EMRFD Message Archive 13638
Message Date From Subject 13638 2017-03-02 01:01:23 Baris Super VXO?
With 3 or more crystals range is slowly increasing but then the output becomes unstable. I tried the both the colpitts circuit with a discrete transistor and with the internal oscillator of a SA612, there's no significant difference.I recently got lots of 7.000 and 14.000 MHz crystals from China, I'd love to build a SA612 VXO with them that covers at least some part of the bands.Any advice would be appreciated.Baris13639 2017-03-02 06:09:31 wb8yyy_curt Re: Super VXO?
these quartz crystals are very stubborn - which is why they work so well to stabilize oscillators.
for a single crystal VXO, please note that adding an inductor to the circuit tends to stretch the tuning range a small additional amount. usually this inductor is best to have a low Q -- a tiny inductor tends to work better then a well-implemented large toroid in this application.
the best VXO I have is in the 38 Special -- around 22 MHz crystal, with a low Q inductor -- I am remembering nearly 20 kHz tuning range. this circuit also uses a varactor diode, perhaps a large rectifier diode.
the super VXO benefits some from lowering the overall Q with a couple crystals in parallel -- and I suspect it is best that they be separated a little in frequency. the 'problem' is that these modern crystals may be more precise than older ones.
I am remembering that K8IQY built a 'super' VXO so perhaps his documentation in on line at 4state QRP (or perhaps he has time to answer your email).
I am thinking that using 2 or maybe 3 crystals provides a small stretch in frequency. the best uses of VXO have been in mixing a higher frequency crystal with a lower frequency IF - as in the SST and 38 special.
I am reading that amateurs about 50 or more years ago used a VXO in make VHF signals tunable -- yes a rather high frequency multiplication value.
Meanwhile a few kHz at 7 MHz could be quite useful in a simple rig -- use a small 'choke' size inductor (the kind about the size of a 1/4th watt sized resistor or smaller) -- perfect your circuit for one crystal before adding a second crystal.
enjoy your experiments and radiations
Curt13640 2017-03-02 06:36:46 wb8yyy_curt Re: Super VXO?
see -- http://www.4sqrp.com/kits/SAVXO/savxoassemblymanualv1.0b-02262012.pdf
please note that opposite of what I suggested, the crystals should be from the same batch -- rather close in frequency.
I do see that K8IQY is getting 8-10 kHz tuning at 7 MHz, while the experimenter from Japan is getting 40 kHz at 10.1 MHz. Very likely these crystals are very different to get this larger frequency span. I see that K8IQY is using 3 crystals, and at least 1.2 uH inductance.
It would be interesting to evaluate the frequency tuning with 1, 2 and 3 crystals. I am thinking that a large portion of the range could be attained with only one crystal (based upon the 38 Special at 22 MHz).13641 2017-03-02 08:26:12 Tayloe, Dan (Noki... Re: Super VXO? The Q of crystals tends to go up has the frequency is lowered. Thus, a 7 MHz VXO will tend to have a smaller percentage change than a higher frequency crystal such as a 22 MHz VXO.
In addition, a 1% change on 7 MHz is roughly 3x smaller than a 1% change at 22 MHz. Thus you have two strikes working against you when running a VXO at 7 MHz compared to higher frequencies.
- Dan, N7VE
13642 2017-03-02 09:37:21 wb8yyy_curt Re: Super VXO?
Not of any help here -- I may have saved some alternate material crystals from the trash that a co-worker once experimented with (maybe lithium niobate). These have a higher coupling coefficient (k-squared) that might help in a VXO -- but since I am confident they cost many times more than a PC type quartz crystal - of no practical value here to our community.
CUL
Curt
13643 2017-03-02 09:41:16 Raj Re: Super VXO?
Emacs!
At 02-03-2017, you wrote:
>[Non-text portions of this message have been removed]
13644 2017-03-02 10:00:11 Jim Kortge Re: Super VXO?
13645 2017-03-02 11:02:31 wb6ogd Re: Super VXO?
Have any of you VXO guys tried "crystal penning"? Your can lower crystal frequencies by
hundreds of KHz and put them exactly where you want them. I was shocked how well
it worked when I heard about it years ago.
See:
http://www.hanssummers.com/penning.html
73,
Gary
WB6OGD13646 2017-03-02 13:08:03 Roelof Bakker Re: Super VXO?
Last year I have been experimenting with the same circuit on 7 MHz.
I reasoned that stray capacitances might reduce the tuning range.
Hence, except for the crystals and tuning capacitor, only SMD parts
were used. A small single sided PCB was milled to mount the parts.
