I'm hoping that we've got a few RF engineers floating around that can help
me out with this. I'm a digital systems type person, myself, but I'm
working in a place that needs me to do work on RF circuitry, so I'm a bit
stuck.
I have a relatively good hybrid coupler that was built as a power splitter.
It has -41dB isolation and -3.08dB ouput ports.
I want to use it to measure scattering parameters of a preamplifier but I
don't know how to connect it. I know that the HP transmission/reflection
test set, which I am trying to emulate with this device, is a 4-port unit,
and that 2 of the ports go to a network analyser input, 1 port that get RF
in, and 1 port that goes to the DUT. I do not, however know what the 2
ports that go to the inputs are.
My directional coupler has the following ports: In, 50ohms, 0 degrees and 90
degrees. What ports do I connect to the network analyser?
I assume that the IN port gets the RF out from the network analyser, and
that the 50-ohm port goes to one of the inputs (but don't know which one),
but I don't know whether the 90-deg or the 0-deg port goes to the DUT.
I, uh, have always used 'directional couplers'
to sample the RF in each direction in order
to test and characterizs devices.
Have you got some new technique that you plan to use
the hybrid in to do the same thing?
As to 'four ports' on a HP transmission/reflection
test unit -
- the old 8475A "S-Param Test Set" I employ has five RF
'ports':
1) One RF input Type-N. This must actually deliver RF to both
the 'Reference' input channel on the 8410B/8411A as well as
RF to the 'test' channel through a suitable a RF 'splitter'.
2) Two ports (APC-7)to the 8410B/8411A harmonic converter
(which is basically a two-channel amplitude and phase
measuring 'receiver').
These are outputs (I have to presume) are from directional
couplers that a) sample the RF from an RF signal generator
and also sample the RF on the 'test' channel as per the
S parameter to be measured.
3) Two (APC-7) ports to the DUT for transmision (S21) and Refl (S11).
Internal directional couplers in the 8475A do the 'magic' in
sampling the RF to/through/reflected from the DUT and direct this
RF to the 8410B/8411A's 'Test' channel.
I use I only deal with three ports - the RF input port and the
two APC-7 ports to the DUT. The 8410B/8411A harmonic converter
'bolts' into position in an area aboard the 8475A S-Parm Test
Set.
This is rather old gear today - but the basics are there!
Brendan,
I have never heard of using a 90 degree hybrid for making S
parameter measurements. They are usually used for comparing
impedences. I don't see any practical way to do it.
Lacking a proper S parameter test set, as described by another
poster, the hook-up for using a separate power splitter and
directional coupler is as follows:
The RF output from the network analyzer goes to the input of the
power splitter. One output from the power splitter goes to the
Reference input of the analyzer. When measuring an active device,
such as your preamp, the other output from the power splitter goes
through an attenuator to the input of the directional coupler and the
non-coupled output goes to the input of the DUT. The coupled
output of the directional coupler goes to either the A or B input on
the analyzer. The attenuator is to reduce the input voltage so it will
not overload the DUT while still providing sufficient Reference signal
for the analyzer. The other, A or B, input of the analyzer goes to
the output of the DUT, might require an attenuator depending on
the gain of the device. This set-up will measure S-11 and S-21, to
measure S-12 and S-22 reverse the DUT. Make sure there is no
DC voltage on the input and output connectors of the DUT, if there
is, a DC block or bias T must be used to isolate it from the
analyzer detectors.
HTH,
Bill, W3NMK
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Thanks Bill & Jim
I guess I assumed that I had something that might work, but when I tried to
find info on using directional couplers, I pretty much came up blank. So,
how does a directional coupler for low RF (10-20MHz) actually work?
I've got directional couplers that are 'effective'
as low as 100 MHz - amd as most of my work in
the past has dealt with this range I've been
okay.
I've got some on-going work (a filter) in the range
of .5 to 5 MHz right now.
I am able to measure S21 (Insertion Loss) using a
spectrum analyzer and signal generator, but shortly
I've got to measure S11 (Return Loss or 'VSWR')
as well.
I'm going to use a crude 'bridge' in combination with
a dual-trace scope or a Vector Voltmeter to accomplish
this in a round-about fashion (measure complex Z and
then compute the Return Loss/S11).
