Hi,

Has anyone a pointer to the RF output impedance of the 74HC
series gates (at about 10MHz)? I have a few inverters/buffers
left over in a package and thought of using them stacked to drive
an output port via a simple matching network. I've seen this done
before and have simulated it myself but would like it spelled
out.


Cheers - Joe, G3LLV

Hi,

I've been able to answer my own question. One minute of
googling produced AN-393 at -


http://www.fairchildsemi.com/


Apologies for wasting the bandwidth.


73 de Joe, G3LLV

In article , Joe McElvenney
writes:

Has anyone a pointer to the RF output impedance of the 74HC
series gates (at about 10MHz)? I have a few inverters/buffers
left over in a package and thought of using them stacked to drive
an output port via a simple matching network. I've seen this done
before and have simulated it myself but would like it spelled
out.

Fairchild's application note AN-393 is an informative one from the
standpoint of _transient_ switching analysis. Analysis or simulation
for impedance matching, presuming matching at a single frequency,
leans towards _steady_state_ analysis.

"Impedance matching" of very fast transition time rectangular
waveforms must be done over a very wide bandwidth to insure
fidelity of the rectangular waveform at the far end of the transmission
line. Digital devices are characterized under near-total On or Off
current source drivers...working into very non-linear active device
loads.

To get into the theory of a narrowband source impedance condition
is possible. Some of that has already been done with the "Class E"
amplifiers using certain SCR devices at 40 meters, such as several
examples at Cal Tech, Pasadena, CA, groups. A search on Class E
material might turn up something that would fit your application.

Class C tube or transistor operation can be considered a sort of "soft
switching" condition similar to On-Off current sources but that is
complicated by the fact that the On state of the active device varies in
current carrying capability depending on a narrowband load... hence the
"soft switching" label. Triode to pentode vacuum tubes were never very
good switches in On-Off operation compared to transistors.over the
very wide bandwidths necessary for fast-transient digital waveforms.

While it might be an interesting intellectual exercise in analysis, it will
take considerable time. Far more time than geting up a small
breadboard test circuit and TRYING IT OUT. Nearly all of the "74" (or
"54") family components are relatively cheap...small-scale integrated
circuit packages cost less than a dollar each in single quantities.

Len Anderson
retired (from regular hours) electronic engineer person

Len,

Fairchild's application note AN-393 is an informative one from the
standpoint of _transient_ switching analysis. Analysis or simulation
for impedance matching, presuming matching at a single frequency,
leans towards _steady state_ analysis.

You are right about AN-393, and the non-linearities, of course. However
when I first posted I needed a ballpark figure and the app-note gave it me. My
application is a rather mundane, fixed frequency, 10MHz frequency standard
required to drive a counter/signal generator combo and whose output should be
reasonably sinusoidal so as not to spray harmonics around the place. The
buffered or inverted output coming off the VCXO has to supply about 1V pk-pk
into 50 ohms. A 74HC04 or '07 followed by a single-section, image-parameter, pi
-section filter appears to do the job but it was just my day for being
scientific :-(


Cheers - Joe

Make sure to keep the capacitor leads in the filter very short, and
build the filter so there isn't a sneak path around it. HF filters often
lose their lowpass characteristics at VHF and above due to stray
impedances and resonances. The 74HC series switches very fast, so it
generates some really high frequency stuff you really need to
effectively filter.

Roy Lewallen, W7EL

Joe McElvenney wrote:

You are right about AN-393, and the non-linearities, of course. However
when I first posted I needed a ballpark figure and the app-note gave it me. My
application is a rather mundane, fixed frequency, 10MHz frequency standard
required to drive a counter/signal generator combo and whose output should be
reasonably sinusoidal so as not to spray harmonics around the place. The
buffered or inverted output coming off the VCXO has to supply about 1V pk-pk
into 50 ohms. A 74HC04 or '07 followed by a single-section, image-parameter, pi
-section filter appears to do the job but it was just my day for being
scientific :-(


Cheers - Joe

Roy Lewallen wrote in message ...
Make sure to keep the capacitor leads in the filter very short, and
build the filter so there isn't a sneak path around it. HF filters often
lose their lowpass characteristics at VHF and above due to stray
impedances and resonances. The 74HC series switches very fast, so it
generates some really high frequency stuff you really need to
effectively filter.

often, u might find a need for a diplexer at the output of the 74HC
series! quite like terminating the HEXFET power amps. I would suggest
that u include a 22 ohms resistor in series with the output and about
20-30 of capacitor to smoothen out the jitter on the square wave
edges. we used to need this to prevent the memory IC from blowing up
when driven by fast TTL chips with a large fan out.

- farhan

Roy Lewallen wrote in message ...
Make sure to keep the capacitor leads in the filter very short, and
build the filter so there isn't a sneak path around it. HF filters often
lose their lowpass characteristics at VHF and above due to stray
impedances and resonances. The 74HC series switches very fast, so it
generates some really high frequency stuff you really need to
effectively filter.

often, u might find a need for a diplexer at the output of the 74HC
series! quite like terminating the HEXFET power amps. I would suggest
that u include a 22 ohms resistor in series with the output and about
20-30 of capacitor to smoothen out the jitter on the square wave
edges. we used to need this to prevent the memory IC from blowing up
when driven by fast TTL chips with a large fan out.

