PD or EMF output on my Sig Gen

[GSBX1220]

Hi guys

Possibly a silly question, but I'm about to re cal the S meter on my Yaesu FT-736 as it's reading quite high. Manual states that output from Sig Gen should be set in dbu, but it does not state if that is PD or EMF.

My Marconi 2022D can be set for either. My guess is PD, but I thought I would check first.

Cheers

Richard

[PETERg0rsq]

Yeasu define 0dBu as 0.5uV

So some calculation/conversion is needed so you can set the 2022D output correctly.

e.g.

S9 = 20dBu = 5uV = 14dBuV = -93dBm in a 50 ohm system!

Plenty of tables/calculators on the internet to do the conversions.

http://www.giangrandi.ch/electronics/radio/smeter.shtml


The 2022D can be set to display output directly in dBmV or dBuV or dBm.

Using the above outputs, you shouldn't need to choose between EMF of PD.

EMF is the generator's source voltage, with NO load, and ZERO current.

PD is the voltage at the output terminals taking into consideration the source and load impedances.

[GSBX1220]

Hi

Thanks for your reply. I think I misread the Yaesu manual on 2 counts. Firstly I missed the bit about 0.5uV being equal to 0dbu. I also read the dbu as dbuV.

I have attempted to calibrate the S meter, but it's not reading correctly across the scale, just at the 2 calibration points in the manual. On FM the manual wants you to inject 60dbu which is 5uv or -33dbm to adjust for full scale and then 10dbu or 1.6uv / -105 dbu for S7. An injected -117dbm does not show S5, but around S1 and -93dBm shows greater than S9.

However, the original issue of the needle slamming off the endstop with strong signals has gone.

[PETERg0rsq]

Richard

dB calculations can get complicated, especially with a "special" figure like 0dBu = 0.5uV

On an S-Meter 0dB normally = 5uV


The difference between the Yeasu reference voltage, and the "normal" reference voltage is..20dB

So to convert between Yeasu dBu and standard S-Meter dB:-

add 20dB to the normal S-Meter figures to get Yeasu dBu

subtract 20dB from Yeasu dBm values to get standard S-Meter dB figures


so (from the table I linked to):-

60dBu = S9+40 = 40 dB = 500 μV = 54 dBμV = 5.0 nW = –53 dBm

not 5mV or -33dBm as you stated!

and

10dBu = S7 = –12 dB = 1.3 μV = 2 dBμV = 32 fW = –105 dBm

note it doesnt quite work for 10dBu as S-points usually go in 6dB steps, not 10dB. Where did 10dBu come from?

Personaly I would forget the yeasu dBu values, and substitute dBm or dBuV values from the table for each S-Point you need to set up.

So for S7 just set the signal generator to -105dBm
for S9 just set output to -73dBm

forget dBu!!!

I have no idea how the S-Meter is calibrated, but surely you need to make adjustments in the main reciever AND each band module?

and

are you sure your signal generator is calibrated?

[GSBX1220]

Hi

Thanks. The -33dbm was a typo. The 10dbu comes from the service manual and they use 10dbu to define S7. I carried out the cal procedure using the values from the link you sent me.

I have attached the page from the manual for reference. The cal procedure is only done once and not per band module.

With regards to the sig gen, I cannot say how much in or out of cal it is. I cannot measure the output level at the values we are discussing here as I only have a Tektronix 2465A scope. The lowest I can measure is -30 dBm and I'm seeing 20mV P/P which works out at 7.07mV or -30dBm. Levels below this, I have no idea.

I have done the cal as per procedure attached. The S7 and full scale are set. It's just the bits in between that don't read correctly. Maybe I'm expecting too much accuracy from the meter and over analysing this, and maybe Yaesu did not set up the S meter as the way the link of S meter values says.

All I know now is that I have done what the manual asks, and as to the accuracy of the S meter, who knows!

[PETERg0rsq]

Doing the maths was driving me mad, so here is the revised S-Meter table, including dBu as used by Yeasu and others

S-Point..dB relative to S9.....dBu..........Vrms......dBμV..........dBm
S1........–48 dB..................-27 dBu.....20 nV.....–34 dBμV....–141 dBm
S2........–42 dB..................-22 dBu.....40 nV.....–28 dBμV....–135 dBm
S3........–36 dB..................-16 dBu.....79 nV.....–22 dBμV....–129 dBm
S4........–30 dB..................-10 dBu....160 nV....–16 dBμV....–123 dBm
S5........–24 dB...................-4 dBu.....320 nV....–10 dBμV....–117 dBm
S6........–18 dB....................2 dBu.....630 nV......–4 dBμV....–111 dBm
S7........–12 dB....................8 dBu.....1.3 μV........2 dBμV....–105 dBm
S8..........–6 dB..................14 dBu.....2.5 μV........8 dBμV.....–99 dBm
S9...........0 dB..................20 dBu......5.0 μV......14 dBμV.....–93 dBm
S9+10....10 dB..................30 dBu.......16 μV......24 dBμV......–83 dBm
S9+20....20 dB..................40 dBu.......50 μV......34 dBμV......–73 dBm
S9+30....30 dB..................50 dBu.....160 μV.......44 dBμV......–63 dBm
S9+40....40 dB..................60 dBu.....500 μV.......54 dBμV......–53 dBm
S9+50....50 dB..................70 dBu.....1.6 mV.......64 dBμV......–43 dBm
S9+60....60 dB..................80 dBu.....5.0 mV.......74 dBμV......–33 dBm

0dBu = 0.5 μVrms

Your 2022 uses a stepped attenuator to give lower outputs, so if the calibration is good at 0dBm then should be good through the attenuator range.

You can test the attenuator if you have a good 10dB attenuator. Substitute it in each 10dB step range and see if you get the same output as the next step down.

Do not forget any test equipment you use to measure voltage out of the signal generator must have a 50 ohm input setting, otherwise pad the input with a 50 ohm resistor to ground.

Suprised you do not need to adjust each module, as if the 2M module was deaf, then the S-Meter readings would be wrong for the other modules!

Finaly how accurate do you need the S-Meter?

Just give 5 and 9...like everyone else

[GSBX1220]

Hi Peter

Thanks for that chart.

I'm not going to go back through that calibration procedure as like I said I'm confident I have done it as per manual. The needle now no longer slams off the endstop when receiving the local 6m& 70cm repeaters.

The sig gen is OK at 0dBm and my 'scope has a 50 Ohm input. The 0 dBm measured on the scope is flat across the frequency range, until I approach the upper bandwidth of the scope.

Thanks for your help and advice

Richard