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Author Topic: Preamplifier limitations  (Read 2925 times)
WD4ELG
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Posts: 863




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« on: December 28, 2012, 09:45:24 PM »

OK, back to basics.

Preamplifier purpose - A preamplifier is required to boost the desired signal power while adding as little noise and distortion as possible so that the retrieval of this signal is possible in the later stages in the system.  http://www.rfglobalnet.com/doc.mvc/Preamplifiers-And-System-Noise-Figure-0001

So for a pre-amp, one would look for a low noise figure (lower is better), and a good Third Order Intercept value (higher is better). http://www.w8ji.com/pre-amplifiers.htm

Question: what is the limitation of pre-amp use?  For example, why not use 2 pre-amps in sequence to really bring up the weak signals...say on a beverage antenna?  At what point do I start raising the noise floor and decrease the effectiveness of the preamp?  Or is this on a case-by-case basis that must be measured for each antenna installation and for each rig?
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WB6BYU
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Posts: 13045




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« Reply #1 on: December 28, 2012, 10:05:42 PM »

When the noise figure of a receiver is higher than the background noise
picked up by the antenna, a preamp will help (assuming it has a lower
noise figure than the receiver does.)

When the received signal is already limited by back ground noise picked
up by the antenna, a lower noise figure won't help you, because the
receiver noise is no longer the limiting factor.

Over much of the HF range, with a good antenna, the background
noise will override the receiver noise.  A preamp can be useful when you
are using a lossy receive antenna, such as a small loop or beverage.
The easy check is this:  if the received noise with no signal increases
when you connect an antenna to the receiver, it can hear the background
noise, so no improvement to the receiver will improve the signal-to-noise
ratio.  If you plug in the antenna and there is no increase in received
noise, then a preamp may help. 
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KZ1X
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Posts: 3227




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« Reply #2 on: December 28, 2012, 10:31:44 PM »

Let's say a common small signal amplifier (useable as a preamp) is set up in a lab, connected to a calibrated good quality RF sinewave signal generator.  

The amplifier is configured for 50 ohms impedance in and out, matching the specs of the signal generator.

The gain of the preamp is set to be 20 dB.  You power up the generator and the amplifier.

The signal generator is set to put out a signal of -70 dBm; in this example ... by the way, this is a tiny voltage ...  0.0000707 VRMS !  (70.7 microvolts) but would be a fairly strong signal for most receivers.

The output of the lab example preamp is put into a spectrum analyzer, also having a 50 ohm impedance.

The analyzer shows a 20 dB stronger signal ... -50 dBm.  Voltage-wise, this is now 0.000707 V RMS.  Good; we've confirmed the preamp is working to spec.  In the lab, anyway.

You can see where this is going ... if you kept adding amplifier stages, wouldn't you eventually be able to generate tremendous amounts of power?  

Well, perhaps, but you would also not be talking about small-signal amplifiers anymore!  Power levels, noise, distortion, and linearity issues become readily apparent variables to deal with ... quickly.

In the lab example, it may or may not be true that adding another identical preamp in series with the first one would result in a signal of -30 dBm.  

There would be many factors to consider, beyond the scope of this simple example.  Amplifiers of this sort are definitely NOT created equal!

And, in the real world, a preamplifier generally doesn't have the luxury of amplifying just one signal like the one in the lab example above.  It is usually faced with seeing 'all' the radio signals an antenna might supply to it.  This is often a complex stew of RF energy and how the amplifier responds to such is often the subject of a good bit of RF engineering.

You asked about raising the noise floor.  That's an interesting additional discussion.  Question:  wouldn't a suitable antenna preamplifier, without any input filtering, not raise the entirety of input energy by the same amount, whether that energy was desired signal or noise?
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WD4ELG
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« Reply #3 on: December 28, 2012, 11:20:39 PM »

WB6BYU/Dale, thanks.

KZ1M/Steve, thanks very much for that analysis.  Helps put clarity to it all!

 It is late and my brain is not working, but I will take a stab at the additional discussion point about raising the noise floor...if I remember correctly from analog circuits about 20 years ago, the nature of the noise is (normally) more broadband than the signal in question (unless the noise is a point source).  So the preamp *should* be increasing the signal to noise ratio...because the noise is spread out over the spectrum.  Something in the cobwebs of my mind is recalling some formula to the power of 2, and it involves the passband bandwidth in that formula.  (I really need to dust off those books from 20 years ago!)