- farhan

Make sure to keep the capacitor leads in the filter very short, and
build the filter so there isn't a sneak path around it. HF filters often
lose their lowpass characteristics at VHF and above due to stray
impedances and resonances. The 74HC series switches very fast, so it
generates some really high frequency stuff you really need to
effectively filter.

Roy Lewallen, W7EL

Joe McElvenney wrote:

You are right about AN-393, and the non-linearities, of course. However
when I first posted I needed a ballpark figure and the app-note gave it me. My
application is a rather mundane, fixed frequency, 10MHz frequency standard
required to drive a counter/signal generator combo and whose output should be
reasonably sinusoidal so as not to spray harmonics around the place. The
buffered or inverted output coming off the VCXO has to supply about 1V pk-pk
into 50 ohms. A 74HC04 or '07 followed by a single-section, image-parameter, pi
-section filter appears to do the job but it was just my day for being
scientific :-(


Cheers - Joe

In article , Joe McElvenney
writes:

You are right about AN-393, and the non-linearities, of course. However
when I first posted I needed a ballpark figure and the app-note gave it me.
My
application is a rather mundane, fixed frequency, 10MHz frequency standard
required to drive a counter/signal generator combo and whose output should be
reasonably sinusoidal so as not to spray harmonics around the place. The
buffered or inverted output coming off the VCXO has to supply about 1V pk-pk
into 50 ohms. A 74HC04 or '07 followed by a single-section, image-parameter,
pi -section filter appears to do the job but it was just my day for being
scientific :-(

No problem here. :-)

As a first approximation for the application, it should be intuitive that
the peak-to-peak voltage out of the gate/inverter into a resistive load
would yield the beginning of data on RF power output capability. The
rest can be figured out on the bench.

More eleborate data sheets for particular devices have specs for
driving different loads, usually as the logic 1 level voltage minimum
for a given resistive load. Depends on the manufacture's budget for
data sheet preparation, I would guess, since those vary in content.

If others are trying such an application at maximum power handling
capability and need lowpass filtering, a caution. Some elliptic filter
input impedances can vary widely in stopband frequencies. That
might, but not necessarily, result in edge spikes above breakdown
at the gate/inverter output. Butterworth response filters are the
most forgiving of such but also have the lowest stopband attenuation.
The 74HC family ought to be rather robust on that and I don't think
it is a great concern.

I did encounter some problems with 74S devices driving filters some
years ago when using a gate as a digital mixer for an optical
interferometer phase measurement instrument. That was with old
74S family devices before the newer incorporated protection
devices were introduced a decade or so ago.

"Digital mixers" work very well indeed when both RF inputs are at
similar high levels common to digital circuits. It's quite easy to get
difference frequency output and to lowpass filter it to remove the
input frequency components. Never tried it for sum-output mixing,
probably need a spectrum analyzer handy for such circuits.

Len Anderson
retired (from regular hours) electronic engineer person

In article , Joe McElvenney
writes:

You are right about AN-393, and the non-linearities, of course. However
when I first posted I needed a ballpark figure and the app-note gave it me.
My
application is a rather mundane, fixed frequency, 10MHz frequency standard
required to drive a counter/signal generator combo and whose output should be
reasonably sinusoidal so as not to spray harmonics around the place. The
buffered or inverted output coming off the VCXO has to supply about 1V pk-pk
into 50 ohms. A 74HC04 or '07 followed by a single-section, image-parameter,
pi -section filter appears to do the job but it was just my day for being
scientific :-(

No problem here. :-)

As a first approximation for the application, it should be intuitive that
the peak-to-peak voltage out of the gate/inverter into a resistive load
would yield the beginning of data on RF power output capability. The
rest can be figured out on the bench.

More eleborate data sheets for particular devices have specs for
driving different loads, usually as the logic 1 level voltage minimum
for a given resistive load. Depends on the manufacture's budget for
data sheet preparation, I would guess, since those vary in content.

If others are trying such an application at maximum power handling
capability and need lowpass filtering, a caution. Some elliptic filter
input impedances can vary widely in stopband frequencies. That
might, but not necessarily, result in edge spikes above breakdown
at the gate/inverter output. Butterworth response filters are the
most forgiving of such but also have the lowest stopband attenuation.
The 74HC family ought to be rather robust on that and I don't think
it is a great concern.

I did encounter some problems with 74S devices driving filters some
years ago when using a gate as a digital mixer for an optical
interferometer phase measurement instrument. That was with old
74S family devices before the newer incorporated protection
devices were introduced a decade or so ago.

"Digital mixers" work very well indeed when both RF inputs are at
similar high levels common to digital circuits. It's quite easy to get
difference frequency output and to lowpass filter it to remove the
input frequency components. Never tried it for sum-output mixing,
probably need a spectrum analyzer handy for such circuits.

Len Anderson
retired (from regular hours) electronic engineer person