Single FET 40 meters

[ka1tdq]

I now have all the major components to finish the rig.  I'm going to use a cookie sheet as a mock-up to make sure the transmitter works before I put everything all together in a nice chassis.  From the cookie sheet picture you can see the layout:

Left:  160 uF capacitor/.15 H choke for the Heising circuit
Left near RF deck:  3-diode keep alive circuit/5 volt regulator
Center:  Antek modulation transformer
Right:  28 volt transformer for carrier and 6 volt transformer for the 3-diode keep alive circuit
Center-top:  40 meter RF deck

I've tested the output power using the 28 volt transformer and I'm getting 20 watts. 

Jon
KA1TDQ

[AB2EZ]

Jon

Okay!

Please read before proceeding:

1. Start by re-testing the transmitter with the 0.15H choke in series with the B+... but with the 160uF capacitor and the Antek transformer not yet connected. Verify that the transmitter still works okay, except that the DC drain voltage will be lower, due to the series resistance of the choke.

2. Next, add the 160uF capacitor (double check the polarity... the + side should connect to the drain of the FET) between the drain of the FET and one end of the secondary of the Antek transformer. Verify that the two primary windings of the transformer are wired properly, and the two secondary windings of the transformer are wired properly (i.e. + of one winding to - of the other winding, if one winding, of a pair of windings, is in series with the other winding, or + to + and - to - if the windings, of a pair of windings, are in parallel). Connect the unconnected side of the secondary winding of the Antek transformer to ground. Leave the primary winding of the Antek transformer disconnected (no audio amplifier connected yet). Verify that the transmitter still works.

3. Next, turn off the transmitter. Hook up the audio amplifier to the primary winding of the Antek transformer. Keep the output level of the audio amplifier set to a low value. Turn on the transmitter (first), then turn on the audio amplifier. Then verify that the transmitter is putting out the expected carrier level RF power. Then talk into the microphone, and adjust the amplifier's audio output level to obtain the desired modulation index.

4. Additional things to do (okay to wait until after the initial testing):

a. Add a bleeder resistor (wire wound is okay) across the 160uF capacitor... with a value of around 10,000 ohms (i.e. 160uF x 10,000 ohms = 1.6 seconds, so the capacitor will be mostly discharged 5 seconds after you turn off the power supply). The dissipation rating of this resistor should be 1W or more. Without this bleeder resistor, the 160uF capacitor may stay charged to the B+ voltage long after you have turned off the power supply (depending upon whether the existing bleeder resistor, across the output of the power supply itself, is still connected between the 0.15H choke and ground, when the power supply is turned off).

Keep us informed of your progress.

Stu

[ka1tdq]

Thanks for the testing procedure. I'll do just that.

The capacitor is non-polarized (and not cheap either... $67 from Digi-Key). 

I like this transmitter design.  It's one FET, two DC power supplies, and passive components. Not much to go wrong. It's also heavy metal for one FET.

The front will only have an on/off switch.

Jon
KA1TDQ

[ka1tdq]

I finished it this morning and tested over-the-air.  I listened to myself on the headphones and it sounds good to me.  I'm getting 25 watts carrier and with the audio on half-way I get 70 watts peak by going "Ahhhhh" into the microphone.  

The next step is to rig this up so that I can switch easily between RX and TX for an actual QSO to get some real audio reports.  If that all goes well, I'll disassemble it and reassemble it into a nice chassis that I picked up at a hamfest.  This will be my main rig and I'll put the single tube transmitter on a shelf.

Jon
KA1TDQ

[AB2EZ]

Jon

Congratulations!

I look forward to hearing the results of your on-air testing.

Note:

Most power meters... even those that claim to measure "peak power"... do not actually capture and display peak power. They are unintentionally or intentionally* designed to produce a reading that does not capture the peak envelope power of a voice-modulated AM (or SSB) signal. Therefore, while your power meter may indicate nearly a tripling of the peak RF output power when you say "Ahhhhh" into the microphone... your actual peak envelope power (corresponding to an audio waveform peak) is probably significantly higher.

