Hi,

Was wondering if a wideband preamp might have a better noise figure if
its input impedance is greater than the antenna's. It also has a
couple of head to tail 1N4148 diodes across the input for protection.
Would removing the diodes extend the bandwidth or is the extra 8 pF
stray diode capacitance become lumped in with the coax capacitance?
Surely the preamp input has some capacitance too, so how does this
affect the voltage delivered? Please, answer at your convenience.

A*s*i*m*o*v

.... Ok, I pulled the pin. Now what?.....Hey! Where are you all going?

It also has a couple of head to
tail 1N4148 diodes across the input for protection.

What those diodes will do is make horrible intermod if you've got a
transmitter anywhere nearby. Especially if it's wideband with no
filtering on the input.

Was wondering if a wideband preamp might have
a better noise figure if its input impedance is greater than
the antenna's.

Freaking around with impedance mismatches just to get a better noise
figure will only work for marketroids. Has no benefit and only
penalties in real life.

To quote H&H: "Big fallacy: don't try to improve things by adding a
resistor in series with a signal source to reach a region of minimum
NF. All you're doing is making the source noisier to make the
amplifier look better!"

Most RF amps spec noise figure for Z_in = 50 ohms and don't even try
marketroid stuff anymore.

Tim.

On Thu, 29 Sep 2005 13:35:17 GMT, "Asimov"
wrote:

Hi,

Was wondering if a wideband preamp might have a better noise figure if
its input impedance is greater than the antenna's. It also has a
couple of head to tail 1N4148 diodes across the input for protection.
Would removing the diodes extend the bandwidth or is the extra 8 pF
stray diode capacitance become lumped in with the coax capacitance?
Surely the preamp input has some capacitance too, so how does this
affect the voltage delivered? Please, answer at your convenience.


The diode capaitance is low and really not a factor in preamp gain or
noise. They are non conducting and the capacitance is easily absorbed
into fees and matching systems.

The exact match for any preamp to antenna is a complex as it depends
on device, circuit topology and frequency. Generally with most fets
the ideal transducer impedence for best gain and low noise is higher
than 50 ohms. For most bipolar transistors it's less than 50 ohms.
Getting a reasonable match and incurring low losses in that matching
network are challenges. Every DB of loss at the input contributes
negatively to system noise figure. This includes feedline [coax]
between the amplifier and the antenna.

With all that... Wide band amps generally are of the feedback type
and unless they are of the Norton topology they are noisy. Added to
that is wideband noise has more total power than a narrow band
segment of noise fro the same source. So it goes to say that narrow
band amplifiers generally have the lowest noise.

For HF this is rarely a real problem as most amps have low enough
noise to be useful as the environment is very noisy. As we
transistion to low VHF and higher this case changes greatly. It's
also at VHF and higher that low noise amplification starts to become
significant and more difficult to achieve. Modern high performance
devices however may make this appear simple. For example a wide
band mimic MAR-6 achieves 20db gain 3db noise figure to upper UHF.
However 3db is only ok, as a good Gasfet at 430mhz will be less than
..8db noise figure and even fair number of cheap bipolars can achive
sub 1db with ease.

So it's really an issue of what noise figure you really need and what
bandwidth you need it to cover. For ham purposes this is an issue
for practical consideration for 10m and above weak signal work or
space comms such as EME. For serious weak signal work in the
VHF/UHF realm you have to look at the antenna system and feed
system as part of that noise figure.

Allison
KB1GMX

NO NO NO NO NO...............

LISTEN to TIM!!!!!!!!!!!!! Tim is correct, you are WRONG.

While the diodes might be benign in the presence of small and medium
sized out of band signals, they do exactly what Tim said in the
presence of large input signals like you might find near transmitters.
Remember, the 200 KW FM broadcast station at the end of the block, or
the 5 KW AM broadcast band transmitter down the road from you. Even
though these signals are no where near the frequency you are trying to
listen to, the diodes will see them and act as a non linearity and
produce IMD all throughout the radio spectrum.

Remember, these diodes are not protected by a frequency selective
tuned circuit.....so any big signal that comes down the antenna is
gonna make a problem for you , especially if you're trying to listen
to a weak signal.

Tim is correct.

Years ago I was working for a tv station as a chief engineer. I had to
oversee the installation of the new 900 foot tower and make sure the
tower monkeys aligned the dish properly for the STL. So, I spent quite
a few days at the site. The guys putting up the antenna had just
purchased new ICOM HT's and had them modified for out of band
operation so they could use them for communication between the tower
people and the ground crew running the wench.

