Transmitter Output Impedance
 

On Apr 25, 6:07*pm, Jim Lux wrote:
Sal M. Onella wrote:
This group has presented members with valuable lessons in antennas and
transmission lines, like how to measure, how to match, etc.

Something I haven't seen is a discussion of the source impedance of
the transmitter. *My curiosity was piqued today as I took some baby
steps into EZNEC. *A particular antenna had such-and-such VSWR if fed
with a 50-ohm cable and a different value if fed with a 75-ohm cable.

While this is hardly news, it got me wondering whether a 75-ohm cable
will load the transmitter the same. *Doesn't seem like it.

My point: *Using 75-ohm cable to improve the match at the antenna
won't help me *... IF ... I suffer a corresponding loss due to
mismatch at the back of the radio. *My HF radios, all solid state,
specify a 50 ohm load. As necessary, I routinely use an internal
autotuner and either of two external manual tuners. *(I'm aware of the
published 1/12 wavelength matching method.)

Wisdom in any form would be appreciated. *Thanks.

"Sal"
(KD6VKW)

I suspect that most ham transmitters do NOT have a 50 ohm output
impedance. *What they do have is a specification that they will
adequately drive a 50 ohm load (and some sort of internal circuitry that
detects an "unacceptable" output condition and turns down the drive).

After all, your transmitter could have an output impedance of zero ohms
(a "stiff" voltage source), and adequately drive your transmission line
and antenna at 50 ohms (yes, this is not the optimum power transfer, but
nobody ever said that ham transmitters are designed for optimum power
transfer... maybe they're perfectly happy with less transfer, but still
operating within their safe area)

ON9CVD made some simple measurements using a couple of resistors and
foudn that a TS440 has a Zout somewhere around 15-40 ohms (depending on
frequency and output power).http://sharon.esrac.ele.tue.nl/~on9c...impedantie.htm

Grant Bingeman also has words on this:http://www.km5kg.com/loads.htm

I agree with Jim. While it's true that if a source (transmitter) is
tuned for maximum output, the output impedance must necessarily be the
conjugate of the load impedance, it is NOT generally the case that the
transmitter is tuned for maximum output. Rather, the transmitter is
tuned for an output that won't destroy the output devices and will
result in acceptable distortion (in the case of a linear amplifier).
There are plenty of cases of sources designed to be loaded with an
impedance far different from their output (source) impedance: the AC
power line, audio amplifiers, ... .

A while back, I set up a couple precision high power directional
couplers so I could measure the output impedance of a couple different
ham rigs. In the case of the rig with vacuum tube output stage, if I
operated the output stage with limited grid drive and tuned the plate
tank for maximum output power, indeed the output impedance was 50
ohms, within the tolerance of my ability to adjust the output for
maximum. But if I increased the grid drive for solid class-C
operation and tuned for the rated output power (which is no longer the
maximum possible power), the impedance seen at the output dropped. If
you work through the pi-network transformation back to the vacuum tube
plates, it's apparent that the plates under those operating conditions
represent a considerably higher source impedance than when things are
tuned for maximum available power (as first described).

But coming back to "Sal's" original question, it's always made sense
to me given the availability of inexpensive 75 ohm line with low loss
to go ahead and use it to feed antennas that have a feedpoint
impedance closer to 75 ohms than to 50 ohms. If you need to provide a
bit of matching at the transmitter end so that the transmitter is
operating correctly, it should be straightforward to do that. But
whether the actual source impedance of the transmitter is one value or
another is really of very little importance. The only time I can
think that it would matter is if you're trying to transmit a very
broadband signal and you don't want power that's reflected at the
transmission-line:antenna interface to re-reflect from the
transmitter:transmission-line interface and go back to the antenna,
delayed by enough to cause a "ghost" (in a television picture), for
example. In such a case, you'll be well served by insuring that the
antenna is well matched to the transmission line so there is an
insignificant reflection there anyway.

Cheers,
Tom

Transmitter Output Impedance
 

On Apr 26, 2:49*pm, Wimpie wrote:
On 26 abr, 22:21, Jim Lux wrote:



Measuring the output impedance (for relative small change in load) is
possible, but is not a simple task. Very likely other people will
comment on this.

ON9CVD's website I linked to has a very simple technique.. 50 ohm dummy
load and a 220 ohm resistor you can switch in.

At 100W (into 50 ohms), the 220 ohms would only dissipate 22W. *You
could get some of those non-inductive resistors from Caddock and series
them up to do something like this.

BTW, this is a simplified version of what's called a "load pull" test....
which makes me wonder if one could cobble up a quick test set that could
be controlled by a computer to do automated output Z measurements of an
HF transceiver over a reasonably wide range... One approach would be to
use a RS-232 controlled antenna tuner and, maybe, a antenna relay box
with several different load resistances).

