Topband: Input Impedance of the FT1000/S-Meter Calibration issues

[Sinisa Hristov]

Hi Tod,


> I have found your reflector comments useful and interesting.

Thank you.


> You said, ".. No, it cannot be fixed this way. This will only ensure that
> constant power is delivered to the  external attenuator, but the original
> problem repeats itself  between external attenuator and RX input. "

> Sinisa, When you say "..it cannot be fixed this way.." should I interpret
> that to mean that it can be fixed, but not the way suggested or that it
> cannot be fixed at all. (I am not actually sure what we are fixing so maybe
> someone can clarify that).

It certainly cannot be fixed that way,
although its external manifestation can be masked out.
In theory, it could be fixed by redesigning some 
of input circuits, but I don't think it's worth the effort.

Doing so could marginally improve sensitivity, but FT1000MP
generally does not suffer from insufficient sensitivity.
Of course, sensitivity measurements would be more meaningful.


> This part of the S-Meter thread began with Eric, K8LV, posting a note on his
> effort to measure the input impedance of the FT1000 receiver. If I
> understood correctly, Eric asserted that " S-meters essentially measure the
> voltage present at the RX input".

That's what we would like them to measure :-)
Or, to be more precise, the power at RX input.

However, taken literally - S meters indicate AGC control voltage,
i.e. the control voltage used to automatically decrease RX gain
in order to keep the audio volume reasonably constant.
This voltage has a complex relationship to the input signal level,
especially in the case of time-varying signals, as most of them are.

Therefore, S meter reading can be used to indicate signal
level trends, but they are hardly usable for real measurements.


> Eric's measurements suggest that the 'black box' I refer to has variable
> performance characteristics depending upon the selections made while using
> the receiver and that these changes are sufficiently significant to make the
> S-meter indication vary even though the input signal remains "constant".

Eric's measurements specifically address the ability of the receiver
to accept the power available from a 50-Ohm source. In other words
- the quality of RX input match to 50-Ohm source. Eric's numbers
can be used to calculate SWR at RX input:

  given:  input impedance  Z = R + j * X
          system impedance Z0 = 50 Ohms

  calculate:   A = SQRT ( (R + Z0) ^ 2 + X ^ 2)
               B = SQRT ( (R - Z0) ^ 2 + X ^ 2)
               SWR = (A + B) / (A - B)

Effect of SWR at RX input is the same as in any other case:
a part of the available power is reflected back towards the source,
and therefore not used to drive the receiver.

When measuring sensitivity of a high SWR RX, such as FT1000MP,
(if no correction is made) the receiver will appear as less
sensitive than it really is, i.e. requiring more power
from the source for the nominal output. Actually, the source
will deliver more power, but only a part of it will enter RX,
rest of it will be reflected back to the source.



73,

Sinisa  YT1NT, VA3TTN