[50mhz] [Yaesu_FT-450] Preamp and receiver question

[Paul Pollock]

Hi John AA5JG,

Since nobody has responded, I'll give it a go <grin>.

Four things need discussion (some of this you already know, but we need common ground, and others are reading <grin>):

1). S-Meter calibration standards (or not)
2). Gain Factors/actual noise
3). User Perceptions
4). Noise Blankers

1). S-meter Calibration Standards
	Without trying to prejudice the conversation, I'm going to state a little secret. The Collins Spec of S-9 being at 50-uv is, for general purposes, meaningless. And for further discussion let's assume that the S-meter of any radio we talk about is properly calibrated for the radio under test, by the manufacturer.

The S-meter voltage is derived from the AGC amplifier(s) of the radio, normally from a late stage of the radio. While there was a lot of importance put on the Collins spec, a long time ago, we have to remember that this happened to be the point at which the AGC voltage in the design radio just began to roll off. This is why anything over S-9 is calibrated as DB-over-S9 , because while the logarithm up to S-9 tends to be fairly consistent (for a specific radio), the roll-off progress is fairly non-linear and operates on a different logarithm (because at this point and further, the actual device simply does not wish to amplify anymore). This is not based on the design stage calibration, it is based on the gain characteristics of the devices involved that are actually producing the AGC voltage and thence to the actual S-meter voltage.

The actual start of the roll-off point varies from device to device. Doesn't matter whether we talk about the tube responsible for AGC voltage in a Collins, or we are talking about a gain-fet in a 706mk2g, or the dsp in a FT-450; for each device responsible for S-meter voltage, the start of roll-off (which can vary by the device curve) can be very different (in the case of the FT-450, the firmware engineer can actually design the AGC math constants into the radio). Last time I read anything about S-9 on a 706m2g, S9 occurred someplace about 39uv (I could be wrong, but that's what I recall off the top of my head, based on a QST review some years ago). The FT-450 S9 is a little lower than that (and both the 706 and FT-450 specs were only printed for normal HF, 6-m was not reported for S-meter).

But, here things get dicey. The next factor we get to talk about is, at what point does the receiver just discern faint signal vs the noise floor? Because this is where the math for the logarithm to S9 begins to ramp. This also varies by the device (either an IF-Amp, or when enabled, the first stage preamp basic gain above noise). The noise of the preamp is normally a stable constant (sometimes measured in a fraction of a db), and dealing in your stated case, to overload the preamp (to the point of raising the noise floor of the preamp stage itself) would require signals that push the meter well past S-9, and neither of the radios you stated are reading anything like that.


2). Gain Factors/actual noise
	After all the previous is on the table, most of the data is already present. The gain logarithm of any radios' AGC is based on the math curve started at the MDS (minimum discernible signal) and progressing to 60-db (for the specific device) to give us S-9. The rest of the meter calibration is based on the curve from S-9 to the number of DB over S-9 the engineer wants to display;  normalized for the AGC amplifiers' gain curve over S-9 (found by actual signal response testing, by the folks at the factory).


3). User Perception
	The problem I always have, is the same one you are having. I keep trying to normalize all actual receiver math, with what happens at the S-meter, as if there were some kind of common ground between one radio design and another; at the S-meter. This is never the case. Engineers try to find some equivalency, but there is rarely a good match between one AGC stage and another (in discreet component radios, the AGC can very some even between two identical model radios).

More importantly, sub-$1000 radios that have 6-m in them are often a compromise for 6-meters. 6-meters almost has to be evaluated as a completely separate subject versus the rest of the shortwave band. In expensive radios (IC-7600 or better), it is not uncommon to find a separate gain circuit specific to the 6-m band, just to correct for the shortcomings of a more universal (and cheaper) design.
	The FT-450 is known by many, to work quite well in 6-m, but its' gain is not linear band-to-band from lower shortwave vs 6-m. So the S-meter will tend to be lower, than some other radios. The 6-m performance in the 706 (somebody is going to yell at me, I'm sure <grin>) while adequate, and will produce higher s-meter readings, may in fact recover less discernible signals than the FT-450 (but not having both radios you may find that signal recovery may be acceptable in the 706). The S-meter curve for each radio may be totally different, and the recovered signal output can be higher on the lower S-meter radio.
	As a rule of thumb, I've found that the basic signal-to-noise ratio of the FT-450 is very low compared to many other radios (putting aside the idling audio amplifier racket [which I fixed by adding a 20-ohm resistance to my external speaker]), which might tend to produce better and more readable signals versus some other product like the 706, regardless of S-meter output.


