Using the IC-7700 as a bench spectrum analyser

by Adam Farson, VA7OJ/AB4OJ

Fig.1: Spectrogram of transmitter 2-tone IMD test at 100W PEP.
Fig.1: Spectrogram of transmitter 2-tone IMD test. Image: VA7OJ.

As we have seen from my recent test report, the IC-7700 spectrum scope has a minimum resolution bandwidth (RBW) of 100 Hz and accurate vertical amplitude tracking. These two properties create an additional application for the IC-7700 as a bench spectrum analyser.

The 5 kHz span setting (200 Hz RBW) and the 2.5 kHz setting (100 Hz RBW) are useful for IMD measurement of 2-tone and multi-tone  RF signals. To ensure an accurate amplitude display, the SLOW sweep setting should always be selected (SCOPE SET menu).

In this example, the transceiver under test was connected via a Bird 4273 line sampler to a Bird 43 power meter terminated in a 50Ω 100W load,  The BNC port of the line sampler was initially connected to the Y-input of an oscilloscope via a 50Ω termination.

The transceiver under test was first set up for 100W CW output, as described here. The two-tone test signal (pure sinusoidal 700 Hz and 1700 Hz tones of equal amplitude) was then applied to the baseband (MOD) input of the radio under test, and the baseband level adjusted for 100W PEP as shown in Fig.2.

Fig.2: 100W PEP 2-tone envelope.
Fig.2: 100W PEP 2-tone envelope. Image: VA7OJ.

The sampler's BNC port was then connected to ANT1 on the IC-7700 via a 40 dB RF attenuator. The sampler was adjusted for a display as shown in Fig. 1. This corresponds to an S9 + 30 dB (-42 dBm) input power level. For this test, the IC-7700 Preamp and ATT were off.

Care should be taken not to apply more than -10 dBm to the IC-7700's antenna inputs. Note that the IC-7700 can also be used to measure the 2-tone IMD of a distant station's transmitter if the operator puts up a 2-tone test signal for this purpose.

Table 1 gives the measured IMD values:

Table 1: Measured TX IMD.
0 dBc = 2-tone PEP
IMD3 IMD5 IMD7 IMD9
-31 dBc -36 dBc -48 dBc -56 dBc
Copyright  2007 A. Farson VA7OJ/AB4OJ. All rights reserved. 
Last updated: 06/03/2017

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