The challenge (having actually looked at doing this with a LDG AT200PC)
is that the Z of the tuner isn't very well defined. *It's a pretty big
calibration project in itself.

Maybe, though, one could build a few test dummy loads.. say a 25 ohm and
a 75 or 100 ohm, along with your vanilla 50 ohm, and the antenna switch
(like an RCS-8V). *Basically, you're building a "high power resistor
substitution box"

You'd want some sort of nice inline watt meter (like an LP100) to make
the measurements.

Hello Jim,

Other method is injecting a slightly off-carrier frequency signal into
the amplifier (this emulates a constant small VSWR shown to the PA
(wtih 50 Ohms load), but with continuous varying phase). Because of
the difference in frequency, one can measure the forward (towards the
PA) and reverse (reflected by PA) signal with a two channel VSA.
This will give you the PA's complex output impedance.

Tom (K7ITM from my head) did this with a HP89410 with couplers.

With kind regards,

Wim
PA3DJSwww.tetech.nl

Yep, 'twas me. Using the 89410, I can resolve a signal removed from
the transmitter's output by a very small frequency offset, so it's
well within the bandwidth of the transmitter. With synchronous
averaging, the injected signal could be one or two Hz away from the
transmitted carrier, though it helps a lot to have a transmitter
running from a really "clean" (low phase noise) source if you're going
to do that. The injected signal can be many tens of dB lower in
amplitude than the transmitted signal. As I recall, I was using a
signal a kHz or so away from the carrier, still well within the
bandwidth of the RF power amplifier. One thing you have to be careful
about is either disabling ALC, or operating well outside the ALC loop
bandwidth; you don't want the ALC screwing up your results.

As Wim points out, this setup presents a load to the RF power
amplifier that's indistinguishable from an R+jX load that's
continuously varying, tracing a path around a little circle on a Smith
chart (a very tiny circle, when using signals that are very small
compared with the transmitted power). The rate the circles are traced
out is just the frequency offset between the transmitter and the test
signal.

Cheers,
Tom
K7ITM

Transmitter Output Impedance
 

On 27 abr, 03:44, Cecil Moore wrote:
On Apr 26, 4:49*pm, Wimpie wrote:

Other method is injecting a slightly off-carrier frequency signal into
the amplifier (this emulates a constant small VSWR shown to the PA
(wtih 50 Ohms load), but with continuous varying phase). Because of
the difference in frequency, one can measure the forward (towards the
PA) and reverse (reflected by PA) signal with a two channel VSA.
This will give you the PA's complex output impedance.

Unfortunately, the impedance encountered by the off-carrier frequency
signal is probably not the same as the impedance encountered by the
carrier frequency so the results don't correlate and are not very
useful. The carrier frequency has interference components that the off-
carrier signal doesn't encounter.
--
73, Cecil, w5dxp.com
"Halitosis is better than no breath at all.", Don, KE6AJH/SK

Helo Cecil,

Depending on the frequency resolution of your VSA, the frequency of
the injected signal can be well within 1 kHz of the carrier, so LC
filters in the PA will not distort the measurement. In case of a 100W
PA and injection of about 100 mW, the difference in wanted signal and
signal to be rejected is 30 dB (not that large).

If you don't have a VSA, you can still do it (with some more
calculations) by using a diode detector, this will give you the
difference frequencies directly and you can observe the phase
differences on an oscilloscope.

There can be a difference between a very slow load variation (for
example manually changing loads and noticing current and voltage
[including phase]) and the VSA method. This is because of decoupling
capacitors in the power supply or bias circuits (for example RC
combination in the grid circuitry to limit the grid current). If you
vary the load about 10..100 times/s, bias and supply voltages don't
have the time to settle to their steady state.

If the VSA method is basically wrong, I would love to hear why.

With kind regards,


Wim
PA3DJS
www.tetech.nl

Transmitter Output Impedance
 

On Apr 25, 9:07*pm, Jim Lux wrote:
Sal M. Onella wrote:
This group has presented members with valuable lessons in antennas and
transmission lines, like how to measure, how to match, etc.

Something I haven't seen is a discussion of the source impedance of
the transmitter. *My curiosity was piqued today as I took some baby
steps into EZNEC. *A particular antenna had such-and-such VSWR if fed
with a 50-ohm cable and a different value if fed with a 75-ohm cable.

While this is hardly news, it got me wondering whether a 75-ohm cable
will load the transmitter the same. *Doesn't seem like it.

