Dumb question 'bout signal routing (RF)
 

Hi,

Let's say I'm trouble-shooting a circuit board with several stages of
RF amplification in addition to a primary source in the form of a
VCXO. I'd like to be able to bypass certain parts of the signal path
by the use of a jumper wire. This wire may need to be up to 4 inches
in length to have sufficient reach between stages. Can anyone see a
problem with this? I mean, rather than a single wire, should I use
some sort of grounded outer lead like coax to carry the signal?

at HF
at VHF
at UHF
--

"Windows [n.], A thirty-two bit extension and GUI shell to a sixteen bit patch
to an eight bit operating system originally coded for a four bit
microprocessor and produced by a two bit company."

The main problem with it is that the two stages being connected with the
wire will no longer see the same impedances they see when operating.
This can cause a multitude of changes in their operation, including
modifying their frequency responses and gains. It can also cause
instability that's not present when the intermediate stage is in place.
The amount of the changes depends greatly on the nature of the cicuits
being connected, as well as the frequency. The changes will almost
certainly be severe at UHF and considerable at VHF. At HF, you might get
by with it and you might not, depending on the circuits.

It's good practice to design each stage for a standard input and output
impedance, with 50 ohms being overwhelmingly the most common choice for
RF circuitry. That is, each stage should be designed for correct
operation when driven from a 50 ohm source and terminated with a 50 ohm
load. The great benefit to this approach is that it allows independent
testing and characterization of each stage with a standard set of test
equipment. A secondary benefit is that any stage can be replaced with a
jumper of 50 ohm coax without changing the impedances seen by the
connected stages.

Roy Lewallen, W7EL

Paul Burridge wrote:
Hi,

Let's say I'm trouble-shooting a circuit board with several stages of
RF amplification in addition to a primary source in the form of a
VCXO. I'd like to be able to bypass certain parts of the signal path
by the use of a jumper wire. This wire may need to be up to 4 inches
in length to have sufficient reach between stages. Can anyone see a
problem with this? I mean, rather than a single wire, should I use
some sort of grounded outer lead like coax to carry the signal?

at HF
at VHF
at UHF

The main problem with it is that the two stages being connected with the
wire will no longer see the same impedances they see when operating.
This can cause a multitude of changes in their operation, including
modifying their frequency responses and gains. It can also cause
instability that's not present when the intermediate stage is in place.
The amount of the changes depends greatly on the nature of the cicuits
being connected, as well as the frequency. The changes will almost
certainly be severe at UHF and considerable at VHF. At HF, you might get
by with it and you might not, depending on the circuits.

It's good practice to design each stage for a standard input and output
impedance, with 50 ohms being overwhelmingly the most common choice for
RF circuitry. That is, each stage should be designed for correct
operation when driven from a 50 ohm source and terminated with a 50 ohm
load. The great benefit to this approach is that it allows independent
testing and characterization of each stage with a standard set of test
equipment. A secondary benefit is that any stage can be replaced with a
jumper of 50 ohm coax without changing the impedances seen by the
connected stages.

Roy Lewallen, W7EL

Paul Burridge wrote:
Hi,

Let's say I'm trouble-shooting a circuit board with several stages of
RF amplification in addition to a primary source in the form of a
VCXO. I'd like to be able to bypass certain parts of the signal path
by the use of a jumper wire. This wire may need to be up to 4 inches
in length to have sufficient reach between stages. Can anyone see a
problem with this? I mean, rather than a single wire, should I use
some sort of grounded outer lead like coax to carry the signal?

at HF
at VHF
at UHF

On Mon, 13 Oct 2003 14:05:57 +0100, Paul Burridge
wrote:


Hi,

Let's say I'm trouble-shooting a circuit board with several stages of
RF amplification in addition to a primary source in the form of a
VCXO. I'd like to be able to bypass certain parts of the signal path
by the use of a jumper wire. This wire may need to be up to 4 inches
in length to have sufficient reach between stages. Can anyone see a
problem with this? I mean, rather than a single wire, should I use
some sort of grounded outer lead like coax to carry the signal?

at HF
at VHF
at UHF

Don't hack up the board just to troubleshoot...(if it is a pcb)
make a rf sniffer instead. one method I use is a piece of coax
connected to a spectrum analyzer with a dc block connector.
the other end of the coax is stripped to expose the center conducter
as a probe tip.
course if this is a hobby project you probably don't have a spectrum
analyzer...if that's the case...my heart bleeds...