I once modified a popular digital RF power meter to produce a reading that accurately corresponded to the peak envelope power. Doing this was not a trivial task. The time constants of the peak detector in the stock meter were just too long to capture peak envelope power.  

The best way (in my opinion) to measure the actual modulation levels is with a two channel digital oscilloscope, and an off-air RF sniffer (like the RF sniffer that feeds the AM detector in your off air monitor). You display both the audio input to the modulator, and the sampled RF output signal... using the two scope input channels. You trigger the scope off of the audio input. You can superimpose the audio signal on the top of the RF signal's envelope using the vertical scaling and vertical offset controls of the scope. If all is well, the top of the displayed RF envelope will faithfully track the audio input signal. [Note: this works best with high-level modulation, or with a high-level-modulated transmitter followed by a linear RF amplifier.  It doesn't work well with something like an SDR transmitter... because of the large time delay between the audio input and the modulated RF output].  

To do this on 7MHz, you need a digital oscilloscope with at least 7MHz of vertical channel bandwidth.

You could do something similar with a 2-channel digital oscilloscope having only a 96 kHz sampling rate... employing your computer's stereo sound card, and with oscilloscope application software running on the computer... by using the output of your off air monitor (AM diode detector) instead of the sampled RF signal... to provide a decent (but less accurate) representation of the modulated envelope of your RF output signal. Computer sound cards don't pass DC, so, with this method, you can compare the modulated RF envelope's shape to the modulating audio waveform's shape... but you can't determine the modulation index. I.e. with a DC-coupled A/D converter, you can observe both the modulation of the RF envelope and the carrier level... but, not with a typical computer sound card serving as the A/D converter.

Or, you could do something similar using an REA modulation monitor in conjunction with a computer.

*I once spoke with a fellow who sells digital power meters that use modern sampling techniques to measure the "peak" RF power level of a modulated RF signal. I asked him why he set the time constants (i.e. the averaging time) in his design so that the peak power readings produced by his product would not capture the actual peaks that occur with voice modulation. His (very logical and business-like) reply was that "other brands of power meters (e.g. a Bird 43 meter with a peak power adapter installed, and competing digital display power meters) do not capture and display the actual peaks... and if he set the time constants in his product to capture and display the actual peaks... then no one would buy his product". I.e., most hams who purchase his product want the peak power reading to be significantly less than the true peak envelope power... so that they can operate at higher peak envelope power levels, while still alleging that they are not exceeding the FCC limit.

Stu

[ka1tdq]

I called CQ and W7CK came back to me.  He lives in the same metro area as me, but my signal was a little PW since I was just running 25 watts carrier.  At first, he said my audio was a little down.  I had the audio drive at 50%, so I raised it to 90%.  The copy got better and my Daiwa PEP meter was reading over 100 watts peak.  He said that he didn't hear any distortion (my main concern), but I will try again after I get a decent carrier using my linear.

...on the linear...

I connected it to the new rig and I'm getting 700 watts carrier.  I don't see how a 25 watt rig could drive it that high, but that's was the wattmeter said on the AVG setting. 

There is no way for me to reduce power on the transmitter (It just has an on/off switch).  I'm going to do the brute force method and put in a 2db attenuator on the output going to the linear.  I'm just guessing at 2db, but I might have to try 3 or more to get the carrier around 250 watts. 

I'll build the attenuator using resistors.

Also, at the end of the conversation, W7ISJ chimed in near Tucson to say that he was copying us both at 100%.

Jon
KA1TDQ

[W3GMS]

Jon,

You should be able to reduce the drain voltage to run lower output.  Of course when you do that, you will need less audio.  That seems like a better and more efficient approach that a resistive attenuator network. 

Joe, GMS

[AB2EZ]

Joe

Reducing the drain voltage significantly may result in the need to change the negative peak limiter's threshold voltage... and may also result in undesirable artifacts on negative modulation peaks (if you reduce the negative peak limiter's threshold voltage too much).

In my own experience with a 35W (at carrier) single FET transmitter... with 40V on the plate (at carrier)... and a 5V negative peak limiting threshold... using an attenuator was a better way to go than reducing the drain voltage.