Sucks, eh? For the same project, I've had to design and build a preamp with
a 0.5dB NF and a gain of >= 26dB. Ended up using components with a minimum
frequency of 450MHz. They work though... I just have to fix the matching
now.
Isn't your ambient noise (atmosphereic, man-made, etc)
going to swamp your preamp nose by a good margin?
(Maybe this isn't for use in free-space, eh?)
BTW - if you have to 'fix the matching' - that will,
in most designs, also affect the 'noise'. There is
something called 'best noise match' and it doesn't
always follow the best 'match' or gain point ...
> .5 dB NF?
>
> Why all that trouble for HF?
>
> Isn't your ambient noise (atmosphereic, man-made, etc)
> going to swamp your preamp nose by a good margin?
>
> (Maybe this isn't for use in free-space, eh?)
All goes into a shielding room.
> BTW - if you have to 'fix the matching' - that will,
> in most designs, also affect the 'noise'. There is
> something called 'best noise match' and it doesn't
> always follow the best 'match' or gain point ...
Yeah, I know. However, since there are no s-parameters available for these
parts at this frequency, I'm pretty much up the creek until I can get some
way of measuring s-parameters (I'd just do a straight impedance measurement,
but it's not as reliable), though the interesting thing is that the current
setup is doing adequately without a proper match. I have the feeling that a
good match could give me a few dB extra to play with.
The directional coupler that I use is a KDI Electronics model
DCK-1010. It has 10 dB coupling and is rated for use from
1.0 to 1000 MHz. Minicircuits ( http://www.minicircuits.com)
makes a line of very inexpensive directional couplers that have a
frequency range from 0.2 MHz up with various coupling coefficients.
They also make a line of inexpensive power splitters for the same
range of frequencies. They also have application notes on their
Web site that explain the design and use of directional couplers
Bill, W3NMK
> I guess I assumed that I had something that might work, but when I tried to
> find info on using directional couplers, I pretty much came up blank. So,
> how does a directional coupler for low RF (10-20MHz) actually work?
>.5 dB NF?
>
>Why all that trouble for HF?
>
>Isn't your ambient noise (atmosphereic, man-made, etc)
>going to swamp your preamp nose by a good margin?
>
>(Maybe this isn't for use in free-space, eh?)
>
>BTW - if you have to 'fix the matching' - that will,
>in most designs, also affect the 'noise'. There is
>something called 'best noise match' and it doesn't
>always follow the best 'match' or gain point ...
May I ask your opinion on adjusting input match for best S/N using a
noise bridge and examining the output only (i.e. no expensive equipment) ?
>10 - 20 MHz?
>
>Ouch!
>
>I've got directional couplers that are 'effective'
>as low as 100 MHz - amd as most of my work in
>the past has dealt with this range I've been
>okay.
>
>I've got some on-going work (a filter) in the range
>of .5 to 5 MHz right now.
>
>I am able to measure S21 (Insertion Loss) using a
>spectrum analyzer and signal generator, but shortly
>I've got to measure S11 (Return Loss or 'VSWR')
>as well.
>
>I'm going to use a crude 'bridge' in combination with
>a dual-trace scope or a Vector Voltmeter to accomplish
>this in a round-about fashion (measure complex Z and
>then compute the Return Loss/S11).
>
>http://www.dallas.net/~jvpoll/tech/ph_mag.html
Try to make a bifilar wound directional coupler on a toroid or
snap-together ferrite transformer for 5MHz. It will very likely work for
you. 30 turns at 5MHz would be too much I think. Surely you have a core
like this in the junkbox.
>Thanks Bill & Jim
>I guess I assumed that I had something that might work, but when I tried to
>find info on using directional couplers, I pretty much came up blank. So,
>how does a directional coupler for low RF (10-20MHz) actually work?
The same as for any frequency. A correctly terminated transmission line
does that. The easiest way to understand a d.c. is to imagine that it has
one line terminated with Z0 at both ends and the other is not matched and
fed by a generator. This will cause a standing wave in the unmatched line
and thus in the coupler. Examine what this causes to appear on the
termination resistors of the matched side of the d.c. assuming that its
coupling coefficient is 1. This is really used to measure SWR usually but
it makes for a good example to understand what is going on. Notice that
for a matched d.c. L/lambda is irrelevant but when it is unmatched
L/lambda influences the magnitude of the signals on the matched outputs.
d.c. for HF are usually made by winding bifilar (or more) wire on a
toroid to achieve a resonable proportion of lambda. (L is the electrical
length of the d.c. and lambda the wavelength at the frequency used).
