HF VHF AM Signal Generator

[Wendymott]

Hi Peeps...Since I sold my 2 Marconi 2022's as I replaced them with My Anritsu, I found occasionally two signal sources are required. I bought a "quickie" from Ebay..it was a 0.1 to 470 Mhz With various FM and burst modulations.. However it was not as quick as I needed as it was faulty and broken, so after a lot of too ing and fro ing I reluctantly accepted their offer to close the problem.
However that still left me with no extra signal source.
As I had some spare pcb's from my other projects, and fortunately enough IC's I decided to build my own.
AM Modulation was required and that caused a little difficulty, however after a bit of perseverance, the final result was OK.
I use a MC1496 for the AM Modulator, and the module is bypassed with a relay for carrier only output.
The reason to bypass the modulator module is that I found.. without a modulator signal, I got no or very little RF output, maybe I have applied the application incorrectly but I think not.
At this point I didnt fit an output attenuator, as I have a nice switched one , but it could be added at some point if needed
I had a nice case in stock, given to me recently, so it was an obvious choice.
I include the schematics for the DDS and the modulator, if anyone wants the PIC Controller please request it in comments.
Just a note about the modulator.. It can be seen that I used a LM368 to drive the MC1496 with a sinewave, but as the input impedance is 50 ohms it needed something that could drive 50R, and a LM368 was the easiest option

[Wendymott]

Hi peeps.... one of our members privately noted that the actual modulator schematic was incorrect as the 51R's to pins 1 & 4 were shown as going to neg line instead of ground.
I checked the pcb copper and it is correct. However I repost the Modulator schematic for completeness. Thanks Paula.

[Julesomega]

Wow, beautiful little unit. I'd so love the PIC code to drive the AD9951, with or without a PCB
The MC1496 is balanced so will give DSB with suppressed carrier. I s'pose you can offset with DC bias to get 50% carrier

[Wendymott]

Hi Julian... PM Sent

[Wendymott]

Hi Peeps.. to post the final update... I changed the modulator to a transformer coupled one with a 2N3866 as a mini pa. I discovered that the MAR6 didnt like too much peak voltage and to get 100% Mod, I was driving it too hard.. Thus the 2N3866. The other thing is.. I am using half wave modulation, as I was too lazy to design a push pull driver. Still it works well.. I attach a couple of photo's for completeness.
I revamped the front panel as well to add more button function data.
The mod transformer is a toroid ring about 40mm dia. I made a shuttle to apply the wire and I lost count of the turns. I stopped when I had two layers and the resistance was about 9 ohms. The secondary (audio) again was about two layers and the resistance was 7 ohms approx to give a turns ratio of about 1.3 : 1..To be honest I was totally surprised when it worked first time, as the toroid was unknown, it has a purple coating.. thats all I know.lucky I guess. Still it works well and as far as I am concerned its finished. Unless any member has a better idea.

[Herald1360]

Those 'scope pictures don't look like AM!


It looks like constant amplitude RF with a lot of AF ripple?

[Argus25]

You appear to have the RF carrier "riding on" or superimposed on a sine wave of modulation. So it is not "amplitude modulation" which is essentially a multiplicative function, where one wave modulates the amplitude of the other. Versus "signal addition" which is what you appear to have.

An excellent amplitude modulator (multiplier) is the MC1496.

If you want to go up into the VHF or UHF region with AM, the IC's to get are the TCA820 or the TDA0820T. Both of these are vintage, but spectacularly good and in surface mount packages.

I have attached a photo you might find interesting of amplitude modulation using an IC multiplier. This is generally called "four quadrant" multiplication. If you over-modulate it , the carrier phase reverses at cross over, shown in image D.

These sorts of things are not that important with AM radio, when you don't intend to do that (if you have proper audio compression and peak limiting, which practically no pantry transmitters have except my own), but critically important in PAL color encoders.

[Wendymott]

Hi Guys.. Thanks for the comments....I knew.. or thought something was wrong... but..I was clearing out some old "test" modules and found a VCA822.... which was bought as a voltage controller attenuator/ modulator.
Hooked it up and it is very good...... no photo's yet as the pcb has been made but not stuffed yet.....
The MC1469, was my first choice, as in my original post, but as I said, there was no or very little output when no modulation was applied...
I will post again when it is up and running.. the downside of the VCA822 is it requires dual supplies, but that has been sorted with a DC/DC converter.

[Argus25]

Originally Posted by Wendymott

The MC1469, was my first choice, as in my original post, but as I said, there was no or very little output when no modulation was applied...

Yes, it is normal when the IC is running in a balanced condition, it simply generates DSB. It needs a DC offset applied so that it is unbalanced and has a significant residual carrier output in the unmodulated state.

Its a great IC, good choice.

I've attached a circuit, ignore the split supplies, but you will see the offset applied to pin 4.

[Wendymott]

Ok Argus.. Many thanks.....I have virtually "spit out my dummy" now LOL... I attach my latest version "10" using the VCA822. Just finished the rear panel artwork.
I really do appreciate the inputs from all our fellow enthusiasts.

[Argus25]

That modulation looks wonderful

[Wendymott]

After all the "hard work"it finally paid off today ....Back on my receiver design project, the new generator provided me with the results I needed, namely the VFO injection level to the MC1469. And as you may know the ideal level is 60m/V rms, as proved today...now just to redesign the IF stage as it is as deaf as a post....40db down on my tri band transceiver.....but now I know.. its not LO injection level...back to the drawing board.