If you have a spare ferrite core (of the type used by WA1QIX, and others, to make transformers for class E transmitters), and a 50 ohm dummy load, its easy to make a 4:1 (or other ratio) attenuator. See the information at the bottom of this web page (power divider):

http://mysite.verizon.net/sdp2/id11.html

I do agree, however, that cutting the the HV B+ supply voltage in half works great for reducing the output power (at carrier) of my Ranger from 40W to 10W. With my Ranger, I actually use an external, continuously variable, HV B+ supply for the 6146, when I want to adjust the output power

Stu

Jon,

You should be able to reduce the drain voltage to run lower output.  Of course when you do that, you will need less audio.  That seems like a better and more efficient approach that a resistive attenuator network.  

Joe, GMS

[ka1tdq]

I built a 2.9 db attenuator (the values worked out pretty good for common resistors) and the carrier power out of the linear is now 300 watts.  My only problem now is that when I turn the Radio Shack microphone preamp Master Volume control up above half scale, I get RF hash in the audio.  To properly modulate the rig the volume control needs to go 80-90%-ish.  FYI, I'm using the Radio Shack 4-channel USB mixer (catalog #3200026).

Running the rig barefoot I don't have the problem, but with the linear amp I do.  Of course, the rig isn't in a metal enclosure and neither is the linear.  I'm using plexiglass on the linear for an art-deco look.  But, I didn't have this problem before using my tube rig with the linear.

I've done some troubleshooting and the RF hash goes away when I unplug the XLR balanced cable from the Radio Shack preamp (this is the cable from the microphone to the preamp).  I've ordered some snap-on RF ferrite chokes that I can install over the cable to see if that helps. 

Also, I don't have a ground in the shack that I can use.  I don't know if it would help anyway if I were to install one since I'd be driving the ground rod into Arizona sand. 

Jon
KA1TDQ

[WD5JKO]

Jon,

    With your grounding setup, or lack of, you have a formidable task to stabilize the station. Perhaps a 33' wire will act as a 40m ground; realize that the end will be RF hot though.

   With my Flex 3000 station I am stable 80-6m with my 80m OCF dipole. if I run my loop fed with OWL, I can run 40-15m with the loop. The EFJ Matchbox is 15" from my Audio chain Mic input! Dumb!! When I turn on the Linear amplifier everything goes to hell with the loop, but not the inverted V. My computer USB keyboard and mouse like to lock up. A wireless mouse didn't help. Common mode chokes on the USB lines help on some bands but not others.

    If you have a high power dummy load, try that to see if the problem goes away. It might NOT with the amplifier wide open like that. If your good on the dummy load, then see about eliminating common mode feeder current, and moving the antenna further away (if possible). As I write this, I think you are running an end fed random wire out the window. True? Then were is your return current supposed to go?

  With the amplifier, tune the loading beyond maximum power (less capacitance) until the RF carrier power drops about 20%. Then modulate looking at the carrier versus PEP power. Tune for the highest ratio on your meter. I bet that takes your carrier down to 250 watts or even less.

Jim
Wd5JKO

[W3GMS]

Joe

Reducing the drain voltage significantly may result in the need to change the negative peak limiter's threshold voltage... and may also result in undesirable artifacts on negative modulation peaks (if you reduce the negative peak limiter's threshold voltage too much).

In my own experience with a 35W (at carrier) single FET transmitter... with 40V on the plate (at carrier)... and a 5V negative peak limiting threshold... using an attenuator was a better way to go than reducing the drain voltage.

If you have a spare ferrite core (of the type used by WA1QIX, and others, to make transformers for class E transmitters), and a 50 ohm dummy load, its easy to make a 4:1 (or other ratio) attenuator. See the information at the bottom of this web page (power divider):

http://mysite.verizon.net/sdp2/id11.html

I do agree, however, that cutting the the HV B+ supply voltage in half works great for reducing the output power (at carrier) of my Ranger from 40W to 10W. With my Ranger, I actually use an external, continuously variable, HV B+ supply for the 6146, when I want to adjust the output power

Stu

Jon,

You should be able to reduce the drain voltage to run lower output.  Of course when you do that, you will need less audio.  That seems like a better and more efficient approach that a resistive attenuator network.  

