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Frequency multiplier question




 
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W9BHI
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« on: July 10, 2017, 10:51:57 AM »

Hello all,
Having heard that a square wave is rich in harmonics, wouldn't it be beneficial
to inject a square wave instead of a sine wave from a DDS generator into the crystal socket on a boatanchor transmitter that uses multiplier stages?
Like a Johnson Ranger or Heathkit.
The output of the Johnson and Heathkit VFO's look more like a distorted square wave than a sine wave.
Thanks,
Don W9BHI
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VE3ELQ
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« Reply #1 on: July 10, 2017, 11:42:19 AM »

Don, A square wave is rich in odd order harmonics, 3rd, 5th, 7th etc so it should work if you use the correct odd order sub harmonic.
But if I may suggest many DDS VFOs out there will operate up to 30mhz and some beyond that so why not just inject the freq you are looking for then the DDS freq readout would be correct.  What DDS were you planning on using??

73s Nigel
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W9BHI
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« Reply #2 on: July 10, 2017, 12:08:10 PM »

Nigel,
I was looking at one of those plug in DDS osc modules on Ebay.
They put out sine waves and square waves.
Don
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AB2EZ
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« Reply #3 on: July 11, 2017, 05:26:32 PM »

Nigel

The strategy you propose may not work in a transmitter using two stages (or more) of frequency multiplication.

For example, consider a transmitter that uses the following scheme to operate at 28.2 MHz:

1. A VFO or a crystal oscillator produces a signal whose fundamental frequency is 7.05 MHz. This signal may include harmonics... but that is not important.

2. The output of the oscillator drives a buffer that is biased to operate in Class C. As a result of the Class C operation, the buffer's output current will be rich in harmonics of 7.05 MHz. Because frequency doubling is required, the output tuned circuit of the buffer is adjusted to resonate at 14.1 MHz. The impedance of the output circuit is only high at 14.1 MHz. As a result, the buffer's output voltage waveform is a sine wave at 14.1 MHz.

3. The output of the buffer drives another stage (perhaps the output amplifier) that is biased to operate in Class C. Therefore, the output current of that stage is rich in harmonics of 14.1 MHz. The output tuned circuit of that stage is tuned to resonate at 28.2 MHz... producing a sine wave voltage at the output at the desired final output frequency.

Here is the problem. If the 7.05MHz oscillator is replaced by a 28.2 MHz oscillator, the buffer stage output tuned circuit will not be able to be resonated at that frequency. Therefore the output voltage from the buffer stage will be too low to properly drive the following stages of the amplifier chain.

Stu
 
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VE3ELQ
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« Reply #4 on: July 11, 2017, 06:29:12 PM »

Nigel

The strategy you propose may not work in a transmitter using two stages (or more) of frequency multiplication.

.............

Here is the problem. If the 7.05MHz oscillator is replaced by a 28.2 MHz oscillator, the buffer stage output tuned circuit will not be able to be resonated at that frequency. Therefore the output voltage from the buffer stage will be too low to properly drive the following stages of the amplifier chain.

Stu
 

Stu, thanks for pointing that out.  My zero experience level with these older tube rigs really shows.  I assumed only one buffer/multiplier stage on an odd order harmonic.
Possibly another approach would be a divide by 4 using 2 flip flops as a DDS buffer, that would both square it up and provide the correct dial reading. Just a thought.  Good luck with it Don.
73s  Nigel
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