'cause this technique works, is perfectly
valid AND works from DC to as high as your
construction techniques will allow (making
it largely 'frequency independent') ...
> "Try to make a "
>
>'cause this technique works, is perfectly
>valid AND works from DC to as high as your
>construction techniques will allow (making
>it largely 'frequency independent') ...
Hey, you're the guy who advocates buying suitable equipment from Ebay.
What would you use for this measurement ?
Peter
PS: BTW, I did 'try' and make what I proposed, for 27MHz.
I'm not going to weigh in one way or another on the directional coupler
discussion, but I will say that buying equipment from eBay is great! I've
been working on a microwave project (weather radar) and I went from having
no microwave test equipment to having everything I need for this project
(signal generator, 40W dummy load, 20W power amp, low-loss microwave coax,
power detector, and directional coupler) for only a few hundred dollars.
>Hey, you're the guy who advocates buying suitable equipment from Ebay.
>What would you use for this measurement ?
>
>Peter
>
>PS: BTW, I did 'try' and make what I proposed, for 27MHz.
>
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At 03:23 PM 7/9/02 -0400, you wrote:
>I'm not going to weigh in one way or another on the directional coupler
>discussion, but I will say that buying equipment from eBay is great! I've
>been working on a microwave project (weather radar) and I went from having
>no microwave test equipment to having everything I need for this project
>(signal generator, 40W dummy load, 20W power amp, low-loss microwave coax,
>power detector, and directional coupler) for only a few hundred dollars.
>
>Sean
Peter penned: "What would you use for this measurement ?
Upon further reflection (no pun intended) -
- an HP Vector Impedance Meter.
HP made a very nice one with a top end of 108 MHz.
The circuit I 'dreamed up' over a ten years ago
while moonlighting as a broadcast engineer in the
form of that resistive bridge was a product of
inspiration at the eleventh hour -
- as I was afraid a certain GR Z bridge I had planned
on borrowing might be not be available after all.
The GR Z bridge (or some suitable substitute) was
needed for the measurement of a most critical
parameter: the Z (impedance) as seen at the base
of 240 plus foot 1/4 wave guyed vertical for 1.06 MHz.
I had been contracted at the time to install and
tune a new ATU - Antenna Tuning Unit - at a 10 KW
AM broadcast station.
As it was - I did get to borrow the GR bridge, but
the inspired circuit *still* played a critical
part - I had to re-tune the ATU and part of the
process calls for finding the parallel resonant
point ... my "R" bridge works optimally for this
purpose - as one can easily see the phase change
most radically with frequency as one tunes with a
signal generator through the resonance point on a
tuned circuit (the ATU in this case) - at this point
the composite Z (R+jx) remains fairly constant and
causes little change in the amplitude of a signal, but
the phase stands out and identifies the resonant
point like a sore thumb.
Whilst your suggestion is most likely valid - I doubt
you can assure accurate numerical results without
some difficulty (calibration charts etc) over several
octaves (unto a decade) of bandwidth ...
>I'm going to use a crude 'bridge' in combination with
>a dual-trace scope or a Vector Voltmeter to accomplish
>this in a round-about fashion (measure complex Z and
>then compute the Return Loss/S11).
>
>http://www.dallas.net/~jvpoll/tech/ph_mag.html
Nice page.
Peter
PS: I think that if you substitute a pulse generator for the RF source you
can do cable TDR with your setup ;-)
o Hewlett Packard 4815A Vector Impedance Meter
o Measures impedance magnitude and phase
o 500 kHz to 108 MHz in five bands
o Impedance range 1 Ohm to 100 kohms
o Phase angle 0 to 360 degress in two ranges
o Internal calibration 100 ohms resistive
o Analogue outputs for magnitude and phase indications
Went for a paltry US $137.50 ...