I told them I had the same radio as they did, (which was true). When I
showed them my radio, there were no odd audio sounds coming out of the
speaker. When they turned on their radios, all they could hear was the
overload from the FM station sharing the same tower.

The owner of the tower company was a ham and modified the ICOM's with
back to back diodes at the antenna to keep the front ends from blowing
out because they often worked within 10 or 20 feet of big powerful
antennas. He told me without the diodes he added, he would lose one
radio per day to rf overload and he used to carry spare front end
transistors. Part of his evening ritual was to sit down and remove the
blown front end transistors and replace them. He had done it so many
times he could do it in 5 minutes! But, after he put the diodes in,
the front end didn't blow anymore....but they always had interference
from other transmitters as a result of the diode mod!

He learned the hard way not to introduce non linearities before the
front end tuned circuits! I think you need the same lesson Allison.

I took one of his radios home that evening and modified it. I
basically removed the gasfet front end transistor and threw his back
to back diodes in the trash. I added a 10 db pad to attenuate the
incoming signals across the entire spectrum. They spent about 3 weeks
doing the tower job, and by the end of the job, I had modified all his
radios. They never had another interference problem and they never
lost a single front end. The owner was a good guy and offered to pay
me dearly for tweaking his radios.

With all due respect, the diode capacitance is IS NOT the problem.
It's the non linearities associated with the rectification in the
presence of big out of band signals.

Listen to Tim, he's telling you the truth!

T


The diode capaitance is low and really not a factor in preamp gain or
noise. They are non conducting and the capacitance is easily absorbed
into fees and matching systems.

I put your tirade on the end of this as it's not relevant to the
orininal question.

First: the question, reread please. The question was a about
bandwidth of a wide band amp and noise figure. Overload performance
was NOT addressed. If it were the first thing would be to rid oneself
of those diodes. A good device will usually withstand more than the
diodes (U310 fet in common base amp I've used for Preamp took 25watts
before expiring as a test!).

However I do not need you lesson on that either. I design and build
both professionally and as a ham VHF op. My experience includes one
idiot that runs a KW onto a M2 7 element 800ft from me to work local
repeaters! On an average day I have between +10-20DBm of rf comming
fown the coax when hes operating. I know more than you suspect or
even checked about working in a RF rich environment, about 35years of
experience in landmobile plus commercial.

Now if you can address the question of amp bandwidth and noise figure
and diodes we can help this person. NOT drag the topic off into the
woods.

Further I happen to have posted at the same time as Tim and was not
contradicting him or addressing him. I was answering the author of
the thread.

Allison



On Sun, 02 Oct 2005 15:30:52 -0400, TRABEM wrote:

NO NO NO NO NO...............

LISTEN to TIM!!!!!!!!!!!!! Tim is correct, you are WRONG.

While the diodes might be benign in the presence of small and medium
sized out of band signals, they do exactly what Tim said in the
presence of large input signals like you might find near transmitters.
Remember, the 200 KW FM broadcast station at the end of the block, or
the 5 KW AM broadcast band transmitter down the road from you. Even
though these signals are no where near the frequency you are trying to
listen to, the diodes will see them and act as a non linearity and
produce IMD all throughout the radio spectrum.

Remember, these diodes are not protected by a frequency selective
tuned circuit.....so any big signal that comes down the antenna is
gonna make a problem for you , especially if you're trying to listen
to a weak signal.

Tim is correct.

Years ago I was working for a tv station as a chief engineer. I had to
oversee the installation of the new 900 foot tower and make sure the
tower monkeys aligned the dish properly for the STL. So, I spent quite
a few days at the site. The guys putting up the antenna had just
purchased new ICOM HT's and had them modified for out of band
operation so they could use them for communication between the tower
people and the ground crew running the wench.

I told them I had the same radio as they did, (which was true). When I
showed them my radio, there were no odd audio sounds coming out of the
speaker. When they turned on their radios, all they could hear was the
overload from the FM station sharing the same tower.

The owner of the tower company was a ham and modified the ICOM's with
back to back diodes at the antenna to keep the front ends from blowing
out because they often worked within 10 or 20 feet of big powerful
antennas. He told me without the diodes he added, he would lose one
radio per day to rf overload and he used to carry spare front end
transistors. Part of his evening ritual was to sit down and remove the
blown front end transistors and replace them. He had done it so many
times he could do it in 5 minutes! But, after he put the diodes in,
the front end didn't blow anymore....but they always had interference
from other transmitters as a result of the diode mod!

