SSB Carrier Supression

[Space Varmint]

Sounds about right. Feedback will improve IP3 but at the same time will degrade noise figure. It is a trade off. Best bet is to use an off the shelf monolithic amp like Mini-circuits ERA-3SM which is about 2.8 NF and IP3 of 25 dBm. I seen em for sale on e-bay for 5 bucks.

I did a quick search and did not find that circuit. I just wanted to see what it consisted of. I'm still of the school that IMD is a mixer product. In which case it would be a healthy trade off because you do not want allot of gain into a mixer. In fact I have heard it said that diode rings are often considered the best mixers because of their inherent loss in amplitude. This can be emulated with the introduction of some attenuation and in my receiver I have found it works rather nicely. Even when an interfering signal is right on your frequency. I'm not saying your wrong, it's just that I have never heard this before about the IP3. That would break down to a difference in the transistor substrate being a factor. I've seen comparisons between bipolar and uni-junction transistors. It always boiled down to how you interface with the mixer.

 

[Space Varmint]

note*

I found a cap in the driver circuit of my 3 stage power amp that was way out. I removed it and re-did the output network and I now have full power. Instead of 8 watts I got 40 watts coming from the MRF477.

 

[Space Varmint]

note*

I found a cap in the driver circuit of my 3 stage power amp that was way out. I removed it and re-did the output network and I now have full power. Instead of 8 watts I got 40 watts coming from the MRF477.

Oops scratch that ^^^^. I goofed. I was on the dummy load and didn't notice. I could use some help here. I can't figure this dern Xsistor out. So I get 40 watts pretty as you please and I put it on the antenna and I'm back at 8 watts My SWR is is only about 1.2 to 1. Do I just need more drive?????

 

[Space Varmint]

Sounds about right. Feedback will improve IP3 but at the same time will degrade noise figure. It is a trade off. Best bet is to use an off the shelf monolithic amp like Mini-circuits ERA-3SM which is about 2.8 NF and IP3 of 25 dBm. I seen em for sale on e-bay for 5 bucks.

But for a bit of honesty here. This is the deal. If not for my preselector I would hear brother Stair every late evening and in to the wee hours of the morning. At my defense, I do know that he simulcasts on a few different bands. I believe he does do one around 7 MHz but I think the one that was getting me was a 1/2 harmonic of 3.210MHz. It is a very powerful signal broadcast out of Nashville, TN. The negative feedback has completely eliminated this IMD. I still get a little here and there from frequencies very close to the selected frequency. The attenuating resistance on the front end does allot to eliminate it but I still need to use the preselctor to attenuate it further which I'm sure loses some sensitivity. Finding the sweet spot.

 

[Mikebits]

Oops scratch that ^^^^. I goofed. I was on the dummy load and didn't notice. I could use some help here. I can't figure this dern Xsistor out. So I get 40 watts pretty as you please and I put it on the antenna and I'm back at 8 watts My SWR is is only about 1.2 to 1. Do I just need more drive?????

Strange to say the least. If your getting 40W into a dummy load, then I would expect only a 1% loss into a antenna match of 1.2

 

[kchriste]

The Amp is probably unstable and going spurious due to the little bit of reflected power. You could confirm this by inserting an **broken link removed** between the amp and the antenna. Sometimes simply changing the coax length will work; but remember that your amplifier should be stable for loads up to at least 3:1 or so.

 

[Mikebits]

The Amp is probably unstable and going spurious due to the little bit of reflected power. You could confirm this by inserting an **broken link removed** between the amp and the antenna. Sometimes simply changing the coax length will work; but remember that your amplifier should be stable for loads up to at least 3:1 or so.

Maybe the LPF is a bad match.

 

[Space Varmint]

The Amp is probably unstable and going spurious due to the little bit of reflected power. You could confirm this by inserting an **broken link removed** between the amp and the antenna. Sometimes simply changing the coax length will work; but remember that your amplifier should be stable for loads up to at least 3:1 or so.

Yeah, you got it. I see what's happening now. I started looking at the modulation envelopes. I'm actually only getting about 2 watts clean power out. I definitely need more drive. The extra power was over modulation. She's a way from being done.

 

[Space Varmint]

Maybe the LPF is a bad match.

I'll bet that's part of it.