OK, Steve...I probably just blew that question away and got it wrong.  And embarassed myself in the process.  Such is life and hamming at 2AM local!  But 160 is wide open to EU, so it's not all bad!   Smiley
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G3RZP
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Posts: 4401




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« Reply #4 on: December 29, 2012, 01:27:30 AM »

If the noise figure of the preamp is lower than that of the rx and the gain is high enough, then it will bring signals out of the noise. But it runs the rx into greater dangers of overload and intermodulation. So you ideally want the lowest possible gain prior to the selectivity determining stages while still getting adequate sensitivity.

As 'BYU says, at HF, with a modern rx, a preamp is unlikely to help - in fact, some rigs enable the operator to switch off a pre-amp to reduce gain. With my FT102, and a ' rural' location by ITU standards, on 7 MHz and below, I don't use the pre-amp and frequently use 10 or 15 dB of attenuation in front of the rx. If you can disconnect the antenna and the noise goes down, a preamp won't help. At the quietest, I get over half a microvolt  of noise on 7 MHz.

With old boat anchor receivers, it MAY help, depending on the boat anchor. In some cases of boat anchors, above 10 MHz a preamp with two tuned circuits will help performance by improving image rejection as well as giving extra gain.
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G4IJE
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Posts: 240




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« Reply #5 on: December 29, 2012, 03:29:46 AM »

With respect to 'RZP and 'BYU, I would slightly modify their suggestion by saying you should switch between a 50 ohm resistive load and the antenna. Listen with a sensible SSB bandwidth selected and any DSP options switched OFF. If you hear more noise from the antenna than from the dummy load, you don't need a preamp.
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G3RZP
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Posts: 4401




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« Reply #6 on: December 29, 2012, 05:03:33 AM »

It's generally considered that an HF rx doesn't need a noise figure lower than 10dB and generally, 15dB is adequate.

Switching to a 50 ohm source is desirable, but for most receivers, not necessary for a quick check. But some get unhappy with an open circuit input.

However........if there are intermod or phase noise problems, these can hide the effect of a poor noise figure, although this is usually only on the bands below 10MHz. This can be checked by switching off the AGC, adjusting the manual gain for  fairly low output on the noise while tuned to a clear channel - say 50mW - and then  put a 3dB attenuator in series with the antenna. The AF output should drop by 3dB - if it drops more, then you've got IMD problems.
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KE3WD
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Posts: 5694




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« Reply #7 on: December 29, 2012, 09:04:19 AM »

Simply put, each preamp in series with another must also amplify the thermal noise of the preamp before it.  

And noise is always referenced as a ratio of Signal/Noise.  

And additional thought, just presenting more gain to the input of a receiver may not be the panacea it first appears.  For example, you may encounter overloading and/or more adjacent channel interference when doing it.  

It is important to not allow yourself to become myopic about any one spec here.  They all must work and play well together.  We get high performance by tweaking each associated stage of the thing a little, rather than any one parameter a lot. 

But worth experimenting with, nonetheless, for the learning's sake IMO.


73
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WB6BYU
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Posts: 13045




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« Reply #8 on: December 29, 2012, 09:24:36 AM »

Quote from: WD4ELG

...So the preamp *should* be increasing the signal to noise ratio...because the noise is spread out over the spectrum...



That might be true if the preamp is setting the receive bandwidth - that is, it has a
narrower bandwidth than the receiver does.  But that is rarely the case:  the receiver
might have a 2kHz bandwidth for SSB, and the receive preamp probably has a 500kHz
bandwidth (so you don't have to readjust it as you tune across the band.)

You can still improve the signal-to-noise ratio by narrowing the bandwidth (switching
to 500Hz for CW, for example) but you do that in the receiver, not the preamp.
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WD4ELG
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Posts: 863




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« Reply #9 on: December 29, 2012, 09:42:18 AM »

Thanks for the guidance.  This helps a lot.  My objective is to bring the weak signals out of the noise on 160, but the preamp is clearly not the "silver bullet" solution for this issue.  Unless I was using beverage antennas (I am not), then a pre-amp for 40/80/160 does not seem to make much sense.
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G3RZP
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Posts: 4401




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« Reply #10 on: December 29, 2012, 09:57:41 AM »

Exactly, ELG.

