[AMPS] 4cx10000/imd/sunits/splatter etc

[Peter Chadwick]

Igor says:

> A RECEIVER WITH A THIRD ORDER INTERCEPT POINT OF   +30DBM

A +30dBm intercept point is no use unless the phase noise is at least
 -130dBc/Hz at the offset you're using.

About 16 - 20dBm  intercept at full sensitivity and tailoring the
sensitivity to go down with increasing intercept is the way - it's rare to
have signals at -30dBm with a received noise floor at -130dBm (corresponding
to a 10dB noise figure rx). If switching between a 50 ohm load and the
antenna produces a 5 or 6dB increase in noise (assuming that the receiver is
linear), then that's all the sensitivity that you can use. linearity can be
checked by switching in a 3db pad: if the noise changes by 3db or so, it's
linear. Most of the time, HF receivers have far more sensitivity than they
need, which is why the antenna attenuator can be so useful. When 40m is
solid with big broadcast signals, it generally doesn't have the man made and
natural noise floor low enough to enable the use of very high sensitivity,
so what most people need is a dynamic
range (whatever that is! - there's at least 5 definitions) that can have
it's starting poiint i.e. noise floor, moved to accord with conditions.

In the term 'S meter', 'S' stands for 'suspicious'. If you want an accurate
level measuring set, you build it different to a ham receiver. Accurate
logarithmic amplifiers aren't that easy, and in any case, the only use for
them would be to drive the S meter! 

The 'standard' of 50 microvolts (-73dBm) (is this microvolts PD or EMF, by
the way?) for S9 gives a range of from -122dBm for S1 to -13dBm for for
'S9+60'. That is some awesome dynamic range for a log amp! 

The phase noise limited dynamic range and intermodulation limited dynamic
range should be equal; many receivers are limited by phase nosie form poor
synthesisers. The Direct digital technique can be very good, if designers
don't try to get rid of discrete spurs by jittering them: that just raises
the noise floor. But spurs are a problem, and ideally should be at least
90dB down; that needs a DAC of at least a real 15 bits.  Of course, a VFO
with coils on a 1 inch diameter ceramic former, air spaced ceramic insulated
variable capacitors, and a 6AU6 or similar putting several hundred
milliwatts of RF into the tank is a good way to get phase noise down!  I
have achieved -153dBm at 25 kHz offset on a 35MHz oscillator: that used a
VMOS power FET with a constant current supply to keep output power constant,
a coil of silver deposited on low loss glass, and 4 varactors at $15 each.

73

Peter G3RZP


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