The AM Forum

I think this qualifies as a tech question although it may be a stupid question.
I was on 10 meter AM with some locals today and one of the stations was giving me a S-9 on peaks. Some of the stations could not receive him well so we all switched to FM.
Then the station who was giving me a S-9 on peaks was now 40 dB over S-9?
Now I would think that the signal to noise ratio might improve by going to FM but I would think that the S - Meter reading would show what the peak AM reading was when on FM. Unless, I guess, if the AM modulation was not 100%. I mean peak modulation power should be what the rig does on FM. But 40 dB!
I would think that the receiver sensitivity would be the same between the two modes since only the detector changes.
Maybe the rice box should not be considered a laboratory instrument.
Set me straight. I am very curious as to what is going one here.

Thanks and regards
Q W1QWT
 

One possibility is that your s-meter isn't fast enough to catch actual peak readings...

Same thing happens on a watt-meter.... you may be hitting say 100 watts pep on ssb....
Your meter likely won't read as high as 70 watts...

On FM, there is no (well, ALMOST..) fluctuation in the signal.... so the meter has time to get al the way to peak....



""Maybe the rice box should not be considered a laboratory instrument.""

Yeah, well... that's a good bet as well!!

I thought of that but the 'rice boxes' meter is a LED bar graph not a d'varsonal meter (SP). I would have thought it would have a fast response time.
Maybe the circuit has a slow response.
Regards
Q, W1QWT

Most likely the AGC loop is different between the two modes.  Remember that an FM receiver, in order to strip off the AM components, runs with the IF gain as hot as possible for effective limiting to occur on weak signals.   The AM components are stripped off with the limiter; the remaining information is (hopefully) just the FM component.    There are no "peaks" in an FM transmission.

Sounds like the S meter signal is derived from the detector not the carrier level. When the bandwidth doesn't change the carrier level should remain constant.

Frank uses a Bird Wattmeter as an S Meter - very Scotch.

Mike WU2D

There are very few receivers with a good S meter. #1 R390A and a modern one is the TCI/BR 8174.  The TCI is very accurate.

What are the full carrier values ? Is AM something like 25 watts and FM sumthing higher ???? Even on peaks the power of AM isn't all that great.

Assuming a standard 100 W PEP rice-box,  FM should produce 100W output. 

The S-meter on an AM receiver responds to the carrier level, not the peaks.  So if the power output at the transmitter is the same, the s-meter reading should also be the same.

But unless the transmitter is high level modulated, the carrier output on AM has to be reduced by 6 dB in order to leave enough headroom to accommodate the positive modulation peaks up to 100%.

But there shouldn't be 40 dB's worth of difference.  I suspect the AGC is derived differently on AM than on FM.  As already mentioned, you can't expect to use a typical ricebox as a laboratory instrument.

The AM/SSB   S meter will be derived from the AGC. The FM S meter will almost certainly be derived from the RSSI (recieved signal strength indicator) from a MC3556/7/9 FM IF chip. In theory  it produces  a logarithmic current proportional to signal strength. In practice on a HF rig theres  so much gain ahead of it that its pretty well useless.
                                             Ian VK3KRI

I guess the term S-Meter no longer means much from a measurement standpoint.

Tuning indicator might be a better term. Ricebox Design Goal: Make the meter wiggle with modulation and generally increase with a stronger signal.

I agree with Ian. The FM meter is mostly useless.
 
If we really did need an accurate S-Meter meter, it could be done. There are some wonderful power measuring chips and voltage variable attenuators out there now.

Bring back the Magic Eye!

Mike WU2D

Evently not.  Ever listen to a quarmtest exchange in recent years?  "You're 5-9 in Ohio.  Five-nine.  Fifty-nine. I need a repeat on your callsign.  I can just barely hear you above the background noise and QRN, and you have heavy QRM."

You may want to test the radio for S meter readings for each mode at the same level without modulation. And try several test levels. A calibrated generator, like a HP-8640 or some such would be a good source. In AM and SSB modes, radios usually  display a voltage that is the result of AGC voltage. It may be the AGC voltage itself, or the current in an IF stage. In an FM reciver, there is no AGC but there may be what is called RSS for received signal strength. It is less a calibrated value than S units. It is good for relative strength and beam turning and fox hunting.

Other than that, as stated by others, a typical 100 W PEP SSB rig will do 25 W carrier in AM and 100 W in FM unless there is a power dissipation limitation. It is the same as key down CW.

I think I will test this very thing on my Ft-897.

I have seen this same thing happen on every rice box that I have ever used. It's  not just yours. Whatever that means...........LB

Why not? What's to keep the receiver from overloading without AGC?



It never did. There has always been variation between manufacturers, not to mention the non-linearity of the meter/system of any particular receiver and the gain variations over frequency/band settings.

Even if S-meters were accurate, the readings are largely meaningless in the HF communications context. What is important is signal-to-noise ratio. I'd much sooner listen to a signal (especially on AM) that is S-9 with a noise level at S-1 or 2 (a typical quiet 75 meters at my location) than someone who is 40db over 9 and the noise is 35 dB over.