My point: *Using 75-ohm cable to improve the match at the antenna
won't help me *... IF ... I suffer a corresponding loss due to
mismatch at the back of the radio. *My HF radios, all solid state,
specify a 50 ohm load. As necessary, I routinely use an internal
autotuner and either of two external manual tuners. *(I'm aware of the
published 1/12 wavelength matching method.)

Wisdom in any form would be appreciated. *Thanks.

"Sal"
(KD6VKW)

I suspect that most ham transmitters do NOT have a 50 ohm output
impedance. *What they do have is a specification that they will
adequately drive a 50 ohm load (and some sort of internal circuitry that
detects an "unacceptable" output condition and turns down the drive).

After all, your transmitter could have an output impedance of zero ohms
(a "stiff" voltage source), and adequately drive your transmission line
and antenna at 50 ohms (yes, this is not the optimum power transfer, but
nobody ever said that ham transmitters are designed for optimum power
transfer... maybe they're perfectly happy with less transfer, but still
operating within their safe area)

ON9CVD made some simple measurements using a couple of resistors and
foudn that a TS440 has a Zout somewhere around 15-40 ohms (depending on
frequency and output power).http://sharon.esrac.ele.tue.nl/~on9c...impedantie.htm

Grant Bingeman also has words on this:http://www.km5kg.com/loads.htm

Being aware that this was the nature of my solidstate transceiver I
attempted to use a tuner with one to improve my match to my antenna
system. While I didnt damage my transmitter I did notice that the best
settings of the tuner for TX and RX did not coincide. I was wondering
if anyone else has observed this .

Jimmie

Transmitter Output Impedance
 

On Apr 27, 10:30*am, Wimpie wrote:
Depending *on the frequency resolution of your VSA, the frequency of
the injected signal can be well within 1 kHz of the carrier, so LC
filters in the PA will not distort the measurement. *In case of a 100W
PA and injection of about 100 mW, the difference in wanted signal and
signal to be rejected is 30 dB (not that large).

Would any competent optical physicist suggest that it is valid to
study the conditions associated with interfering coherent light waves
inside an interferometer by introducing an incoherent light source
into the system? Why would any competent RF engineer suggest that the
system source conditions associated with interfering coherent RF waves
can be studied by introducing an incoherent test signal?
--
73, Cecil, w5dxp.com
"Halitosis is better than no breath at all.", Don, KE6AJH/SK

Transmitter Output Impedance
 

On Wed, 27 Apr 2011 09:40:14 -0700 (PDT), JIMMIE
wrote:

Being aware that this was the nature of my solidstate transceiver I
attempted to use a tuner with one to improve my match to my antenna
system.

Hi Jimmie,

This is somewhat cryptic, you would be expected to need a tuner.

While I didnt damage my transmitter

This transcends cryptic. It's like saying that as you came to a stop
sign, you used your brakes and you didn't damage your car.

I did notice that the best
settings of the tuner for TX and RX did not coincide. I was wondering
if anyone else has observed this .

Two different circuits inside, hence two different loads. Two
different loads, hence two different matches. Some receive paths
might be 50 Ohms, but that value is not as essential as for
transmission.

73's
Richard Clark, KB7QHC

Transmitter Output Impedance
 

Hello Cecil,

On 27 abr, 20:13, Cecil Moore wrote:
On Apr 27, 10:30*am, Wimpie wrote:

Depending *on the frequency resolution of your VSA, the frequency of
the injected signal can be well within 1 kHz of the carrier, so LC
filters in the PA will not distort the measurement. *In case of a 100W
PA and injection of about 100 mW, the difference in wanted signal and
signal to be rejected is 30 dB (not that large).

Would any competent optical physicist suggest that it is valid to
study the conditions associated with interfering coherent light waves
inside an interferometer by introducing an incoherent light source
into the system? Why would any competent RF engineer suggest that the
system source conditions associated with interfering coherent RF waves
can be studied by introducing an incoherent test signal?

As this slightly off-carrier frequency signal behaves like a load with
very low VSWR with a cable in between that extends with constant
speed. In other words, the amplifier sees a constant VSWR, but with
changing phase. Small frequency difference results in slow phase
change of VSWR.


Maybe you should read the postings from Tom also (K7ITM)

Wim
PA3DJS
www.tetech.nl


73, Cecil, w5dxp.com
"Halitosis is better than no breath at all.", Don, KE6AJH/SK

Transmitter Output Impedance
 

On 27 abr, 18:40, JIMMIE wrote:
On Apr 25, 9:07*pm, Jim Lux wrote:



Sal M. Onella wrote:
This group has presented members with valuable lessons in antennas and
transmission lines, like how to measure, how to match, etc.