 

[Space Varmint]

I see it perfectly now. Mike you are right on target. I should have been looking at the modulation envelopes. It is much easier to see the distortion. So what is happening is when I adjust my output filter for max power out it clips on one side so then it creates trash, and that is what is being amplified and showing me a higher power reading. Transmitting spurious garbage. I got about 2 watts of clean power out. So actually, I talked to these guys in St Louis during the day on two watts. Not bad. I love HF. So I will have to go back though this amplifier strip with a fine tooth comb.

Now I will also have to probably put a volume control in my audio. Dern audioguru told me the mic was high Z so I put a JFET at the input and it is too sensitive. So sometimes if I talk too load it will clip but that is not the main problem.

I wonder if I got my input bias wrong on the final? Because like I said, it clips on only one side on the output when I up the gain to the antenna.

 

[flat5]

I don't know what I'm talking about - I've never designed a transmitter - but
the bias must be very low if you are clamping it with D2 then feeding the base
through L4 & R9/R1. Am I completely out to lunch?

I wonder if putting L4 on the other side of R9 and making R9 adjustable makes any sense?

 

[Space Varmint]

I don't know what I'm talking about - I've never designed a transmitter - but
the bias must be very low if you are clamping it with D2 then feeding the base
through L4 & R9/R1. Am I completely out to lunch?

I wonder if putting L4 on the other side of R9 and making R9 adjustable makes any sense?

My mistake flat5. Funny you pointed that out. I did not use the diode. I'm with you on that one. I used one on the 80 watter though and really, it does work...lol. Maybe I should use the diode. I've got one of two or both things happening here now that I blew my Saturday working on this thing. Three stages could be a little much in one box. I think I need to do some serious shielding. I notice when it's tuned right and I put some significant audio into it and the power level comes up, I start seeing like fuzzy spikes around the envelopes like it is trying to feedback. That's one part of it, and once I do get it shielded better, I'm hoping I don't need more drive. I have gotten the 40 watts out the antenna, but not any signal you would call communications or intelligence...lol.

 

[Space Varmint]

I am pretty well convinced it is feedback. If I increase the coupling to the final I get flat feedback across the board. If I back it down it will idle fine until I speak into the microphone and the louder I speak the more I get the spikes.
I did want to mention one thing. Without feedback the output is perfectly linear now. What I did is at the input to the 3 amplifier (box) where it is being fed through coax, I put a step down transformer and it evened it right out.

Next step:

Shielding...shielding shielding

 

[flat5]

Proper power supply, impedance matching, good layout and grounding without path problems, bypassing, and shielding. Good luck!

 

[audioguru]

Your RF output transistor is biased into class-C so it is cutoff at idle.
Then you talk into the mic and get no output until you scream into the mic.
The audio bandwidth is so narrow that the distortion harmonics are not heard.

 

[Space Varmint]

Your RF output transistor is biased into class-C so it is cutoff at idle.
Then you talk into the mic and get no output until you scream into the mic.
The audio bandwidth is so narrow that the distortion harmonics are not heard.

U Didn't see the last page. The mic is too sensitive. It picks up everything and transmits it. Also I'm not using the diode. It is biased class A linear.

No, here are the conclusions and progress of what I did yesterday. Help me, this thing is almost done

I was bumping up the power by simply increasing the capacitive coupling to the final. I got a good 20 watts at times but with the fuzzies. So I got to looking at two things. Cleanliness of signal and feedback. I started doing some shielding with copper clad board and it really didn't do much of anything so I took a hard look at the signal again. I've still got too much mixer product in my signal from the up conversion. I did more filtering and in deed even the envelopes look nicer. Well, as all filters go, I also got some insertion loss so I am losing power at the same time. I now see I need some high pass filtering as well. Now I will have to add some more drive.

Oh well, like I said. It's a ways from being done....but getting closer

 

[Space Varmint]

In fact there 'Guru. It is so linear, that I can go back four stages and see the exact same signal at the antenna output as appears at the previous 4 stages. Even the defects. What happens is...as I increase audio the 7 MHz output starts to break apart into what looks like a low frequency harmonic of the two signals combined in the up conversion which are 11MHz & 4MHz and when subtracted is of course is 7MHz.

I'm working on some high pass filtering to attenuate this defect.

 

[audioguru]

Aren't you over-modulating the signal?

 

[Space Varmint]

Aren't you over-modulating the signal?

Hardly, unless I screeam into the mic. The audio is is pretty good but I should put a volume control in it in case I do over modulate it.

 

[audioguru]

A volume control will not stop over-modulation. You need an audio limiter circuit like radio stations have.