He learned the hard way not to introduce non linearities before the
front end tuned circuits! I think you need the same lesson Allison.

I took one of his radios home that evening and modified it. I
basically removed the gasfet front end transistor and threw his back
to back diodes in the trash. I added a 10 db pad to attenuate the
incoming signals across the entire spectrum. They spent about 3 weeks
doing the tower job, and by the end of the job, I had modified all his
radios. They never had another interference problem and they never
lost a single front end. The owner was a good guy and offered to pay
me dearly for tweaking his radios.

With all due respect, the diode capacitance is IS NOT the problem.
It's the non linearities associated with the rectification in the
presence of big out of band signals.

Listen to Tim, he's telling you the truth!

T


The diode capaitance is low and really not a factor in preamp gain or
noise. They are non conducting and the capacitance is easily absorbed
into fees and matching systems.

On Mon, 03 Oct 2005 01:09:33 GMT, wrote:

I put your tirade on the end of this as it's not relevant to the
orininal question.


The original post (by A*s*i*m*o*v) mentioned back to back diodes as
protection, so my reply was quite relevant.

Not sure what ruffled your feathers so badly Allison. But, whatever it
was, I am sorry if I did something that didn't meet with your
approval.

I also worked professionally as an avionics tech, and was in the
business when the first Motorola commercial repeaters using
synthesizers were put up...and they wondered why the other repeaters
on the same tower couldn't hear well. That only took 'em about 10
years to figure out. Then, about 10 years after that, they started
using gasfets, and it took them another 10 years to figure out they
didn't do well in high rf fields. By then, it was the 1980's.....but
at least they got it right eventually.

Maybe you designed some of those early nightmarish radios yourself?

Anyway, I wish you luck. And hope you realize the importance of
keeping the front end clean, which sometimes means dumping the gasfet
and using a bipolar preamp instead.

Actually, I hope the original poster (A*s*i*m*o*v) got the message
about introducing gross non linear back to back diodes across the
antenna terminals.

Have fun.

T

On Mon, 03 Oct 2005 00:13:54 -0400, TRABEM wrote:

On Mon, 03 Oct 2005 01:09:33 GMT, wrote:

I put your tirade on the end of this as it's not relevant to the
orininal question.


The original post (by A*s*i*m*o*v) mentioned back to back diodes as
protection, so my reply was quite relevant.

If it was about overload yes. It was about bandwidth and noise.

Not sure what ruffled your feathers so badly Allison. But, whatever it
was, I am sorry if I did something that didn't meet with your
approval.

You really came in with and attitude. but completely missed the real
subject of the question.

I also worked professionally as an avionics tech, and was in the
business when the first Motorola commercial repeaters using
synthesizers were put up...and they wondered why the other repeaters
on the same tower couldn't hear well. That only took 'em about 10
years to figure out. Then, about 10 years after that, they started
using gasfets, and it took them another 10 years to figure out they
didn't do well in high rf fields. By then, it was the 1980's.....but
at least they got it right eventually.

We are talking presynth. I had my First Class before that.
My first commercial UHF repeater was built from U44 components then
later Motrac as they were current radios For 1971-2 that was pretty
good. The motrac front ends were 6 cavitys a mixer then IF
selectivity, crunchproof. LO was always a crystal for least phase
noise. Anyone that read papers by Rhode understood this in the
early to mid 70s.

Maybe you designed some of those early nightmarish radios yourself?

I take that as insult. Gads no, I thought they were crap too.

I'm not all that thrilled with the ARC308C or worse 324 and the put
them in planes. I know that as I have a 308 as backup. Gag, Ick,
ick. but at 1500 to 2k for a new one I'll keep it till it quits.

Anyway, I wish you luck. And hope you realize the importance of
keeping the front end clean, which sometimes means dumping the gasfet
and using a bipolar preamp instead.

Your being pendantic again.

I'm not big on gasfets untill I get above 1GHz. At 6m a u310 bipolar
fet in common base gets me about 12db. With a good IP and noise
figure. Just enough to get past the image stripping filter before the
Level-7 DBM. That makes for a nice crunchproof frontend that with
sub 3db noise figure. If I could get that noise figure without the
fet I'd not use it but I need the gain. I've tried mrf571 in a norton
push pull configuration, hard to overload but the 100ma standing
current was hard to take for portable ops!

