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Old 5th Jun 2020, 12:09 am   #1
Skywave
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Question R.F. pre-amp.: assessing dynamic range

When it comes to measuring basic specs. of things like R.F. amplifiers, my ability to interpret corresponding published data is a bit limited.

Here, I have an HP 8447A (with option 001) dual-channel R.F. preamplifier. My measurements of its dynamic range (per channel) at 10 MHz produced a linear response (i.e. within the limits of +/- 1dB) between the input limits of 1 mV and 35 mV sinewave, with O/P terminated in 50 Ohms. I was anticipating a larger O/P before significant compression was shown (and perhaps a smaller input level, also).

Signal source: HP 8640B; O/P measurement: Tek. 2465 'scope or Marconi TF2603 mV meter. (The dynamic range of each is greater than that of the E.U.T. I have no reason to doubt the calibrations of this test equipment)

Attached are the specs. for this device - but my ability to interpret the data therein to see if it matches my measurement is beyond my ability.

So, can anyone here help with this, please?
Is my HP 8447A 'up to spec.' ?

Al.
Attached Files
File Type: pdf HP 8447A spec..pdf (188.6 KB, 47 views)

Last edited by Skywave; 5th Jun 2020 at 12:33 am. Reason: Adding attachment using Windows 7 failed. Resorted to Windows XP
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Old 5th Jun 2020, 12:55 am   #2
G0HZU_JMR
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Default Re: R.F. pre-amp.: assessing dynamic range

The datasheet says that the output 1dB compression point is +6dBm (= 4mW).

This is quite a low figure for a preamp and this means the preamp gain will be compressed by 1dB at just 447mVrms output into a 50R load.

So you should easily start to see notable compression effects at around 350-400mVrms at the output. The gain is a nominal 20dB and this means the voltage gain will be about 10 as the device has 50R input and output. So you should begin to see some compression at about 35-40mVrms (-15dBm) output from the sig gen.

This seems fairly close to your measurements and some allowance will be required for the measurement uncertainty of the setup. For example, the input VSWR could be 1.7:1 and your sig gen output VSWR could be 1.5:1. There is about +/- 0.5dB uncertainty just in this part of the test setup. So I would say your measurements show that your amp is OK in terms of the 1dB compression point as long as the preamp gain measures correctly at approximately 20dB.
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Old 5th Jun 2020, 5:29 am   #3
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Default Re: R.F. pre-amp.: assessing dynamic range

If you had a classic spectrum analyser, you would find that it had an ultimate noise floor equivalent to a noise figure of a bit over 20dB (in its lowest band on a microwave model, it jumps higher in the higher (harmonic mixing) bands)

So a preamp with a gain of 20dB and a noise figure of 7dB will create a useful increase in sensitivity. 20dB is just enough gain to let the preamp start to dominate the noise of the preamplified analyser, and not so much gain that it ruins the overload level.

In using an analyser with a preamp, the analyser's input attenuator will be switched right out, and the input goes through a low loss filter and into the mixer. For good spectrum cleanliness, mixers should be run to no more than -20dBm, so a +6dBm 1dB compression point on the output of the preamp is reasonable, its harmonics should be around 60dB down, and third order intermod products around 90dB down.

So your amplifier seems OK, and was actually designed with a purpose in mind.

You could attach it between a spectrum analyser and an antenna and it would allow you to see about 10dB further down. These things were often used in EMC measurement facilities with broadband antennae, but they needed preselector filters to enable them to meet the requirements for overload in the presence of pulse-type interference. R&S did a far better job in this area than HP.

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Old 5th Jun 2020, 1:12 pm   #4
Skywave
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Default Re: R.F. pre-amp.: assessing dynamic range

Gentlemen: thank you.
I cannot recall from where or when I acquired this item: I've owned it for decades. It hasn't seen a great deal of use for that time period.

Z-matching: by using the technique of adding another known 50 Ohm R.F. load in parallel to the required 50 Ohm load to the sig. gen., its O/P Z is confirmed as being 50 Ohms: measured O/P drops by half. Same procedure for the HP pre-amp.: same result. All coax cables & adapters used for interconnects were also known to be 50 Ohms.

The gain of the pre-amp. was measured as 20 dB at 10, 50 and 100 MHz. Obviously all measurements were subject to the usual errors, such as reading meter indications, etc., but overall this pre-amp does appear to be O.K. as regards gain and dynamic range. Its minimum and maximum input for 'reasonable linearity' is approx. 1 mV and 35 mV, corresponding to 10 mV and 350 mV O/P with a 50 Ohm load. Now I do not know what the internal PSU d.c. voltages are for the integrated amps. that are in this pre-amp. If they are at least +5v and -5v., I would have expected a higher O/P than 350 mV, considering that 350 mV across 50 Ohms is about only 2.5 mW.

So to conclude: this item is performing to spec.; thanks for your help.

Al.
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Old 5th Jun 2020, 1:19 pm   #5
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Default Re: R.F. pre-amp.: assessing dynamic range

You can also crudely test the lower limit for the noise performance. If your amplifier has 20dB gain and a typical noise figure of 5dB and a 3dB BW of 700MHz then it will generate a reasonable amount of noise power over this bandwidth. In a 50R system the voltage for this would be in the ballpark of 200uVrms.

If you terminated the input of the amplifier in 50R and fed the output to the wideband mV meter (terminated in 50R) it might be able to display about 200uVrms for the noise voltage. I've never used a TF2603 so I don't know if this is possible. It would typically be visible as a slight increase in the residual noise level seen on a typical scope of similar bandwidth on the 1mV/div setting when the scope input is terminated in 50R.

See below for a quick and dirty excel spreadsheet that you may find interesting to play with. It may contain nuts as I have just typed it in quickly.
There's also a screenshot below to show what the spreadsheet looks like.
Attached Thumbnails
Click image for larger version

Name:	Noise Voltage.jpg
Views:	41
Size:	55.0 KB
ID:	207736  
Attached Files
File Type: xlsx Noise Voltage.xlsx (10.5 KB, 30 views)
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Last edited by G0HZU_JMR; 5th Jun 2020 at 1:33 pm.
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