NE-602 Modulator Build and Measurements Info

[Steve - WB3HUZ]

The NE 602 AM modulator for the FT-102 is indeed free of any overshoot or turn-on surge. It's a simple circuit and it works very well (better than the FM/AM Board for the 102 from Yaesu). The pix below are of one I built on a small perf board (2" x 2" or so). Better component placement would yield a smaller board, but the one I built fit easily in the space where the Yaesu FM/AM Board would go (see second photo - board is in lower right corner of open access panel on bottom of the FT-102).






Some Test Results

FT-102 driving a pair of 3-500Z in GG linear service

200 watts out - clean 180-200% modulation
300 watts out - clean 150% modulation
400 watts out - clean 100 % modulation

This equates to roughly 20, 30 and 40 watts output from the 102 barefoot.

Frequency response 10 Hz - 186 kHz (-3 dB). Yea that's right, 186 kHz on the high end. How ridiculous is that!?! I could be 400 kHz wide if I wanted! KAYMOAN.

Ultimately, the distortion products are defined by the IMD of the 102's final amp: around -40 dB. The modulator itself is much better. Measurements with a spectrum analyzer showed harmonics (by harmonics I mean if I'm modulating with 1 kHz, on the spectrum analyzer I see the "legitimate" sidebands at +/- 1 kHz, and then some other sidebands at +/- 2, 3 and so on) at least -55 dBc (dB down from the carrier level) at 100 % modulation. Most were more than -60 dbc. In fact I could only see the 2nd, 3rd and just barely the fourth on the analyzer since I was up against its dynamic range/noise floor.

At 150 % modulation no harmonic was greater than -50 dBc. Similar results were obtained at 200% modulation. Higher harmonics (4, 5, 6) were close to -70 dBc. Very clean!

Things weren't quite as clean when going through the 102 final amp. But I tweaked the NE602 carrier injection to achieve at least -35 dBc even at 150% modulation. It was -40 dBc at 100% modulation. This is as good or better than almost all SSB setups on the air.

This last point is somewhat critical for users of this modulator. You can produce some pretty ugly looking spectrums if the carrier injection control is not adjusted properly. I can't think of a good/easy way to do this without a spectrum analyzer. I suppose you could do it with a receiver tuned to one of the harmonics (preferably the 2nd or 3rd) on one of the sidebands and then adjust the carrier injection for minimum signal. This would get you pretty close. The reality is that it's a trade-off between the 2nd harmonic and the higher order harmonics, most especially the 3rd (most of the rest are usually so far down the aren't worth worrying about).

[K1JJ]

Way to good, dude!  Looks like a nice clean installation.

I think you, Bill and I are the only ones running it so far in our FT-102's. It's gives real good control of the AM waveform.

T

[Steve - WB3HUZ]

TNX OM. The best part of using this setup is that the carrier is never cut off, even when you exceed 100% modulation in the negative direction. Instead, the signal starts to look like a DSB reduced carrier emission on the scope. This feature alone should cut down on any off-channel splatter. The fact that the setup is so clean should reduce it further.

The only danger is that you can be legitimately W - I - D - E due to the extended high frequency response. So it is wise to do some lowpass filtering in the audio chain before the audio is input to the FT-102.

[K1JJ]

Yep, the little "wavelet"   between the peaks is a tiny replica of the main peaks.  It fills in where the negative pinch off would normally be.  This is DSB with carrier.  It's easy to do 300%+, or 1000%, for that matter, with this system. But 130% is the limit in the real whirl due to diode detectors. Sync detectors wud permit super modulation.  

With that precision chip and -40 IMD linear final, it is cleaner than any plate modulated or class E rig out there.  But not as power efficient of course.

That 455 IF transformer ratio can take some fiddling around to get perfect, but looks liie you figgered it out.

As for bandwidth, just rolling off the EQ sharply after 5-6kc is all that's needed.    I found an additional sharp roll of filter was not needed.  In fact, when I roll the EQ at 4kc I get comments that I have no highs....   :grin:

BTW, adjusting the proper carrier injection level is easy using an audio sine wave and watching the RF envelope on the scope. With a good trained eye for a perfect sine wave, you can see when the correct injection takes place. The wave does get poor with cross over distortion and other artifacts when misadjusted... and also looks poor when a poor match through that homebrew IF transformer takes place..

Can't  wait to hear your new config, OM!

T

[Steve - WB3HUZ]

Yep, the little "wavelet"   between the peaks is a tiny replica of the main peaks.  It fills in where the negative pinch off would normally be.  This is DSB with carrier.  It's easy to do 300%+, or 1000%, for that matter, with this system. But 130% is the limit in the real whirl due to diode detectors. Sync detectors wud permit super modulation.

The image below shows a typical scope display with sine wave modulation. The top is 100% modulation. The middle is overmodulation in a typical AM system (non-balanced modulator system). Notice the carrier cutoff. The bottom is roughly what the scope pattern looks like using the NE602 with the same amount of audio as in the middle waveform. Notice there is now no carrier cutoff.





With speech input, the scope waveform looks pretty normal, except you never see any "sparklies" or bright spots in the negative modulation troughs. Instead, the negative modulation appears to bottom out at about 95%. And the positive peaks can swing way up there if you choose to put the audio maul down.

With that precision chip and -40 IMD linear final, it is cleaner than any plate modulated or class E rig out there.  But not as power efficient of course.

That 455 IF transformer ratio can take some fiddling around to get perfect, but looks liie you figgered it out.

That would be 8.2 MHz vice 455 kHz. I got lucky and was pretty close the first time. I just had to reduce the value of the resonating cap across the primary of the transformer to 100 pF.

As for bandwidth, just rolling off the EQ sharply after 5-6kc is all that's needed.    I found an additional sharp roll of filter was not needed.  In fact, when I roll the EQ at 4kc I get comments that I have no highs....   :grin:

I run mine at 4 or 5 kHz cutoff too. The rolloff is created by the eq sliders and a variable cutoff lowpass filter that is built into the eq.

BTW, adjusting the proper carrier injection level is easy using an audio sine wave and watching the RF envelope on the scope. With a good trained eye for a perfect sine wave, you can see when the correct injection takes place. The wave does get poor with cross over distortion and other artifacts when misadjusted... and also looks poor when a poor match through that homebrew IF transformer takes place..

Hmmm, I'll have to check that out. The next one I build, I will use your method of adjustment and then check it on the spectrum analyzer.

Can't  wait to hear your new config, OM!

T

It's been on the air for about six weeks. I don't think I've heard you on the air in that time.