On Mon, 13 Oct 2003 14:05:57 +0100, Paul Burridge
wrote:


Hi,

Let's say I'm trouble-shooting a circuit board with several stages of
RF amplification in addition to a primary source in the form of a
VCXO. I'd like to be able to bypass certain parts of the signal path
by the use of a jumper wire. This wire may need to be up to 4 inches
in length to have sufficient reach between stages. Can anyone see a
problem with this? I mean, rather than a single wire, should I use
some sort of grounded outer lead like coax to carry the signal?

at HF
at VHF
at UHF

Don't hack up the board just to troubleshoot...(if it is a pcb)
make a rf sniffer instead. one method I use is a piece of coax
connected to a spectrum analyzer with a dc block connector.
the other end of the coax is stripped to expose the center conducter
as a probe tip.
course if this is a hobby project you probably don't have a spectrum
analyzer...if that's the case...my heart bleeds...

In article , Paul Burridge
writes:

Let's say I'm trouble-shooting a circuit board with several stages of
RF amplification in addition to a primary source in the form of a
VCXO. I'd like to be able to bypass certain parts of the signal path
by the use of a jumper wire. This wire may need to be up to 4 inches
in length to have sufficient reach between stages. Can anyone see a
problem with this? I mean, rather than a single wire, should I use
some sort of grounded outer lead like coax to carry the signal?

Paul, offhand I'd say no problem since I've done that on the bench
many a time from LF on up to low UHF. The waveform may not
be optimum but "quick check" circuit jumpering should not be a
source of worry. Not at 4" length.

I'll have to add that my personal experience is based on circuit
boards (including breadboards of the universal kind) all had fairly
good ground planes ("earthed" planes, heh heh).

Digital circuits have rather high frequency content, must have to
carry very fast edge-transient-time signals. I've done a lot of
one-shot applications that were not-yet-formalized production
designs using 4 1/2" square Douglas Electronic circuit cards*
using #26 insulated wires snaking around the card containing up
to 16 DIPs. Fast revisions of wiring and package connections were
done without any problems.

* Douglas Electronics has excellent quality prototype cards but
they are also quite expensive, I'd say too costly for amateur home-
brewing at $15+ or so now.

Len Anderson
retired (from regular hours) electronic engineer person

In article , Paul Burridge
writes:

Let's say I'm trouble-shooting a circuit board with several stages of
RF amplification in addition to a primary source in the form of a
VCXO. I'd like to be able to bypass certain parts of the signal path
by the use of a jumper wire. This wire may need to be up to 4 inches
in length to have sufficient reach between stages. Can anyone see a
problem with this? I mean, rather than a single wire, should I use
some sort of grounded outer lead like coax to carry the signal?

Paul, offhand I'd say no problem since I've done that on the bench
many a time from LF on up to low UHF. The waveform may not
be optimum but "quick check" circuit jumpering should not be a
source of worry. Not at 4" length.

I'll have to add that my personal experience is based on circuit
boards (including breadboards of the universal kind) all had fairly
good ground planes ("earthed" planes, heh heh).

Digital circuits have rather high frequency content, must have to
carry very fast edge-transient-time signals. I've done a lot of
one-shot applications that were not-yet-formalized production
designs using 4 1/2" square Douglas Electronic circuit cards*
using #26 insulated wires snaking around the card containing up
to 16 DIPs. Fast revisions of wiring and package connections were
done without any problems.

* Douglas Electronics has excellent quality prototype cards but
they are also quite expensive, I'd say too costly for amateur home-
brewing at $15+ or so now.

Len Anderson
retired (from regular hours) electronic engineer person

On Mon, 13 Oct 2003 14:05:57 +0100, Paul Burridge, said...

