In article , Paul Burridge
writes:
On 13 Mar 2004 07:33:15 -0800, (Tim Shoppa)
wrote:
Fifth harmonic frequency multipliers do exist, but it's usually much
easier to double and triple your way to the final frequency if possible.
(You just discovered this, I think!)
Yeah, but trying to get the 5th is hardly asking for the moon...
In a way, it IS. Fifth harmonic of an infinitely sharp transition
rectangular waveform is still low in energy compared to the
fundamental. Chances are that a single stage using an active
device as a quintupler will NOT be successful, transistor or tube
(valve).
Let's get to some specifics on this problem -
1. Let us know what you are using to determine whether or not
a 5th harmonic exists. The lack of indication may be due to
whatever it is (not a spectrum analyzer) being used.
2. Describe the multiplier stage in more detail and include an
approximate level and impedance/admittance of the RF
source. That would include supply rails and biasing.
3. Describe whatever is being used to select the 5th harmonic
and inhibit the fundamental and other harmonics. There's
lots of energy at many different frequencies floating around
there and you only want one frequency.
4. Review again with us the output drive level requirements so
we can get a handle on that.
5. If you are using an oscilloscope to measure the fundamental
waveform, estimate the actual risetime/falltime based on the
rise/fall times limits of the oscilloscope. That yields some
basic data that can be applied to a Fourier series to determine
the level of 5th harmonic energy you have to work with. [that
will also reveal the approximate frequency limits of the scope]
The lack of even harmonics is typical of push-pull stages ... if you
are messing around with CMOS gates, you might try using a TTL gate
(which pulls low much stronger than it pulls high) or an open collector
TTL gate, both with smmallish (100-200 ohm) pull-up resistors for
doubling.
I've a reasonably fast Schmitt I'm going to stick in there in place of
the 74HC04 before I resort to anything fancy (same pin-out).
Why not do a x3 followed by a x2 to get 17.2 MHz out of 2.866 MHz?
Because I don't have a rock lying about for that fundamental!
Hint: A mixed 2x and 3x = 5x out if filtered to pass that. No lying-
around rock needed. [not an optimum solution]
Note: As already pointed out, a single PLL IC can do the job in the
same PCB footprint.
When frustration hits a peak, it's time to sit back away from the
problem and do an objective review of what is the overall task and
what you've accomplished so far and what you know about certain
circuits fundamentals. General problem descriptions only result in
general solutions without quantitative values needed for a specific
application.
I've found that NON-linear circuits (a multiplier stage is definietly
in the non-linear category) take rather more development time
than a linear circuit. There's lots of different things going on in a
multiplier circuit and those have to be considered for the whole.
Hopefully some supreme being here will spot a problem with the traces
I've now posted...
Try as we might in the depths of our frustrations, supreme beings
tend not to intervene in us humans' petty affairs. :-)
Len Anderson
retired (from regular hours) electronic engineer person