[Icom] Re: IC756 SWR meter compared with FC902 or DAIWA CN101

[Adam Farson]

Hi Clete,

A few points need clarification:

1. There is already an (spelling corrected by MS Outlook) accurate
reflectometer in the transmit signal path, between the output of the
low-pass filter box and the input of the autotuner. The low-pass filters
following the PA have a 50-ohm input & output impedance. The internal
reflectometer located in the transmit signal path between the LPF output and
the autotuner input sends "Forward Power" and "Reflected Power" voltageto
the CPU, which calculates the difference between the actual load
impedance and 50+j0 ohms. The CPU, in turn, drives the SWR scale of the
meter.

2. The PA, being a broadband transformer-coupled push-pull Class AB
amplifier, does not have a 50-ohm source impedance. It is designed such that
a 50-ohm resistive load will present the correct load resistance to the
collectors (or drains) of the RF output devices for optimum power transfer
at the rated DC supply voltage (Vcc or Vdd) and power output for the PA.
Without any exception that I know of, all modern solid-state HF transmitters
employ this topology.

Please refer to http://www.qsl.net/ab4oj/quadra/sshfamp.html

3. The reason why many modern HF transceivers incorporate an autotuner is to
ensure that a 50-ohm resistive load is presented to the low-pass filters
(whose performance is degraded if they are misterminated), and hence to the
PA. Proper matching to the PA is a prerequisite for optimum linearity. This
point is addressed in the Rockwell-Collins textbook "HF Radio Systems and
Circuits", by Sabin et al., ch. 12, pp.483-484.

4. As I mentioned in a previous posting, if the autotuner is disengaged, an
external SWR/power meter will read approximately the same as the radio's SWR
scale (assuming a short jumper between the radio and the external meter). If
the autotuner is engaged, the radio's SWR scale will indicate the SWR at the
LPF output (autotuner input),
whilst the external meter will read the SWR arising from the load connected
to the antenna socket.

Cheers for now, 73,
Adam VA7OJ/AB4OJ