Modified Heising for Class C transistor final

[steve_qix]

Ok, sounds good!  Hopefully, it will work    (it should!).

Good idea to get the parts - I would stock up on some very common components such as 2n3904 and 2n3906 transistors, assorted resistors containing multiple values, capacitors (1uF, 3.3uF, 10uF, 47uF, 100uF), etc.  The diode in the negative peak limiter is a 1n4007.  These are good diodes to have around.  Also, I'd get a bunch of 7805s, 7812s and 7912s along with some TL074s and TL072s.  Maybe a couple of 25VCT power transformers if there are any to be had - very useful.

Armed with this, you will have flexibility in building analog circuits quickly.

When I first looked at your W1 call, I thought - heck, this guy is local.  NOT!  I checked QRZ....

[ka1tdq]

Yeah, technically I'm a Maine-iac from back in the day. 

If I do .3 volts carrier from 33 volts, that will put me at 10 volts carrier.  Using Vcc^2/(2Power)=Zout, I get 1-ish watts for 50 ohms. 

If I run the numbers for 18.4 ohms, I will get 2.7 watts carrier.  18.4 ohms gives me a .01 capacitor for the first part of the Pi output network (a good even number for a capacitor).

2.7 watts will probably not strap anything, even in Arizona.

[steve_qix]

Yeah, voltage is really king in this kind of amplifier if you want to keep the impedance up.  You could either try to run more current (I figured you'd be running about 12 volts at about 1 amp), or you could substitute one of your IRF740s for the transistor (you may have to change the RF amplifier circuit a bit), and run a whole lot more voltage.  With this, you should be able to increase the power supply voltage to at least 60 volts, and run 20 volts at an amp or so at carrier.  That would be good for driving the linear (which is what I think this thing is for anyway!).

[ka1tdq]

I'm almost finished.  I just need to run in town later today and get two 10uF electrolytics.  From the picture, the smaller blue trimmer is the negative peak limit control (used a 5k instead because that's what I had), and the larger blue trimmer is the carrier adjustment.  I've set both to mid-scale.

Also, I had a 250k smooth pot for the microphone audio adjust and used it instead of the 100k. 

I modified the power supply to give me 50 volts instead.  For testing, I tapped the lower electrolytic tap (around 33 volts) to supply just the RF stage.  I didn't want to tap directly on the 50 volt+ tap since that would probably blow the final NPN. Under load, the HV tap is about 54 volts.  Once I add the audio stage in, that should probably be nicely around 50 volts.

If I set the carrier to .3(50)=15 volts, then the maximum rail voltage (about 45 volts) would be my voice peak limit.  If 100% modulation is 30 volts, then 200% peak modulation would 45 volts, yes?  If that's true, insert a Borat "Very nice!" here.

Also, carrier power at 50 ohms and 15 volts would be about 2.5 watts.  Not a lot of power at all, but it opens up quite a few posibilities for linears.

Jon
KA1TDQ

[steve_qix]

Once you get this working, the next step is higher power!!

Oh, the audio input is not microphone level by any stretch of the imagination.  You probably know that, but just in case - the last post you did use the phrase "microphone audio adjust", and the modulator, as designed takes LINE LEVEL input 

On the modulation question - 200% modulation is 3x the carrier DC in a high level modulated system.  So, your 15VDC carrier, if the modulator supplies 45V on an audio peak would indeed be 200% positive modulation, assuming all of the RF amplifier components, and the amplifier itself as a unit follows a perfect square law (linear) characteristic.

Question:  have you actually used the RF amplifier so you know it works correctly?  I think you have, but I don't remember 

[ka1tdq]

Yes, I tested the RF section before I started the modulator this morning. 

I've attached a copy of the RF output waveform I'm getting with the carrier adjusted to 15.0 vdc.  It looks like an 18 volt pk/pk wave.  Also, I measured the HV during keyed state, and I'm still getting 51.4 volts loaded (plenty for 200% modulation).

While looking for the screws for the cover I found a couple non-polarized 10uF/25v caps that I had pulled from a tv some time ago.  I threw them in and it should be fine.

I need to replenish my audio stuff.  All I have left is a cheap Goodwill microphone that I picked up for a couple dollars and no "good" line level audio stuff.  This will give me plenty of time to get an amplifier going.

[AB2EZ]

Jon

If you get a chance, can you re-do your last post with the B+ adjusted to 7.5V (instead of 15V) to see if the peak-to-peak amplitude of the RF output changes by a corresponding factor of 2.

Stu

[steve_qix]

Hey, if you get a chance, post a scope picture of the collector waveform at a known DC (just tell us what it was/is when you take the picture).  That would be very interesting !

Regards,

Steve

[ka1tdq]

I've attached a couple scope pictures, both taken with the carrier adjusted for 7.5 volts.  The one taken at the collector seems to show to waves, one small one really large.  It's not there after the filter in the other photo.

[AB2EZ]

Jon

What is the peak-to-peak amplitude of the RF output signal when the B+ is 7.5V? If the oscilloscope scale has not been changed... then it appears to be more than half of what the peak-to-peak amplitude was (18V p-p) when the B+ was set to 15V.

Stu

[steve_qix]

That's pretty neat.  I wonder if it is possible to increase the sweep rate and show, say, 3 or thereabouts waveforms on the screen so we can see what is really looks like - and increase the volts-per-division so that none of the waveform is off the screen.  How many volts peak are on the transistor's collector with a known DC voltage - that's a really important number.

Very interesting!

Regards,  Steve

[ka1tdq]

I always thought that there was one simple wave.  I guess there's more.

I've attached a photo of the finished product.  To get the 200% positive peak capability and set the voltage on the final at 15 vdc, I'm getting an 18 volt pk/pk output wave.  If my math is correct for a 50 ohm load, that's 6.5 watts. 

Thanks to everyone who helped me complete this.  I bought the enclosure at a local hamfest for $1 and was debating even then.  I'm glad I bought it and the project morphed into this.  I'm going to have lots of fun with this radio! And Steve, thanks especially for the modulator design.

I'm going to post a nice picture on my QRZ profile sometime this weekend to put a bow on this thing.

Jon
KA1TDQ

[AB2EZ]

Jon

18V p-p  is equal to 9 volts peak.

9 volts peak across a 50 ohm load corresponds to 0.81W into the load [I.e. 0.5 x 9V x 9V / 50 ohms]

Assuming the output RF amplitude is linearly proportional to the value of the modulated B+... then:

At 100% positive peak modulation (for example) your peak envelope power will be 0.81W x 4 = 3.24W

At 150% positive modulation peaks (for example) your peak envelope power will by 0.81W x 6.25 = 5.06W

Stu