Something I haven't seen is a discussion of the source impedance of
the transmitter. *My curiosity was piqued today as I took some baby
steps into EZNEC. *A particular antenna had such-and-such VSWR if fed
with a 50-ohm cable and a different value if fed with a 75-ohm cable.

While this is hardly news, it got me wondering whether a 75-ohm cable
will load the transmitter the same. *Doesn't seem like it.

My point: *Using 75-ohm cable to improve the match at the antenna
won't help me *... IF ... I suffer a corresponding loss due to
mismatch at the back of the radio. *My HF radios, all solid state,
specify a 50 ohm load. As necessary, I routinely use an internal
autotuner and either of two external manual tuners. *(I'm aware of the
published 1/12 wavelength matching method.)

Wisdom in any form would be appreciated. *Thanks.

"Sal"
(KD6VKW)

I suspect that most ham transmitters do NOT have a 50 ohm output
impedance. *What they do have is a specification that they will
adequately drive a 50 ohm load (and some sort of internal circuitry that
detects an "unacceptable" output condition and turns down the drive).

After all, your transmitter could have an output impedance of zero ohms
(a "stiff" voltage source), and adequately drive your transmission line
and antenna at 50 ohms (yes, this is not the optimum power transfer, but
nobody ever said that ham transmitters are designed for optimum power
transfer... maybe they're perfectly happy with less transfer, but still
operating within their safe area)

ON9CVD made some simple measurements using a couple of resistors and
foudn that a TS440 has a Zout somewhere around 15-40 ohms (depending on
frequency and output power).http://sharon.esrac.ele.tue.nl/~on9c...impedantie.htm

Grant Bingeman also has words on this:http://www.km5kg.com/loads.htm

Being aware that this was the nature of my solidstate *transceiver I
attempted to use a tuner with one to improve my match to my antenna
system. While I didnt damage my transmitter I did notice that the best
settings of the tuner for TX and RX did not coincide. I was wondering
if anyone else has observed this .

Jimmie

Hello Jimmie,

I noticed this also when I was experimenting with CB equipment and
simple antenna experiments.

For several CB transceivers I could get more output by slightly
mismatching the load as seen by the PA (but in many cases with too
much increase in current consumption).

I tried to use a matcher/tuner (and later a high Q resonator) to
reject other stations close by and then I figured out that when
applying mismatch to the receiver, the S-meter moved further.

The above isn't strange. As PA's are mostly not designed to show 50
Ohms, many receivers are also not designed to show 50 Ohms. I am not
discussing wide band receivers (for example digital or analog video).

Most active devices have lowest noise figure when driven from a source
impedance that is far from the input impedance of the active device.
If you want them to be equal, you need to use feedback and that
complicates the design.

Also filters with significant pass band ripple show, even when
designed for 50 Ohms, significant input reflection when referenced to
50 Ohms

With kind regards,

Wim
PA3DJS
www.tetech.nl

Transmitter Output Impedance
 

On Apr 27, 1:43*pm, Wimpie wrote:
In other words, the amplifier sees a constant VSWR, but with
changing phase. Small frequency difference results in slow phase
change of VSWR.

From the IEEE Dictionary:

"impedance -

(1)(A) The corresponding impedance function with p
replaced by jw in which w is real. Note: Definitions
(A) and (B) are equivalent.

(1)(B) The ratio of the phasor equivalent of a steady-
state sine wave voltage ... to the phasor equivalent
of a steady-state sine wave current ...

(1)(C) A physical device or combination of devices
whose impedance as defined in definition (A) or (B)
can be determined. Note: This sentence illustrates
the double use of the word impedance ... Definition
(C) is a second use of 'impedance' and is independent
of definitions (A) and (B)."

The pinging experiment seems to be measuring a physical impedance (1)
(C) the nature of which is unclear. When the amplifier is outputting
power, it seems that the source impedance would be a V/I ratio (1)(B)
which doesn't respond to incoherent signals. Seems to me, you guys are
pinging something other than the source impedance.
--
73, Cecil, w5dxp.com
"Halitosis is better than no breath at all.", Don, KE6AJH/SK

Transmitter Output Impedance
 

JIMMIE wrote:

Being aware that this was the nature of my solidstate transceiver I
attempted to use a tuner with one to improve my match to my antenna
system. While I didnt damage my transmitter I did notice that the best
settings of the tuner for TX and RX did not coincide. I was wondering
if anyone else has observed this .


Not surprising at all.

Consider the whole black art of adjusting the input match for lowest
noise figure, which may or may not coincide with the largest output
signal for a given input.

All those theorems about matching always have an asterisk about the
assumption that they're reciprocal linear devices with constant
impedances, etc. Start putting nonideal active devices in the mix, and
life gets interesting.