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

[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.