Many modern HF transceivers and solid-state amplifiers are fitted with an internal T-network autotuner. If the autotuner is left in the RF signal path on receive as well as on transmit, it will provide some additional preselection. (The same applies to an external manual or auto ATU, as long as it is in the receive signal path.)
A swept-frequency test was conducted on the internal autotuner of a Yaesu Quadra 1kW HF/6m amplifier. The purpose of the test was to measure the tuner's amplitude/frequency response on each amateur band from 160m to 6m, so as to evaluate its usefulness as a preselector.
The Quadra autotuner was first tuned into a 50Ω resistive termination on a midband frequency in each band in turn, with the PA stage in standby. Then, a swept RF signal at +10 dBm was applied to the Quadra INPUT 1 socket from an HP8601A sweep generator.
The Quadra ANT1 output was then connected to a Wavetek D150 50Ω RF detector, whose DC output was in turn connected to the vertical input of a Tektronix 455A/B oscilloscope. The HP8601A sweep output drove the scope's horizontal input. On the HP8601A, the 5 MHz markers and return-trace blanking were ON.
Measurements were taken for all amateur bands, and the oscilloscope screen images were captured on a digital camera. The screenshots were marked up with amplitude and frequency reference points.
It will be seen that the passband is asymmetrical, with a sharp cutoff below the tuned frequency and a gentler roll-off above it. This is to be expected, as the T-network tuner is in effect a high-pass filter. Typically, on 20m the -3dB point is at -2 MHz on the low side, and at +6 MHz on the high side. This is admittedly a rather modest sort of preselector, but I have found it to be helpful under strong-signal conditions.
Copyright © 2006 A. Farson VA7OJ/AB4OJ (incl. images). Last revised: 09/04/2006.