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VK5QX > KENWOO 28.04.03 13:49l 136 Lines 5527 Bytes #999 (0) @ WW
BID : DE0616VK5QX
Read: DG2KBC GUEST VE7HFY
Subj: Re: S9 = 40dB ???????????
Path: DB0ZKA<DB0SAA<DB0TTM<DB0FP<DB0SON<DB0MRW<OK0PKL<OK0PPL<RZ6HXA<SP7MGD<
ON0BEL<ON0AR<VK6HGR<VK5UJ<VK5BRC<VK5SPG<VK5LZ
Sent: 030428/0629Z @:VK5LZ.#ADL.#SA.AUS.OC #:62291 [Elizabeth] $:DE0616VK5QX
From: VK5QX@VK5LZ.#ADL.#SA.AUS.OC
To : KENWOO@WW
TO dB OR NOT TO dB ??
Col VK3JCW queried matters concerning calibration of "S" Meters.
He expressed concern regarding a comparison between his TS520 and TS50
transceivers, as well as (in part) making the following comment:-
__________
" I always thought that regardless of the type or make of an S-Meter,
they should read the same for any given signal. "
__________
Well, I believe that Col can follow whichever way he wishes without having
to concern himself too much.
I can well remember a series of discussions on this same subject when back
some years ago several of us technical types at a well known Federal
Government research facility decided to implement a general coverage
receiver building project.
This was one where we went to a great deal of effort to each produce a
receiver that offered a wide range of options. It was to be capable of
reception from VLF through to 30 MHz with separately selectable 1 MHz
bands, CW, AM, SSB with suitable switchable filters etc. etc.
We based the receiver on a design utilising a Wadley Phase Locked Loop
front end and a tuneable IF covering from around 3 to 4 MHz. As a receiver
it worked very well and was extremely stable. the British "RACAL" company
eventually manufactured a number of receivers, both valve and solid state,
using this principle.
Eventually it came around to the matter of "Signal Strength" meter setting
and calibration for our receivers.
Let me enlighten you with just a few conclusions.
1. The setting of "S" meters readings is VERY MUCH arbitrary.
2. If you carry out some close checks you will most likely find that, with
a comparison of two of the same type and make of receiver, readings will
be different, often to quite some degree.
3. Some manufacturers use a 6dB step for each "S Unit". Others use a 4dB
step per "S Unit" whilst some seem to use almost any old figure.
In other words, and generally speaking, it really does not matter to any
great degree and usually a larger reading on the meter represents a
stronger signal.
However, that may not necessarily be the case where something strange
happens and the AGC system possibly reacts in an unusual way to an
overload signal or malfunctions produce some other result.
Another aspect facing you is that "rarely" will you find an AGC system
that operates in a linear fashion, thus a calibration of the "S Meter" can
result in a most unsatisfactory display as regards the various points
across the scale.
(You may note that here I am referring to the more common method of "S
Meter" operation where the indication of signal level is initially derived
from the receiver AGC system.)
For the purpose of our receiver project referred to above, we settled on
an arbitrary approach where we set a figure of 128 microvolts across 50
ohms as representing a received Signal Strength of "S9".
Having determined this point we then calibrated the meter downward using a
change in level of 6 dB per "S Unit". Decibel markings above "S9" were set
using divisions of 10, 20, 30 and 40 dB etc.
This seemed to turn out fairly well, however, there was never any
guarantee that you could use the "S Meter" for other than comparative
observations as to signal strength.
To sum the matter up. The issue of signal levels and variations in signal
strength can become very subjective.
Finally, a couple of other general comments.
Comparative measurements can also be rather misleading when it comes to
trying to obviate errors due to path fluctuations. Signal levels can
suffer quite sharp dips due to propagation factors.
(Have you ever run across a case where an operator with his 100 foot tower
and multi element stacked beams becomes very disgruntled when he receives
a report that his signal is only about 4 dB better than another nearby
station using only a dipole at around 25 feet in height ?)
Quite often these dips in received signal level are not easy to cater for
and can even go relatively unnoticed when trying to make a comparison.
Thus you can easily finish up with some major errors compared to what
really happens to be the true facts regarding signal levels.
That this is the case was more easily seen when we carried out calibration
measurements over a long, but single, hop HF path using accurately set
transmitting levels, antennas with measured gain and recording periods of
no less than 5 minutes or more for a particular frequency.
This series of measurements continued for a total period of around 18
months and included measurements taken over complete 24 hour comparative
periods.
However, these measurements make up yet another story.
I would say to Col to go ahead and set up his receiver "S Meters" to
provide him with what, to him, appear to be reasonable comparative
indications.
It does not matter that you cannot say with any certainty that the signals
being received from another station are exactly equivalent to a certain
number of microvolts etc.
Incidentally, a somewhat better way of comparing received signals levels
at any particular instant is to set the indicated level to a specific
reference point and then to insert more or less attenuation using a well
calibrated stepped attenuator so as to determine the various differences
in level.
I hope that this information will be of some help in allaying any fears
that Col VK3JCW may have regarding his "S Meter" indications.
Regards,
Ian
__________
73 de Ian, VK5QX
@ VK5LZ.#ADL.#SA.AUS.OC
28 Apr 2003
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