Hi,

Let's say I'm trouble-shooting a circuit board with several stages of
RF amplification in addition to a primary source in the form of a
VCXO. I'd like to be able to bypass certain parts of the signal path
by the use of a jumper wire. This wire may need to be up to 4 inches
in length to have sufficient reach between stages. Can anyone see a
problem with this? I mean, rather than a single wire, should I use
some sort of grounded outer lead like coax to carry the signal?


i don't usually have to bypass anything to shoot. you look for last
known good, first known bad, with your RF probe/scope/sniffer/whatever
and go from there. lift an input component leg, terminate an output,
check signal, if good, plug it back in. find the stage that's bad and
check transistor DC levels first.

at HF
a wire might work
at VHF
getting dangerous. 50 ohm systems and coax looking good
at UHF
same as above but remove "getting"

you don't mention max allowed power out and freq. i know what your up
to, but can't remember the rules.

have you considered Mini-Circuits MMIC amps? 50 ohms in and out. if you
need more power, just put them in parallel fed with a wilkinson divider
and flip the divider around on the output for a combiner. alternately,
if the divider legs are too long at your freq, use the coil version. you
could probably get away with tearing open those 75 ohm CATV splitters
and hacking them. you'll loose a little power, but it's doable. a quick
solution.

too much time on radios and not enough on bot killing weapory. quick
solution for that... shotgun?

HTH,
mike

On Mon, 13 Oct 2003 14:05:57 +0100, Paul Burridge, said...

Hi,

Let's say I'm trouble-shooting a circuit board with several stages of
RF amplification in addition to a primary source in the form of a
VCXO. I'd like to be able to bypass certain parts of the signal path
by the use of a jumper wire. This wire may need to be up to 4 inches
in length to have sufficient reach between stages. Can anyone see a
problem with this? I mean, rather than a single wire, should I use
some sort of grounded outer lead like coax to carry the signal?


i don't usually have to bypass anything to shoot. you look for last
known good, first known bad, with your RF probe/scope/sniffer/whatever
and go from there. lift an input component leg, terminate an output,
check signal, if good, plug it back in. find the stage that's bad and
check transistor DC levels first.

at HF
a wire might work
at VHF
getting dangerous. 50 ohm systems and coax looking good
at UHF
same as above but remove "getting"

you don't mention max allowed power out and freq. i know what your up
to, but can't remember the rules.

have you considered Mini-Circuits MMIC amps? 50 ohms in and out. if you
need more power, just put them in parallel fed with a wilkinson divider
and flip the divider around on the output for a combiner. alternately,
if the divider legs are too long at your freq, use the coil version. you
could probably get away with tearing open those 75 ohm CATV splitters
and hacking them. you'll loose a little power, but it's doable. a quick
solution.

too much time on radios and not enough on bot killing weapory. quick
solution for that... shotgun?

HTH,
mike

On Tue, 14 Oct 2003 15:30:54 GMT, Active8
wrote:


i don't usually have to bypass anything to shoot. you look for last
known good, first known bad, with your RF probe/scope/sniffer/whatever
and go from there. lift an input component leg, terminate an output,
check signal, if good, plug it back in. find the stage that's bad and
check transistor DC levels first.

Agreed. And that's what I'd do in practice.
I'm just curious, however as to what effect jumping RF from point A in
a circuit to point B with just a single wire has on the signal. IOW,
will the integrity of the signal be preserved by this quick and simple
expedient? The great and the good here seem to believe it shouldn't be
a problem up into VHF.

you don't mention max allowed power out and freq. i know what your up
to, but can't remember the rules.

Mike, I do do *other* stuff apart from build killer robots! :-D

too much time on radios and not enough on bot killing weapory. quick
solution for that... shotgun?

Actually it's been the other way around lately. But the new
disemboweler is quite impressive, though I do say so myself. I'll
upload some pix of it when they're available.
--

"Windows [n.], A thirty-two bit extension and GUI shell to a sixteen bit patch
to an eight bit operating system originally coded for a four bit
microprocessor and produced by a two bit company."