• To anyone looking to acquire commercial radio programming software:

    Please do not make requests for copies of radio programming software which is sold (or was sold) by the manufacturer for any monetary value. All requests will be deleted and a forum infraction issued. Making a request such as this is attempting to engage in software piracy and this forum cannot be involved or associated with this activity. The same goes for any private transaction via Private Message. Even if you attempt to engage in this activity in PM's we will still enforce the forum rules. Your PM's are not private and the administration has the right to read them if there's a hint to criminal activity.

    If you are having trouble legally obtaining software please state so. We do not want any hurt feelings when your vague post is mistaken for a free request. It is YOUR responsibility to properly word your request.

    To obtain Motorola software see the Sticky in the Motorola forum.

    The various other vendors often permit their dealers to sell the software online (i.e., Kenwood). Please use Google or some other search engine to find a dealer that sells the software. Typically each series or individual radio requires its own software package. Often the Kenwood software is less than $100 so don't be a cheapskate; just purchase it.

    For M/A Com/Harris/GE, etc: there are two software packages that program all current and past radios. One package is for conventional programming and the other for trunked programming. The trunked package is in upwards of $2,500. The conventional package is more reasonable though is still several hundred dollars. The benefit is you do not need multiple versions for each radio (unlike Motorola).

    This is a large and very visible forum. We cannot jeopardize the ability to provide the RadioReference services by allowing this activity to occur. Please respect this.

Spectrum Analyzer Amplifier Noise Analysis

Status
Not open for further replies.
M

mnovia

Member
Joined
Jun 29, 2017
Messages
5
I have been struggling with this for a few days and can't seem to figure it out.

I am looking at a broadband rf amp (20mhz wide). With no signal fed into the system, I get a nice fat hump tucked nicely inside the filters:
...................
l l
........l l.............

I believe that his hump consists of 3 things:

the amplifier gain (which is 80dBm)
the noise figure of the amp (which is speced at 4 by the manufacturer)
the noise figure of the spec a (which I calculate to be 28 without the internal pre-amp, 16 with the pre-amp)

No matter how I do the math, my measurements never add up.

I am starting to think some of my basic assumptions are incorrect.

What is the math that drives the size / power of this hump????

Using the marker to noise floor option I get:
Peak to peak measurements: noise floor -135, amplifier -90

Channel power measurements: noise floor -61, amplifer -16

I believe the difference of 45dB is the same because the hump is very uniform.

How does 80dBm gain + any noise figure = 45dB???
NOTE: I used the external attenuation option on the Spec A to factor all cables /adapters out of the equation.
 
prcguy

prcguy

Member
Joined
Jun 30, 2006
Messages
15,344
Location
So Cal - Richardson, TX - Tewksbury, MA
I doubt if your amplifier has 80dB gain and gain is measured in dB, not dBm. How did you calculate the noise figure of the spectrum analyzer? 28dB or even 16dB is really high.

If your amp actually had 80dB of gain, then the slightest noise figure would create enough internal noise for the amp to put out a good amount of noise power.

Typically amps I would use in front of a spectrum analyzer might range from 10dB to maybe 35dB gain at the top most end. Adding a 10dB gain amp has little effect on the noise floor of a spectrum analyzer and a low noise figure amp (less than 2dB) with 35dB gain will raise the noise floor some but probably less than 10dB.

If your amplifier is very frequency specific and rolls off above or below its rated spec then you may see a noise hump within its band and the noise floor will return to "normal". Are you just connecting the amp with no antenna and seeing this noise hump or is there an antenna connected to the amp? What is it that your trying to do?
prcguy


mnovia said:
I have been struggling with this for a few days and can't seem to figure it out.

I am looking at a broadband rf amp (20mhz wide). With no signal fed into the system, I get a nice fat hump tucked nicely inside the filters:
...................
l l
........l l.............