HP 4191A RF Impedance Analyzer
1 - 1000 MHz (this instrument seems like it's more suitable
for component testing)
>As it was - I did get to borrow the GR bridge, but
>the inspired circuit *still* played a critical
>part - I had to re-tune the ATU and part of the
>process calls for finding the parallel resonant
>point ... my "R" bridge works optimally for this
>purpose - as one can easily see the phase change
>most radically with frequency as one tunes with a
>signal generator through the resonance point on a
>tuned circuit (the ATU in this case) - at this point
>the composite Z (R+jx) remains fairly constant and
>causes little change in the amplitude of a signal, but
>the phase stands out and identifies the resonant
>point like a sore thumb.
I aggree but as you would surely say, it takes cojones to do that with
what amounts to diy equipment and then turn on the juice (even slowly).
100W RF has pretty impressive built-in fault indicators afaik. 10kW from a
tube PA more so ...
>Whilst your suggestion is most likely valid - I doubt
>you can assure accurate numerical results without
>some difficulty (calibration charts etc) over several
>octaves (unto a decade) of bandwidth ...
Of course not. You need to spot-check the d.c. itself from time to time at
the working frequency. However the d.t. is a SWR meter and it takes a
couple of seconds (and four ganged good quality 100ohm cermet trimmers).
This is harder to explain than to do. I got the right kind of wire in 3
tries. The best kind of winding for wideband is with progressively more
spaced-out turns, but this does not work on a d.c. so I just put in the
turns (deliberately disordered winding).
>HP 4815A Vector Impedance Meter
>
>http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=1722056183
>
>http://www.tucker.com/NCCatalogs/SrchExp/sexpnc372.asp
>
>http://www.e-test.com/TABL1-10.HTM
>
>o Hewlett Packard 4815A Vector Impedance Meter
>o Measures impedance magnitude and phase
>o 500 kHz to 108 MHz in five bands
>o Impedance range 1 Ohm to 100 kohms
>o Phase angle 0 to 360 degress in two ranges
>o Internal calibration 100 ohms resistive
>o Analogue outputs for magnitude and phase indications
>
>Went for a paltry US $137.50 ...
>
>
>
>HP 4191A RF Impedance Analyzer
>1 - 1000 MHz (this instrument seems like it's more suitable
>for component testing)
>
>http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=1747099222
>
>www.testequipmentdepot.com/usedequipment/hewlettpackard/impedanceanal
>yzers/4191a.htm
>
>Weighing in currently at US $900
>
>
>or
>
>
>HP 4193A RF Impedance Analyzer
>400 KHz to 110 MHz (a replacement for the 4815A)
>None currently on eBay ...
>http://www.testequity.com/products/149/
>
>Jim
>
>
(Jim?) does anybody know how small a signal can be while the HP4815A still
reads right ? I.e. can it be used to measure verly low signals (say 10mV
pk-pk) and give meaningful results ?
I checked the specs for the 4815A (in the '73 catalog)
and it doesn't say - the specs for the 4193A regarding
output appears thusly:
"Test Level: constant current source"
and shows a small chart describing the RF currents used
on each.
Briefly, the max voltage appears to be 1 V RMS for the
100K Z range (given a constant RF I of 10 uA) -
- to 1 mV RMS for a current of 100 uA on the 10 Ohm Z range.
Of course, when measuring the low-level input Z of a
pre-eamp *any* setup of equipment has to be verified
NOT to be driving the pre-amp into non-linear region
of operation ...
I measured the input Z of some popular 2-Meter rigs once
upon a time over 20 years ago using an HP 8405A - and
kept the RF level as low as possible to avoid over-driving
the first RF stage. As I recall - the input match was horrid!
On the order of 10 to 11 dB RL at 146 MHz! (On a Yaesu FT-227
"Memorizer" I think it was.)
Perhaps I should refer to what I am actually trying to do here. I have this
same preamp. The input is really not much of a question since the source
for it is a >5k + j0 ohm coil. It's tough to match, but I have something
that works. Once again, magnetic components for this particular project are
a nono. It also needs to be small in size. This eliminates the use of
large value inductors, and large trimmer caps. It's the output that's
giving me grief. I need to get a measurement of the current impedance
before I can do much to fix it, since the only specs I have for the active
component are for a long way outside of my freq. range.
The test equipment I have available is as follows:
1xHP4395A network analyser
1xHP54645A DSO
1xHP43961A Impedance test set
1x Nice function generator.
1x Not so nice function generator.
1x Hybrid power splitter
With that, and something I can build easily, how can I measure s-parameters
or impedances accurately?
