[TenTec] OMNI VII vs. K3 ? Opinions invited.,

[CSM(r) Gary Huber - AB9M]

Thanks for the detailed explanation Jerry.  The TT folks at Dayton have told 
me the Corsair II has one of the best if not the best receivers they've 
built.  In fact they discouraged me from buying a couple of their newer PLL 
frequency synthesized radios because of their phase noise, because they 
said, "you're used to working the weak ones down at S-1 or S-2 with the 
Corsair and this new radio won't be able to do the same."

With all the discussions on the K3 and Omni VII (getting back on the thread) 
and with recent discussions on the Omni VI+ logic board being non-repairable 
at this point; I am thinking about replacing my working Omni VI+ with a Omni 
VII.  I've been operating TT radios exclusively for over 30 years and I'd 
have a difficult time learning a different philosophy of radio techniques. 
The Corsair II and external VFO (dual RX) stay anyway, it's a good and true 
companion and I've worked a lot of DX with it.

73,

Gary - AB9M

--------------------------------------------------
From: "Dr. Gerald N. Johnson" <geraldj at weather.net>
Sent: Monday, April 26, 2010 1:59 AM
To: <tentec at contesting.com>
Subject: Re: [TenTec] OMNI VII vs. K3 ? Opinions invited.,

>
>
> On 4/25/2010 11:08 PM, CSM(r) Gary Huber - AB9M wrote:
>> Is there any phase noise in an analog LC PTO? I believe the only digital
>> circuit in the Corsair II is the frequency counter.
>
> Every oscillator has phase noise. In the analog oscillator it comes from
> phase modulation of the sinewave by the 1/f and flicker noise of the
> active device and is usually better with a bipolar transistor than a
> FET. That's because a FET inherently has far greater 1/f and flicker
> noise than a good bipolar transistor. Phase noise is least with a high Q
> crystal in the best of circuits, like the HP 10811 oscillator. Phase
> noise in a PLL can be high from multiple sources. First the inherent
> phase noise of the wide range VCO is often large, made worse by a large
> frequency agility per volt of control voltage that often means kHz shift
> per microvolt and its really hard to keep analogue voltages noise free
> at the microvolt level, just the random noise from resistances in the
> circuit contribute much noise in the control loop. Then phase detectors
> and dividers add phase noise. Amplifiers can add a little phase noise
> too. Oscillators also have amplitude noise from the device noise, but
> generally good mixers (double balanced) ignore amplitude noise so its
> usually neglected. Oscillator design has to trade off power dissipation
> and the resulting drift with signal to noise ratio in the oscillator.
>
> Oscillators not in a phase locked loop tend to have rapidly decreasing
> phase noise near the signal, often dropping 20 dB per decade of offset
> until reaching levels lower than -120 dBc/Hz, then they begin to flatten
> off with good oscillators reaching -160 dBc/Hz at MHz plus offsets. The
> wide band phase noise tends to have a flat spectrum from the broad band
> noise of the active devices. This spectrum was first characterized by
> Leeson and so is referred to as a Leeson phase noise spectrum. A few
> really fine oscillators reach -180 dBc/Hz but they are uncommon.
>
> Wide range VCOs as used in some synthesizers can have open loop phase
> noise spectra much like the best oscillators except 40 to 80 dB
> stronger. Often they are controlled with a wide band (up to a MHz)
> control loop that typically gives the PLL a flat spectrum out to the
> control bandwidth, then it slopes down. There is much more power in that
> shape of spectrum than in the Leeson spectrum.
>
> In the perfect world phase noise increases when the signal is multiplied
> by a factor of 20 log10 n where n is the multiplication factor.
> Multiplying a 100 MHz crystal to 10 Ghz where the multiplier is 100,
> increases the phase noise by 40 dB. That's a minimum increase, practical
> circuits increase the phase noise more. Conversely, dividing reduces the
> phase noise almost as much as 10 log10 n where n is the fraction. And
> that is used in the Omni V and VI where the synthesizer runs in the 400
> MHz region and is divided down to 5 MHz.
>
> I claim, and there is beginning to be some evidence though I plan a
> detailed experiment some day, that at frequencies where the outside
> noise is not the limiting factor (UHF and higher) that the reciprocal
> mixing of broad band RF circuit noise by the phase noise sidebands of
> the LO affects the minimum discernible signal. This effect is less
> significant than that of reciprocal mixing if there are any strong
> signals in the RF input and I claim its more significant for narrower
> bandwidths and lower noise figure systems. I have to prove that. In the
> past year since I brought up that theory, W1GHZ has made MDS
> measurements at 10 GHz comparing a crystal LO and a synthesized LO and
> found a 2 dB difference in MDS. Since the test signal for noise figure
> measurements is broad band (generally 10 MHz to 18 GHz or more)
> comparing NF measurements for different levels of phase noise finds no
> difference in NF. There are many articles talking about the MDS effect
> in general terms but none put numbers to the effects. Not even all the
> articles and books about phase noise by Ulrich Rohde put numbers to the
> phenomena.
>
> Reciprocal mixing from phase noise is very serious on the lower HF
> bands, far more than the effects on MDS because those bands (especially
> 40m in Europe) have many more STRONG signals and nearly all receivers
> are limited by antenna noise, not MDS. To add complications those strong
> signals also have phase noise sidebands.
>>
>> When I've had my Corsair II running with either a Paragon or a Omni VI+ 
>> as
>> its AUX RX, the Corsair II was always quieter. Weak signal detection is
>> about the same or too small a difference to hear. Only in close in (2 
>> KHz)
>> weak DX, strong station (multiple S9+ signals) CW pileups does the OMNI 
>> VI+
>> prevail.
>>
>> 73,
>>
>> Gary - AB9M
>>
>>
> I had the opportunity to compare Corsair II, Cubic 103 and Omni V on the
> same weak 10m signals with rapid antenna and radio switching in WA0ROI's
> shack one day and the differences between Corsair II and Omni V were
> barely detectable, less than changes in propagation minute to minute,
> but the Cubic 103 heard better. I don't know how the Cubic does in
> strong signal situations. Its rare and mostly unobtainable anyway. I
> chose the Corsair II because for VHF operations it runs CW on USB on 28
> MHz where the Omni V and VI run CW on LSB, copying a bad ICOM idea. That
> makes it impossible to run mixed mode QSOs on VHF which is common in
> contests.
>
> 73, Jerry, K0CQ
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