I believe that his hump consists of 3 things:

the amplifier gain (which is 80dBm)
the noise figure of the amp (which is speced at 4 by the manufacturer)
the noise figure of the spec a (which I calculate to be 28 without the internal pre-amp, 16 with the pre-amp)

No matter how I do the math, my measurements never add up.

I am starting to think some of my basic assumptions are incorrect.

What is the math that drives the size / power of this hump????

Using the marker to noise floor option I get:
Peak to peak measurements: noise floor -135, amplifier -90

Channel power measurements: noise floor -61, amplifer -16

I believe the difference of 45dB is the same because the hump is very uniform.

How does 80dBm gain + any noise figure = 45dB???
NOTE: I used the external attenuation option on the Spec A to factor all cables /adapters out of the equation.
Click to expand...
 
M

mnovia

Member
Joined
Jun 29, 2017
Messages
5
Figured it out

I think I figured this one out.

The amp is indeed 80dB. It is not a pre-amp, it is a big honking donor amplifier for cellular bands.

The formula is:

N{Out} = N{Thermal} (dBm/Hz) + Gain (dB) + NF (dB)

I am getting around 8dB for the noise figure, 4dB higher than specs. I can live with that.

My assumptions were mostly correct. The hump does consist of the amplifier gain and the amplifier noise figure. However, the spectrum analyzer contribution is minimal. Use Friis cascading amplifier theory, the 1st stage amp dominates the noise figure of the amplifier chain.

https://en.wikipedia.org/wiki/Friis_formulas_for_noise#The_Friis_formula_for_noise_factor

I believe the noise figure measurements on the spec a were accurate. At RBW 10khz, you expect a thermal noise floor of -134. Put a 50 ohm load on the RF IN port, and the difference between -134 and your reading is the spec A noise floor.
 
prcguy

prcguy

Member
Joined
Jun 30, 2006
Messages
15,344
Location
So Cal - Richardson, TX - Tewksbury, MA
If it really has 80dB of gain then its a power amp with driver. Many amps of this type have drive levels around 0dBm, but if that were the case here your amp would be trying to put out 100,000 watts with 1mw drive, which we know is not the case. Even if its a power amp made for -30dBm input, with 80dB of gain its going to try and put out 100 watts with -30dBm in.

If its a big amp with big heatsinks I would not connect that directly to a spectrum analyzer. Any input signal within its working frequency range will smoke your analyzer. Do you have a make and model on the amp?
prcguy

mnovia said:
I think I figured this one out.

The amp is indeed 80dB. It is not a pre-amp, it is a big honking donor amplifier for cellular bands.

The formula is:

N{Out} = N{Thermal} (dBm/Hz) + Gain (dB) + NF (dB)

I am getting around 8dB for the noise figure, 4dB higher than specs. I can live with that.

My assumptions were mostly correct. The hump does consist of the amplifier gain and the amplifier noise figure. However, the spectrum analyzer contribution is minimal. Use Friis cascading amplifier theory, the 1st stage amp dominates the noise figure of the amplifier chain.

https://en.wikipedia.org/wiki/Friis_formulas_for_noise#The_Friis_formula_for_noise_factor

I believe the noise figure measurements on the spec a were accurate. At RBW 10khz, you expect a thermal noise floor of -134. Put a 50 ohm load on the RF IN port, and the difference between -134 and your reading is the spec A noise floor.
Click to expand...
 
Status
Not open for further replies.

Similar threads

BC2001
Amplifier Questions
Replies
1
Views
397
Ubbe
U
niceguy71
really more of a ham question but... question for the Quad 5 guys and ham operators.
2
Replies
31
Views
3K
DeeEx
D
niceguy71
truck install finally done
Replies
6
Views
1K
slowmover
slowmover
kb5udf
Pre-amp upgrade?
2
Replies
31
Views
2K
kb5udf
kb5udf
slowmover
Big Truck Radio 12V Source Upgrade
2
Replies
22
Views
2K
slowmover
slowmover
Top