4). Noise Blankers
	Too many people talk about NB's as if they are supposed to work against ambient racket (hash). Normally not so. They are designed to work on pulse noises. The basic design is a storage noise gate that shunts the IF signal to ground for the duration of a circuit-discerned pulse which is strong enough to fire the trigger circuit. Many people know that engaging the NB can often lower the S-meter noise level on stuff where pulses are not apparent to the listener. This is caused by spikes hi-enough to trigger the NB, but so short that the ear does not hear it. But the S-meter sees it, and the NB can work on it. With careful listening (on conditions like this) the sound output is somewhat easier to listen to, but the mind does not perceive a difference <grin>.
	Sometimes, a NB can cause distortion of audio on strong signals, because the signal strength is so strong that the NB trigger point is reached. Many radios are often not calibrated with a high enough threshold to prevent this (very common while listening to strong, well modulated, AM broadcast signals; but less common on SSB). I hear this distortion a lot on my FT-450 when listening to Radio Taiwan International, it is radiated in Florida and is quite loud in most of the U.S., so I turn the NB off for this kind of problem.


I realize that I've painted with broad strokes at points, and I hope I've not confused you. Just be aware that many radio engineers wish the average radio operator would ignore the S-meter most of the time. What is more important, is what kind of sounds your ear can hear.

Have fun and good luck!

-Paul Pollock
  KD7BWB



On Jun 29, 2011, at 9:52 AM, John Geiger wrote:

> I have a receiver question for those on the list who know a lot more about
> this stuff than I do, at least from the electronics standpoint. I know more
> about receiver performance and less about how we get there.
> 
> I have an Icom 706-had it over 3 months now which is approaching a record
> for me HI HI. On 6 meters I have some noise source to my southeast (right
> at the Carribean) that really raises the noise level on it. It hits S8 or
> so if the beam is pointed in the right direction. I didn't remember
> the FT450 I had being that noisy, so I had a friend bring his FT450 over,
> and it was showing a noise level of S2 or so in the same direction.
> 
> Now I know that different variables are in play. Checking the QST review
> for each radio, the 706 (preamp on) has a 3db advantage in sensitivity. The
> MDS for it is -139dbm vs -136dbm for the FT450 (preamp on). That should be
> 1/2 S unit difference or so in picked up noise. The S meter calibration is
> very different. For the 706 with the preamp on on 6 meters, a S9 signal
> takes 3.3 microvolts, which is insanely liberal, but that is what it is.
> For the FT450 under same conditions (preamp on, 6 meters) it takes a 33
> microvolt signal for S9. That is a factor of 10, which should show up as a
> 10db difference on the meter-correct? If that is correct, we have a 13db
> difference in readings between the 706 and 450 (3 db for increased MDS, 10db
> for the meter difference). That should be 2 S units or maybe 3-4 given the
> non linear calibration of most meters. But I am seeing a 7 S unit
> difference or so.
> 
> So here is my big question-can the preamp device in the 706 (or any other
> radio for that matter) become overloaded and start to generate increased
> noise? I think it can, correct? If you put enough signal, local noise,
> whatever into the receiver, won't the preamp at some point become swamped
> and start to create noise on its own? Could that be where this extra noise
> is coming from?
> 
> You really can't run the 706 original on 6m without the preamp as it is
> pretty deaf without it. The noise blanker on either rig doesn't do much on
> this noise, so it isn't that the NB in the FT450 is taking it out while the
> 706 isn't.
> 
> 73s John AA5JG
> 
> [Non-text portions of this message have been removed]
> 
> __._,_.___
> Reply to sender | Reply to group | Reply via web post | Start a New Topic
> Messages in this topic (1)
> RECENT ACTIVITY: New Members 8 New Photos 10
> Visit Your Group
> Http://www.foxtango.org
> Switch to: Text-Only, Daily Digest • Unsubscribe • Terms of Use
> .
>  
> __,_._,___