The tuning capacitor is 4 - 40 pF, the inductors are 1210 size and I
used 2 x 10uH and 1 x 22uH. The crystals are 7050 kHz and were
bought at the Funk-Amateur On-line Shop in Berlin, Germany.
The base-emitter capacitor is 180 pF and the capacitor from emitter
to ground is 360 pF.
One Crystal:
L = 0, F = 7049.8 - 7055.7 kHz, delta F = 5.9 kHz
L = 10uH, F = 7048.9 - 7055.4 kHz, delta F = 6.5 kHz
L = 20uH, F = 7047.8 - 7055.1 kHz, delta F = 7.3 kHz
L = 42uH, F = 7044.3 - 7054.0 kHz, delta F = 9.7 kHz
Two crystals:
L = 0, F = 7051.0 - 7058.5 kHz, delta F = 7.5 kHz
L = 10uH, F = 7049.5 - 7058.1 kHz, delta F = 8.6 kHz
L = 20uH, F = 7047.2 - 7057.8 kHz, delta F = 10.3 kHz
L = 42uH, F = 7037.9 - 7056.7 kHz, delta F = 18.8 kHz
I also tested a pair of 7030 kHz crystals. These can be tuned from
7017 to 7036 kHz. The output varies about 1.6 dB and this happens
between 7030 and 7036 kHz. From 7017 to 7030 kHz the output is very
flat. With a supply voltage of 6V the output is about 0 dBm in 50
ohm.
The waveform is not clean; the second harmonic is only 11 dB down.
However, with a 7 element low pass filter this figure is > 50 dB.
I hope this helps.
73,
Roelof Bakker, pa0rdt13647 2017-03-02 20:13:38 vk3hn Re: Super VXO? 13648 2017-03-02 20:19:28 Raj Re: Super VXO?
Raj, vu2zap
At 03-03-2017, you wrote:
Its been a hard sell before but...
Have any of you VXO guys tried "crystal penning"? Your can lower crystal frequencies by
hundreds of KHz and put them exactly where you want them. I was shocked how well
it worked when I heard about it years ago.
See:
http://www.hanssummers.com/penning.html
73,
Gary
WB6OGD13649 2017-03-03 08:08:12 winston376 Re: Super VXO? 13654 2017-03-05 00:09:37 Nick Tsakonas Re: Super VXO? 13655 2017-03-05 06:09:29 Baris Re: Super VXO? 13656 2017-03-05 07:23:34 Clutter Re: Super VXO?
Novice Rig Roundup in late February with my Knight T-60 transmitter
and crystal control, I got motivated to cobble together a 'super VXO'
for 40 meters. Although I never got finished with that circuit before
the 9 day contest ended, I was so pleasantly surprised at my results
that I plan to finish it for use with that T-60 and similar radios.
I used as a starting point the SAXVO by Jim Kortge, K8IQY. This
proved to have been a good choice. Looks like Jim spent some time
perfecting his circuit. Mine fired right up, and it gave me some fairly
surprising results. WB8YYY posted a link to get that schematic
earlier in this thread. You should be able to find it by searching for
the Four States QRP group under 'Kits'. The SAXVO is a retired kit
but the documentation remains available online.
I have a trio of old, military surplus HC-6/U crystals at 7127.778 KHz.
I would guess that these were probably made to fairly good standards,
i.e. 'high Q'. Two oscillate on essentially the same frequency and the
3rd unit is within about 100Hz or better. Using an ~440pF air variable
capacitor, I was able to pull the trio from ~7130 down to ~7105. That's
on 40 meters, folks! According to my oscilloscope the stability is good
for the first ~15KHz or so, and it appears to be quite usable over the
entire ~25KHz range. I was pretty surprised. Two other single HC-6
crystals at ~7117 and 7105 each pulled about 5 KHz in the same test
circuit. My series inductor was a "Wee 33 uH", the old yellow ones
sometimes seen in surplus. The original circuit specified an 18uH
inductor. I found that a little more was better.
I will need to investigate further, as my counter indicates some
startup drift of about 200 Hz downward over 20-30 seconds. This
may indicate my circuit is a little 'chirpy' right now. It's just cobbled
together- no voltage regulator and no attempt to optimize any of
the caps or the inductor so far... I did use C0G ceramics for the 47pF
caps in my circuit.
One should *not* expect similar results with FT-243 holder types
from what I have heard, BTW. The plated crystals in metal holders
are said to be the most pullable.
One other neat idea I picked up during this experiment was that
you can use old 'loctal' tube sockets rather than formal HC-6
sockets for your crystals. The HC-6 sockets have gotten way
overpriced on the auction site IMO. I got some nice ceramic loctals
online for just over 1 USD each, and that will socket *two* of the
HC-6/U crystals.
73, David K3KY
Derwood, MD