I would like to demonstrate how even the internal digital channel combine function can be quite useful for two tone tests as long as we're not dealing with (very) high level components.
This is certainly true for low frequencies like the 7040kHz example. At this frequency, it's no problem to use output levels of +13dBm, which is a quite common maximum level for RF signal generators. In this case, third order intermodulation products are at 68dBc and worst case third order intercept point is +47dBm.
See attached screenshot SDG6000X_IMD_7040kHz_d20kHz_13dBm_int_RBW10Hz and the corresponding peak table:
Frequency | Amplitude | Delta Frequency | Delta Amplitude |
7.049999 MHz | 13.03 dBm | 0.000000 Hz | 0.00 dB |
7.029998 MHz | 13.02 dBm | -20.001200 kHz | -0.01 dB |
7.069996 MHz | -55.22 dBm | 19.997491 kHz | -68.25 dB |
7.009996 MHz | -55.35 dBm | -40.002399 kHz | -68.38 dB |
Let's check the situation at a lower output level of -5dBm, which will be used for a practical test later. Third order intermodulation products are now even lower at 78dBc and worst case third order intercept point is +34dBm.
See attached screenshot SDG6000X_IMD_7040kHz_d20kHz_-5dBm_int_RBW10Hz and the corresponding peak table:
Frequency | Amplitude | Delta Frequency | Delta Amplitude |
7.049999 MHz | -4.97 dBm | 0.000000 Hz | 0.00 dB |
7.029998 MHz | -4.98 dBm | -20.001200 kHz | -0.01 dB |
7.069998 MHz | -82.88 dBm | 19.999345 kHz | -77.91 dB |
7.009998 MHz | -83.64 dBm | -40.000545 kHz | -78.67 dB |
Finally the most critical situation at the upper end of the SDG6052X freqeuncy range, 480MHz have been used for this test. Maximum output level with internal combiner is 6dB lower than without, so we get -6dBm, which happens to suit my praxis test quite fine. Third order intermodulation products are now rather high at 37dBc and worst case third order intercept point is +12.3dBm.
See attached screenshot SDG6000X_IMD_480MHz_d20kHz_-6dBm_int_RBW10Hz and the corresponding peak table:
Frequency | Amplitude | Delta Frequency | Delta Amplitude |
480.009893 MHz | -6.03 dBm | 0.000000 Hz | 0.00 dB |
479.989892 MHz | -6.03 dBm | -20.001200 kHz | -0.01 dB |
479.969893 MHz | -42.80 dBm | -40.000545 kHz | -36.77 dB |
480.029892 MHz | -44.33 dBm | 19.999345 kHz | -38.30 dB |
Now for the practical test with a Marconi Instruments TF2175 linear amplifier. Here are the relevant specifications:
Bandwidth: 2 ~ 500 MHz +/-1 dB
Maximum linear power: >300 mW (24.8 dBm)
Input for 300 mW output: 0.6 mW (-2.2 dBm)
Gain: 27 dB +/-1 dB
Harmonic Distortion: <30 dBc at 200 mW
3rd Order Intermodulation Intercept Point: +35 dBm typical
Noise Figure: <9.5 dB (8 dB typical)
Dynamic Range: >80 dB
We are interested in the 3rd order intermodulation intercept point. As long as this is lower than the TOI of our two-tone signal source at the required output level, we can get meaningful test results when measuring the TOI of the amplifier.
There's one trap: the specified TOI is for the amplifier output, but it's the input TOI that matter for our test. For the Marconi TF2175, this is Output TOI - Gain = 35dBm - 27dB = +8dBm. This means that the SDG6000X with internal channel combine function should be perfectly usable for measuring the TOI of the TF2175 amplifier over its full frequency range.
First test at 7040kHz.
Generator level is -5dBm, TF2175 output level is +21.6dBm. Third order intermodulation products are at 26.5dBc and worst case third order intercept point is +34.86dBm, which matches the specification quite nicely.
See attached screenshot SDG6000X_IMD_7040kHz_d20kHz_-5dBm_int_RBW10Hz_TF2175 and the corresponding peak table:
Frequency | Amplitude | Delta Frequency | Delta Amplitude |
7.049999 MHz | 21.59 dBm | 0.000000 Hz | 0.00 dB |
7.029998 MHz | 21.58 dBm | -20.001200 kHz | kHz,-0.01 dB |
7.069998 MHz | -4.98 dBm | 19.999345 kHz | -26.56 dB |
7.009998 MHz | -5.07 dBm | -40.000545 kHz | -26.65 dB |
Finally the test at 480MHz.
Generator level is -6dBm, TF2175 output level is +20.75dBm. Third order intermodulation products are at 28.5dBc and worst case third order intercept point is +34.99dBm, which once again almost perfectly matches the specification.
See attached screenshot SDG6000X_IMD_480MHz_d20kHz_-6dBm_int_RBW10Hz_TF2175 and the corresponding peak table:
Frequency | Amplitude | Delta Frequency | Delta Amplitude |
480.009891 MHz | 20.75 dBm | 0.000000 Hz | 0.00 dB |
479.989892 MHz | 20.73 dBm | -19.999345 kHz | -0.02 dB |
479.969893 MHz | -7.79 dBm | -39.998690 kHz | -28.54 dB |
480.029892 MHz | -8.64 dBm | 20.001200 kHz | -29.39 dB |