Joe, GMS

Stu,
I do fully realize there are limitations on how much the drain voltage can be reduced, but with the 3 db or so required here, it should be doable.  Of coarse one needs to examine the drain switched waveform to assure it looks as it should.  As far as changes to the negative peak limiter, that should also be easy. 

One of the advantages of Class D or E is the efficiency and I realize we are dealing with very low power here, but it seems contradictory to do it with pads when other solutions are available.

Joe, GMS

[AB2EZ]

Jon

To reduce the RF being coupled into the microphone via the balanced microphone output cable:

1. Slip a ferrite core (the same type and size as used to wind transformers) over the microphone cable. Run the cable around the ferrite core two or three times. This will require the removal of the cable/connector at one end; but it is more effective than a snap-on ferrite.

2. Connect the shield of the cable to ground at the end that plugs into the preamplifier, but leave the shield floating (not connected to the microphone shell) at the microphone end (unless the microphone requires phantom power). This will create a Faraday shield around the microphone cable, and will eliminate the common mode path (on the outside of the shield) between the microphone and the preamplifier.

Stu

I built a 2.9 db attenuator (the values worked out pretty good for common resistors) and the carrier power out of the linear is now 300 watts.  My only problem now is that when I turn the Radio Shack microphone preamp Master Volume control up above half scale, I get RF hash in the audio.  To properly modulate the rig the volume control needs to go 80-90%-ish.  FYI, I'm using the Radio Shack 4-channel USB mixer (catalog #3200026).

Running the rig barefoot I don't have the problem, but with the linear amp I do.  Of course, the rig isn't in a metal enclosure and neither is the linear.  I'm using plexiglass on the linear for an art-deco look.  But, I didn't have this problem before using my tube rig with the linear.

I've done some troubleshooting and the RF hash goes away when I unplug the XLR balanced cable from the Radio Shack preamp (this is the cable from the microphone to the preamp).  I've ordered some snap-on RF ferrite chokes that I can install over the cable to see if that helps.  

Also, I don't have a ground in the shack that I can use.  I don't know if it would help anyway if I were to install one since I'd be driving the ground rod into Arizona sand.  

Jon
KA1TDQ

[AB2EZ]

Joe

I think you are correct. I did not fully understand the physics of a MOSFET.

Jon is using 28V on the drain of the single FQA 11N90. To reduce the power by 3dB, he would need to reduce the drain voltage to 28V x 0.707 = 20V.

For -90% negative modulation peaks, the drain voltage would be down to 2V, which seemed kind of "iffy", to me, considering the gate-to-source threshold voltage is specified as 3-5 volts.

However, upon review of the information on the web, and also reminding myself that the D-S voltage drops to zero on each RF cycle... there is no problem with a low D-S voltage, provided it is positive or zero. A negative D-S voltage would be a problem, because of the diode that exists between the drain and the source in a MOSFET. The specification sheet for the FQA11N90C indicates that the device will work okay, in the "on" state, with drain-to-source voltages below 0.1V.

So, it appears that a low D-S voltage is not a problem, provided the negative peak limiter's threshold is adjusted accordingly, to prevent negative D-S voltages on negative modulation peaks.


Stu

Joe

Reducing the drain voltage significantly may result in the need to change the negative peak limiter's threshold voltage... and may also result in undesirable artifacts on negative modulation peaks (if you reduce the negative peak limiter's threshold voltage too much).

In my own experience with a 35W (at carrier) single FET transmitter... with 40V on the plate (at carrier)... and a 5V negative peak limiting threshold... using an attenuator was a better way to go than reducing the drain voltage.