I forgot to mention that the circuit which Jim showed looks useful. I'm
going to try it.
> With that, and something I can build easily, how can I measure
s-parameters
> or impedances accurately?
It looks to me like you've got a lot of
what you need in that "HP 4395A network
analyzer".
I would take my active device and build a
small fixture and simply characterize it
using that HP 4395A at my frequency of
interest.
Does that particular 4395A have any options
(like the Impedance Measurement Function
and RF Impedance Test Kit)? (I don't own
one - I'm looking at it in the HP catalog ...)
Oh - I see you have the HP43961A Impedance test
set. (Too bad you don't have the S-Parameter
Test Set for the 4395A.)
With the Impedance Test Set you can get the Z
looking into a 'network' (like your pre-amp).
Now you need a test set-up to measure Insertion
Loss (and Phase change) and come up with gain
(or loss) of a network.
What are those four ports on the HP 4395A:
1) RF Output
2) R(ef) Input?
3) A and
4) B inputs?
I think you are closer than you think to a
solution!
> It looks to me like you've got a lot of
> what you need in that "HP 4395A network
> analyzer".
>
> I would take my active device and build a
> small fixture and simply characterize it
> using that HP 4395A at my frequency of
> interest.
Ah, yes, it is that fixture that seems to be the problem
> Does that particular 4395A have any options
> (like the Impedance Measurement Function
> and RF Impedance Test Kit)? (I don't own
> one - I'm looking at it in the HP catalog ...)
It has impedance analyser, spectrum analyser and network analyser modes.
> Oh - I see you have the HP43961A Impedance test
> set. (Too bad you don't have the S-Parameter
> Test Set for the 4395A.)
> With the Impedance Test Set you can get the Z
> looking into a 'network' (like your pre-amp).
>
> Now you need a test set-up to measure Insertion
> Loss (and Phase change) and come up with gain
> (or loss) of a network.
>
> What are those four ports on the HP 4395A:
>
> 1) RF Output
> 2) R(ef) Input?
> 3) A and
> 4) B inputs?
Correct
> I think you are closer than you think to a
> solution!
I expect I am. However, this, unfortunately, falls under the "so close, yet
so far" range, I think.
I have just been having trouble figuring out how to measure reflection
coeficients. Transmission loss is not a big concern to me, since that one's
easy to measure with a network analyser.
Brendan - you've got the capability to measure
the reflection coefficient - the Impedance Test
Set you've got can be used to measure Input
Impedance numbers that can then be converted into
S-Parameter values (if that's what's needed for
design).
As to characterizing the device - I would build
a quickie little breadboard with the ability
to 'bias-up' the part (including the Drain) and
then terminate the output (AC-wise) with a
50 Ohm term and then measure the input Z (which
can then be converted into an actual S11 value
if need be). You will still have to be careful - this
device will *still* have gain across a wide range
of frequencies (and it may want to oscillate!).
You might need an inductor (RF choke) on the
drain in order to provide DC feed while making
that point isolated for AC - then AC couple (using
a capacitor) into a 50 Ohm load (or one of the
channels of the NA for S21 values!).
>DIY weather radar or a commercial product?
>
>-J
>
>At 03:23 PM 7/9/02 -0400, you wrote:
>>I'm not going to weigh in one way or another on the directional coupler
>>discussion, but I will say that buying equipment from eBay is great! I've
>>been working on a microwave project (weather radar) and I went from having
>>no microwave test equipment to having everything I need for this project
>>(signal generator, 40W dummy load, 20W power amp, low-loss microwave coax,
>>power detector, and directional coupler) for only a few hundred dollars.
>>
>>Sean
>
>Jeremy S. Walworth
>"I fly, I sail, I throw caution to the wind" -Jimmy Buffett
>http://www.jeremyanddarcy.com
>
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>
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The measurement examples and corresponding S-parameter
matrices shown in Figure 6 are based on a vector network
analyzer and S-parameter test set having 50-W characteristic
impedance.
If you connect a 25-W resistor to port 1 and leave port 2
unused (Figure 6a), the S-parameter matrix reduces to a
single parameter (the resistor is a one-port network requiring
a 1x1 matrix), which represents the reflection coefficient.
