1645	2008-05-22 10:07:11	Pat Bunn	VXO with 74HCT74 divider circuit
Message	Date	From	Subject
I just built a circuit board for the 6 meter VXO (the dsb transciever). I plan to use it with a phasing circuit for a simple ssb transmitter. Problem is that instead of getting odd homonics - I am getting even order harmonics. I noticed after finishing the board, that all the other VXO examples using this configuration have a 2K pullup resistor at the 74HC74 output - the 6 meter circuit does not. I get a square wave at the output with some ringing. I have not tried adding a pulluop - I just noticed it was included on the 20 meter circuit. The spectrum analyser has nice 42 and 56 mhz spurs but the desired 6 meter signal is in the noise. Anyone run into this?._spurs but the 49 mhz signal is in the noise_,_._,___ [Non-text portions of this message have been removed]
1646	2008-05-22 13:52:29	Wes Hayward	Re: VXO with 74HCT74 divider circuit
Hi Pat, et al, I assume that the DSB rig you are talking about it that in section 6.11 of EMRFD, starting on page 6.90, again of the book. In looking at the schematic in Fig 6.158, I'm not sure that the first filter is properly designed. It looks like the filter was designed to be looking for a moderately high impedance, perhaps 1K. A lower value would be a better assumption. You say you are seeing a square wave, yet you say you are only seeing even harmonics. The two are not consistent. I have often used a pull-up when using a HC D Flip flop, but that was not well founded. Example: Fig 5.49 of EMRFD. The pull-up was mainly an hold over from using TTL in old forms in earlier projects. The output Z of the HC parts tends to be pretty low. I measured the output Z of a 74HC04 gate at about 40 Ohms when delivering power to 50 Ohms. Paralleling more gates pulls the impedance down. That measurement is post-EMRFD publication, so is not reflected in the text. Generally though, I don't think that the pull-up is required. Be careful of the really high speed variations of the high speed CMOS families. While I have found the straight HC stuff easy to use, an experiment with a faster variation produced unexpected results. I've since heard similar results from others. For those of you without any print to examine, this discussion regards a frequency multiplier scheme used where a signal is divided with a divide by 2 (I've mainly used 74HC74 D-FFs) to produce a square wave. Then odd harmonics of this output are extracted with bandpass filters. If you divide the output of a 14.318 MHz oscillator, which is a standard TV color burst crystal, you end up with a 7.159 square wave signal. The 7th harmonic is 50.113 MHz. VXOing this just a bit and you can easily hit the calling frequency
1647	2008-05-22 14:30:45	Bill Noyce	Re: VXO with 74HCT74 divider circuit
It sounds as if the divide-by-two isn't happening. Is the square wave on pin 5 ("Q") of the 74HC74 running at 7 MHz (as it should), or 14 MHz? What are the voltages on the Set and Reset pins (pins 4 & 1 for secti
1648	2008-05-22 18:39:27	wimmie262000	Re: VXO with 74HCT74 divider circuit
That would be really odd. Digital clock signals have very strong odd harmonics. Are you sure your crystal is operating at 14.3Mz? If it would be 16.x you might be looking at the 5th and 7th harmonic. Whereas with a 14.3MHz crystal should give about 35-36Mhz at 5th harmonic. Did you count the harmonic number of your 42 and 56? Cheers, Joop - pe1cqp
1650	2008-05-23 07:29:50	Pat Bunn	Re: VXO with 74HCT74 divider circuit
The divide by twi is hapening. I get a square wave with some ringing on the edges on my 400 mhz scope but the amplitude is only 3 volts with a 5 volt supply on the chip. I think the problem is with the filter although I am puzzled at the amplitude of the even harmonics with respect to the odd ones. The even harmonics are larger than the odd ones which isn't possible with a good square wave. The pull up resistor makes no difference. I need toi keep working on it. I have designed a lower input z filter and will try that. ----- Original Message -----
1655	2008-05-23 09:43:48	Kenneth Stringham	Re: VXO with 74HCT74 divider circuit
I'm not surprised by the 3v amplitude you mentioned, but I am surprised that you are reading even harmonics rather than odd harmonics. My first question would be how are you mounting your components for these experiments? You will need very short connections to all components and be careful with your bypassing. I have used all kinds of logic gates to drive filters, but I ways used 200 ohm loads at the output of the gates. I would design my filters for 200 ohm terminations and then use a broadband transformer to step the 200 ohms down to 50 ohms. My second question would be how you are measuring your harmonics? I would hope that you have a spectrum analyzer. All square waves will have high odd harmonics and low even harmonics. Perfect square waves will only odd harmonics, but this is not going to happen in the real world. You mention that there is ring on the output square wave. This is an indication that you have a mismatch somewhere. Do you see the save behavior, if you use another HCT gate as the load? Ken - AE1X
1660	2008-05-23 17:46:48	David Smith	Re: VXO with 74HCT74 divider circuit
Hi Pat, One thing that I didn't know until recently is that the level of the individual harmonics for a square wave depend on the duty cycle of the waveform. A 50:50 square wave gives max level for 3rd, 5th, 7th etc harmonics. However, if the waveform is changed to say 40:60, the 5th harmonic level drops to zero. Have a look here: http://www.wenzel.com/pdffiles1/pdfs/choose.pdf What I suspect is that your waveform is not 50:50. Unfortunately, I don't yet have the book so I'm not familiar with the circuit. If there's a sine to square wave conversion using a TTL gate somewhere, I would look at the gate input switching point and try to adjust it with a divider - perhaps adding a coupling capacitor also. Regards, Dave VK3HZ
1667	2008-05-24 08:45:03	Bob Macklin	Re: VXO with 74HCT74 divider circuit
At low frequncies ie, below 50MHz the symetry od a HTC74 is for all practiacl purposes 50%. The ringing is probably your scope connection. Bob Macklin Seattle, Wa. David Smith <david@smithfamily.net.au> wrote: Hi Pat, One thing that I didn't know until recently is that the level of the individual harmonics for a square wave depend on the duty cycle of the waveform. A 50:50 square wave gives max level for 3rd, 5th, 7th etc harmonics. However, if the waveform is changed to say 40:60, the 5th harmonic level drops to zero. Have a look here: http://www.wenzel.com/pdffiles1/pdfs/choose.pdf What I suspect is that your waveform is not 50:50. Unfortunately, I don't yet have the book so I'm not familiar with the circuit. If there's a sine to square wave conversion using a TTL gate somewhere, I would look at the gate input switching point and try to adjust it with a divider - perhaps adding a coupling capacitor also. Regards, Dave VK3HZ ------------------------------------ Yahoo! Groups Links [Non-text portions of this message have been removed]
1672	2008-05-24 18:47:46	Kenneth Stringham	Re: VXO with 74HCT74 divider circuit
I did exactly this in a source for a clock for system I worked on for military project. All I had to do was place voltage divider value near the near the switching point. I set the divider resistor values to set input impedance as well for matching to the signal source impedance. Ken
1684	2008-05-26 17:41:04	Pat Bunn	Re: VXO with 74HCT74 divider circuit
I am using a 400 mhz scope. The ring is not significant and is not evident on a 100 mhz scope. The filter is the problem, I believe. I am using a spectrum analyser . With no filter I get both odd and even harmonics. The even harmonics are higher than I would have suspected. I need to experiment with the filter further. ----- Original Message -----