just want to remind, that a good circuit should be a good compromise both in parameters and price.  I think it is a nonsense to use a cheap DDS, such as  AD9834 which can be had for $3, and then try modulate it using a $8 OTA or a $12 four quadrant multiplier.  It might solve my problem fully, but then it does not count as an appropriate solution - in my little enginerding opinion.

I might try giving a try going the second route, i.e. modulating the DAC current and bringing this solution to perfection.  If it happens to fulfill the requirements on signal quality, then it might be the way to go, as making a cheap wideband AM modulator does not simply happen (which is also a result)

[MT]

And how is this circuit supposed to work as a VCA? It is just an extremely non-linear trimpot driven something, which looks like a T-pad.

Apparently it operates as a VCD (damper) not VCA ,you asked for something and got something, i didnt say its fantastic but if you dont even want to consider it i regret the deepest i even mentioned it.  Now im going out to the shed and cry!

Anyhow, the cheapest method is to fiddle with Iset/Fs adjust current via NPN/PNP sink/source or digital pot leaving all the external multiplier debacle out..

[Richard Crowley]

This is the 21st century, the Digital Age.
What about simply PWM the carrier frequency?
Then filter the harmonics and PA up to the desired power?
I believe there are MW broadcast transmitters that use this method.

[Yansi]

Any kind of signal chopping does not cut the mustard, when the modulator must be WIDE BAND.  You would need a metric shit ton of filters to cover a range from 100k to 30MHz.


On the other hand... look what I have found! https://www.analog.com/media/en/reference-design-documentation/reference-designs/CN0156.pdf

I have never noticed this appnote. That looks very usable, as the Rset resistor just pulls current through some internal current mirror, placed against the internal VREF of 1.2V.

So, first thing I thought of, was just feeding the AC modulation signal with a DC offset to the Rset resistor directly, see the image below.

But there's the drawback of the voltage drop accross the current mirror being very likely non linear, so it would be better to convert the modulation signal to current and feed the FSADJ pin directly.

I just need a decently linear programmable current sink. I think that could be done easily.

[Yansi]

So how about this one?

A programmable current sink. The full scale current, according to typical values from the datasheet, is 1.15V / 6k8 = 169uA.  So I set the current sink to half that. 0.25V / 83.3uA, which is the half of the full scale current. 

Then the full scale input voltage of 1Vpp gets divided by two, to be a 0.5Vpp or 250mV peak, which fits nicely with the modulation range.

I have used the internal 1.2V reference from the DDS to obtain a repeatable bias at half the full scale. I don't think those 21uA drawn by the divider will do any harm to the DDS.  C11 is used to prevent modulation signal reaching the REFOUT pin.

Bandwidth of the current loop (R8, C9) was set as 70kHz.

Do you think it could work like this?

[MT]

What max frequency is going to be applied on X1?

[Yansi]

Audio, so 20kHz at most, probably I will limit it to 9kHz with a steep low pass filter, for it to produce similar spectrum as AM broadcast stations.

[CJay]

Will be interesting to see how you get on with modulating FS

[Yansi]

I have designed my circuit based on this and some common sense reading the AD9834 datasheet, to get an idea, what might be sitting at the FSADJ pin - as I've said, probably a cirrent mirror, sourcing current from the REFOUT internal reference. Supported by the fact, that the voltage at FSADJ pin is slightly below the internal ref voltage suggest this may be true.

AS the FSADJ current SHOULD scale linearly with output current, one probably might use this to obtain the AM.

I will try making (yet another) test PCB for this.

I just need to figure out how to implement the output (filter + buffer). Filter probably a 7th order elliptical @ 30MHz, but I don't know whether to use the differential output of the DDS, or whether not. And where and how is it best to obtain the impedance transformation from 200 to 50 ohm.

If I have calculated correct, the DDS should produce about -12dBm maximum with 3mA full scale current set (4mA is the limit by the datasheet). (full scale current being 18 times what is pulled from the FSADJ pin).

[MT]

I have designed my circuit based on this and some common sense reading the AD9834 datasheet, to get an idea, what might be sitting at the FSADJ pin - as I've said, probably a cirrent mirror, sourcing current from the REFOUT internal reference. Supported by the fact, that the voltage at FSADJ pin is slightly below the internal ref voltage suggest this may be true.

As the FS adj current SHOULD scale linearly with output current, one probably might use this to obtain the AM.

It will swing between 32mV and 624mV with 200ohm load on Dac outs within the given current min-max range at Fs pin.  RELAX! it has been done before, many times! AD9102 uses switched resistor network on the Fs Iset..

[Yansi]

In which way should i RELAX?  I am kind of a non-believer when it comes to strange hacks like this one. What has been done before specifically?