How do you design a modulation scheme for signal generator?

[Chenleine]

As part of a vintage radio restoration project, I decided to build a simple signal generator for alignment.

The idea was straightforward, a short distance AM transmitter(http://www.kynix.com/Product/Cate/219.html) with a variable carrier frequency in the MW band (at least 550kHz to 1.6MHz) modulated by a fixed low frequency audio tone (2kHz ). In my naivety, I decided to build the oscillators first: a JFET based Hartley for the carrier and a gain stabilized Wien bridge for the audio tone. So far, so good and both oscillators perform very well.

I thought the final step of modulating the audio tone would be simple. Boy, was I wrong. I have spent several days of trial and error with various (suitable) AM modulation schemes with very little to show. The montage below shows some of the modulators I've come across and I seem to be stumbling on the same issue with most of them: the band-pass filter! This makes sense, of course since most modulators are designed for transmission on a single carrier frequency. However, my requirement is that I can adjust the carrier across the broadcast band.

So, my question is this: how exactly do you design a modulation scheme for signal generator where the carrier frequency is adjustable?

Other notes:

One option I came across after I'd built the oscillators was explained (here). In this case the author varies the supply to the carrier oscillator itself which solves the problem of needing a second band pass filter as part of a separate modulator. However, I experimented with my JFET based Hartley and found that doing this led to a considerable amount of distortion (probably because the tuned circuit is at the JFET emitter).

I discovered that a major problem with many (all?) of these schemes is that even if you manage to get a half-decent AM output, when you adjust the carrier frequency you get large amplitude changes to the modulated output. It's almost like you'll need some kind of AGC control in place, even if you did manage to alter the band-pass filter in line with the carrier adjustment.

I considered using the second (smaller capacitance) section of my air variable tuning capacitor to form the band-pass that would change alongside the carrier frequency section. The result was a mess. It was too difficult to pick a suitable inductance to track the changing carrier. So, a good idea that proved difficult to achieve in practice.

[Station X]

It would have been nice if you'd responded to those who tried to help you in this unconnected thread:-

https://www.vintage-radio.net/forum/...d.php?t=136479

[lesmw0sec]

I built a DDS signal generator and used a wide-band VCA for amplitude modulation. The details and circuit are shown as part of a power-point talk which is listed here:
http://www.mw0sec.co.uk/amateur.html
Scroll down to 'Projects'.
Les.

[G8HQP Dave]

If you want good AM don't modulate the oscillator. Modulate a gain stage or attenuator after the oscillator. The modulator does not need a tuned circuit, unless it is nonlinear - in which case it may be a bad modulator. An exception is Class C, but you won't be using Class C in a sig gen gain stage.

[Al (astral highway)]

Hi there,

The modulator illustrated in the bottom of your infomatic there is generically similar to one in this thread.

There was a giant discussion about it.

It's pretty straightforward and works well as a modulator. Just ignore the loop aerial for your purposes.

[Argus25]

Its dead easy to make a linear amplitude modulator. You simply use a 4 quadrant multiplier IC. If you are dealing with low level signals an MC1496 is handy.

For larger level signals, multiplier IC's like the AD734 are useful. The AD734 makes a good amplitude modulator for audio modulating a carrier up to about 7MHz.

You can also use these multiplier IC's as level controllers/attenuators if you feed DC from a pot into one of the inputs. They have multiple other uses too.

If you look up the application notes for IC's like the MC1496 and AD734 you will see example circuits. Essentially the AD734 takes the two inputs and multiplies them together and divides the output typically by 10 to keep it inside the power supply limits.

For most of my pantry transmitters I use an MC1496 as the amplitude modulator, being fed with audio from a compressor circuit and the RF comes from a separate crystal oscillator.

In general I agree with the remark, that it is best not to modulate the oscillator. However, with the right design, it is possible to attain fairly linear modulation by modulating the emitter current of a single transistor oscillator, but the setup is a little more tricky than using a multiplier IC. Though Tek did this in one of their sine wave generators for amplitude control, but they were masters of discrete transistor design.

[Argus25]

If you don't want to go down the IC road, I have attached a simple circuit I designed which was published in Silicon Chip, June 2016.

This amplitude modulator was placed inside an FM radio, the idea being to get the audio out from the detector of the FM radio and modulate it onto a 1400kHz carrier so an AM car radio could receive it, the output being coupled to the AM radio's antenna wire.

As you can see the 1400kHz RF is coupled into the base of Q2 , while the audio modulation is coupled into Q4. The output is taken from the collector of Q3.

[kalee20]

I'd definitely agree with Argus, use a linear multiplier IC. I had misgivings about performance at RF, but up to 1MHz should be fine. Check manufacturer's data to be sure.

Modulating an oscillator's supply voltage can be done, and can work well - but it is prone to giving an amount of FM too. For aligning radios, you can get weird effects such as the response curve slope-detecting the FM, which interferes with the already-existing AM, sometimes constructively, sometimes destructively.

Using a stable oscillator, and trying to modulate a following amplifier, you then have problems with distortion of the waveform - unless you use a tuned circuit to restore it. In a transmitter, that's fine (fixed frequency). In a variable signal generator, you're into a tracking problem with two or more tuned circuits.

You avoid all this with the multiplier.

[Argus25]

Originally Posted by kalee20

I had misgivings about performance at RF, but up to 1MHz should be fine.