Indeed, an attenuator may be more use if there's any IMD problems, and it's quite likely that depending where you live and your equipment, a high pass filter cutting off below 1750kHz could help.
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KE3WD
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Posts: 5694




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« Reply #11 on: December 29, 2012, 01:19:58 PM »

If its Top Band, been my experience that filtering out those strong AM Broadcast stations nearby almost always makes a difference. 

Well, maybe not using a compromise antenna along with the above...

But think about this long and hard, Top Band users:  All those AM b'cast stations are using a compromise antenna anway.  Check out the length of the tower used as vertical to see what I mean.  Some are down around 1/10th wavelength.  Yet can often be heard from rather long distances at night when condx are right. 


73
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AA4HA
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Posts: 1378




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« Reply #12 on: December 29, 2012, 02:12:19 PM »

A neat amplifier is the "Norton amplifier". I ran across it while reading about pennant antennas back when Dallas Lankford still left his stuff available for us to read. One commercialized version of the amp can be found here;

http://www.cliftonlaboratories.com/norton_amplifier.htm

Some other really neat low noise amps are based upon the N Channel JFET, U310;

http://www.ham-radio.com/n6ca/50MHz/50appnotes/U310.html

Or for something that can work up into the VHF/UHF. The PHEMT GaAs FET's;

http://www.anglelinear.com/gaasfet/2x_phemt.html

Some of the specs on PHEMT GaAsFET's are incredible;

Quote from web site:
"Noise figure is typically 0.5 dB below 500 MHz, 0.6 dB to 750 MHz and 0.7 db to 1 GHz.

Gain is typically: 37 db at 300 MHz; 34 db at 400 MHz; 32 dB at 500 MHz; 30 db at 600 MHz; 27 dB at 800 MHz; and 26 dB at 950 MHz. Input and output return loss are typically greater than 10 dB."[/i]

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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
AC5UP
Member

Posts: 3825




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« Reply #13 on: December 29, 2012, 03:14:34 PM »

If its Top Band, been my experience that filtering out those strong AM Broadcast stations nearby almost always makes a difference.

This is an important consideration in that building a good preamp for low band use is not that hard today... There are low cost, low noise op-amps that are stable up to 4 MHz and offer 100 dB of clean gain. Where the difficulty appears is when there's a local big signal AM BCB station that drives the preamp into non-linear overload (AKA: Never-Never Land) and you start hearing NewsTalk 1320 all over the dial.

I'm enough of a purist to say that a BC-348 is groovy enough as a BC-348 since part of the Boatanker Mystique involves the vintage performance spec that comes with it. If I was serious about preamping one I'd go light on the gain and give serious consideration to a preselector up front. Harder to build, one more knob to adjust, but that's the way it was done back in the day and there's a good reason for it... Anything that reduces broadband noise reduces noise.

And noise isn't what we're listening for.   Tongue

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WB2WIK
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Posts: 20547




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« Reply #14 on: December 29, 2012, 04:03:00 PM »

Thanks for the guidance.  This helps a lot.  My objective is to bring the weak signals out of the noise on 160, but the preamp is clearly not the "silver bullet" solution for this issue.  Unless I was using beverage antennas (I am not), then a pre-amp for 40/80/160 does not seem to make much sense.

Wait a sec.  You don't need a Beverage, necessarily, to improve S/N on 160m -- although that's a common approach.

The problem on 160m is usually not so much that signals are so weak, but noise is stronger.  So anything you can do to reduce noise slightly more than reducing signals works. 

I don't have room for Beverages here, but a simple receiving loop, which has probably 30 dB loss, often works just fine and doesn't take up much space at all: Mine is only about 42" in diameter, and rotatable (it's very directional).  Because it's so lossy, a preamp helps a lot...the only goal is for signals to be stronger than noise, so it doesn't really matter if you lose 30 dB of signal strength, if you coincidentally reduce noise by 36 dB: You just gained a 6 dB improvement in what you can hear.
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