I've been playing with Russian GI-7B power triodes, which seem to have a reputation for poor linearity / intermodulation distortion so I decided to try to see what the distortion might be on one of the amps that I built.

Here's the test setup that I started with: I used a 40db tap element for my bird watt meter that I fed into my spectrum analyzer (HP 141T frame with the 8553? 10-110MHz RF module) through an external 20db attenuator. I really don't want to damage my analyzer, so I'm being really cautious to not overload the front end.

The amp is 50mhz so I'm driving it with a Yaesu 726R and terminating the output of the amp in a cantenna. Unfortunately, I don't have a two-tone generator to use for the tests, which is where the homebrew may come in. By talking into the mike, I can only see the carrier frequency on the analyzer screen, so I'm probably doing something fundamentally wrong. The bandwidth of the analyzer is set for 3Khz.

I'm thinking I need two distinct tones to drive the audio on the 726R. Is there a standard method for performing these tests and if so, what are the tone frequencies that should be used? I have a pair of the XR2206 chips that was discussed in a previous thread as a VFO, but they should also work well for audio signwave tone generators.

One other thing that crossed my mind is that with the 8 watts I get out of the 726R, I don't nearly drive the amp to full output power. I expect the IMD will increase significantly as drive increases, approaching the upper limit of the amp's power rating. When testing for IMD, should the test setup run the amp within the linear range or should it be approaching overdriven?

arnie

You will need two separate tones within the 3 KHz bandwidth of the 726R in either USB or LSB and a bandwidth setting on the analyzer of a few hundred Hz tops to be able to see both tones as separate frequencies. I'm not sure what to suggest for tones or how you are going to get them.

You have to not overdrive the 726R itself or all you are going to see is its IMD rather than the amplifier.

Curtis Eickerman

This might help http://www.cliftonlaboratories.com/imd_measurements.htm

Thanks for the input and references. I was able to configure my spectrum analyzer along the lines described by the link and see more than my orignial setup. I still need to put together a two tone tester - they're suggesting 1000hz and 1750hz.

I did make one intersting observation though. When I talk loudly into the microphone, I can see distortion products 10 or 20 khz away - that's without the amp running, just the 726R. The 726R has a pretty agressive ALC that does a pretty good job when it has time to react. Quick, large excursions allow distortion before the ALC can react and back off the gain. With a two-tone tester, the transceiver ALC should be able to ensure that the signal coming from the transceiver is clean - maybe Then any distortion products should be coming from the amp.

arnie

Everything is going to have 'some' IM so, without knowing the characteristics of the rest, I'd suggest you first measure IM without the amp and then with. The difference should then be what the amp is adding.

Arnie,
The standard ARRL and manufacturers tests cherry pick the tone frequencies so that the IMD products fall outside the filter passband.

The only accurate test is swept noise as used in the telecommunications industry for multi channel microwave products and Im sure the military. The products I was involved with at the design level before retiring required all IMD products to be -55dB or better.

There has been considerable discussion on some ham forums about utilizing those procedures.

You coming North in about 10 days?

Carl

Thanks for the info Carl. Yes, I'll be going up on Friday - don't know about Saturday. Perhaps we can do an eyeball and discuss this a bit more. I'd like to have a repeatable procedure that I can use for all the amps I have to get a good comparison.

arnie