The following equation (Ref. 3) describes the reflection
coefficient due to a load impedance ZL with respect to
characteristic (system or line) impedance Z0:
I really don't know the answer but maybe you could use a spectrum analyzer
and a noise source. This will not tell you the impedance but you should be
able to improve your circuit by trial and error. It may also show you any
resonances in band, and noise peaks. A dense 'fence rail' signal may
replace the noise generator and make the noise floor easier to see on the
specan when you tweak the circuit.
>Perhaps I should refer to what I am actually trying to do here. I have this
>same preamp. The input is really not much of a question since the source
>for it is a >5k + j0 ohm coil. It's tough to match, but I have something
>that works. Once again, magnetic components for this particular project are
>a nono. It also needs to be small in size. This eliminates the use of
>large value inductors, and large trimmer caps. It's the output that's
>giving me grief. I need to get a measurement of the current impedance
>before I can do much to fix it, since the only specs I have for the active
>component are for a long way outside of my freq. range.
>
>The test equipment I have available is as follows:
>1xHP4395A network analyser
>1xHP54645A DSO
>1xHP43961A Impedance test set
>1x Nice function generator.
>1x Not so nice function generator.
>
>1x Hybrid power splitter
>
>With that, and something I can build easily, how can I measure s-parameters
>or impedances accurately?
>
>--Brendan
>
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>
>
>
> I really don't know the answer but maybe you could use a spectrum analyzer
> and a noise source. This will not tell you the impedance but you should be
> able to improve your circuit by trial and error. It may also show you any
> resonances in band, and noise peaks. A dense 'fence rail' signal may
> replace the noise generator and make the noise floor easier to see on the
> specan when you tweak the circuit.
Well, I have been developing a sort of a theory of operation when working
with analogue circuits: Never use trial and error where calculations will
do the job. So, I admit that I don't particularly like the idea of using
trial and error to solve this problem.
Aside from that, I didn't list the noise figure meter because I didn't think
it was relevant. The problem I've had with it, though, is it measures
consistently higher than I expect.
For example: I have an antenna pre-amp with a rated NF of 0.5dB. It has
50-ohm in and 50-ohm out ports. When I hook it up to the NF meter, (which I
have calibrated using a noise source and the same 2 cables off the noise
source, only with a F-BNC to F-BNC connector, that I use with the antenna
pre-amp) It gives me 11dB or so. Hardly an acceptable error.
It's primarily for that reason that I didn't mention the NF meter.
>> I really don't know the answer but maybe you could use a spectrum analyzer
>> and a noise source. This will not tell you the impedance but you should be
>> able to improve your circuit by trial and error. It may also show you any
>> resonances in band, and noise peaks. A dense 'fence rail' signal may
>> replace the noise generator and make the noise floor easier to see on the
>> specan when you tweak the circuit.
BTW the noise peaks and the noise floor slope can let you infer something
useful about the imaginary part of the input. If you are careful you
should be able to read at least the modulus of the imaginary component
from the specan if not the phase.
>> I really don't know the answer but maybe you could use a spectrum analyzer
>> and a noise source. This will not tell you the impedance but you should be
>> able to improve your circuit by trial and error. It may also show you any
>> resonances in band, and noise peaks. A dense 'fence rail' signal may
>> replace the noise generator and make the noise floor easier to see on the
>> specan when you tweak the circuit.
>
>Well, I have been developing a sort of a theory of operation when working
>with analogue circuits: Never use trial and error where calculations will
>do the job. So, I admit that I don't particularly like the idea of using
>trial and error to solve this problem.
If you don't have the parameters for the circuit the next best thing is to
get some real world data to give you a starting point.
>Aside from that, I didn't list the noise figure meter because I didn't think
>it was relevant. The problem I've had with it, though, is it measures
>consistently higher than I expect.
>For example: I have an antenna pre-amp with a rated NF of 0.5dB. It has
>50-ohm in and 50-ohm out ports. When I hook it up to the NF meter, (which I
>have calibrated using a noise source and the same 2 cables off the noise
>source, only with a F-BNC to F-BNC connector, that I use with the antenna
>pre-amp) It gives me 11dB or so. Hardly an acceptable error.
>
>It's primarily for that reason that I didn't mention the NF meter.
>
>Anyways, got any clues on that one?
Nope. I know too little about your circuit to be able to help. Sorry.
mitvma.mit.edu
