When signal strength meters were added to receivers they were set up measure the reduction in plate current flow, typically in an AGC controlled IF stage. The AGC voltage was a high impedance circuit and could hardly drive a milliamp meter. Since plate current was typically several milliamps, it was fairly easy to come up with a cheap circuit to add to deflect a meter. Some receivers had the meter directly in the cathode leg, but these required special reverse deflecting meters so that maximum current drove the meter leftwards to zero. Others, like the R4 have a bridge circuit in the plate circuit comparing current drian of stages with AGC control to those without. All depend upon a logarithmic relation between AGC and stage gain. Manufacturers arbitrarily set S9 at half scale and determined the 10 dB marks above mid scale by tests on the prototype. The lower half was just arbitrarily divided into S units. Eventualy the 50 uV for S9 "standard" was set to try and settle the argument. As to the actual values of S1 to S9 it basically absurd on a radio as it was totally subjective in the first place. The S units from original CW work were totally subjective in the first place.
As to the value of a S unit, 6 dB makesd most sense to me, but what the heck do I know. Drake deicded on 5 dB. this is probably because the prototypes worked out that way....nothing else. As a matter of personal perception, what is the difference between S1 and S2? Or, for that matter S8 and S9? If S1 originally mean "barely peceptable," doesn't that mean just above the noise floor? Wouldn't that imply something like 10 dB S+N / N ???? For my R4, 10 dB S=N / N for SSB is at .18 uV. That is about 9 dB below .5 uV or S-1 if S1 is .5 uV. Whatever the left hand scale edge may have printed on it, S1 or S0, it is really the point of AGC threshold. That is, the signal level for which the AGC just starts to reduce gain, as determined by some stage having its bias current start to fall. This is actually somewhere around what used to thgouht of as S3...or was that S4....S2? As a final thought, I had thought about changing the S meter circuit by adding a FET meter drive circuit to actually measure the AGC voltage itself. Another consideration was to also add a PIC microcontroller to not only measure the voltage but also correct it and drive the meter movement and calibrate the thing. rob - K2CU >Herbert Schulz <[EMAIL PROTECTED]> wrote: >Months ago, i have read in a publication, that "S9" in the US >corresponds to 50[�V] into 50[Ohms] and that one S-unit is >5[dB]. In Europe, "S9" is equal to 100[�V] into 50[Ohms] and >one S-unit corresponds to 6[dB]. I have to double check, if my >remembrance according this article is right, but i think so. > >Now, assume three cases (let's assume, that the S-meters are >displaying a perfect log scale, which is almost never the case >in reality; furthermore let's assume, that Drake's "S9 = 30[�V] >into 50[Ohms]" statement is based on the "5[dB] per S-unit" >definition): > >- S9 = 100[�V] into 50[Ohms] / 6[dB]: S1 then equals to 0.39[�V] >- S9 = 50[�V] into 50[Ohms] / 5[dB]: S1 then equalt to 0.50[�V] >- S9 = 30[�V] into 50[Ohms] / 5[dB]: S1 then equals to 0.30[�V] > >The manual of my R-4C states a sensivity of better than 0.25[�V] >at a noise ratio (S+N)/N = 10[dB] in the ham bands and better than >0.50[�V] at noise ratio (S+N)/N = 10[dB] on other frequencies. > >As S1 normally is a indication of the receivers sensivity at its >noise limit, and this readout should be given at the receivers worst- >case sensivity value (which is "0.50[�V] on other frequencies" accor- >ding to the R-4C specification), i would suggest to use the US-defini- >tion "S9 = 50[�V] into 50[Ohms] at a 5[dB] per S-unit scale" here. > >Regards and 73 from > > >Herbert, DG7MCC > > > > >mailbox55122 schrieb: > >> Hi friends, >> on the R4C manual is stated that S9 on the S meter means 30 microvolts of RF signal >> at the antenna input. It looks that the standard S9 signal are 100 microvolts of RF >> signal (on 50 ohm)..... >> >> Where is the true? Or better, what is the real S9 value? >> >> 73 >> de >> IOKPL, Pietro >> >> >> >> ---------------------------------------------------------------------- >> Submissions: [EMAIL PROTECTED] >> Subscribe: [EMAIL PROTECTED] - subscribe drakelist in body >> Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body >> Hopelessly Lost: [EMAIL PROTECTED] - help in body of message >> Brought to you courtesy of TLCHost.net http://www.tlchost.net/ >> ---------------------------------------------------------------------- > __________________________________________________________________ Introducing the New Netscape Internet Service. Only $9.95 a month -- Sign up today at http://isp.netscape.com/register Netscape. Just the Net You Need. New! Netscape Toolbar for Internet Explorer Search from anywhere on the Web and block those annoying pop-ups. Download now at http://channels.netscape.com/ns/search/install.jsp ---------------------------------------------------------------------- Submissions: [EMAIL PROTECTED] Subscribe: [EMAIL PROTECTED] - subscribe drakelist in body Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost: [EMAIL PROTECTED] - help in body of message Brought to you courtesy of TLCHost.net http://www.tlchost.net/ ----------------------------------------------------------------------