If your interested, ops here are QRP weak signal 6m SSB and 2M ssb
and occasionally CW using solar power and battery. Power consumption
is a consideration as well as perfomance. Overload performance with
a KW 800ft away and in band is well challenging. The challenge is to
get enough gain to ovecome the mixer noise without having a watt of
power at the mixer.

Actually, I hope the original poster (A*s*i*m*o*v) got the message
about introducing gross non linear back to back diodes across the
antenna terminals.

That is true. However think about it for more than a second. Diodes
or not hes talking about gain, bandwidth and noise so even without
the diodes hes already running wide and risking overload at the next
stage. Without selectivity you get cascade overload. The classic
problem of weak reciever fix of stacking gain before it and watching
the input overload threshold wander in to the -40dbm (or worse)
region. Bad way to fix the problem.

Allison
KB1GMX

"TRABEM" bravely wrote to "All" (03 Oct 05 00:13:54)
--- on the heady topic of " Preamp"

TR From: TRABEM
TR Xref: core-easynews rec.radio.amateur.homebrew:88137

TR On Mon, 03 Oct 2005 01:09:33 GMT, wrote:

I put your tirade on the end of this as it's not relevant to the
orininal question.


TR The original post (by A*s*i*m*o*v) mentioned back to back diodes as
TR protection, so my reply was quite relevant.
[,,,]
TR Actually, I hope the original poster (A*s*i*m*o*v) got the message
TR about introducing gross non linear back to back diodes across the
TR antenna terminals.

TR Have fun.

I have followed both your points with great interest. My guess is that
the diodes are benign in a weak signal area. I expect the original
design used them more for static rather than for overload protection
because the input was intended to expect microvolt levels.

My choice would be not to use anti-parallel diodes in a design but this
is what I found in the circuit so some engineer thought about it enough
to use them. I would have preferentially used a reverse biased diode
across the active device's junction because it is usually the reverse
polarity in a pulse that damages a junction.

In this question I was more interested on the effect of the protection
diodes' capacitance because as you know it is greatest at zero bias.
This wideband pre-amp's input stage uses feedback to define the input
impedance and the diodes' capacitance is at the node point, so I
wondered if there was little effect. As you know, in an inverter the
node point is ideally a voltage null.

Thank you both for the great debate!

A*s*i*m*o*v

.... As I suspected, you're a rank sentimentalist! --Cpt. Louis Renault

On Mon, 03 Oct 2005 16:11:05 GMT, "Asimov"
wrote:

My choice would be not to use anti-parallel diodes in a design but this
is what I found in the circuit so some engineer thought about it enough
to use them. I would have preferentially used a reverse biased diode
across the active device's junction because it is usually the reverse
polarity in a pulse that damages a junction.

Generally a good design and a robust device will be tougher than the
diodes frequenty used.

For example I've found JFETS will easily stand 40+volts on the gate
before failing, MOSFETS (dual gate 3n200 types and BFR998) 60-80V
were needed to kill them. Even a lowly 2n5176 bipolar will be grossly
overloaded in a wide band amp before possibility of destruction.

Remember .5V PP (I'll assume silicon diodes) is at or around -1 to
0DBm. Thats one milliwatt of RF. the real issue is with wide band
signals you can have multiple signals within the bandwidth that at
some point in the cycle are additive and as we've all said those
diodes will do bad things under those conditions.

In this question I was more interested on the effect of the protection
diodes' capacitance because as you know it is greatest at zero bias.
This wideband pre-amp's input stage uses feedback to define the input
impedance and the diodes' capacitance is at the node point, so I
wondered if there was little effect. As you know, in an inverter the
node point is ideally a voltage null.

The capacitance if of limited significance. Its' effects will be
tempered by topology and impedences at the point of use.
As pointed out, they must not see conduction or "bad things happen".

_IF_ you really must protect the input with diodes put a healthy
(6-12V) of reverse bias on them. That should adequately protect
the first device but still absorb transients.

For instrumentation applications (high impedence) they are a
commonplace thing as the source could be anything or known.

For RF use I tend to leave them out as most devices once in circuit
are robust as is. And usually uf there is an event big enough to fry
the device then protection diodes are just two more fried devices.

One exception: series diodes on the supply for reverse polarity
protection and Zeners(or transzorbs) on the supply for overvoltage
protection. Often destruction didn't come down the antenna but in
the back door via the power or other control lines.

Thank you both for the great debate!

As in most electronics there are subtle and gross effects. Being
aware of both are of importance. Hope it helps.


Allison
KB1GMX