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Author Topic: Modern VFO for Boat Anchors  (Read 1074 times)
W1JTO
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Posts: 118




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« on: January 14, 2019, 07:40:03 PM »

Over the past few years there have been a number of Boat Anchor VFO's described in ARRL's QEX and elsewhere that use at there core AD 9850's or SI 5351's. There seem to be definite advantages to each.  The 9850 based VFO's seem very simple and in fact there are semi-commercial units available, Electronic Specialty Products, for example.

Recently there have been some units described using 9950's that will go above 30mhz and others using 5351's that can be GPS synchronized and/or utilize TCXO's.

The 9850 based designs output a sine wave that can easily be amplified to 5-8v P-P and inserted into the BA via the crystal socket.

A recent 5351 design (outlined in a September 18 QEX article)  produces a square wave - Could this also be fed into the crystal socket or would this need to be converted to a sine wave first ?.  It is also said that there is a fair amount of phase noise on this device, would this make it a poor choice for a VFO ?.

Any thoughts or experience on the use of these or other designs would be very welcome

73 John W1JTO





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G3RZP
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Posts: 1041




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« Reply #1 on: January 15, 2019, 02:18:44 AM »

Square wave drive can be useful but it depends on the mixer that it is driving. One point to watch with DDS designs is the number and amplitude of non-harmonically related spurious signals in the output. Depending on their frequency and level, they can provide significant degradation of the effective selectivity of a receiver.

How much phase noise depends on what the application is. For serious contesting or DXing at HF, you really need phase noise at -120dBc/Hz or better at 2kHz offset, although for many people, external noise will limit the performance you can actually use.
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W1BR
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« Reply #2 on: January 15, 2019, 08:31:32 AM »

Depends on how many times the VFO is multiplied in the transmitter to reach the operating frequency.

For straight through the waveform makes littler difference, if it is multiplied than you need to consider that a perfect square wave contains only odd-integer harmonic frequencies.  Sometimes a triangular or distorted sine wave is better. 

In practice, the multiplier stages are not linear; so I'd expect the waveform is not overly critical.

I'd worry more about the phase noise performance of the cheaper DDS units.

Pete
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AB5NI
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« Reply #3 on: January 29, 2019, 04:09:30 PM »

I follow the "Pete Juilano" channel (N6QW) on YouTube, and says that the phase noise from the SI5351 is more than acceptable at HF. Look through his videos until you find the ones that mention the SI5351 and watch those.

The person that started all of this SI5351 stuff is NT7S. You can find a lot of useful info here: https://github.com/etherkit/Si5351Arduino .

73,

Randy AB5NI
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I eat spaghetti code out of a bit bucket while sitting at a hash table, and I pay for the meal with cache!
KX4OM
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« Reply #4 on: January 29, 2019, 04:19:28 PM »

For lower phase noise in a VFO using the Si5351A, I recall reading that generating the RF at higher frequencies and dividing it down results in lower phase noise.

Ted, KX4OM
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G3RZP
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« Reply #5 on: January 30, 2019, 02:49:44 AM »

That was the technique used in the Hewlett Packard 8640 series signal generators in the mid 1970s. The wide band noise performance becomes determined by the dividers used. Plessey Semiconductors worked Marconi Instruments in the 1980s to produce some very low phase noise ECL dividers. Interestingly, GaAs devices were not particularly good because of the poor 1/f noise in GaAs, and possibly the surface leakage in small geometry CMOS could offer a similar problem.
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K9WW
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Posts: 111




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« Reply #6 on: March 20, 2019, 02:03:59 PM »

Hello,

What type rig do you want to put the vfo into? I ask because for a long time I used a Heathkit HW-100 with poor dial design and associated backlash. I subsequently bought and built a Heathkit SB-303 receiver with a vfo that covered the same 5.0 to 5.5 MHz as the HW-100. I used a spare jack on the '303 back chassis and wired the vfo output to it. I ran the signal through a homebrew tube buffer to a switch on the '101 so that I could use the second RX or the '101 to tune or work split. Another workaround could be the Heathkit HG-10 vfo that has several bands in a similar fashion (without the split RX capability). HG-10s are often available online at very reasonable prices. Hope this might help. 73.

Kirk  K9WW
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