|
|
Power Amplifier Noise Figure - RF Cafe Forums
|
John
|
Post subject: Power Amplifier Noise Figure
Posted: Mon Aug 01, 2005 10:03 pm
|
|
|
Can a Noise Figure Meter such as the old HP 8970B
be used to measure the noise generated by a power
amplifier in the same way as a LNA's Noise Figure
is measured. I want to know how much spectral noise
is generated by a PA with no input signal.
Thanks in advance,
John
|
|
|
|
|
Joe |
Post subject: NF
Posted: Fri Aug 05, 2005 4:19 pm
|
|
|
Yes, you can measure any amplifier. (Converters
also)
Check the output power of the power
amp with a 50 ohm load on the input to be sure you
won't damage the NF meter. Measuring the power amp
with no input should work. The exception to this
would be class C amplifiers which won't turn on
without a strong enough signal at the input.
If the gain is high or just to protect the NF
meter you can add an attenuator after the amplifier.
On the 8970, use 34.4 Special Function, "Enter loss
after DUT" to compensate for the attenuator.
|
|
|
|
|
Guest |
Post subject: Re: NF
Posted: Mon Aug 08, 2005 3:45 pm
|
|
|
Joe wrote:
Yes, you can measure any amplifier. (Converters
also)
Check the output power of the power
amp with a 50 ohm load on the input to be sure
you won't damage the NF meter. Measuring the
power amp with no input should work. The exception
to this would be class C amplifiers which won't
turn on without a strong enough signal at the
input.
If the gain is high or just to
protect the NF meter you can add an attenuator
after the amplifier. On the 8970, use 34.4 Special
Function, "Enter loss after DUT" to compensate
for the attenuator.
Joe thanks for the reply and sorry for
the delay. If I'm trying to calculate the kTB power
in dBm then do I need to place a filter on either
the input or the output of the PA to simulate the
operating BW? I'm only operating in a narrow bandwidth
compared to its total operating frequency range.
Thanks again, John
|
|
|
|
|
Joe |
Post subject: NF Meter
Posted: Tue Aug 09, 2005 7:55 pm
|
|
|
You don't need to add any filters. The reading is
a spot noise figure at individual frequency. I am
not sure what the internal bandwidth is for the
8970B, but there are internal filters.
I
would measure several frequency points, depending
on your operational bandwidth.
You can calculate
the noise power using the operating bandwidth. You
want noise figure and gain measurements for use
in the noise power equation. These should be bandwidth
independent.
|
|
|
|
|
John |
Post subject: Power Amplifier Noise Figure
Posted: Tue Aug 09, 2005 10:44 pm
|
|
|
Joe thanks again for responding.
Doesn't
Noise Power equal;
NP=10Log(k) + 10Log(T)
+ 10Log(B) + NF + Gain ?
Based on above I
can calculate a NP in a 1Hz Bandwidth but I'd also
like to determine the Noise Power in dBm in it's
operating Bandwidth. If I don't have an operating
bandwidth filter after my Unit Under Test (UUT)
then the Bandwidth is either the bandwith of the
instrument or the bandwidth of the UUT which ever
is less.
Am I correct in this assumption?
John
|
|
|
|
|
Joe |
Post subject: Noise Power
Posted: Wed Aug 10, 2005 3:07 pm
|
|
|
John,
The equation you have shown will give
you noise power in dBm over your operating bandwidth.
The 10log(B) portion of the equation changes the
units of the equation from dBm/Hz to dBm. You need
to calculate your operating bandwidth and use it
in the equation. If your system is filtered, then
the bandwidth is reduced, otherwise it will be the
full bandwidth of the amplifier. The noise produced
and transmitted is not only limited by the bandwidth
of the amplifier. Although you are only using a
narrow portion of the amplifier bandwidth, it will
still amplify noise over its full bandwidth. The
only system improvement you may see is an equivalent
bandwidth reduction from the noise contribution
of earlier components if they operate at a reduced
bandwidth. If you don't care about the noise outside
your operating bandwidth calculate the noise power
based on your operating bandwidth.
As for
the measurements, if you measure the noise power
with a power meter it will not be band limited.
You would need a filter to measure the noise power
within your bandwidth. Unless the noise power is
high, this may not be a good measurement because
you may be operating at or below the noise floor
of the power sensor. A measurement with a spectrum
analyzer will have the resolution bandwidth of the
analyzer determining the bandwidth of the measurement.
Again this measurement is limited by the noise floor
of the analyzer. The noise figure measurement is
not determined by bandwidth. It is a unitless number
describing the increase in noise due to the effects
of the amplifier.
In the equatiion:
NP=10Log(k) + 10Log(T) + 10Log(B) + NF + Gain
NP will be in dBm provided you convert to mW
when using Boltzmann's constant.
The units
are:
k in J/degree K or W/Hz/degree K
T in degree K b in Hz
Converting from
watts to mW and 10log() converts mW to dBm. The
noise figure and gain are in dB and have no other
units.
I hope this clears things up rather
than add more confusion.
Joe
|
|
Posted 11/12/2012
|
|
|