If you have a spare ferrite core (of the type used by WA1QIX, and others, to make transformers for class E transmitters), and a 50 ohm dummy load, its easy to make a 4:1 (or other ratio) attenuator. See the information at the bottom of this web page (power divider):

http://mysite.verizon.net/sdp2/id11.html

I do agree, however, that cutting the the HV B+ supply voltage in half works great for reducing the output power (at carrier) of my Ranger from 40W to 10W. With my Ranger, I actually use an external, continuously variable, HV B+ supply for the 6146, when I want to adjust the output power

Stu

Jon,

You should be able to reduce the drain voltage to run lower output.  Of course when you do that, you will need less audio.  That seems like a better and more efficient approach that a resistive attenuator network.  

Joe, GMS

Stu,
I do fully realize there are limitations on how much the drain voltage can be reduced, but with the 3 db or so required here, it should be doable.  Of coarse one needs to examine the drain switched waveform to assure it looks as it should.  As far as changes to the negative peak limiter, that should also be easy.  

One of the advantages of Class D or E is the efficiency and I realize we are dealing with very low power here, but it seems contradictory to do it with pads when other solutions are available.

Joe, GMS

[ka1tdq]

Yeah, my negative peak limiter has a 7805 regulating the voltage to 5 volts.  I cranked up the audio really high and the one QSO I had said that he didn't detect any distortion on my signal.

I spoke too soon on the RF hash source being the microphone cable.  I unplugged the XLR cable from the preamp and ran the test again.  The hash showed up again when the volume level went above 50%.  I did take a class E ferrite core (for the lack of a better name) and put it over the RCA cable which feeds my car audio amplifer from the preamp.  Bam!  Hash gone.

However, when I plug the XLR microphone cable in and put the volume above 50%, I again get hash.  So I'm back to putting cores on the XLR cable. 

For simplicity, I think I will just try the snap-on ferrite cores for now when they arrive.  My XLR cable has commercial, pre-molded ends and I don't want to cut them if I don't have to.  I'll try to get at least two wraps of cable through the strap-on.  If the hash is still there, I'll make up my own new cable and ends to your specifications.

Jon
KA1TDQ

[K4RT]

Jon - It sounds like you're on your way.  I have found it interesting to follow this thread and your progress.  Keep us updated.

73
Brad

[ka1tdq]

I just got my 3 snap-on chokes and added them one by one onto the microphone cable.  I added the first one next to the preamp and it didn't seem to help too much.  I put the next one next to the microphone and things quieted down alot!  I added the third in the middle of the cable and my carrier got totally silent.  I listened to myself in the headphones over the air and it sounds super clean.

I also changed my attenuator from a 2.9 db reduction to a 4.15 db (again, convenient values for resistors).  Out of the linear, my carrier is 210 watts.  This seems like a nice level with enough headroom for voice peaks.  

It feels good.  This transmitter is only class C, but I get warm fuzzies talking through a mosfet rather than a tube.  

I just got off the air with AC0RL in Kansas, but it was hard to get good audio reports due to band conditions being noisy.

Jon
KA1TDQ

[AB2EZ]

Jon

Congratulations again!

What's your next project?

Stu

[ka1tdq]

I'm going to rest for a bit after I get this rig permanently mounted in its chassis.  I still need to powder coat the outside and get the steel for the front and bottom.

But, I could do a medium power class E rig.  I know how to use analog drive for the gate and I can use this same modulator design.  With the components I used, they could easily do 100 watts carrier.  I'd just need to get a larger car audio amplifier to drive it.  The only thing new would be the output circuit for the drain. 

It'll have to wait a while though.  My wife is already asking me where I plan to put new things that I build.  I've run out of space in the shack, and I need to find a shelf for my tube transmitter. 

-or-

Build a solid state 40 meter linear.  This will totally get me off of tubes. 

I'm enclosing a last look at my final.  Look, no filament wires!

Jon
KA1TDQ

[KB2WIG]

" My wife is already asking me where I plan to put new things that I build.  I've run out of space in the shack. "

Well, there is always the space inside partition walls. A good 3 inches....

And as Tuco might say, "I feel a man like you can manage it."


klc

[DMOD]

It feels good.  This transmitter is only class C, but I get warm fuzzies talking through a mosfet rather than a tube.

I only get the warm fuzzies when my tube filaments consume at least 150W per tube. 

Congrats on your project!

Phil