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

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

On Sun, 12 Oct 2003 15:23:22 +0100, Joe McElvenney
wrote:

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.

How about using an open collector device like the '06 or '07 and put a
parallel resonant circuit from the output to V+ (which might be Vcc or
some lower or higher voltage depending on your need and buffer voltage
rating) and analyze this as a class C amplifier stage ?

If you have multiple OC inverters, run two out of phase into a centre
tapped transformer (CT to V+) and put the resonant circuit on the
secondary. Again, V+ could be adjusted to give required impedance and
power levels. This circuit would have good symmetry, since the
impedance would be the same on both half cycles, thus reducing the
even harmonics, thus, the third harmonics would be the first one to
worry about, if you need sinusoid output.

Paul OH3LWR

On Sun, 12 Oct 2003 15:23:22 +0100, Joe McElvenney
wrote:

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.

How about using an open collector device like the '06 or '07 and put a
parallel resonant circuit from the output to V+ (which might be Vcc or
some lower or higher voltage depending on your need and buffer voltage
rating) and analyze this as a class C amplifier stage ?

If you have multiple OC inverters, run two out of phase into a centre
tapped transformer (CT to V+) and put the resonant circuit on the
secondary. Again, V+ could be adjusted to give required impedance and
power levels. This circuit would have good symmetry, since the
impedance would be the same on both half cycles, thus reducing the
even harmonics, thus, the third harmonics would be the first one to
worry about, if you need sinusoid output.

Paul OH3LWR