I found the AD734 is fine (very linear) to at least a carrier frequency of 7MHz used in an audio amplitude modulator which is about what one would expect from its specs.

There are some fantastic small and vintage surface mount IC's around from the 1980's that have essentially the same crossed collector (Gilbert cell) configuration as the MC1496, they were used in the Philips PM5519 pattern generator in the RF modulator, they are the TCA820 , which got replaced in later models by the TDA0820T. They make wonderful modulators right up into the TV UHF spectrum and make an MC1496 look sluggish!

[camallison]

Originally Posted by Station X

It would have been nice if you'd responded to those who tried to help you in this unconnected thread:-

https://www.vintage-radio.net/forum/...d.php?t=136479

Seems to be going for a double.

Colin

[Wendymott]

The DDS AD9850 can be amplitude modulated via pin 12. Connect a 2N7000 N type enhancement fet to ground, a 1K5 resisor in the source to deck. From pin 12 a 510pf cap to deck. Audio to the gate of the fet. Unfortunately you need to add a 1:1:1 transformer to pins 20/21 to get a full balanced modulated output. If used single ended then you only get the top half of the modulated carrier.
Not actually tried it yet, but it is in the application notes for the 9850. One of the many jobs.,
LES... MW0SEC. I have looked at your generator with great interest.... but I cannot find the data sheet for the 822.... what is the prefix or manufacture please.

[lesmw0sec]

Wendy, you really have me head-scratching on this one! I built the thing quite a few years back. I bought the VCA from RS components, the part number being 459-231 and the actual device code is VCA8221DG4.

The odd thing is that when I enter the part no. in that dreadful RS 'search' facility it comes up with a similar looking device by Texas, but the pin numbers do not match up! Tried a google search for the full device number, but nothing shown.

Either the specific part is now obsolete, or I may have had a mental breakdown when drawing the circuit. My only other alternative is to take the covers off to see how it actually is wired...

[Wendymott]

Hi Les. I found the Texas one and ordered from RS.... £5 ish plus tax ordered two.... I will make a test pcb from the pin outs in the data sheet. Thanks for the feedback though.
As you may have seen, I am making a SSB Generator 80 to 10m but those bands only, not a full frequency range, and this device will help the output level settings.
I will let you know progress.

[Radio Wrangler]

If using a four quadrant multiplier to create AM, then it's important to know to add some DC offset to the audio in order to create a carrier. Otherwise you make DSBSC (Double Sideband, Suppressed Carrier) which doesn't work on AM receivers.

David

[Argus25]

Exactly right.

On all my circuits, using AD734 or MC 1496 I call this the "axis" control. Just a pot to bias the modulator so that there is carrier output in the absence of audio modulation, otherwise you get DSBSC if its balanced. To show the OP how an MC1496 is set up for this application I have attached one of my circuits that uses it. (The output of this circuit passes via a link cable to a 1400KHz power amplifier and then to a loop antenna)

I have attached some scope images from the MC1496 amplitude modulator, to show the OP. The carrier is 1400KHz. A & B are close to 100% modulation, C shows the carrier modulated out and D shows the effect with over-modulation as the modulation goes to the opposite quadrant.

The big problem with amplitude modulating a carrier from audio sources is the dynamic range of the source material, not say just within the song itself, but between different songs from different original sources. It is important not to over-modulate the carrier.

The minimal requirement in my view is to prevent over modulation with a soft clipping circuit (one attached) prior to the audio entering the modulator. Mine also having a peak indicator LED corresponding to the level which would be 90% modulation. Result with the soft clipping circuit attached too.

The ultimate luxury though, is to level the volume of the source material (typically music). By cascading two stages of the NE571 compander IC it is possible to get volume leveling so good you would swear it was coming from a real radio station (one stage is not enough despite volume compressors being sold with this IC and saying that it is, where often its two circuits are used for R & L stereo volume compression).

Cascading the two stages of the NE571 for mono results in the output being proportional to the 4th root of the signal.

As my pantry transmitters have advanced over the years, I always process the audio this way, with both the volume leveling circuit and soft peak clipping circuit, before I let the audio signal get anywhere near the modulator IC.

Also, since the topic of modulating an oscillator directly came up, one of my pantry transmitters is a modified Tek sine wave generator. I use this for radio alignment. In this unit, Tek made a clever oscillator design with just one transistor,where the constants that determine the frequency are relatively independent of the transistor's DC setup. When the emitter current is modulated it produces excellent AM with virtually no FM component. So in this unit I simply added a PCB containing the volume leveler and soft peak limiter, modified the circuit a little and presto, a variable output frequency AM transmitter.

[Argus25]

I've added another couple of photos. The transmitter in the orange die-cast housing is constructed with three boards stacked on each other with spacers, the lower one contains a push pull transistor power amplifier, the one in the middle the power supply parts, chokes & caps etc and the one on the top the amplitude modulator and peak limiter/soft clipper (photo) .

Unfortunately, when I first designed & built this unit I didn't understand the value and importance of the volume leveler for listening to music received on my Hacker Sovereign (using the iPod as the music source for the transmitter) and I only had the soft peak clipping circuit prior to the modulator .

So as can be seen I had to add the NE571 compander IC/leveler circuit later as an additional pcb screw in module.(I'm not fond of add ons) So I learnt my lesson and I now incorporate the volume leveler into all of my AM transmitters since.