Peter, please help me:
Peter Chadwick wrote:
>
> A +30dBm intercept point is no use unless the phase noise is at least
> -130dBc/Hz at the offset you're using.
Why?
> 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).
Do you mean tailoring the gain to go down with increasing intercept?
A received noise floor specified as a power does not correspond to any
receiver noise figure. For example, increasing the filter bandwidth from
2 kHz to 200 kHz increases the received noise level from -130 to -110
dBm. What is the receiver's noise figure now? Still the above 10 dB.
> 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.
My 10 GHz system produces a decrease of 6 dB when the load at 290 K
ambient is
disconnected and the antenna at 20 K is connected. Now what? Presumably
you mean that when the load at 290 K is substituted with the HF antenna
at 10,000 K, any further decrease in receiver noise temperature is
pointless as the signal-to-noise ratio of the system is determined
largely by the temperature of the antenna? Would you agree that
"sensitivity" of a system is defined by antenna gain and receiver noise
power ratio (noise figure)?
> linearity can be
> checked by switching in a 3db pad: if the noise changes by 3db or so, it's
> linear.
This is more like the definition of non-linearity. Introducing increased
transmission line loss, at constant temperature (the 3 dB pad),
increases the system noise temperature, and decreases the
signal-to-noise ratio more than the expected 3 dB, due to the additional
3 dB loss in the propogation path and the increased transmission line
temperature. Presumably you mean that if the input voltage is still
amplified 3 dB by the gain stages of the receiver, then the receiver is
not driven into gain compression and is still linear, or do you mean the
s-meter, if it drops 3 dB, is linear. Or is the noise
figure changing by 3 dB, methinks not?
> 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.
Do you mean that HF receivers have far more gain than they need, as the
active stages are driven into gain compression, hence the use of the
attenuator?
>
> 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.
How does one jitter discrete spurs without introducing the mother and
father of FM onto the carrier?
> 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!
High power keeps down phase noise?
> 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.
Varactors are bad news in oscillator circuits, one is enough surely?
73,
Ian ZS6BTE
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|